A method for reducing edge cracking in the cold rolling process of 1300 mpa square tube steel

Abstract

The application relates to a method for reducing edge cracks in the cold rolling process of 1300Mpa square tube steel, and belongs to the technical field of steelmaking processes. The method comprises the following steps: obtaining a slab with a set chemical composition; heating the slab and then performing high-pressure water descaling; performing rough rolling on the slab after high-pressure water descaling to obtain an intermediate billet; performing finish rolling on the intermediate billet to obtain a semi-finished strip; performing laminar cooling on the semi-finished strip and then coiling; slowly cooling the coiled semi-finished strip; and cold rolling the slowly cooled semi-finished strip to obtain a finished strip. Based on the characteristics of the steel grade composition system of the 1300Mpa square tube steel, each link such as heating, descaling process, rolling process and cooling process is accurately controlled, the edge crack problem of the 1300MPa square tube steel in the cold rolling process is effectively reduced, and the quality and yield of the steel are improved.

Description

Technical Field

[0001] This application relates to the field of steelmaking process technology, and in particular to a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel. Background Technology

[0002] The use of lightweight materials is currently the mainstream technology in automotive lightweighting. While ensuring the safety performance of buses, replacing ordinary products with high-strength square tubing can significantly reduce the overall weight of the bus, representing a future trend in high-end, premium buses.

[0003] The hot rolling process of high-strength square tube steel faces numerous challenges. A key issue affecting product yield is the frequent edge cracking during the cold rolling of 1300 MPa square tube steel, with an incidence rate as high as 90%, more severe at the beginning and end than in the middle, and the deepest edge cracks reaching nearly 40 mm. Edge cracking also easily induces strip breakage during cold rolling, damages the rolls, increases production costs, and prolongs the product certification and promotion cycle.

[0004] Shougang Group has published a patent, "A Method for Eliminating Edge Cracks in Cold-Rolled Strip Steel," to address the edge cracking problem in its DP980 series steel grades. The patent focuses on controlling the side water spray volume during the finishing rolling process to ensure the temperature difference between the center and edge of the strip in the width direction is ≤50℃. The hot rolling process employs a low-temperature coiling control approach to refine the MA island dimensions. Cold rolling of the hot-rolled strip, with a cold rolling reduction rate controlled to ≤60%, can achieve edge crack control. However, field experiments have shown that these process adjustments have no significant effect on suppressing edge cracking in 1300MPa square tube steel during cold rolling, necessitating the development of more refined process methods based on the characteristics of the steel grade's composition. Summary of the Invention

[0005] This application provides a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel, in order to solve the following technical problem: how to reduce the edge cracking phenomenon that occurs in 1300 MPa square tube steel during the cold rolling process.

[0006] This application provides a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel, the method comprising:

[0007] A slab with a specified chemical composition is obtained;

[0008] The slab is heated and then descaled with high-pressure water.

[0009] The slab after high-pressure water descaling is rough rolled to obtain an intermediate slab.

[0010] The intermediate billet is precision rolled to obtain semi-finished strip steel;

[0011] The semi-finished strip steel is subjected to laminar flow cooling and then coiled.

[0012] The coiled semi-finished steel strip is then slowly cooled.

[0013] The semi-finished strip steel after slow cooling is cold rolled to obtain finished strip steel.

[0014] Optionally, the specified chemical composition of the slab, by mass fraction, includes: C: 0.22%–0.26%, Cr: 0.75%–0.85%, Mn: 1.65%–1.75%, Si: 0.35%–0.45%, Nb+Ti: 0.05%–0.07%, where Nb+Ti represents the sum of the contents of Nb and Ti.

[0015] Optionally, the total furnace time for heating is 160 min to 200 min, and the furnace exit temperature is 1230℃ to 1250℃.

[0016] Optionally, the water pressure for the high-pressure water descaling is ≥30 MPa.

[0017] Optionally, the roughing mill adopts a 1+5 rolling mode, the roughing mill R1 uses 1 pass for descaling, the roughing mill R2 uses 1, 3, 4 and 5 passes for descaling, and the thickness of the intermediate billet is 34mm to 40mm.

