Ultra-thin-gauge high-strength steel and method for producing ultra-thin-gauge high-strength steel on basis of discontinuous quenching line

Abstract

Disclosed in the present application are an ultra-thin-gauge high-strength steel and a method for producing an ultra-thin-gauge high-strength steel on the basis of a discontinuous quenching line. The method comprises: providing molten steel, and subjecting the molten steel to converter smelting, external refining and Si-Ca treatment during the external refining, so as to obtain molten steel of high-strength steel, wherein a soft blowing treatment needs to be performed on the molten steel during the Si-Ca treatment, and the Ca / S ratio of the molten steel after the Si-Ca treatment is 1.0-3.0; continuously casting the molten steel of high-strength steel to obtain a continuously cast slab; heating the continuously cast slab to obtain a first heated slab; subjecting the first heated slab to rough rolling and finish rolling to obtain finish-rolled steel, wherein the rough rolling temperature is 1060°C to 1200°C, and the finishing temperature of the finish rolling is 860°C to 900°C; subjecting the finish-rolled steel to laminar cooling to obtain cooled steel, wherein the cooling rate is 40°C / s to 80°C / s; subjecting the cooled steel to pickling and cold rolling to obtain cold-rolled steel; subjecting the cold-rolled steel to levelling and quenching to obtain quenched steel; and subjecting the quenched steel to straightening and tempering to obtain ultra-thin-gauge high-strength steel having a thickness of less than or equal to 3 mm and a yield strength of greater than or equal to 960 MPa.

Description

Ultra-thin high-strength steel and a method for producing ultra-thin high-strength steel based on a discontinuous quenching line Technical Field

[0001] This application belongs to the field of steelmaking technology, and in particular relates to an ultra-thin high-strength steel and a method for producing ultra-thin high-strength steel based on a discontinuous quenching line. Background Technology

[0002] As construction machinery develops towards high-end applications, lightweighting and high-strength thinning of the entire vehicle are attracting increasing attention from industry professionals. The previously used 700MPa grade high-strength steel has been partially upgraded to 960MPa grade. Among these, ultra-thin high-strength steel with a thickness ≤3mm and a yield strength ≥960MPa is mainly used in the final boom sections of small cranes, aerial work platforms, and truck-mounted cranes, playing a crucial role in the high-end and customized development of construction machinery.

[0003] High-strength steel products with a thickness ≤3mm and a yield strength ≥960MPa are generally produced using cold rolling or continuous quenching lines. Cold rolling results in a long production cycle, high product strength but also high springback, and poor assembly precision during the forming process. Therefore, continuous quenching lines are the optimal choice, as the strip is uniformly stressed under the tension rollers, resulting in good shape control. However, continuous quenching lines can only produce quenched plates with a thickness ≤6mm, while the annual output of ultra-thin plates with a thickness ≤3mm is relatively small, making the economic benefits of investing hundreds of millions of yuan in building such a production line low. Using existing discontinuous quenching lines to produce ultra-thin high-strength steel means the quenching rollers can no longer constrain the steel plate, exceeding the line's design limits. This results in poor shape for ultra-thin high-strength steel, hindering the development of this type of product. Summary of the Invention

[0004] This application provides an ultra-thin high-strength steel and a method for producing ultra-thin high-strength steel based on a discontinuous quenching line. It can produce high-strength steel products with a thickness ≤3mm and a yield strength ≥960MPa that meet the requirements through a simple preparation process based on a discontinuous quenching line.

[0005] In a first aspect, this application provides a method for producing ultra-thin high-strength steel based on a discontinuous quenching line, comprising:

[0006] Molten steel is provided and subjected to converter smelting, ladle refining, and Si-Ca treatment during the ladle refining process. The Si-Ca treatment requires soft blowing of the molten steel. After Si-Ca treatment, the Ca / S ratio in the molten steel is 1.0–3.0, resulting in high-strength steel. The high-strength steel is composed of the following chemical components by mass percentage: C 0.16 wt.%–0.21 wt.%, Si < 0.10 wt.%, Mn 1.60 wt.%–1.90 wt.%, and P ≤ 0.015 wt.%. The composition of the chemical components is as follows: S ≤ 0.004 wt.%, Cr 0.20 wt.%~0.50 wt.%, Nb 0.020 wt.%~0.040 wt.%, Ti 0.010 wt.%~0.040 wt.%, Mo 0.30 wt.%~0.50 wt.%, B 0.0010 wt.%~0.0025 wt.%, N ≤ 0.004 wt.%, Als 0.010 wt.%~0.060 wt.%, with the remainder being Fe and unavoidable impurities. The above chemical composition also satisfies the following relationship:

[0007] 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1);

[0008] Molten high-strength steel is continuously cast to obtain continuously cast slabs;

[0009] The continuously cast slab is heated to obtain the first heated slab.

[0010] The first heated slab is subjected to rough rolling and finish rolling in sequence to obtain finished steel; wherein the rough rolling temperature is 1060℃~1200℃ and the finish rolling temperature is 860℃~900℃.

[0011] Laminar flow cooling is applied to the finished rolled steel at a cooling rate of 40℃ / s to 80℃ / s to obtain cooled steel.

[0012] Cold-rolled steel is obtained by pickling and cold rolling cooled steel.

