Steel having high surface quality and ultrahigh hole expansion ratio, and manufacturing method therefor

Abstract

A steel having a high surface quality and an ultrahigh hole expansion ratio, and a manufacturing method therefor. The steel comprises the following chemical components in percentage by weight: C: 0.020-0.080%; Si≤0.10%; Mn: 0.40-0.80%; Al: 0.015-0.050%; Ti: 0.030-0.080%; N≤0.0070%; at least one of Nb: 0.001-0.050%, V: 0.002-0.060%, Cr: 0.002-0.600%, Ca: 0.0005-0.0050%, RE: 0.0005-0.0050% and B: 0.0002-0.0030%; and the balance being Fe and other inevitable impurities. The components are required to satisfy: Tieff>0.015%, and Tieff＝Ti-3.42×N-3×S. In the present invention, a low carbon-low silicon-low manganese component design is utilized, thereby increasing the hole expansion ratio of a steel plate and ensuring a high surface quality while ensuring the strength thereof; and the steel having a high surface hole expansion ratio has a yield strength Rel greater than or equal to 305 MPa, a tensile strength Rm greater than or equal to 440 MPa, an elongation A greater than or equal to 34% and a hole expansion ratio λ greater than or equal to 110%.

Description

A high surface quality ultra-high hole expansion steel and its manufacturing method Technical Field

[0001] The invention belongs to the field of hole-expanding steel, and in particular relates to high-surface-quality ultrahigh hole-expanding steel and a manufacturing method thereof. Background Art

[0002] In recent years, with the rapid development of the automotive industry, the market and users have also put forward higher requirements for the performance of automobiles. This requires the use of better steel plates in the manufacture of automobiles, such as hot-rolled plates and hot-rolled pickled plates, which have been widely used in the manufacture of chassis and wheel structural parts of passenger cars, and account for a quarter of the steel used in automobiles.

[0003] In the actual production of passenger car parts, steel often requires hole expansion and partial flanging to achieve the high strength, thinness, and lightweighting goals required by the automotive industry. Research has found that the hole expansion performance of steel plates is related to the steel's composition, strength, and microstructure uniformity. Traditional 440MPa steel grades, mainly carbon-manganese solid solution-strengthened steels and low-alloy precipitation-strengthened steels, have hole expansion rates between 50% and 70%.

[0004] As the requirements of automobile design for chassis structure are increasing, the preparation of current automobile parts forming is more complicated, and automobile manufacturers' requirements for the flanging and hole expansion performance of steel plates are also constantly increasing. The existing traditional carbon-manganese solid solution strengthened steel and low alloy precipitation strengthened steel structural steel plates can no longer meet the current automobile chassis and cantilever parts forming requirements. High hole expansion steel has become one of the important varieties of automobile steel plates.

[0005] At present, in response to the demand for high-hole expansion steel in the existing automotive field, many researchers have begun to research and develop high-hole expansion steel plates.

[0006] Chinese patent application CN103667880A discloses a "440 MPa tensile strength high-hole expansion steel plate and its manufacturing method." Its chemical composition by weight is as follows: C: 0.05-0.1%, Si: 0.1-0.6%, Mn: 0.9-1.8%, P ≤ 0.02%, S ≤ 0.005%, Al: 0.015-0.060%, Ca < 0.0050%, with the remainder being Fe and unavoidable impurities. ([C] × [Mn]) ≤ 0.1; ([P] + 10 [S]) ≤ 0.04; and the product of the steel plate's strength and hole expansion ratio, TSxλ, ≥ 44,000 MPa. However, this technical solution adds a large amount of Si to the chemical element composition, resulting in poor surface quality of the steel plate, failing to meet the high performance requirements of users.

[0007] Japanese patent application JP2006063394A discloses a hot-rolled high hole expansion steel, which has C: 0.20-0.48%, Si: less than 0.1%, Mn: 0.20-0.60%, P: less than 0.02%, S: less than 0.01%, Al: less than 0.1%, N: less than 0.005%, B: 0.001-0.005%, Cr: 0.05-0.3%, and a tensile strength of ≥440MPa, but its hole expansion rate is only ≥70%, and annealing treatment at 640°C is required after hot rolling.