[0018] Optionally, the entry temperature of the finishing mill is 1050℃~1070℃, the finishing mill's final rolling temperature is 910℃~930℃, and the thickness of the semi-finished strip is 2.5mm~3mm.

[0019] Optionally, the finishing mill equipment includes six finishing mill stands F1 to F6, with the cooling water operation ratio between stands F1 to F3 being 50% to 80%, and the cooling water operation ratio between stands F4 to F6 being 0% to 30%.

[0020] Optionally, the laminar flow cooling water pipe is partially shielded to shut off the laminar flow cooling water at the edges.

[0021] Optionally, the winding process adopts a front-end cooling mode, and the head and tail 100m of the cooling layer is not turned on.

[0022] Optionally, the winding temperature is 730℃~750℃.

[0023] Optionally, before the slow cooling, the temperature of the outer ring of the semi-finished strip is ≥600℃.

[0024] Optionally, the cold rolling reduction rate is 50% to 60%.

[0025] The technical solutions provided in this application have the following advantages compared with the prior art:

[0026] This application provides a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel. The method includes: obtaining a slab with a set chemical composition; heating the slab and then performing high-pressure water descaling; rough rolling the descaled slab to obtain an intermediate slab; finishing rolling the intermediate slab to obtain a semi-finished strip; laminar cooling the semi-finished strip and then coiling it; slow cooling the coiled semi-finished strip; and cold rolling the slow-cooled semi-finished strip to obtain a finished strip. Based on the characteristics of the steel composition system of 1300 MPa square tube steel, by increasing the final rolling temperature, increasing the coiling temperature, and performing slow cooling, the edge microstructure of the produced strip is made of ferrite and pearlite. Simultaneously, combined with process measures to control excessively rapid temperature drop at the edges, the cooling rate between the edges and the center of the strip is not significantly different, avoiding the formation of more hard phase structures due to rapid edge cooling, which could cause edge cracking during cold rolling. This effectively controls the edge cracking defect during cold rolling. Attached Figure Description

[0027] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0028] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a flowchart illustrating a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel, as provided in an embodiment of this application.

[0030] Figure 2 Microstructure of the edge of the hot-rolled coil provided as a comparative example in this application.

[0031] Figure 3 Microstructure of the edge of a hot-rolled coil provided in an embodiment of this application. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0033] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range such as 1, 2, 3, 4, 5, and 6, regardless of the range; in addition, whenever a numerical range is indicated herein, it means including any referenced number (fraction or integer) within the indicated range.

[0034] In this document, terms such as “comprising” mean “including but not limited to”. Relational terms such as “first” and “second” are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. “And / or” describes the relationship between related objects, indicating that there can be three relationships, for example, A and / or B can mean: A exists alone, A and B exist simultaneously, or B exists alone; where A and B can be singular or plural. “At least one” means one or more, “more” means two or more; “at least one,” “at least one of the following,” or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, “at least one of a, b, or c,” or “at least one of a, b, and c,” can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0035] Figure 1 A schematic flowchart illustrating a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel, provided for embodiments of this application;

[0036] like Figure 1 As shown in the embodiment of this application, a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel is provided, comprising:

[0037] S1. Obtain a slab with a set chemical composition;

[0038] In some embodiments, the specified chemical composition of the slab, by mass fraction, includes: C: 0.22%–0.26%, Cr: 0.75%–0.85%, Mn: 1.65%–1.75%, Si: 0.35%–0.45%, Nb+Ti: 0.05%–0.07%, wherein Nb+Ti represents the sum of the contents of Nb and Ti.

[0039] Controlling the chemical composition of the slab to meet standard requirements can reduce edge cracks caused by material problems.

[0040] The positive effects of controlling the C content to 0.22%–0.26% include: ensuring the strength grade of the steel and forming a martensitic structure after continuous annealing and quenching; too low a content results in insufficient strength, while too high a content deteriorates welding and forming properties. For example, the C content can be 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, etc.