[0013] Cold-rolled steel is leveled and quenched to obtain quenched steel.

[0014] By straightening and tempering quenched steel, ultra-high strength steel with a thickness ≤3mm and a yield strength ≥960MPa is obtained.

[0015] According to an embodiment of the first aspect of this application, the soft blowing treatment of molten steel during the Si-Ca treatment process includes blowing argon gas into the molten steel between 5 minutes before the Si-Ca treatment and 8 minutes after the Si-Ca treatment.

[0016] According to an embodiment of the first aspect of this application, the continuous casting of high-strength steel molten steel includes electromagnetic stirring of the crystallizer. The electromagnetic stirring adopts an alternating forward and reverse stirring mode with an alternation time of 50s to 60s, an electromagnetic stirring current of 360A to 480A, and an electromagnetic stirring frequency of 12Hz to 20Hz, so as to obtain a continuously cast slab with a thickness of 210mm to 230mm.

[0017] According to an embodiment of the first aspect of this application, the heat treatment of the continuously cast slab includes setting the heating temperature of the second heating section and the soaking section to 1160°C to 1200°C.

[0018] According to an embodiment of the first aspect of this application, the sequential roughing and finishing rolling of the first heated slab includes, during the roughing rolling of the first heated slab, the pressure of the dephosphorizing water used between the roughing mill stands is 110 bar to 130 bar.

[0019] According to an embodiment of the first aspect of this application, the method for producing ultra-thin high-strength steel based on a discontinuous quenching line further includes: coiling the cooled steel at a temperature of 600°C to 650°C to obtain coiled cooled steel.

[0020] According to an embodiment of the first aspect of this application, pickling of cooled steel includes pickling in pickling tanks with acid concentrations of 70g / L to 120g / L, 130g / L to 170g / L, and 170g / L to 210g / L. The temperature of the acid in the pickling tank is 60℃ to 90℃, and the pickling speed is 40m / min to 70m / min.

[0021] According to an embodiment of the first aspect of this application, the cold rolling process of cooled steel includes using a single-stand rolling mill to cold roll the pickled cooled steel, with 2 to 4 cold rolling passes, a single-pass reduction rate of ≤15%, a front tension of 4KN / mm to 10KN / mm, and a back tension of 11KN / mm to 16KN / mm, to obtain cold-rolled steel.

[0022] According to an embodiment of the first aspect of this application, the leveling and quenching treatment of cold-rolled steel includes: leveling and rough straightening and leveling and fine straightening of cold-rolled steel, wherein the pressure of rough straightening is 6000KN to 9000KN and the pressure of fine straightening is 15000KN to 18000KN, thereby obtaining straightened steel.

[0023] The straightened steel was quenched at a heating temperature of 900±20℃ and a holding time of 20±3 minutes. The ratio of water volume in the upper and lower parts of the quenching process was 3:4, and the cooling rate was 35℃ / s~50℃ / s, resulting in quenched steel.

[0024] According to an embodiment of the first aspect of this application, the straightening and tempering treatment of quenched steel includes: coarse straightening and fine straightening of quenched steel, with a coarse straightening pressure of 6000KN~9000KN and a fine straightening pressure of 15000KN~18000KN, to obtain a straightened steel strip with a bent-down shape after straightening, and a flatness ≤6mm / m;

[0025] The straightened steel strip is subjected to tempering treatment. The heating temperature for tempering treatment is 600±20℃, the holding time is 40±3 minutes, and air cooling is performed to obtain tempered steel.

[0026] Tempered steel is subjected to secondary coarse straightening and fine straightening. The coarse straightening pressure is 4000KN~7000KN, and the fine straightening pressure is 12000KN~15000KN, to obtain ultra-thin high-strength steel.

[0027] Secondly, this application provides an ultra-thin high-strength steel, which is produced by a method based on a discontinuous quenching line; the ultra-thin high-strength steel, by mass percentage, is composed of the following chemical composition: C 0.16wt.%~0.21wt.%, Si <0.10wt.%, Mn 1.60wt.%~1.90wt.%, P ≤0.015wt.%, S ≤0.004wt.%, and Cr 0.20wt.%~0. 50 wt.%, Nb 0.020 wt.%~0.040 wt.%, Ti 0.010 wt.%~0.040 wt.%, Mo 0.30 wt.%~0.50 wt.%, B 0.0010 wt.%~0.0025 wt.%, N≤0.004 wt.%, Als 0.010 wt.%~0.060 wt.%, with the remainder being Fe and unavoidable impurities; the above chemical composition simultaneously satisfies the following relationship:

[0028] 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1).

[0029] According to an embodiment of the second aspect of this application, the steel plate of ultra-thin high-strength steel produced by a discontinuous quenching line has a flatness ≤3mm / m and a thickness ≤3mm, and its mechanical properties satisfy: yield strength R eL ≥960MPa, tensile strength R m ≥980MPa, elongation A≥12%; d=3a, 180° cold bending qualified.

