Method for preparing high-strength steel plates with improved cold bending properties and high-strength steel

Abstract

This application discloses a method for preparing high-strength steel plates with improved cold bending performance and the high-strength steel itself. The method includes providing a hot-rolled coil as a substrate; using the hot-rolled coil as the substrate, heating and holding the substrate in a steel plate quenching furnace to austenitize the substrate microstructure before quenching; and then subjecting the quenched substrate to low-temperature tempering in a steel plate tempering furnace to obtain a high-strength steel plate. The method of this application, by selecting a hot-rolled coil as the substrate, first heating and holding the substrate in a steel plate quenching furnace for a period of time to austenitize the substrate microstructure before quenching, and then subjecting the quenched steel plate to tempering in a steel plate tempering furnace, transforms the quenched martensite of the substrate into a tempered martensite microstructure with dispersed precipitates and low internal stress through appropriate tempering temperature and time. The steel plate with this microstructure exhibits good cold bending performance and high strength, resulting in a high-strength steel plate with improved cold bending performance that meets national standards.

Description

Technical Field

[0001] This application belongs to the field of materials processing technology, and in particular relates to a method for preparing high-strength steel plates with improved cold bending performance and high-strength steel. Background Technology

[0002] Cold bending performance is one of the important processing properties of steel plates. Cold bending of steel plates using a steel plate bending machine is a common method for processing steel plates into components. However, cold bending performance usually decreases as the strength of the steel plate increases. Therefore, cold bending performance is usually a challenge in the development of high-strength steel products with a yield strength of 960 MPa and above. Summary of the Invention

[0003] This application provides a method for preparing high-strength steel plates with improved cold bending performance. This method can improve the cold bending performance of high-strength steel plates with a yield strength of 1100MPa (hereinafter referred to as 1100MPa steel plates). The improved cold bending performance of 1100MPa steel plates is achieved by reducing the current mandrel diameter of 6a to 4a without cracking during a 90° cold bending test.

[0004] In a first aspect, this application provides a method for preparing a high-strength steel plate with improved cold bending performance, comprising:

[0005] S1. Provide hot-rolled coil as the substrate;

[0006] S2. Hot-rolled coil is used as the substrate. The substrate is heated and kept at a temperature in a steel plate quenching furnace to austenitize the substrate structure before quenching.

[0007] S3. The quenched substrate is then subjected to low-temperature tempering in a steel plate tempering furnace to obtain a high-strength steel plate.

[0008] The method for preparing high-strength steel plates with improved cold bending performance according to the embodiments of this application involves selecting hot-rolled coils as substrates, first heating and holding the substrate in a steel plate quenching furnace for a period of time to austenitize the substrate microstructure, then quenching it, and then subjecting the quenched steel plate to low-temperature tempering in a steel plate tempering furnace. By using appropriate tempering temperature and tempering time, the substrate obtains a tempered martensite microstructure with dispersed precipitates and low residual internal stress. This microstructure has good cold bending performance and high strength, resulting in a high-strength steel plate with improved cold bending performance that meets national standards. Attached Figure Description

[0009] 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.

[0010] Figure 1 This is a morphology image of the precipitates of an LG1100QT hot-rolled steel plate with a thickness * width * length of 7 * 2000 * 5000 mm provided in one embodiment of this application, after being quenched at 870°C for 25 minutes and tempered at 220°C for 20 minutes.

[0011] Figure 2 This is a morphology image of the precipitates of an LG1100QT hot-rolled steel plate with a thickness * width * length of 7 * 2000 * 5000 mm provided in one embodiment of this application, after being quenched at 870°C for 25 minutes and tempered at 220°C for 35 minutes.

[0012] Figure 3 This is a morphology image of the precipitates of an LG1100QT hot-rolled steel plate with a thickness * width * length of 7 * 2000 * 5000 mm provided in one embodiment of this application, after being quenched at 870°C for 25 minutes and tempered at 220°C for 70 minutes.

[0013] Figure 4 The results of a transverse cold bending performance test were obtained by sampling a LG1100QT hot-rolled steel plate with a thickness * width * length of 10 * 2000 * 12000 mm provided in one embodiment of this application after quenching at 870℃ for 29 min and tempering at 220℃ for 39 min. The cold bending sample had a thickness * width * length of 10 * 20 * 160 mm and a support roller spacing of 46 mm. The sample was bent at 90° with a bending mandrel diameter of 1.6a (16 mm). There were no cracks on the outer arc surface of the bent part. The width was reduced from the actual 19.5 mm to 15.5 mm. The width of the outer arc surface was reduced by 4.0 mm, which is a reduction of 20.51%.

[0014] Figure 5 This application describes the results of a transverse cold bending performance test on a 10*2000*12000 mm thick * width* length LG1100QT hot-rolled steel plate provided in one embodiment of the present application. The plate was quenched at 870℃ for 29 minutes and then tempered at 220℃ for 39 minutes. The cold-bending sample had dimensions (thickness * width * length) of 10*100*160 mm, a roller spacing of 70 mm, and was bent 90° with a 4a (40 mm) bending mandrel diameter. The outer arc surface of the bent portion showed no cracks, and the width decreased from the actual 100.0 mm to 98.5 mm, a reduction of 1.5 mm (1.5%). This cold-bending sample... Figure 4The cold bending test specimens were taken from 1 / 4 of the width of the same LG1100QT finished steel plate. When the width-to-thickness ratio of the cold bending test specimen increased from 2 to 10, the diameter of the mandrel that did not crack in the 90° cold bending test increased from 1.6a to 4a, that is, the 90° cold bending performance decreased from 1.6a to 4a, indicating that the width-to-thickness ratio of the cold bending test specimen has a significant impact on the cold bending test results.