[0008] It can be seen that the 440MPa grade high hole expansion steel plate currently developed and designed with existing technology still has the problems of insufficient hole expansion rate and poor surface quality. Summary of the Invention

[0009] The present invention aims to provide high-surface-quality, ultra-high-hole-expansion steel and a method for manufacturing the same. The resulting steel exhibits both high surface quality and a high hole-expansion ratio. The steel exhibits a yield strength Rel ≥ 305 MPa, a tensile strength Rm ≥ 440 MPa, an elongation A ≥ 34%, and a hole-expansion ratio λ ≥ 110%. These steels meet the performance requirements of applications such as automotive chassis and complex stamping parts, demonstrating promising prospects for widespread application. In this context, high surface quality refers to the absence of red iron oxide on the surface.

[0010] To achieve the above purpose, unlike the existing high hole expansion design which mainly adopts the addition of Si and high Mn, the technical solution of the present invention adopts a low-carbon component design to achieve an increase in hole expansion rate, avoids poor coating caused by red iron sheet through low Si design, and effectively guarantees strength by combining low manganese with micro-alloy elements.

[0011] Specifically, the high surface quality ultra-high hole expansion steel of the present invention has the following chemical composition weight percentages: C: 0.020-0.080%, Si≤0.100%, Mn: 0.40-0.80%, Al: 0.015-0.050%, Ti: 0.030-0.080%, N≤0.007%; and at least one of Nb: 0.001-0.050%, V: 0.002-0.06%, Cr: 0.002-0.6%, Ca: 0.0005-0.0050%, RE: 0.0005-0.0050%, B: 0.0002-0.0030%, and the rest includes Fe and other unavoidable impurities, and it is necessary to meet Ti at the same time. eff >0.015%, Ti eff =Ti-3.42×N-3×S.

[0012] In some embodiments, Ti eff In the range of 0.017~0.056%.

[0013] Preferably, N≤0.0065%.

[0014] Furthermore, the balance is Fe and other inevitable impurities.

[0015] Preferably, the inevitable impurities include P≤0.025%, S≤0.0060%, and preferably S≤0.0030%.

[0016] The microstructure of the high surface quality ultra-high hole expansion steel of the present invention is ferrite + bainite, wherein the volume ratio of ferrite is ≥ 90%. In some embodiments, the volume ratio of ferrite is 90-96%.

[0017] The high surface quality ultra-high hole expansion steel of the present invention has a yield strength Rel≥305MPa, a tensile strength Rm≥440MPa, an elongation A≥34%, and a hole expansion ratio λ≥110%.

[0018] In some embodiments, the yield strength Rel of the hole-expanding steel is ≥340 MPa. In some embodiments, the yield strength Rel of the hole-expanding steel is ≥360 MPa. In some embodiments, the yield strength Rel of the hole-expanding steel is 305-400 MPa.

[0019] In some embodiments, the tensile strength Rm of the hole-expanding steel is ≥ 460 MPa. In some embodiments, the tensile strength Rm of the hole-expanding steel is 440-505 MPa.

[0020] In some embodiments, the hole expansion ratio λ of the hole expansion steel is ≥ 115%. In some embodiments, the hole expansion ratio λ of the hole expansion steel is ≥ 120%. In some embodiments, the hole expansion ratio λ of the hole expansion steel is 110-145%.

[0021] In the composition design of the high surface area and ultra-high hole expansion steel of the present invention:

[0022] C: In the high-surface-quality, ultra-high hole-expanding steel described herein, adding an appropriate amount of C forms sufficient carbide-reinforcing phases to ensure the steel's strength level. If the C content is too low, the steel's strength will not meet the required level. However, if the C content is too high, the resulting carbide particles will be large, adversely affecting the steel's hole-expanding performance. Furthermore, if the carbon and manganese content is too high, coarse pearlite and pearlite banding can occur, which can affect the steel's performance. Therefore, the present invention controls the C content to between 0.020% and 0.080%.

[0023] Si: In the high-surface-quality, ultra-high-hole-expansion steel described herein, Si acts as a solid-solution strengthening agent in carbon-manganese steel. However, excessive Si content in the steel should not be too high. Excessive Si content can easily cause surface defects such as red iron scale on the steel plate, and the surface quality may not meet the high demands of users. Therefore, the present invention controls the Si content to ≤ 0.10%. In some embodiments, the Si content is 0.030 to 0.090%.