[0030] This application describes a method for producing ultra-thin high-strength steel using a discontinuous quenching line. The method increases the C and Mn content and decreases the Si content in the ultra-thin high-strength steel. The main reason is that because the steel sheet is too thin, normal purging airflow cannot be used during quenching to prevent the steel sheet from being blown into the roller gap. When the Si content is high, water on the strip surface is not completely purged before entering the tempering furnace, leading to severe red rust formation. Increasing the C and Mn content not only avoids the strength loss caused by the reduced Si content but also prevents the low strength caused by the decarburization layer on the strip surface during quenching. By increasing the C and Mn content, the solid solution strengthening effect of the steel sheet is improved, and the carbon content in the martensite is increased to enhance strength, thereby ensuring the strength of the ultra-thin high-strength steel. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 is an actual product image of ultra-thin high-strength steel prepared by the method of preparing ultra-thin steel based on a discontinuous quenching line according to an embodiment of this application.

[0033] Figure 2 is an actual product image of the ultra-thin high-strength steel provided in Example 2.

[0034] Figure 3 is an actual product image of the ultra-thin high-strength steel provided in Embodiment 5 of this application. Detailed Implementation

[0035] The features and exemplary embodiments of various aspects of this application will be described in detail below. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain this application and not to limit it. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples.

[0036] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes said element.

[0037] To address the problems in the prior art, this application provides an embodiment of ultra-thin high-strength steel produced based on a discontinuous quenching line and its preparation method.

[0038] The method for producing ultra-thin high-strength steel based on a discontinuous quenching line, as provided in the embodiments of this application, will be described below. Figure 1 shows a schematic flowchart of the method for producing ultra-thin high-strength steel based on a discontinuous quenching line, as provided in the embodiments of this application.

[0039] As shown in Figure 1, the method for producing ultra-thin high-strength steel based on a discontinuous quenching line includes: providing molten steel, performing converter smelting, ladle refining, and Si-Ca treatment during the ladle refining process. During the Si-Ca treatment, the molten steel undergoes soft blowing treatment. After Si-Ca treatment, the Ca / S ratio in the molten steel is 1.0–3.0, resulting in high-strength steel. The high-strength steel is composed of the following chemical components by mass percentage: C 0.16 wt.%–0.21 wt.%, Si < 0.10 wt.%, and Mn 1.60 wt.%. %~1.90wt.%, P≤0.015wt.%, S≤0.004wt.%, Cr 0.20wt.%~0.50wt.%, Nb 0.020wt.%~0.040wt.%, Ti 0.010wt.%~0.040wt.%, Mo 0.30wt.%~0.50wt.%, B 0.0010wt.%~0.0025wt.%, N≤0.00wt.4%, Als 0.010wt.%~0.060wt.%, with the remainder being Fe and unavoidable impurities; the above chemical composition simultaneously satisfies the following relationship:

[0040] 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1);

[0041] Molten high-strength steel is continuously cast to obtain a continuously cast slab; the continuously cast slab is heated to obtain a first heated slab; the first heated slab is then subjected to rough rolling and finish rolling to obtain finished steel; the rough rolling temperature is 1060℃~1200℃, and the finish rolling temperature is 860℃~900℃; the finished steel is subjected to laminar flow cooling at a cooling rate of 40℃ / s~80℃ / s to obtain cooled steel; the cooled steel is then pickled and cold rolled to obtain cold-rolled steel; the cold-rolled steel is then leveled and quenched to obtain quenched steel; the quenched steel is then straightened and tempered to obtain ultra-thin high-strength steel with a thickness ≤3mm and a yield strength ≥960MPa.

[0042] This application describes a method for producing ultra-thin high-strength steel using a discontinuous quenching line. The method increases the C and Mn content and decreases the Si content in the ultra-thin high-strength steel. The main reason is that the steel plates of ultra-thin high-strength steel are too thin, making it impossible to use normal purging airflow during quenching to prevent the steel plates from being blown into the roller gap. When the Si content in the steel is high, water on the strip surface is not completely purged before entering the tempering furnace, leading to severe red rust formation on the surface. Increasing the C and Mn content not only avoids the strength loss caused by the reduced Si content but also prevents the low strength caused by the decarburization layer on the strip surface during quenching. By increasing the C and Mn content to improve the solid solution strengthening effect of the steel plate and increasing the carbon content in the martensite to increase strength, the strength of the ultra-thin high-strength steel is guaranteed.

[0043] It should be noted that the discontinuous quenching line refers to a regular quenching line, that is, a production line in which steel plates are quenched one sheet or one piece at a time. The method for producing ultra-thin high-strength steel using the discontinuous quenching line in this application is based on such a production line.

[0044] In the preparation method of this application, in formula (1), [%C] represents the mass content of carbon; [%Si] represents the mass content of silicon; [%Mn] represents the mass content of manganese; [%Mo] represents the mass content of molybdenum; and [%Cr] represents the mass content of chromium.

[0045] Although the mechanism is not yet clear, the applicant unexpectedly discovered that the mechanical properties of molten steel that meets the formula (1) meet the requirements for use. When the value of formula (1) exceeds 1000, the strength exceeds the upper limit and the carbon equivalent is too high, affecting the weldability of the steel. When the value of formula (1) is less than 800, the strength of the steel cannot meet the target requirements.