[0015] Figure 6 The results of a transverse cold bending performance test were conducted on a 10*2000*12000 specification LG1100QT hot-rolled steel plate provided in one embodiment of this application after quenching at 870℃ for 29 min and tempering at 220℃ for 39 min. Three cold-bending samples with a thickness*width*length of 10*20*160 mm were cold-bent 90° with a bending mandrel diameter of 1.6a (16 mm) and support roll spacing of 46 mm, 41 mm and 36 mm respectively. The cold-bending sample with a support roll spacing of 46 mm had no cracks; the cold-bending sample with a support roll spacing of 41 mm had cracks; and the cold-bending sample with a support roll spacing of 36 mm broke, with a crack depth exceeding 2 / 3 of the thickness. This indicates that the support roll spacing has a significant impact on the cold bending test results of heat-treated high-strength steel plates. Detailed Implementation

[0016] 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.

[0017] 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.

[0018] As noted in the background section, the cold bending performance of steel plates is a challenge in the development of high-strength steel plates. The embodiments of this application provide a method for preparing high-strength steel plates with improved cold bending performance.

[0019] LG1100QT is a hot-rolled heat-treated steel plate with a yield strength of 1100MPa recently developed and produced by the applicant. It uses hot-rolled coils as raw material and undergoes quenching and low-temperature tempering treatment. It is mainly used in engineering machinery and other fields, such as for bending and forming crane booms. The "QT" in the grade designation indicates quenching and tempering. The basic requirement for the cold bending performance of LG1100QT steel plate in actual use is that when the mandrel diameter D = 8a (where a is the nominal thickness of the steel plate, and 'a' in the following text refers to the nominal thickness of the steel plate), the steel plate should not crack when bent transversely at 90° (the bend line is parallel to the rolling direction). Some users require that the steel plate should not crack when bent transversely at 90° when the mandrel diameter D = 6a. Therefore, how to improve the actual cold bending performance of LG1100QT products has become a problem that must be solved.

[0020] Method for preparing high-strength steel plates with improved cold bending properties

[0021] The provided process path for hot-rolled coils includes operations such as blast furnace ironmaking, hot metal pretreatment, converter, LF-RH refining, continuous casting of slabs, slab heating, rough rolling, finish rolling, coiling, and flattening.

[0022] The basic standard for this product is GB / T 280909-2012 "Heat-Treatable Steel Plates for Ultra-High Strength Structures". The mechanical properties required by this standard are: yield strength ≥1100 QT, tensile strength 1200~1550 MPa, impact energy at -40℃ ≥27 J, and elongation ≥9%.

[0023] The heat treatment process of steel is closely related to its composition. The chemical composition of LG1100QT steel plate is: C 0.1-0.20%, Si 0.1-0.5%, Mn 1.0-1.5%, Cr 0.1-0.7%, Mo 0.2-0.7%, Al 0.02-0.1%, P≤0.0012%, S≤0.005%, with the remainder being Fe and unavoidable residual and impurity elements from the smelting process. In terms of composition, the element content must also meet the requirement of CEV (carbon equivalent) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cr + Ni) / 15 ≤ 0.82%.

[0024] LG1100QT steel plate uses hot-rolled coil as the base plate. The base plate is heated to 860℃~930℃ in a continuous steel plate quenching heating furnace with gas burning in a burner in a radiant tube as the heat source and nitrogen as the protective atmosphere in the furnace. After austenitization, it is quenched by a roller press quenching machine and then tempered in a steel plate tempering furnace to obtain a high-strength steel plate.

[0025] When heating a substrate in a continuous heating furnace using coal gas combustion in a radiant tube as a heat source and nitrogen as the protective atmosphere, the holding time is calculated from the moment the furnace temperature rises to the quenching temperature after the substrate enters the furnace. The calculation formula is as follows:

[0026] t1=a1k1D (1);

[0027] In equation (1),

[0028] t1 is the quenching heating time, in minutes;

[0029] a1 is the heating coefficient when hot-rolled coil is heated in a continuous heating furnace with gas burning in radiant tubes through burners as the heat source and nitrogen as the protective atmosphere inside the furnace. The unit is minutes / mm. The value is 1.1 to 1.3 for carbon steel and 1.4 to 1.7 for alloy steel.

[0030] D is the thickness of the substrate, in millimeters;

[0031] k1 is the substrate loading correction factor, which is 1.0 when the substrates are not stacked.

[0032] To ensure that the strength of the high-strength steel plate meets the requirements, a low-temperature tempering process is used to temper the quenched substrate.

[0033] Quenching temperature refers to the set temperature of the quenching heating furnace. Currently, the control accuracy of the quenching heating furnace temperature is usually no less than ±15℃ of the set temperature. When the furnace temperature is no less than 15℃ below the set quenching temperature after the steel plate enters the furnace, the holding time is calculated.