[0024] Mn: In the high-surface-quality, ultra-high hole-expanding steel described herein, Mn is a solid-solution strengthening element. When the Mn content in the steel is less than 0.40%, the steel will suffer from insufficient strength. Similarly, the Mn content should not be too high. Excessive Mn addition increases the pearlite phase ratio and causes manganese segregation in the slab, which is one of the main causes of banded structure in the steel plate. This banded structure degrades the hole-expanding performance of the steel plate, and the alloying cost of Mn is also high. Therefore, to ensure excellent hole-expanding performance, the present invention controls the Mn content in the steel to 0.40-0.80%.

[0025] Al: In the high-surface-quality, ultra-high-hole-expansion steel described herein, Al serves as a deoxidizing element. Adding an appropriate amount of Al effectively reduces oxide inclusions and purifies the steel. Therefore, the Al content is controlled within a range of 0.015% to 0.050%.

[0026] Ti: In the high-surface-quality, ultra-high hole-expansion steel described herein, Ti has a strong affinity for nitrogen, oxygen, and carbon. Ti is an excellent deoxidizer and degasser, and an effective element for fixing nitrogen and carbon. When present as a solid solution in ferrite, Ti effectively enhances ferrite strength. Therefore, the Ti content is controlled within the range of 0.030% to 0.080%.

[0027] It should be noted that in the high surface quality and ultra-high hole expansion steel designed in the present invention, in addition to the aforementioned C, Si, Mn, and Al, at least one element from among Nb, V, Cr, Ca, RE, and B may be further added to the steel according to specific requirements to improve the steel's performance. In some embodiments, at least three, or at least four, of the following elements may be further added to the steel: Nb, V, Cr, Ca, RE, and B.

[0028] Nb: In the high-surface-quality, ultra-high hole-expansion steel described herein, Nb primarily strengthens the steel through grain refinement and dispersion strengthening. Niobium reacts with carbon and nitrogen in the steel to form stable carbides and carbonitrides, and it also disperses the carbides, resulting in a steel with refined grains. Therefore, the Nb content is controlled within a range of 0.001% to 0.050%.

[0029] V: In the high-surface-quality, ultra-high-hole-expansion steel described herein, the addition of V effectively refines the steel's microstructure and grain size, raising the grain coarsening temperature, thereby reducing the steel's overheating sensitivity and effectively improving its strength and toughness. Therefore, the V content is controlled within a range of 0.002% to 0.060%.

[0030] Cr: In the high surface quality ultra-high hole expansion steel of the present invention, chromium can form a continuous solid solution with iron and improve the strength and hardness of the steel. Therefore, the present invention controls the Cr element content to 0.002-0.6%.

[0031] Ca: In the high-surface-quality, ultra-high-hole-expansion steel described herein, Ca modifies the morphology of sulfides and improves the plasticity and toughness of the steel plate. Therefore, the molten steel designed in the present invention can be treated with calcium to control the Ca content to 0.0005% to 0.0050%.

[0032] RE: In the high-surface-quality, ultra-high-hole-expansion steel described herein, rare earth elements (RE) can be added to the steel to form spherical rare earth sulfides or oxysulfides, replacing the elongated manganese sulfide inclusions. This allows for complete control of sulfide morphology and improves the steel's toughness and ductility. Therefore, the RE element content is controlled within a range of 0.0005-0.005%.

[0033] B: In the high-surface-quality, ultra-high-hole-expansion steel described in the present invention, adding an appropriate amount of element B can effectively increase the hardenability of the steel and inhibit austenite recrystallization. Therefore, the present invention controls the B content to between 0.0002% and 0.0030%.

[0034] Furthermore, in the high surface quality ultrahigh hole expansion steel of the present invention, among the inevitable impurities, P≤0.025%, S≤0.0060%; preferably, S≤0.0030%.

[0035] In the above technical solution, P and S are both impurity elements in steel. If technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the material should be reduced as much as possible.

[0036] P: In the present invention, the P element will aggravate the segregation of slab components during the continuous casting process, resulting in uneven structure. Therefore, the P element content is controlled to ≤ 0.025%.