[0046] In some embodiments of this application, Si-Ca treatment during ladle refining refers to adding silicon-calcium wire to the molten steel during ladle refining to achieve a Ca content of 20 ppm to 30 ppm. Si-Ca treatment of the molten steel during ladle refining can improve the morphology of MnS inclusions in the molten steel, changing them from banded to spherical, thereby improving the cold forming performance of ultra-thin high-strength steel. The calcium content in the molten steel can be determined using methods for measuring the composition of molten steel. When the calcium content in the molten steel reaches 20 ppm to 30 ppm, the Si-Ca treatment during ladle refining can be considered complete. Exemplarily, the calcium content in the molten steel reaches 22 ppm, 25 ppm, 27 ppm, 29 ppm, and 30 ppm.

[0047] In some embodiments of this application, the soft blowing treatment of molten steel during the Si-Ca treatment includes blowing argon gas into the molten steel between 5 minutes before the Si-Ca treatment and 8 minutes after the Si-Ca treatment, so that the surface of the molten steel is slightly agitated.

[0048] In some embodiments of this application, the continuous casting of high-strength steel molten steel includes electromagnetic stirring of the crystallizer. The electromagnetic stirring adopts an alternating forward and reverse stirring mode with an alternation time of 50s to 60s, an electromagnetic stirring current of 360A to 480A, and an electromagnetic stirring frequency of 12Hz to 20Hz, so as to obtain a continuously cast slab with a thickness of 210mm to 230mm.

[0049] In some embodiments of this application, the heating treatment of the continuously cast slab includes setting the heating temperature of the second heating section and the soaking section to 1160°C to 1200°C.

[0050] In some embodiments of this application, the sequential roughing and finishing rolling of the first heated slab includes, during the roughing rolling of the first heated slab, using a dephosphorization water pressure of 110 bar to 130 bar between the roughing mill stands.

[0051] In some embodiments of this application, the method for producing ultra-thin high-strength steel based on a discontinuous quenching line further includes: coiling the cooled steel at a temperature of 600°C to 650°C to obtain coiled cooled steel.

[0052] In some embodiments of this application, pickling of cooled steel includes pickling the cooled steel in pickling tanks with acid concentrations of 70g / L to 120g / L, 130g / L to 170g / L, and 170g / L to 210g / L. The temperature of the acid in the pickling tank is 60℃ to 90℃, and the pickling speed is 40m / min to 70m / min.

[0053] In the preparation method of this application, pickling is employed to remove iron oxide scale from the surface of the cooled steel. This not only benefits single-stand rolling but also facilitates shape control during subsequent quenching. The presence of iron oxide scale on the steel surface affects the temperature uniformity during heating. Ultra-thin high-strength steel, being strip-shaped and very thin, is prone to deformation due to uneven heating in the furnace, exacerbating deformation during quenching. Therefore, pickling of the cooled steel is necessary.

[0054] In some embodiments of this application, the cold rolling process of the cooled steel includes using a single-stand rolling mill to cold roll the pickled cooled steel, with 2 to 4 cold rolling passes, a single-pass reduction rate of ≤15%, a front tension of 4KN / mm to 10KN / mm, and a back tension of 11KN / mm to 16KN / mm, to obtain cold-rolled steel.

[0055] In the preparation method of this application, a single-stand rolling mill is used to cold roll the cooled steel, which not only improves the flatness of the original plate shape but also reduces the thickness of the steel plate. As is well known, hot rolling production lines face difficulties in controlling the plate shape when producing thin-gauge hot-rolled coils, and also require steel plates of different thicknesses for transition, resulting in a large number of undesirable products. However, using single-stand rolling allows 3mm hot-rolled raw materials to be directly rolled into a 2mm thickness, reducing production costs.

[0056] In some embodiments of this application, the leveling and quenching treatment of cold-rolled steel includes: leveling and rough straightening and leveling and fine straightening of cold-rolled steel, wherein the pressure of rough straightening is 6000KN~9000KN and the pressure of fine straightening is 15000KN~18000KN, to obtain straightened steel; and quenching treatment of straightened steel, wherein the heating temperature of quenching treatment is 900±20℃, the holding time is 20±3 minutes, the ratio of water volume between the top and bottom layers is 3:4, and the cooling rate is 35℃ / s~50℃ / s, to obtain quenched steel.

[0057] In the preparation method of this application, the quenching process adopts a cooling water strategy of "smaller volume at the top and larger volume at the bottom," with an upper to lower water volume ratio of 3:4 and a cooling rate of 35℃ / s to 50℃ / s. Because the cold-rolled steel undergoing quenching is strip-shaped and very thin, a smaller volume at the top and larger volume at the bottom is necessary; otherwise, uneven cooling will lead to severe deformation of the cold-rolled steel. Similarly, because the cold-rolled steel is very thin, a high cooling rate will easily cause deformation of the strip-shaped cold-rolled steel, while a low cooling rate will prevent the formation of martensite. Therefore, the quenching strategy of this application ensures a good match between the shape and strength of the strip-shaped cold-rolled steel.

[0058] In some embodiments of this application, the straightening and tempering treatment of quenched steel includes: rough straightening and fine straightening of the quenched steel, with a rough straightening pressure of 6000KN~9000KN and a fine straightening pressure of 15000KN~18000KN, to obtain a straightened steel strip with a downward-curving shape and a flatness ≤6mm / m; tempering the straightened steel strip, with a tempering heating temperature of 600±20℃, a holding time of 40±3 minutes, and air cooling, to obtain tempered steel; and second rough straightening and fine straightening of the tempered steel, with a rough straightening pressure of 4000KN~7000KN and a fine straightening pressure of 12000KN~15000KN, to obtain ultra-high strength steel.