[0034] Difference from traditional quenching heating processes

[0035] The formula for calculating the holding time when quenching steel parts in a traditional box-type resistance furnace is:

[0036] t2=a2k2D (2);

[0037] In equation (2), t2 is the heat preservation time, in minutes;

[0038] a2 is the heating coefficient, in minutes per millimeter; for carbon steel, it is 0.9 to 1.1, and for low-alloy steel (alloy structural steel), it is 1.2 to 1.5.

[0039] k2 is the furnace loading coefficient for the workpiece, and is taken as 1.0 when the steel workpiece is not stacked.

[0040] From the values ​​of the heating coefficients calculated by formulas (1) and (2), it can be seen that in the method for preparing high-strength steel plates with improved cold bending performance, the values ​​of the heating coefficients differ to some extent when the hot-rolled coils are heated in a continuous heating furnace with gas burning in a radiant tube through a burner as the heat source and nitrogen as the protective atmosphere inside the furnace. k1 is about 0.2 higher than k2. The main reason for the difference is the difference in furnace type.

[0041] Low temperature tempering

[0042] Tempering temperature refers to the set furnace temperature of the tempering furnace. Low-temperature tempering requires far less heat than high-temperature tempering. In actual production, in continuous low-temperature tempering furnaces, except for the first section where the furnace temperature is approximately 20°C lower than the set temperature, the furnace temperatures of other sections can be controlled within ±15°C of the set temperature. Since continuous steel plate tempering typically involves more than 10 heating sections, and all sections in the low-temperature tempering process use the same set temperature, the furnace time ≈ tempering time = A*D+K. Here, the set temperature is 200°C~230°C.

[0043] This application uses a continuous steel plate tempering furnace for tempering hot-rolled coils. The tempering time is calculated from the time the furnace temperature rises to the tempering temperature after the substrate enters the furnace. The formula for calculating the tempering time is as follows:

[0044] t3=A3*D+K3 (3);

[0045] In equation (3), t3 is the tempering time, in minutes;

[0046] K3 is the tempering time base value, in minutes, ranging from 15 minutes to 35 minutes, with a preferred value of 25 minutes; D is the substrate thickness, in millimeters; A3 is the tempering time coefficient, ranging from 1.2 to 1.6, with a preferred value of 1.4.

[0047] Differences from traditional low-temperature tempering process

[0048] For traditional discontinuous tempering furnaces, such as box-type resistance furnaces, the tempering time is calculated from the time the furnace temperature rises to the tempering temperature after the substrate is placed in the furnace. Current low-temperature tempering times range from 1 to 3 hours. An empirical formula for determining the tempering time is:

[0049] t4=A4*D+K4 (4);

[0050] In equation (4), t4 is the tempering time, in minutes;

[0051] K4 is the tempering time base value, in minutes; when performing low-temperature tempering at temperatures below 300℃ in a box-type resistance furnace, the value of K4 ranges from 60 minutes to 180 minutes, with a recommended value of 120 minutes; D is the thickness of the substrate, in millimeters; A4 is the tempering time coefficient, with a value of 1.0 for box-type resistance furnaces.

[0052] In the traditional formula (4) for calculating the tempering time of a box-type resistance furnace, since the value of K4 ranges from 60 minutes to 180 minutes, with a recommended value of 120 minutes, the tempering time is usually at least 2 hours. The continuous tempering time base K3 of this application is 15 minutes to 35 minutes. The tempering time base of this application is significantly lower than that of the traditional low-temperature tempering process. The main reason for this is related to the composition of the steel and the main purpose of this application is to optimize the cold bending performance. The main purpose of the traditional low-temperature tempering process is to perform low-temperature tempering treatment on quenched parts to reduce residual internal stress and improve toughness and plasticity, without considering cold bending performance.

[0053] In the formula (3) for calculating the tempering time in this application, the tempering time coefficient is 0.4 higher than that of the traditional box-type resistance furnace, which is mainly related to the type of heating furnace.

[0054] Based on the above, embodiments of this application provide a method for preparing high-strength steel plates with improved cold bending performance, comprising:

[0055] S1. Provide hot-rolled coil as the substrate;

[0056] S2. Hot-rolled coil is used as the substrate. The substrate is heated and kept at a temperature in a steel plate quenching furnace to austenitize the substrate structure before quenching.

[0057] S3. The quenched substrate is then subjected to low-temperature tempering in a steel plate tempering furnace to obtain a high-strength steel plate.

[0058] The method for preparing high-strength steel plates with improved cold bending performance according to embodiments of this application involves selecting hot-rolled coils as substrates. The substrate is first heated and held at a certain temperature in a steel plate quenching furnace for a period of time to austenitize the substrate microstructure, followed by quenching. The quenched steel plate is then subjected to low-temperature tempering in a steel plate tempering furnace. Through appropriate tempering temperature and time, the quenched martensite microstructure of the substrate is transformed into tempered martensite microstructure with dispersed precipitates and low residual internal stress. This microstructure exhibits excellent cold bending performance and high strength, resulting in heat-treated high-strength steel plates with improved cold bending performance that meet national standards. In addition to improving the cold bending performance of heat-treated high-strength steel plates, the method of this application provides ample mechanical properties and significantly improves the production efficiency of the tempering process by significantly shortening the tempering time.

[0059] The process path for providing hot-rolled coils includes blast furnace ironmaking, hot metal pretreatment, converter, LF-RH refining, continuous casting of slabs, slab heating, rough rolling, finish rolling, coiling, and plate cutting.