[0037] S: In the present invention, sulfur is an impurity element in steel. S easily forms MnS in steel. The amount and morphology of sulfides in steel directly affect the hole expansion rate of the steel plate. Furthermore, the amount and morphology of inclusions significantly impact the hole expansion performance of the steel plate. In particular, stripe-shaped sulfide inclusions can easily cause cracking during deformation. Therefore, the S content in the present invention must be ≤ 0.0060%, and preferably, ≤ 0.0030%.

[0038] Nitrogen: Nitrogen has a similar impact on steel properties as carbon and phosphorus. As nitrogen content increases, it significantly increases steel strength, but also significantly reduces plasticity, especially toughness, worsens weldability, and exacerbates cold brittleness. It also increases aging tendency, cold brittleness, and hot brittleness, impairing the steel's weldability, cold bending properties, and hole expansion performance. Therefore, the present invention requires that the nitrogen content be ≤ 0.0070%, and preferably, ≤ 0.0065%.

[0039] Ti eff >0.015%, Ti eff =Ti-3.42×N-3×S. This formula is based on the atomic ratio of elements that form compounds of Ti and N, and the atomic ratio of elements that form compounds of Ti and S, to control Ti eff >0.015%, the Ti content is required to form an effective precipitation strengthening effect. It ensures that the strength and hole expansion rate are improved based on the low carbon composition design.

[0040] The method for manufacturing high surface quality ultra-high hole expansion steel of the present invention comprises the following steps:

[0041] 1) Smelting and casting

[0042] Smelt and refine according to the above composition, control the S content to ≤ 0.0060%, and cast into billets;

[0043] 2) heating;

[0044] 3) Hot rolling

[0045] The rough rolling temperature is 990-1100℃; the finishing rolling temperature is 820-900℃; then laminar cooling is carried out, using a two-stage cooling method: the first stage cooling rate is 6-20℃ / s, the cooling time is 3-8s, the second stage is 60-120℃ / s, the cooling time is 2-4s, and then coiling is carried out;

[0046] 4) Pickling

[0047] Cool naturally to below 50℃ and pickle.

[0048] Preferably, in step 1), the S content is controlled to be ≤ 0.0030%.

[0049] Preferably, in step 2), the heating temperature is 1100-1260°C, and the heating time in the furnace is 160-250 minutes.

[0050] Preferably, in step 3), the coiling temperature is 500-580°C.

[0051] In the manufacturing method of the present invention:

[0052] During the smelting process, the steel composition must meet the chemical composition design requirements of the present invention. Deep desulfurization treatment in a conventional LF furnace can be used to control the sulfur content of the molten steel to ≤ 0.0060%, preferably ≤ 0.0030%. After smelting, the molten steel can be continuously cast to produce slabs.

[0053] In step 4), the steel coil obtained by hot rolling can be naturally cooled to below 50° C., and then the steel plate is removed from the iron oxide scale in a hot hydrochloric acid solution by a pickling unit; and after the pickling is completed, the steel plate obtained by pickling can be further rinsed, squeezed, dried and then oiled.

[0054] In step 2), the heating temperature is controlled to be 1100-1260° C. and the heating time in the furnace is 160-250 min to achieve austenitization of the slab and effective solid solution of the alloy elements.

[0055] In step 3), laminar cooling adopts two-stage cooling, i.e., two-stage cooling rate control:

[0056] The first stage cooling rate is 6-20°C / s, and the cooling time is 3-8s. The first stage low cooling rate realizes ferrite structure transformation and precipitation strengthening of Ti element, and controls the volume content of ferrite to be above 90%.

[0057] The cooling rate in the second stage is 60-120°C / s, and the cooling time is 2-4s. At the high cooling rate, the remaining austenite is converted into bainite, avoiding the formation of structures such as pearlite that degrade the hole expansion rate.