[0041] The positive effects of controlling the Cr content to 0.75%–0.85% are as follows: A high Cr content system is used to ensure hardenability during cold rolling. Too low a Cr content results in insufficient hardenability, making it impossible to obtain martensite at slow cooling rates, and a critical cooling rate is required. Too high a Cr content reduces plasticity and increases alloy cost. For example, the Cr content can be 0.75%, 0.77%, 0.81%, 0.83%, 0.85%, etc.

[0042] The positive effects of controlling the Mn content to 1.65%–1.75% are as follows: Mn is a solid solution strengthening element. When heated in the critical region, it dissolves in austenite, which is beneficial for improving the hardenability of austenite and for obtaining martensitic structure and increasing the strength of martensite. For example, the Mn content can be 1.65%, 1.67%, 1.69%, 1.72%, 1.75%, etc.

[0043] The positive effects of controlling the Si content to 0.35%–0.45% include: Si is a solid solution strengthening element that can improve hardenability, facilitate the formation of fine and uniformly distributed martensite, and expand the critical zone. However, a high Si content is detrimental to weldability; conversely, a low Si content results in poor solid solution strengthening and affects hardenability. For example, the Si content can be 0.35%, 0.37%, 0.39%, 0.42%, 0.45%, etc.

[0044] Nb+Ti represents the sum of the contents of Nb and Ti. Controlling the Nb+Ti content to 0.05%–0.07% has the positive effect of adding trace alloying elements for precipitation strengthening. Too low a content reduces strength, while too high a content increases cost and affects plasticity. For example, the Nb+Ti content can be 0.05%, 0.055%, 0.06%, 0.07%, etc.

[0045] S2. The slab is heated and then descaled with high-pressure water.

[0046] Heating is used to bring the slab to a suitable rolling temperature, increasing its plasticity and reducing rolling force. Heating also helps remove moisture and impurities from the slab surface. High-pressure water descaling is performed after heating to remove iron oxide scale and other impurities from the slab surface, preventing them from being forced into the steel during rolling and causing defects.

[0047] In some embodiments, the total furnace time for heating is 160 min to 200 min.

[0048] The positive effect of controlling the total furnace time of the heating to be 160 min to 200 min is that it ensures the complete dissolution of Ti and Nb precipitated on the surface during the continuous casting process. For example, the total furnace time of the heating can be 160 min, 165 min, 170 min, 180 min, 200 min, etc.

[0049] In some embodiments, the furnace exit temperature after heating is 1230°C to 1250°C.

[0050] The positive effects of controlling the furnace exit temperature to 1230℃~1250℃ are as follows: Since Cr+Si>1% in the slab and the Cr / Si ratio is around 2, the enrichment of Cr on the surface can inhibit the high-temperature oxidation of Si to a certain extent, thus a higher furnace exit temperature is adopted. For example, the furnace exit temperature can be 1230℃, 1235℃, 1240℃, 1245℃, 1250℃, etc.

[0051] In some embodiments, the water pressure for the high-pressure water descaling is ≥30 MPa.

[0052] The positive effects of controlling the water pressure for high-pressure water descaling to ≥30 MPa: The steel grade has a high Si content, which can effectively remove the influence of the olivine phase in the furnace pig iron, ensuring the surface quality of the steel plate. For example, the water pressure for high-pressure water descaling can be 30 MPa, 31 MPa, 32 MPa, 33 MPa, 34 MPa, etc.

[0053] S3. The slab after high-pressure water descaling is rough rolled to obtain an intermediate slab;

[0054] Rough rolling is a preliminary plastic deformation of a slab, which reduces its thickness and width, making its shape closer to the final product.

[0055] In some embodiments, the roughing mill adopts a 1+5 rolling mode, the roughing mill R1 uses 1 pass for descaling, the roughing mill R2 uses 1, 3, 4 and 5 passes for descaling, and the thickness of the intermediate billet is 34mm to 40mm.

[0056] In some implementations, a sheet roll box is used after rough rolling to enhance the scale-breaking effect.