[0060] According to an embodiment of this application, a method for preparing a high-strength steel plate with improved cold bending performance is provided, step S1 comprising providing a hot-rolled coil having the following elemental contents: C 0.1–0.20%, Si 0.1–0.5%, Mn 1.0–1.5%, Cr 0.1–0.7%, Mo 0.2–0.7%, Al 0.02–0.1%, P ≤ 0.0012%, S ≤ 0.005%, with the remainder being Fe and unavoidable residual and impurity elements from the smelting process. Residual elements refer to elements that cannot be completely removed.

[0061] According to the preparation method for preparing high-strength steel plates with improved cold bending performance according to the embodiments of this application, the element content of the hot-rolled coil provided in step S1 also satisfies: CEV (carbon equivalent) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cr + Ni) / 15 ≤ 0.82%.

[0062] According to an embodiment of this application, step S2 uses a hot-rolled coil as a substrate, heats and holds the substrate in a steel plate quenching furnace to austenitize the substrate structure, and then quenches it, including:

[0063] Hot-rolled coils are used as the substrate;

[0064] A continuous steel plate quenching furnace is used to heat and hold the substrate before quenching in order to obtain an austenitic substrate. The holding time of the continuous steel plate quenching furnace is calculated from the time the furnace temperature rises to the quenching temperature after the substrate enters the furnace.

[0065] The austenitized substrate is then quenched.

[0066] In the embodiments of this application, the step of heating and holding the substrate before quenching using a continuous steel plate quenching furnace includes:

[0067] In a continuous steel plate quenching furnace, using coal gas combustion in a radiant tube via burners as the heat source and nitrogen as the protective atmosphere, the steel plate is heated and held at 860℃~930℃ before quenching. That is, the quenching temperature is 860℃~930℃.

[0068] In one embodiment of this application, step S2 uses a roller press quenching machine for quenching at a temperature of 860°C to 880°C.

[0069] In the embodiments of this application, step S2 uses a continuous quenching furnace to heat and hold the substrate before quenching. The holding time is calculated from the time the furnace temperature rises to the quenching temperature after the substrate enters the furnace. The formula for calculating the holding time is:

[0070] t1=a1k1D (1)

[0071] In equation (1), t1 is the quenching heating and holding time, in minutes;

[0072] a1 is the heating coefficient when hot-rolled coil is heated in a continuous heating furnace with gas burning in radiant tubes through burners as the heat source and nitrogen as the protective atmosphere inside the furnace. The unit is minutes / mm. The value is 1.1 to 1.3 for carbon steel and 1.4 to 1.7 for alloy steel.

[0073] D is the thickness of the substrate, in millimeters;

[0074] k1 is the substrate loading correction factor, which is 1.0 when the substrates are not stacked.

[0075] According to an embodiment of this application, step S3 involves tempering and holding the quenched substrate in a steel plate tempering furnace to obtain a heat-treated steel plate, comprising:

[0076] The quenched substrate is then tempered in a continuous steel plate tempering furnace using coal gas combustion through burners as the heat source, with a tempering temperature of 200℃~230℃, to obtain a heat-treated high-strength steel plate with improved cold bending performance.

[0077] In one embodiment, the preferred tempering temperature is 215°C to 225°C.

[0078] In one embodiment, the tempering temperature is preferably 220°C.

[0079] To ensure that the strength of the high-strength steel plate meets the requirements, a low-temperature tempering process is used to temper the substrate.

[0080] In the preparation method prior to the improvements in this application, step S3 involves tempering in a continuous steel plate tempering furnace using gas passing through a burner as a heat source. The tempering time is calculated using the tempering time of a traditional discontinuous tempering furnace, specifically a box-type resistance furnace, starting from when the furnace temperature rises to the tempering temperature after the substrate enters the furnace. The calculation formula is as follows:

[0081] t4=A4*D+K4 (4)

[0082] In formula (4), t4 is the tempering time in minutes; K4 is the tempering time base in minutes; D is the thickness of the substrate in millimeters; and A4 is the tempering time coefficient, which is 1.0.

[0083] When performing low-temperature tempering in a box-type resistance furnace at temperatures below 300℃, the value of K4 should range from 60 minutes to 180 minutes, with a recommended value of 120 minutes.

[0084] In one embodiment, the tempering temperature is 220°C, K4 is 63 minutes, A4 is 1.0, D is 7 mm, and the tempering time t4 = 1.0 * 7 + 63 = 70 minutes.

[0085] The method described in this application can improve the cold bending performance of steel plates while shortening the tempering and holding time by approximately one hour, effectively improving production efficiency. The continuous tempering furnace comprises multiple sections. Each section in the low-temperature tempering process uses the same furnace temperature setting. Except for the first section, which is approximately 20°C lower than the setting temperature, the actual furnace temperatures of the other sections can be controlled within ±15°C of the setting temperature. Unless otherwise specified, tempering temperature refers to the set temperature of the tempering furnace, and tempering time refers to the holding time during tempering. Tempering time is calculated starting when the actual furnace temperature is not lower than the set temperature minus 15°C. Since the continuous tempering furnace typically has more than 10 sections, the tempering time of the steel plate during low-temperature tempering is approximately equal to the time spent in the furnace.

[0086] In one embodiment, the tempering temperature in step S3 is 200℃~230℃, and the tempering time is the tempering time when K3 is 10 minutes to 35 minutes in formula (3).