[0059] In the preparation method of this application, straightening treatment is performed both before and after tempering, each with its own function. The first straightening produces a slightly downward-curving plate shape because the strip-shaped quenched steel will shrink during air cooling after tempering, and the slight downward curvature of the original plate shape can offset the deformation caused by the shrinkage. The second straightening uniformly distributes the internal stress of the steel plate and adjusts the final plate shape.

[0060] Therefore, the method for producing ultra-thin high-strength steel based on a discontinuous quenching line in this application employs a reasonable composition design. By increasing the C and Mn content and decreasing the Si content, it ensures that the material strength meets performance requirements and improves the surface quality of the steel. Furthermore, this application uses pickling and single-stand rolling mill processes to obtain quenched raw materials with thickness, shape, and surface quality all meeting requirements. A strategy combining double straightening and quenching is employed to obtain ultra-thin high-strength steel with good shape, and its flatness is ≤3mm / m. The yield strength R... eL ≥960MPa, tensile strength R m ≥980MPa, elongation A≥12%; d=3a, 180° cold bending qualified. This application describes a method for preparing ultra-thin steel based on a discontinuous quenching line. As a novel manufacturing process, it enables the production of high-strength, ultra-thin steel that meets the required specifications using a discontinuous quenching line.

[0061] Secondly, this application provides an ultra-thin high-strength steel produced using a discontinuous quenching line, which is prepared by a method for producing ultra-thin high-strength steel using a discontinuous quenching line. The ultra-thin high-strength steel, by mass percentage, has the following chemical composition: C 0.16wt.%~0.21wt.%, Si <0.10wt.%, Mn 1.60wt.%~1.90wt.%, P ≤0.015wt.%, S ≤0.004wt.%, and Cr 0.20wt.%... wt.%~0.50wt.%, Nb 0.020wt.%~0.040wt.%, Ti 0.010wt.%~0.040wt.%, Mo 0.30wt.%~0.50wt.%, B 0.0010wt.%~0.0025wt.%, N≤0.004wt.%, Als 0.010wt.%~0.060wt.%, with the remainder being Fe and unavoidable impurities; the above chemical composition simultaneously satisfies the following relationship:

[0062] 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1).

[0063] In some embodiments of this application, the mechanical properties of ultra-thin high-strength steel produced based on discontinuous quenching lines meet the following requirements: steel plate unevenness ≤ 3 mm / m, thickness ≤ 3 mm, and yield strength R eL ≥960MPa, tensile strength R m ≥980MPa, elongation A≥12%; d=3a, 180° cold bending qualified.

[0064] The technical solution for preparing ultra-thin high-strength steel using the above-mentioned composition in this application is based on the following considerations: C: As an interstitial atom in steel, C plays a very important role in improving the strength of steel, having the greatest impact on the yield strength and tensile strength of steel, and is an important element determining the strength and hardness of the material. C can stabilize austenite and control the transformation of ferrite during laminar air cooling. To obtain high-strength steel with a tensile strength of 980 MPa, a certain carbon content must be ensured; however, too high a carbon content will affect weldability. Therefore, the C content in this application is set at 0.16 wt.%~0.21 wt.%.

[0065] Si: Si is a solid solution strengthening element, and adding Si to steel has many advantages. However, when Si ≥ 0.10 wt.%, it will form a highly adhesive iron oxide scale on the surface of the strip steel, affecting its descaling effect. Therefore, the Si content in this application is set to < 0.10 wt.%.

[0066] Mn: Mn is a substitutional element that plays a role in solid solution strengthening. It can expand the austenite phase region, reduce the critical quenching rate of steel, stabilize austenite, refine grains, and delay the transformation of austenite to pearlite. To ensure a tensile strength ≥980MPa, the Mn content should be controlled above 1.60wt.%. If the Mn content is too low, the supercooled austenite is not stable enough and is prone to transforming into a pearlite-type structure during air cooling; if the Mn content is too high, it will affect the alloy cost. Therefore, the Mn content in this application is set at 1.60wt.%~1.90wt.%.

[0067] P and S: As harmful inclusions in steel, P and S have a significant detrimental effect on the steel's formability, low-temperature toughness, weldability, and fatigue performance. This application aims to reduce production costs and improve product quality by controlling the P content to ≤0.015wt.% and the S content to ≤0.004wt.%, thereby reducing the impact of P and S on formability to a low level.

[0068] Cr: Cr can improve the hardenability of steel and reduce the adhesion of iron oxide scale on the surface of strip steel, reduce the pulverization of iron oxide scale on the surface of steel plate, and improve the surface quality of steel plate. Therefore, the Cr content in this application is set to 0.20wt.%~0.50wt.%.

[0069] Nitrogen (Nb): Nb can inhibit austenite recrystallization, precipitate NbC to refine ferrite grains, and improve strength and toughness. Nb can improve the tempering stability of steel and reduce its temper brittleness. In the solid solution state, Nb can effectively inhibit the transformation of austenite to ferrite, pearlite, and bainite, improving the hardenability of steel plates. However, Nb is a precious metal element, so an Nb content of 0.020 wt.% to 0.040 wt.% is more suitable.