[0057] Coil boxes are mainly used in hot strip mills, placed between the roughing and finishing mills. They take the intermediate slabs rolled from the roughing mill, wind them into coils using a coreless coiling method, and then immediately uncoil them in the opposite direction, trimming the ends before sending them to the finishing mill. This allows for simultaneous uncoiling and rolling. This technology solves the problems caused by excessive temperature differences between the ends of the rolled strip due to the long distance between the roughing and finishing mills, resulting in large temperature drops in the intermediate slabs of hot-rolled strip, uneven temperature distribution along the entire length, high load on the finishing mill, and inconsistent mechanical properties and thickness of the finished strip. The coreless coiling method keeps the hotter strip end at the center of the coil box before uncoiling, turning the cooler strip end into a new head that is then sent to the finishing mill for rolling.

[0058] The positive effects of controlling the thickness of the intermediate billet to 34mm-40mm include: increasing rolling deformation during roughing, reducing rolling deformation during finishing, and preventing excessive temperature drop at the edge during finishing. For example, the thickness of the intermediate billet can be 34mm, 35mm, 36mm, 37mm, 40mm, etc.

[0059] S4. The intermediate billet is precision rolled to obtain a semi-finished strip steel;

[0060] Finish rolling is a further rolling process on the intermediate billet to achieve the required dimensions and surface finish. The finished steel is a semi-finished strip, ready for subsequent processing. In the finish rolling process, double-pass descaling is used to ensure the surface quality of the hot-rolled coil.

[0061] In some embodiments, the entry temperature of the finishing mill is 1050℃~1070℃, the finishing mill finishing temperature is 910℃~930℃, and the thickness of the semi-finished strip is 2.5mm~3mm.

[0062] The positive effects of controlling the inlet temperature of the finishing mill to be between 1050℃ and 1070℃ are as follows: Ensuring a higher inlet temperature ensures that the edge temperature remains within the single-phase austenite region during hot rolling. For example, this inlet temperature can be 1050℃, 1055℃, 1060℃, 1065℃, 1070℃, etc.

[0063] The positive effects of controlling the final rolling temperature of finishing rolling to 910℃~930℃ include: ensuring that the edge position during hot rolling is above the phase transformation temperature, thus avoiding issues such as mixed crystal structure induced by excessively low temperatures. For example, the final rolling temperature can be 910℃, 915℃, 920℃, 925℃, 930℃, etc.

[0064] The positive effects of controlling the thickness of the semi-finished strip steel to 2.5mm to 3mm are as follows: By ensuring a thinner thickness during hot rolling, the compression ratio during cold rolling is reduced, thus avoiding severe edge work hardening and cracking during cold rolling. For example, the thickness of the semi-finished strip steel can be 2.5mm, 2.6mm, 2.8mm, 2.9mm, 3mm, etc.

[0065] In some embodiments, the finishing mill equipment includes six finishing mill stands F1 to F6, with the cooling water between stands F1 to F3 operating at 50% to 80% capacity and the cooling water between stands F4 to F6 operating at 0% to 30% capacity.

[0066] The positive effects of having the cooling water in the F1-F3 stands open at 50%-80% are as follows: opening the water in the first stand can reduce surface temperature, increase rolling speed, and avoid problems such as overheating of the rolls. For example, the opening ratio of the cooling water in the F1-F3 stands can be 50%, 60%, 70%, 75%, 80%, etc.

[0067] The positive effects of setting the cooling water opening ratio between stands F4 and F6 to 0%–30% are as follows: Reducing the cooling water opening ratio in the later stages is mainly to avoid situations such as excessive surface temperature drop in the later stages of the finishing rolling process, which could cause the edges to fall into the two-phase region. For example, the cooling water opening ratio between stands F4 and F6 can be 5%, 10%, 20%, 25%, 30%, etc.

[0068] S5. The semi-finished strip steel is subjected to laminar flow cooling and then coiled.

[0069] Laminar flow cooling is used to control the cooling rate and temperature distribution of steel to obtain the desired microstructure and properties. Coiling involves rolling the cooled semi-finished steel strip into coils for easy storage and transportation.

[0070] In some embodiments, the laminar cooling water pipes are edge-shielded to shut off the edge laminar cooling water.