[0087] The inventors' research revealed:

[0088] 1) The optimal tempering temperature for LG1100QT steel plate is 220℃;

[0089] 2) When the tempering temperature is 220℃, the K3 value in formula (3) is best when the cold bending performance is 25 minutes. When the K3 value is 10 minutes to 20 minutes, the transverse impact toughness reaches the maximum value. However, the K3 value with the best cold bending performance and the K3 value with the greatest transverse impact energy are significantly lower than the traditional low temperature tempering process.

[0090] Example: Five steel plates with a thickness * width * length of 7 * 2000 * 5000 mm were continuously cut from a hot-rolled coil of LG1100QT with coil number 1A29029600. These plates were quenched on-site using the same quenching process: quenching temperature 870℃, holding time 25 minutes, water quenching. The quenched steel plates were then subjected to low-temperature tempering at 220℃ to test the effect of different tempering times on mechanical properties and cold bending properties. The cold bending property test conditions were: the cold bending sample direction was transverse (perpendicular to the rolling direction); the cold bending sample dimensions were: thickness * width * length = 7 * 100 * 200 mm; and the spacing between the cold bending test rolls = mandrel diameter + 3a. The effect of tempering time on mechanical properties is shown in Table 1 below, and the effect of tempering time on cold bending properties is shown in Table 2 below.

[0091] Table 1. Effect of tempering time on mechanical properties

[0092]

[0093] Remark:

[0094] 1) Steel plates No. 1, 2, 3, 4, and 5 are all heated and kept warm in a continuous steel plate heating furnace with coal gas burning in the radiant tube through burners as the heat source and nitrogen as the protective atmosphere inside the furnace. After the substrate structure is austenitized, it is quenched by water spraying using a roller press quenching machine. Tempering is carried out in a continuous steel plate tempering furnace with coal gas burning in the burners as the heat source.

[0095] 2) The tempering time of No. 5 steel plate adopts the tempering time of the traditional non-continuous tempering heating furnace, that is, the tempering time of the box-type resistance furnace. The calculation formula is shown in formula (4). The tempering time base K4 in formula (4) is 63 minutes, the tempering time coefficient A4 is 1.0, and the tempering time = 1.0 * 7 + 63 = 70 minutes.

[0096] When the tempering temperature is 220℃, Table 1 shows that: the tempering time of 20 to 30 minutes has little effect on the transverse impact toughness, with a variation of no more than 2%; after the tempering time is greater than 30 minutes, the transverse impact toughness shows a decreasing trend. When the tempering time is extended from 30 minutes to 70 minutes, the transverse impact energy decreases from 71J to 56J, a decrease of 21%; the tempering time of 20 to 70 minutes has little effect on the yield strength and tensile strength, with a variation of no more than 4%.

[0097] Table 2. Effect of tempering time on cold bending performance

[0098]

[0099] Remark:

[0100] 1) Steel plates No. 1, 2, 3, 4, and 5 are all heated and kept warm in a continuous steel plate heating furnace with coal gas burning in the radiant tube through burners as the heat source and nitrogen as the protective atmosphere inside the furnace. After the substrate structure is austenitized, it is quenched by water spraying using a roller press quenching machine. Tempering is carried out in a continuous steel plate tempering furnace with coal gas burning in the burners as the heat source.

[0101] 2) The tempering time of No. 5 steel plate adopts the tempering time of the traditional non-continuous tempering heating furnace, that is, the tempering time of the box-type resistance furnace. The calculation formula is shown in formula (4). In formula (4), the tempering time base K4 is 60 minutes and the tempering time coefficient A4 is 1.0, that is, tempering time = 1.0 * 7 + 63 = 70 minutes.

[0102] Table 2 shows that, at a tempering temperature of 220℃, the tempering time should be between 20 and 35 minutes to obtain good cold bending performance, with the preferred K3 value being 25 minutes, meaning the preferred tempering time is 35 minutes.

[0103] Comparing Tables 1 and 2, it can be seen that the optimal K3 value for transverse impact toughness is 10-20 minutes, while the optimal K3 value for transverse cold bending performance is 25 minutes. Therefore, it can be concluded that the optimal K3 value for transverse impact toughness is not the same as the optimal K3 value for cold bending performance.

[0104] From Tables 1 and 2, it can be seen that when K3 is set to 10 minutes, for a steel plate with a thickness of 7 mm, the tempering time = 1.4 * 7 + 10 = 20 minutes. (See attached table.) Figure 1 As shown, at this time, the tempering time is too short. Although the precipitates are small, the optimal cold bending performance cannot be obtained because most of the internal stress cannot be eliminated.

[0105] When K3 is 25 minutes, the corresponding tempering time is 35 minutes. Figure 2 As shown, at this point, the precipitation of precipitates is relatively sufficient, but still fine and dispersed, and the elimination of quenching internal stress is more complete than when the tempering time is 20 minutes. When the K4 value is 63 minutes, a 7 mm thick steel plate is tempered in a continuous tempering furnace according to the tempering time required for low-temperature tempering in a traditional box furnace. At this time, the tempering time t4 = 1.0 * 7 + 63 = 70 minutes. Figure 3 As shown, although the tempering time t4 is significantly longer than the tempering time t3 in this application, the internal stress relief is more complete than when tempering for 35 minutes, but the precipitates have become significantly coarser, leading to a decrease in cold bending performance. The initial precipitates formed during low-temperature tempering of quenched martensite are coherent with the parent phase. However, if the tempering time is too long, the precipitates will aggregate and grow, transforming into precipitates incompatible with the parent phase, resulting in a significant decrease in the bonding force with the parent phase and thus a decrease in cold bending performance.