[0070] Ti: Ti has certain grain refinement and precipitation strengthening effects. A small amount of Ti can also improve welding performance. In this application, the main focus is on the grain refinement and welding performance of Ti, therefore the Ti content is set to 0.010 wt.%~0.040 wt.%.

[0071] Mo: Mo can refine grains, improve strength and toughness. In steel, Mo exists in both solid solution and carbide phases; therefore, molybdenum-containing steel exhibits both solid solution strengthening and carbide dispersion strengthening. Mo can shift the C-curve of steel to the right, significantly improving hardenability and tempering stability. Mo can also improve the high-temperature brittleness of alloy tempered steels and enhance the toughness of tempered sorbite. During high-temperature tempering, Mo₂C precipitates in situ at dislocations and remains coherent with the matrix, preventing agglomeration and growth, resulting in a strong secondary hardening effect. Mo dissolves in ferrite, increasing the self-diffusion activation energy of iron and raising the recovery and recrystallization temperatures of steel. However, Mo is a noble metal element; therefore, the Mo content is set at 0.30 wt.%~0.50 wt.%.

[0072] B: B is an element with a strong hardenability in steel. Even a trace amount of B can greatly improve the hardenability of steel. Therefore, the B content in this application is set to 0.0010 wt.%~0.0025 wt.%.

[0073] N: N is a harmful element in steel. Controlling the N content to ≤0.004wt.% can reduce the risk of forming coarse TiN inclusions.

[0074] Als: Als plays a deoxidizing role in steelmaking, improving the purity of molten steel. Furthermore, Als can fix nitrogen in steel and form stable compounds with it, effectively refining the grain size. Therefore, the Als content in this application is set at 0.010 wt.% to 0.060 wt.%.

[0075] The technical solution of this application will be further described below through specific embodiments and comparative examples.

[0076] Examples 1-9 and Comparative Examples 1-3

[0077] Each method for producing ultra-thin high-strength steel based on a discontinuous quenching line is provided, including providing molten steel from Examples 1-9 and Comparative Examples 1-3, and subjecting the molten steel to converter smelting, ladle refining, and Si-Ca treatment by adding a silicon-calcium wire. After Si-Ca treatment, the Ca / S ratio in the molten steel is controlled between 1.0 and 3.0, and the calcium content in the molten steel reaches 25 ppm. During the Si-Ca treatment, argon gas is blown into the molten steel for soft blowing treatment to obtain high-strength steel. The molten steel obtained from Examples 1-9 and Comparative Examples 1-3 is shown in Table 1.

[0078] Table 1. Chemical composition and mass percentage of molten steel obtained in Examples 1-9 and Comparative Examples 1-3

[0079]

[0080] High-strength steel is continuously cast using electromagnetic stirring, which employs alternating forward and reverse stirring modes with an alternation time of 55 seconds. The electromagnetic stirring current is 360A~380A, and the stirring frequency is 15Hz~16Hz, resulting in a continuously cast slab. The continuously cast slab is then heated to 1160℃~1200℃ in the second heating section and soaking section, resulting in a first heated slab. This first heated slab is then subjected to rough rolling and finish rolling to obtain finished steel. The rough rolling temperature is 1060℃~1200℃, and the rough rolling mill uses a stirring pattern between stands. The dephosphorization water pressure is 120-125 bar, and the finishing rolling temperature is 860-900℃. The finished steel is then subjected to laminar flow cooling at the rates shown in Table 2, resulting in cooled steel. The cooled steel is then coiled and stacked at the coiling temperatures shown in Table 2, yielding coiled cooled steel. The coiled cooled steel undergoes pickling and cold rolling. The temperature of the pickling solution in pickling tanks #1-#3 is 75℃, and the cold rolling passes are 2-4 times. The initial tension is 4-10 KN / mm, and the subsequent tension is 11-16 KN / mm, resulting in cold-rolled steel. The cold-rolled steel... The leveling and quenching process includes: coarse straightening and fine straightening of the cold-rolled steel, with a coarse straightening pressure of 6000KN~9000KN and a fine straightening pressure of 15000KN~18000KN, resulting in straightened steel; quenching of the straightened steel, with the quenching temperature shown in Table 2, a holding time of 20±3 minutes, a water-to-water ratio of 3:4, and a cooling rate shown in Table 2, resulting in quenched steel; and straightening and tempering of the quenched steel, including: coarse straightening and fine straightening, with a coarse straightening pressure of 6000KN~9000KN. The straightening pressure is 15000KN~18000KN, resulting in a straightened steel strip with a downward-curving shape and a flatness ≤6mm / m. The straightened steel strip is then tempered, with the heating temperature shown in Table 2 and the holding time being 40±3 minutes, followed by air cooling to obtain tempered steel. The tempered steel is then subjected to secondary coarse straightening and fine straightening, with coarse straightening pressure of 4000KN~7000KN and fine straightening pressure of 12000KN~15000KN, to obtain ultra-thin high-strength steel with a thickness ≤3mm and a yield strength ≥960MPa. The production process of ultra-thin high-strength steel in Examples 1-9 and Comparative Examples 1-3 is described in Table 2.