[0071] During the strip cooling process, due to differences in thickness and heat dissipation conditions between the edge and center, the edge is prone to overcooling, which is a significant factor leading to crack formation. Edge shielding can effectively reduce the cooling water flow in the edge area, thereby lowering the edge temperature and preventing excessive temperature differences with the center. When implementing edge shielding, the scope and degree of shielding need to be precisely controlled to avoid adversely affecting the cooling effect on the center. When the strip width is 1200mm, edge shielding can be used to shut off the cooling water within 40mm of the edge to increase the strip edge temperature before coiling.

[0072] In some implementations, the winding process employs a front-end cooling mode, where the head and tail sections are not cooled by a layer of cold water.

[0073] Because the head and tail sections are susceptible to various factors during rolling, the final rolling temperature differs from that of the middle section. By not using the laminar flow cooling system at the head and tail sections, excessive cooling in these areas can prevent a rapid temperature drop. Overcooled head and tail sections are prone to thermal stress concentration during coiling, increasing the risk of cracking. By not using the laminar flow cooling system, the cooling rate in these areas can be reduced, minimizing thermal stress concentration and thus preventing crack formation.

[0074] In some embodiments, the winding temperature is 730°C to 750°C.

[0075] The positive effects of controlling the winding temperature to 730℃~750℃ are: using a higher winding temperature can avoid the formation of bainite or martensite at the edges, which can affect the microstructure and cause cracking during the cold rolling process. For example, the winding temperature can be 730℃, 735℃, 740℃, 745℃, 750℃, etc.

[0076] S6. Slowly cool the coiled semi-finished strip steel.

[0077] After the strip steel comes off the production line, it is bundled to prevent the ends from loosening and then sent to a slow cooling pit for slow cooling.

[0078] A slow cooling pit is an important piece of equipment used to reduce residual thermal stress and improve the mechanical properties of metal parts. During cold rolling, the steel is still at a relatively high temperature after final rolling and coiling. If it is directly and rapidly cooled at this point, it may generate significant thermal and structural stresses within the steel, leading to cracks. A slow cooling pit, however, provides a relatively slow and uniform cooling environment, allowing the steel to fully release internal stresses during cooling and preventing crack formation.

[0079] In some embodiments, the temperature of the outer ring of the semi-finished strip is ≥600°C before the slow cooling.

[0080] The positive effects of controlling the outer ring temperature of the semi-finished strip steel to ≥600℃ are: avoiding the formation of hardened structures, and temperatures above 600℃ are conducive to the formation of ferrite + pearlite structures. For example, the outer ring temperature can be 600℃, 610℃, 620℃, 625℃, 630℃, etc.

[0081] S7. The semi-finished strip steel after slow cooling is cold rolled to obtain finished strip steel;

[0082] Cold rolling is a rolling process carried out at or below room temperature, which can further reduce the thickness of steel and improve its surface finish and dimensional accuracy. The steel that has undergone cold rolling is the finished strip steel, which meets specific mechanical properties and dimensional requirements.

[0083] In some embodiments, the cold rolling reduction rate is 50% to 60%.

[0084] The positive effects of controlling the cold rolling reduction rate to 50%–60%: The cold rolling reduction rate is one of the important factors affecting edge cracking. Controlling the cold rolling reduction rate within a reasonable range can effectively reduce the occurrence of edge cracking. Excessively high reduction rates increase the internal stress and deformation resistance of the material, thus leading to edge cracking. For example, the reduction rate can be 50%, 52%, 54%, 58%, 60%, etc.

[0085] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards; if no corresponding industry standard exists, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0086] The following will describe in detail a method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel provided in this application, in conjunction with embodiments, comparative examples, and experimental data. The method includes:

[0087] S11. Obtain a slab with a set chemical composition;

[0088] S21. The slab is heated and then descaled with high-pressure water.

[0089] S31. The slab after high-pressure water descaling is rough rolled to obtain an intermediate slab;

[0090] S41. The intermediate billet is precision rolled to obtain a semi-finished strip steel;

[0091] S51. The semi-finished strip steel is subjected to laminar flow cooling and then coiled.