[0106] Cold bending properties of high-strength steel plates and their testing methods

[0107] GB / T232-2010 specifies the following requirements for the width of bending test specimens for sheet metal products: The specimen width shall comply with the requirements of the relevant product standard. If not specifically specified, it shall comply with the following requirements:

[0108] a) When the product width is not greater than 20mm, the sample width is the original product width;

[0109] b) When the product width is greater than 20mm:

[0110] —When the product thickness is less than 3mm, the sample width is (20±5)mm;

[0111] —When the product thickness is not less than 3mm, the sample width should be between 20mm and 50mm.

[0112] GB / T232-2010 specifies the following requirements for the thickness of bending test specimens for sheet metal products: For sheet metal, the specimen thickness should be the original product thickness. If the product thickness is not greater than 25 mm, the specimen thickness can be machined to a minimum of 25 mm, while retaining the original thickness on one side. During the bending test, the retained surface of the specimen should be located on the side of tensile deformation.

[0113] GB / T232-2010 requires the following spacing l for the sample support rollers when performing bending tests on sheet metal products: l = bending core diameter + 3a ± a / 2, where a is the thickness of the sample.

[0114] The standard implemented by LG1100QT is GB / T 280909-2012 "Heat-treated steel plates for ultra-high strength structures". This standard does not specify the cold bending performance of the product or the width of the cold bending test specimen. As a result, the cold bending performance of this product could only be tested according to the method specified in GB / T232-2010 "Metallic materials bending test method".

[0115] However, when samples of heat-treated high-strength steel plates with a thickness of 6mm to 16mm and a yield strength of 960MPa or higher (the specifications of the cracked steel plates are: thickness * width * length = 5~16 * 900~2060 * 5000~14000 mm) were taken and subjected to bending performance tests according to GB / T232-2010 with the same mandrel diameter, bending angle, and support roller spacing as the user, no cracking occurred. Therefore, the bending test method for high-strength steel plates needs to be improved to better evaluate the actual bending or cold bending performance of the steel plates and guide the improvement of high-strength steel plate production processes. LG1100QT high-strength steel plates are mainly used to manufacture crane booms, with a bending angle of 90°. To ensure that the steel plate's transverse cold bending performance meets the user's requirements, achieving no cracking when cold-bent at 90° with a mandrel diameter of 6a (a being the nominal thickness of the steel plate, i.e., the nominal thickness of the cold-bending sample), the cold bending performance method used in this application was adopted.

[0116] 1) For steel plates with a nominal thickness of less than 10 mm, the width of the cold bending test specimen is 100 mm;

[0117] 2) For steel plates with a nominal thickness of 10 to 25 mm, the width of the cold-bending specimen shall be ≥ 10 times the nominal thickness of the steel plate;

[0118] 3) Cut transverse cold bending specimens using wire cutting;

[0119] 4) The spacing between the support rollers in the bending test is l = bending core diameter + 3a = 4a + 3a;

[0120] 5) Bending angle 90°;

[0121] 6) The bend line is parallel to the rolling direction of the steel plate;

[0122] 7) Any matters not covered herein shall be handled in accordance with GB / T232-2010.

[0123] The criteria for determining the cold bending performance of LG1100QT high-strength steel plate are as follows: When using the cold bending performance test method of this application, no visible cracks are found when bending 90° without bending the core diameter to a value not greater than 5a.

[0124] The inventor discovered through research that:

[0125] The width-to-thickness ratio of the cold-bending LG1100QT steel plate specimens has a significant impact on the test results, and the spacing between the support rollers also has a noticeable impact on the test results.

[0126] Transverse cold-bending specimens of different widths were cut from the 1 / 4 width portion of an LG1100QT finished steel plate with the same thickness * width * length = 10 * 2000 * 12000 mm. The specifications of the cold-bending specimens were: width * length * thickness of 20 / 40 / 60 / 80 / 100 / 120 / 140 * 160 * 10 mm. Using the cold-bending test method used in this application, cold-bending tests were conducted on a SANS BHT5106 bending testing machine and a UBB-500 / 3200D steel plate bending machine manufactured by Ruitie CNC Machine Tool Co., Ltd. The test results are shown in Tables 3 and 4 below:

[0127] Table 3. Influence of cold-bending specimen width on test results

[0128]

[0129] Note: The test equipment used in Table 3 is the UBB-500 / 3200D steel plate bending machine from Ruitie CNC Machine Tool Co., Ltd.

[0130] Table 4. Influence of cold-bending specimen width on test results

[0131]

[0132] Note: The testing equipment used in Table 4 is the SANS BHT5106 bending testing machine.

[0133] In Tables 3 and 4, 1.6a, 2a, 3a, 4a, and 5a represent the mandrel diameters during the 90° cold bending test. For a given cold bending specimen, when the specimen width b < 10a, the 90° cold bending test results of the steel plate all changed from no cracks to cracks as the width-to-thickness ratio increased, indicating that the width has a significant impact on the cold bending performance test results. However, when the specimen width b ≥ 10a, there was no significant effect on the 90° cold bending test results of the steel plate as the width-to-thickness ratio continued to increase, indicating that the width b ≥ 10a has no effect on the cold bending performance test results of the steel plate.