[0081] Table 2. Preparation process parameters of ultra-thin high-strength steel in Examples 1-9 and Comparative Examples 1-3

[0082]

[0083] Table 2 (Continued) shows the preparation process parameters of ultra-thin high-strength steels in Examples 1-9 and Comparative Examples 1-3.

[0084]

[0085] The high-strength steels of Examples 1-9 and Comparative Examples 1-3 were subjected to performance tests, and the test results are recorded in Table 3 below:

[0086] Table 3 Performance test results of high-strength steels in Examples 1-9 and Comparative Examples 1-3

[0087]

[0088] Comparing Tables 1-3, it can be seen that the element content of Comparative Example 1 does not conform to the composition limit range of this application, nor does it conform to the relationship (1) of this application. Even though its preparation process is the same as that of this application, and the strength and flatness of the high-strength steel product prepared therefrom meet the requirements of this application, its bending performance is very poor and cannot meet the requirements of d=3a and 180° cold bending qualification. The element content of Comparative Example 2 basically conforms to the relationship (1) of this application, and the hot rolling process also conforms to the limit of the preparation method of this application. However, the pickling speed is too fast, the iron oxide scale on the surface of the steel plate is not completely removed, and it is not heated evenly in the quenching furnace. As shown in Figure 2, the steel plate is warped when it is taken out of the furnace. Moreover, the water ratio between the top and bottom layers during quenching does not conform to the range limited by this application. The amount of water in the upper layer is large and the cooling rate is too large, resulting in the average flatness of the steel plate reaching 7mm / m, which is much higher than the flatness of the ultra-thin high-strength steel of this application. Comparative Example 3's composition meets the element content limits of this application, but it was not pickled or single-stand rolled, and the iron oxide scale on the steel plate surface severely affected its uniform heating, causing severe warping during heating. Although a water-to-cooling ratio of smaller at the top and larger at the bottom was adopted during quenching, and the cooling rate also met the limits of this application, it still could not compensate for the warping during heating, resulting in a very poor final plate shape with a flatness of 11 mm / m. It is evident that at least one of the steel elements and their content, or the preparation process, is outside the limits of this application, making it difficult for high-strength steel produced based on a discontinuous quenching line to meet the requirements for ultra-thin high-strength steel.

[0089] As can be seen from the test results of the ultra-thin high-strength steel in Examples 1-9, the method for producing ultra-thin high-strength steel based on a discontinuous quenching line according to the embodiments of this application can produce plates with a flatness ≤3mm / m, a thickness ≤3mm, and a yield strength R eL ≥960MPa, tensile strength R m ≥980MPa, elongation A≥12%; d=3a, qualified ultra-thin high-strength steel with 180° cold bending. As shown in Figure 3, the ultra-thin high-strength steel of Example 5 of this application has small unevenness and a smooth surface.

[0090] This application's embodiment describes a method for producing ultra-thin high-strength steel using a discontinuous quenching line. It employs a reasonable chemical composition ratio and tempering process to ensure the material's mechanical properties meet requirements. Through pickling and single-stand rolling, it ensures the steel has a good initial shape before quenching, and the steel plate surface is free of iron oxide scale, ensuring uniform heating during the heating process. Furthermore, this method, using a water ratio that is smaller at the top and larger at the bottom, along with a slower quenching speed, guarantees a good shape of the steel after quenching. Two straightening processes ensure the final flatness of the steel plate meets a requirement of 3mm / m. This achieves a good balance between ultra-thin dimensions, high strength, and good shape.

[0091] The above description is merely a specific implementation of this application. Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here. It should be understood that the protection scope of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the protection scope of this application.

Claims

1. A method for producing an ultra-thin gauge high-strength steel based on a non-continuous quenching line, characterized in that, include: Molten steel is provided and subjected to converter smelting, ladle refining, and Si-Ca treatment during the ladle refining process. The Si-Ca treatment requires soft blowing of the molten steel. After Si-Ca treatment, the Ca / S ratio in the molten steel is 1.0–3.0, resulting in high-strength steel. The high-strength steel is composed of the following chemical components by mass percentage: C 0.16 wt.%–0.21 wt.%, Si < 0.10 wt.%, Mn 1.60 wt.%–1.90 wt.%, and P ≤ 0.015 wt.%. The composition of the chemical components is as follows: S ≤ 0.004 wt.%, Cr 0.20 wt.%~0.50 wt.%, Nb 0.020 wt.%~0.040 wt.%, Ti 0.010 wt.%~0.040 wt.%, Mo 0.30 wt.%~0.50 wt.%, B 0.0010 wt.%~0.0025 wt.%, N ≤ 0.004 wt.%, Als 0.010 wt.%~0.060 wt.%, with the remainder being Fe and unavoidable impurities. The above chemical composition also satisfies the following relationship: 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1); The molten steel of the high-strength steel is continuously cast to obtain a continuously cast slab; The continuously cast slab is heated to obtain a first heated slab; The first heated slab is subjected to rough rolling and finish rolling in sequence to obtain finished steel; wherein the rough rolling temperature is 1060℃~1200℃ and the finish rolling temperature is 860℃~900℃. The finished steel is subjected to laminar flow cooling at a rate of 40°C / s to 80°C / s to obtain cooled steel. The cooled steel is pickled and cold rolled to obtain cold-rolled steel. The cold-rolled steel is leveled and quenched to obtain quenched steel. The quenched steel is straightened and tempered to obtain ultra-thin high-strength steel with a thickness ≤3mm and a yield strength ≥960MPa.