[0092] S61. Slowly cool the coiled semi-finished strip steel.

[0093] S71. The semi-finished strip steel after slow cooling is cold rolled to obtain finished strip steel. The process parameters of each embodiment and comparative example are listed in Table 1.

[0094] Table 1. Process parameters for reducing edge cracking during cold rolling of 1300 MPa square tube steel.

[0095]

[0096] As can be seen from the data in Table 1, the process parameters of the embodiment are within the required range of the present invention, and the microstructure of the edge of the hot roll is of the ferrite + pearlite type, with a microhardness of less than 300 HV.

[0097] Appendix Figure 2-3 Detailed explanation:

[0098] Figure 2 Microstructure of the edge of the hot-rolled coil provided as a comparative example in this application; such as Figure 2 As shown, the microstructure of the edge of the hot-rolled coil provided in the comparative example is bainite.

[0099] Figure 3 Microstructure of the edge of the hot-rolled coil provided in the embodiments of this application; such as Figure 3 As shown, the edge microstructure of the hot-rolled coil provided in the embodiment is ferrite + pearlite.

[0100] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0101] 1. Improve product quality: Edge cracking is one of the main problems affecting the surface quality of cold-rolled steel. By solving the edge cracking problem, the surface smoothness and consistency of square tubes can be significantly improved, reducing the generation of defects and substandard products.

[0102] 2. Improve production efficiency: Edge cracking often leads to production line interruptions, increasing scrap rates and repair costs. By resolving edge cracking, downtime and repetitive work caused by product defects can be reduced, thereby improving overall production efficiency.

[0103] 3. Reduce production costs: Solving the edge cracking problem not only reduces the scrap rate and downtime, but also reduces the raw material and labor costs incurred due to scrap disposal and repeated production.

[0104] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for reducing edge cracking during the cold rolling process of 1300 MPa square tube steel, characterized in that, The method includes: A slab with a specified chemical composition is obtained; The slab is heated and then descaled with high-pressure water. The slab after high-pressure water descaling is rough rolled to obtain an intermediate slab. The intermediate billet is precision rolled to obtain semi-finished strip steel; The semi-finished strip steel is subjected to laminar flow cooling and then coiled. The coiled semi-finished steel strip is then slowly cooled. The semi-finished steel strip after slow cooling is cold rolled to obtain finished steel strip; The specified chemical composition of the slab, by mass fraction, includes: C: 0.22%–0.26%, Cr: 0.75%–0.85%, Mn: 1.65%–1.75%, Si: 0.35%–0.45%, Nb+Ti: 0.05%–0.07%, where Nb+Ti represents the sum of the contents of Nb and Ti; The total furnace time for heating is 160 min to 200 min, and the furnace exit temperature is 1230℃ to 1250℃. The water pressure for high-pressure water descaling is ≥30 MPa; The roughing mill adopts a 1+5 rolling mode. The roughing mill R1 uses 1 pass for descaling, and the roughing mill R2 uses 1, 3, 4 and 5 passes for descaling. The thickness of the intermediate billet is 34mm to 40mm. The entry temperature of the finishing mill is 1050℃~1070℃, the finishing mill finishing temperature is 910℃~930℃, and the thickness of the semi-finished strip is 2.5mm~3mm. The finishing mill equipment contains 6 finishing mill stands F1 to F6. The cooling water opening ratio between stands F1 to F3 is 50% to 80%, and the cooling water opening ratio between stands F4 to F6 is 0% to 30%.

2. The method according to claim 1, characterized in that, The laminar flow cooling water pipes are partially shielded to shut off the laminar flow cooling water at the edges.

3. The method according to claim 1, characterized in that, The winding process employs a front-end cooling mode, with the head and tail sections not using chilled water; and / or, The winding temperature is 730℃~750℃; and / or, Before the slow cooling process, the temperature of the outer ring of the semi-finished strip steel is ≥600℃.

4. The method according to claim 1, characterized in that, The reduction rate of the cold rolling is 50% to 60%.

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