[0058] In step 3), the coiling temperature is controlled to be 500-580° C., and the residual austenite is controlled to achieve bainite transformation.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] Different from the conventional high hole expansion steel which mainly adopts the design of adding Si and high Mn, the present invention adopts low carbon composition design in composition design to achieve the improvement of hole expansion rate, and at the same time combines with the addition of Ti element and controls Ti eff>0.015%, achieving effective precipitation strengthening, thereby avoiding the reduction in mechanical properties associated with reduced carbon content. Low Si design prevents poor coating due to red iron scale, while low manganese combined with microalloying elements effectively ensures strength. This results in a steel with excellent mechanical properties, a high hole expansion rate, and a high surface quality. Its yield strength Rel ≥ 305 MPa, tensile strength Rm ≥ 440 MPa, elongation A ≥ 34%, and hole expansion rate λ ≥ 110% meet the performance requirements of steel for applications such as automotive chassis and complex stamping parts, and has excellent prospects for widespread application.

[0061] Based on the composition design, the present invention widens the hot rolling process window and combines the control of the subsequent two-stage laminar cooling to make the microstructure of the steel more than 90% ferrite and the rest less than 10% second phase bainite, thereby ensuring that the prepared steel has high mechanical properties and high hole expansion performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG1 is a metallographic structure photograph of the steel of Example 1 of the present invention observed under an optical microscope after being corroded by nitric acid. DETAILED DESCRIPTION

[0063] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0064] The composition of the steel of the embodiment of the present invention is shown in Table 1. Table 2 shows the production process parameters of the steel of the embodiment of the present invention. Table 3 shows the mechanical properties of the steel plate of the embodiment of the present invention.

[0065] The steel samples of the embodiments of the present invention and the comparative examples were respectively sampled and their surface quality was observed. It was found that the surface of the embodiment had no defects such as red iron oxide that affect coating. The microstructure was measured, and the yield strength, tensile strength, elongation and hole expansion rate of the steel plates were detected. The relevant observation and measurement results are detailed in Table 3.

[0066] Tensile test: At room temperature (20°C), the strain rate during yielding was controlled to be between 0.00025 and 0.0025 / s. The test was conducted in accordance with the conditions of GB / T 228. The yield strength Rel, tensile strength Rm, and elongation A of the steel plates of Examples 1-9 and Comparative Examples 1-3 were measured.

[0067] Hole expansion rate test: Take 150×150mm steel plates of each embodiment and comparative example sample and use it as the initial hole diameter d[1] to punch a hole with a diameter of 10mm. Use a conical punch with a vertex angle of 60° to expand the hole. At the same time, measure the hole diameter d[2] when the crack generated in the punched part penetrates the plate thickness. Combine the above hole diameters d[1] and d[2] to calculate the hole expansion rate by the following formula:

[0068] Hole expansion ratio (λ) = [(d[2] - d[1]) / d[1]] × 100%.

[0069] FIG1 shows a metallographic structure photograph of the high surface area and high hole expansion steel of Example 1 observed under an optical microscope after being corroded by nitric acid alcohol.

[0070] As shown in FIG1 , the microstructure of the high surface area and high hole expansion steel prepared in Example 1 is ferrite+bainite, and the volume phase ratio of ferrite is ≥90%.

[0071] As can be seen from Table 3, the comprehensive performance of the high-surface, high-hole expansion steels of Examples 1-9 of the present invention is significantly superior to the comparative steels of Comparative Examples 1-3. The surfaces of the steel plates of the present invention are free of red iron oxide and other coating defects that could affect the finish, resulting in high surface quality and no poor finish due to red iron scale. Furthermore, their microstructures are all composed of ferrite + bainite, with a ferrite volume fraction of ≥90%. In contrast, the ferrite volume fractions of Comparative Examples 1-3 do not meet the hole expansion ratio requirements in Comparative Example 1 and Comparative Example 3; Comparative Example 2 exhibits poor surface quality and a microstructure that does not meet design requirements, consisting solely of ferrite.

[0072] In the present invention, the high surface quality ultra-high hole expansion steel of Examples 1-9 has a yield strength Rel≥305MPa, a tensile strength Rm≥440MPa, an elongation A≥34%, and a hole expansion rate λ≥110%. It not only has good strength and plasticity, but also has a high hole expansion rate and excellent surface quality.

[0073] In the chemical composition design of Comparative Example 1, the element content is not within the required range of the present invention, and the C element content is too high, and the final microstructure obtained is ferrite + bainite. Although the yield strength and tensile strength of the comparative steel plate prepared therefrom can meet the design requirements, its hole expansion rate does not meet the requirements.