[0134] As can be seen from the test results in Tables 3 and 4, the cold bending performance test results conducted according to the current national standard GB / T232-2010 "Metallic Materials - Test Method for Bending" (hereinafter referred to as the standard cold bending test results) are insufficient to detect the actual cold bending performance of steel plate materials. The main reason for this is the unreasonable selection of the specimen width. This standard stipulates that when the steel plate is greater than 20 mm and the thickness is not less than 3 mm, the specimen width should be 20 mm to 50 mm. For steel plate products with a thickness ≤ 25 mm, a full-thickness cold bending specimen should be taken. Since GB / T232-2010 does not specify the width-to-thickness ratio of the cold bending specimen, when testing the cold bending performance of steel plates according to this standard, the intermediate width cold bending specimens within the width range of 20 mm to 50 mm specified in GB / T232-2010 are usually used, i.e., cold bending specimens with a width of 35 mm. At this time, only cold bending specimens of steel plates with a thickness of 3 mm to 3.5 mm meet the requirement of a width-to-thickness ratio ≥ 10 mm proposed in this application.

[0135] To more accurately evaluate the actual cold bending performance of steel plates and guide on-site process improvements, based on the research results in this application, the specimen width must be ≥10a (where a is the nominal thickness of the steel plate). The principle is as follows: when a cold-bending specimen with a width-to-thickness ratio less than 10 undergoes bending deformation, in addition to longitudinal extension deformation, the metal on the outer arc side exhibits significant inward transverse flow, which "fills" the longitudinal extension and prevents cracking. The smaller the width-to-thickness ratio, the more pronounced this filling effect and the less prone to cracking.

[0136] To further verify the influence of the width-to-thickness ratio of the cold-bending specimens on the cold-bending test results, transverse cold-bending specimens with widths of 2a and 10a were cut from the 1 / 4 width portion of the same 10*2000*12000 mm LG1100QT finished steel plate after tempering optimization, and 90° cold-bending tests were conducted. That is, the widths of the cold-bending specimens were 10*20*160 mm and 10*100*160 mm, respectively. The test results are shown below. Figure 4 , Figure 5 .

[0137] Figure 4 The results of a transverse cold bending performance test were conducted on a 10*2000*12000 mm LG1100QT hot-rolled steel plate provided in one embodiment of this application after quenching at 870℃ for 29 min and tempering at 220℃ for 39 min. The cold bending sample specifications (thickness*width*length) were 10*20*160 mm, the support roller spacing was 46 mm, and it was bent 90° with a bending mandrel diameter of 1.6a (16 mm). There were no cracks on the outer arc surface of the bent part, and the width was reduced from the actual 19.5 mm to 15.5 mm, that is, the outer arc surface width was reduced by 4.0 mm, and the outer arc surface width reduction ratio was 20.51%.

[0138] Figure 5 The results of a transverse cold bending performance test were obtained by sampling a 10*2000*12000 specification LG1100QT hot-rolled steel plate provided in one embodiment of this application after quenching at 870℃ for 29 min and tempering at 220℃ for 39 min. The cold bending sample specifications (thickness*width*length) were 10*100*160 mm, the support roller spacing was 70 mm, and it was bent at 90° with a bending mandrel diameter of 4a (40 mm). There were no cracks on the outer arc surface of the bending part. The width of the outer arc surface was reduced from 100.0 mm to 98.5 mm, a reduction of 1.5 mm, which is 1.5%. That is, when the 10a width cold bending sample was bent, in addition to the longitudinal extension deformation, the inward transverse flow of the metal on the outer arc side played a "filling" role on the longitudinal extension, which was significantly weakened and less prone to cracking than the 2a width cold bending sample.

[0139] When the width-to-thickness ratio increases from 2 to 10, the diameter of the bending core that does not crack in the 90° cold bending test increases from 1.6a to 4a, that is, the 90° cold bending performance decreases from 1.6a to 4a. The significant influence of the width-to-thickness ratio of the cold bending specimen on the cold bending test results is verified again, as shown in Tables 3 and 4 for the test results of the 20 mm wide cold bending specimen.

[0140] The inventors' research also found that the spacing between the support rollers in the cold bending sample has a significant impact on the test results of the 90° cold bending performance of LG1100QT high-strength steel plate.

[0141] 20 mm wide transverse cold-bending specimens were cut from a quarter-width section of an LG1100QT finished steel plate with a thickness * width * length = 10 * 2000 * 12000 mm using wire cutting. Cold bending tests of 90° were conducted using a SANS BHT5106 bending testing machine and a UBB-500 / 3200D steel plate bending machine manufactured by Ruitie CNC Machine Tool Co., Ltd., with a mandrel diameter of 1.6a (16 mm). The test results showed that the roller spacing had a significant impact on the cold bending performance test results (see Tables 5 and 6 below). No cracks appeared in the cold-bending specimens when the roller spacing was 46 mm, small cracks appeared when the roller spacing was 41 mm, and the cold-bending specimens broke when the roller spacing was 36 mm. Figure 6 As shown:

[0142] Table 5. Influence of support roll spacing on cold bending test results

[0143]

[0144] Note: The test equipment used in Table 5 is the UBB-500 / 3200D steel plate bending machine from Ruitie CNC Machine Tool Co., Ltd.