2. The method of claim 1, wherein, During the Si-Ca treatment, the molten steel is subjected to soft blowing treatment, which includes blowing argon gas into the molten steel between 5 minutes before the Si-Ca treatment and 8 minutes after the Si-Ca treatment.

3. The method of claim 1, wherein, The continuous casting of high-strength steel molten steel includes electromagnetic stirring of the crystallizer. The electromagnetic stirring adopts an alternating forward and reverse stirring mode with an alternation time of 50s to 60s, an electromagnetic stirring current of 360A to 480A, and an electromagnetic stirring frequency of 12Hz to 20Hz, so as to obtain a continuously cast slab with a thickness of 210mm to 230mm.

4. The method of claim 1, wherein, At least one of the following requirements must be met: The heating treatment of the continuously cast slab includes setting the heating temperature of the second heating section and the soaking section to 1160℃~1200℃; The sequential roughing and finishing rolling of the first heated slab includes, during the roughing rolling process of the first heated slab, using a dephosphorization water pressure of 110 bar to 130 bar between the roughing mill stands.

5. The method of claim 1, wherein, The method for preparing ultra-thin steel based on a discontinuous quenching line further includes: coiling the cooled steel at a temperature of 600℃~650℃ to obtain coiled cooled steel.

6. The method of claim 1, wherein, At least one of the following requirements must be met: The pickling treatment of the cooled steel includes pickling in pickling tanks with acid concentrations of 70g / L to 120g / L, 130g / L to 170g / L, and 170g / L to 210g / L. The temperature of the acid in the pickling tanks is 60℃ to 90℃, and the pickling speed is 40m / min to 70m / min. The cold rolling process of the cooled steel includes using a single-stand rolling mill to cold roll the pickled cooled steel, with 2 to 4 cold rolling passes, a single-pass reduction rate of ≤15%, a front tension of 4KN / mm to 10KN / mm, and a back tension of 11KN / mm to 16KN / mm, to obtain cold-rolled steel.

7. The method of claim 1, wherein, At least one of the following requirements must be met: The leveling and quenching treatment of the cold-rolled steel includes: leveling and rough straightening and leveling and fine straightening of the cold-rolled steel, wherein the pressure of the rough straightening is 6000KN to 9000KN and the pressure of the fine straightening is 15000KN to 18000KN, to obtain straightened steel. The straightened steel is subjected to quenching treatment. The heating temperature for quenching treatment is 900±20℃, the holding time is 20±3 minutes, the ratio of water volume in the upper and lower parts of the quenching process is 3:4, and the cooling rate is 35℃ / s~50℃ / s, to obtain quenched steel.

8. The method of claim 1, wherein, At least one of the following requirements must be met: The straightening and tempering treatment of the quenched steel includes: coarse straightening and fine straightening of the quenched steel, with a coarse straightening pressure of 6000KN~9000KN and a fine straightening pressure of 15000KN~18000KN, to obtain a straightened steel strip with a downward-curving shape after straightening, and a flatness ≤6mm / m; The straightened steel strip is subjected to tempering treatment at a heating temperature of 600±20℃ and a holding time of 40±3 minutes, followed by air cooling to obtain tempered steel. The tempered steel is subjected to secondary coarse straightening and fine straightening. The coarse straightening pressure is 4000KN to 7000KN, and the fine straightening pressure is 12000KN to 15000KN, to obtain ultra-thin high-strength steel.

9. An ultra-thin gauge high-strength steel, characterized by, The ultra-thin high-strength steel is prepared according to the method for producing ultra-thin high-strength steel based on a discontinuous quenching line according to any one of claims 1-8; the ultra-thin high-strength steel, by mass percentage, is composed of the following chemical composition: C 0.16wt.%~0.21wt.%, Si <0.10wt.%, Mn 1.60wt.%~1.90wt.%, P ≤0.015wt.%, S ≤0.004wt.%, and Cr 0.20wt.%~0.5wt.%. 0 wt.%, Nb 0.020 wt.%~0.040 wt.%, Ti 0.010 wt.%~0.040 wt.%, Mo 0.30 wt.%~0.50 wt.%, B 0.0010 wt.%~0.0025 wt.%, N≤0.004 wt.%, Als 0.010 wt.%~0.060 wt.%, with the remainder being Fe and unavoidable impurities; the above chemical composition simultaneously satisfies the following relationship: 800≤4370[%C]+88[%Si]+53[%Mn]+22[%Mo]-30[%Cr]≤1000 (1).

10. The ultra-thin gauge high strength steel of claim 9, wherein, The steel plate of the ultra-thin gauge high-strength steel has unevenness of ≤3 mm / m and thickness of ≤3 mm, and mechanical properties meet: yield strength R eL ≥960 MPa, tensile strength R m ≥980 MPa, and elongation A≥12%. d=3a, 180° cold bending is qualified.

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