[0074] In the chemical composition design of Comparative Example 2, the element content is not within the required range of the present invention, and the Si content is relatively high. The final microstructure obtained is ferrite. Although the yield strength, tensile strength, and hole expansion rate of the comparative steel plate prepared therefrom can meet the design requirements, the excessively high Si content in the steel easily leads to red iron scale, and high surface requirements cannot be achieved.

[0075] In the chemical composition design of Comparative Example 3, the element content is not within the range required by the present invention, and the C element content is relatively high. The final microstructure obtained is ferrite + bainite, but the elongation and hole expansion rate of the comparative steel plate prepared therefrom are relatively low, and do not meet the design requirements of the present invention.

[0076] Table 1 (Unit: weight percentage)

[0077] Table 2

[0078] Table 3

Claims

1. A high surface quality ultra-high hole expansion steel, the chemical composition weight percentage of which is: C: 0.020-0.080%, Si≤0.10%, Mn: 0.40-0.80%, Al: 0.015-0.050%, Ti: 0.030-0.080%, N≤0.0070%; and at least one of Nb: 0.001-0.050%, V: 0.002-0.060%, Cr: 0.002-0.600%, Ca: 0.0005-0.0050%, RE: 0.0005-0.0050%, B: 0.0002-0.0030%, the rest includes Fe and other inevitable impurities, and needs to meet the following requirements at the same time: Ti eff >0.015%, Ti eff =Ti-3.42×N-3×S.

2. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: N≤0.0065％。 3. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: The Si content is 0.03 to 0.09%.

4. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: The hole expansion steel contains at least three of Nb: 0.001-0.050%, V: 0.002-0.060%, Cr: 0.002-0.600%, Ca: 0.0005-0.0050%, RE: 0.0005-0.0050%, and B: 0.0002-0.0030%.

5. The high surface quality ultra-high hole expansion steel according to claim 4, characterized in that: The hole expansion steel contains at least four of Nb: 0.001-0.050%, V: 0.002-0.060%, Cr: 0.002-0.600%, Ca: 0.0005-0.0050%, RE: 0.0005-0.0050%, and B: 0.0002-0.0030%.

6. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

7. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: Among the unavoidable impurities, P≤0.025%, S≤0.0060%, preferably S≤0.0030%.

8. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: The microstructure of the hole-expanding steel is ferrite+bainite, wherein the volume proportion of ferrite is ≥90%.

9. The high surface quality ultra-high hole expansion steel according to claim 1, characterized in that: The yield strength Rel of the hole-expanding steel is ≥305 MPa, the tensile strength Rm is ≥440 MPa, the elongation A is ≥34%, and the hole-expanding ratio λ is ≥110%.

10. The high surface quality ultra-high hole expansion steel according to claim 9, characterized in that: The yield strength Rel of the hole-expanding steel is ≥340 MPa, the tensile strength Rm is ≥460 MPa, and the hole-expanding ratio λ is ≥115%.

11. The high surface quality ultra-high hole expansion steel according to claim 9, characterized in that: The yield strength Rel of the hole-expanding steel is 305-400 MPa, the tensile strength Rm is 440-505 MPa, and the hole-expanding ratio λ is 110-145%.

12. The method for manufacturing high surface quality ultra-high hole expansion steel according to any one of claims 1 to 11, characterized in that: The steps include: 1) Smelting and casting Smelting and refining the components according to any one of claims 1 to 7, controlling the S content to ≤ 0.0060%, and casting into billets; 2) heating; 3) Hot rolling Rough rolling temperature is 990~1100℃; finishing rolling temperature is 820~900℃; then laminar cooling is carried out, using two-stage cooling: the first stage cooling rate is 6~20℃ / s, cooling time is 3~8s, the second stage is 60~120℃ / s, cooling time is 2~4s, and then coiling; 4) Pickling Cool naturally to below 50℃ and pickle.

13. The manufacturing method according to claim 12, characterized in that: In step 1), the S content is controlled to be ≤ 0.0030%.

14. The manufacturing method according to claim 12, characterized in that: In step 2), the heating temperature is 1100-1260° C., and the heating time in the furnace is 160-250 min.

15. The manufacturing method according to claim 12, characterized in that: In step 3), the coiling temperature is 500-580°C.

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