[0145] Table 6. Influence of support roll spacing on cold bending test results

[0146]

[0147] Note: The testing equipment used in Table 6 is the SANS BHT5106 bending testing machine.

[0148] Based on the research on the test methods for the cold bending performance of steel plates mentioned above, it was determined that the width of the specimen for testing the cold bending performance of steel plates needs to be ≥10 times the nominal thickness of the steel plate to be tested or the nominal thickness of the cold bending specimen. To facilitate specimen processing, for LG1100QT steel plate products with thicknesses ranging from 4 mm to 10 mm that have been developed and produced, the width of the cold bending specimen is uniformly specified as 100 mm.

[0149] Furthermore, to eliminate the influence of the support roll spacing on the cold bending performance test results, a support roll spacing of D+3a was used to test the 90° cold bending performance of the steel plate, instead of D+3a±3a as specified in GB / T232-2010. Using the above-mentioned width of the cold bending specimen and a support roll spacing of D+3a, the cold bending performance of the steel plate prepared in the embodiments of the method for preparing high-strength steel plates with improved cold bending performance in this application was tested. Any matters not covered in the cold bending performance test were handled in accordance with the provisions of the current national standard GB / T232-2010 "Metallic Materials - Test Method for Bending".

[0150] Implementation Results and Related Explanations

[0151] The method for preparing high-strength steel plates with improved cold bending performance according to the embodiments of this application results in the following cold bending performance of LG1100QT high-strength steel plates: impact energy at -40℃ is much greater than 27J, reaching at least 56J in the transverse direction and 106J in the longitudinal direction; elongation is greater than 10% in both directions; yield strength in both the transverse and longitudinal directions is greater than 1200MPa; longitudinal tensile strength reaches 1378MPa~1401MPa; and transverse tensile strength reaches 1386MPa~1422MPa. All mechanical properties meet existing national standards and have sufficient margin. Furthermore, it does not crack when the mandrel diameter is 4a in a 90° cold bending test, compared to existing high-strength steels that do not crack when the mandrel diameter is 6a in a 90° cold bending test. This demonstrates that the method for preparing high-strength steel plates with improved cold bending performance according to the embodiments of this application significantly improves the cold bending performance of the steel plates.

[0152] 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 processes of the systems, modules, and units described above can be referred to the corresponding processes 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 preparing high-strength steel plates with improved cold bending properties, characterized in that, include: S1. Provide hot-rolled coil as substrate. Step S1 provides hot-rolled coil with the following elemental content: C 0.1-0.20%, Si 0.1-0.5%, Mn 1.0-1.5%, Cr 0.1-0.70%, Mo 0.2-0.7%, Al 0.02-0.1%, P≤0.0012%, S≤0.005%, with the remainder being Fe and unavoidable residual and impurity elements from the smelting process. The elemental content of the hot-rolled coil satisfies: CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cr+Ni) / 15≤0.82%. S2. Using hot-rolled coil as the substrate, the substrate is heated and kept warm in a steel plate quenching furnace, including: heating and keeping warm at 860℃~930℃ in a continuous steel plate quenching furnace with coal gas burning in a burner in a radiant tube as the heat source and nitrogen as the protective atmosphere in the furnace to austenitize the substrate structure before quenching. S3. The quenched substrate is then subjected to low-temperature tempering in a steel plate tempering furnace. Step S3 includes: the quenched substrate is then subjected to tempering in a continuous steel plate tempering furnace with gas passing through a burner as the heat source. The tempering temperature is 200℃~230℃, to obtain a heat-treated high-strength steel plate with improved cold bending performance.

2. The method according to claim 1, characterized in that... Step S2 includes: Hot-rolled coils are used as the substrate; The substrate is heated and held at a constant temperature before quenching using a continuous steel plate quenching furnace to obtain an austenitic substrate. The holding time for heating and holding in a non-continuous steel plate quenching furnace is calculated from the time the furnace temperature rises to the quenching temperature after the substrate enters the furnace. The quenching temperature is 860℃~930℃. The substrate after austenitization is quenched.

3. The method according to claim 1 or 2, characterized in that, The formula for calculating the heat preservation time is: t1=a1k1D (1) In equation (1), t1 is the quenching heating and holding time, in minutes; a1 is the heating coefficient, in minutes per millimeter; D is the thickness of the substrate, in millimeters. k1 is the substrate loading correction factor, with a value of 1.

0.

4. The method according to claim 3, characterized in that The substrate is made of carbon steel, and the heating coefficient a1 is 1.1 to 1.3; or the substrate is made of alloy steel, and the heating coefficient a1 is 1.4 to 1.

7.

5. The method according to claim 1, characterized in that, In step S3, the tempering time when tempering in a continuous tempering furnace is calculated from the time the furnace temperature rises to the tempering temperature after the substrate enters the continuous tempering furnace.

6. The method according to claim 5, characterized in that, The formula for calculating the tempering time is: t3=A3*D+K3 (3); In equation (3), t3 is the tempering time, in minutes; A3 is the tempering time coefficient; D represents the effective thickness of the substrate, in millimeters; K3 represents the tempering time base, in minutes.

7. The method according to claim 6, characterized in that, The value of A3 is 1.2 to 1.6, and the value of the tempering time base K3 is 15 minutes to 35 minutes.

8. A high-strength steel, characterized in that, It is prepared according to the method for preparing high-strength steel plates with improved cold bending properties according to any one of claims 1 to 7.

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