Methods for determining tempering temperature and tempering time of hot-rolled tempered plates and their applications

Abstract

This application provides a method for determining the tempering temperature and tempering time of hot-rolled tempered steel plates and its application. The method includes the following steps: S10: Measuring the Ac1 of the hot-rolled steel plate to determine the tempering temperature Tc of the hot-rolled tempered steel plate = Ac1 - 20~50℃; S20: Under the condition of tempering temperature Tc, measuring the tempering time required for two or more groups of hot-rolled steel plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering treatment, and calculating the obtained data from the two or more groups according to formula t. i =K+Ad i Perform fitting to determine the K and A values; S30: Based on the determined K and A values ​​and the thickness of the hot-rolled plate to be tempered, according to the above formula t i =K+Ad i The required tempering time for the hot-rolled sheet to be tempered at the tempering temperature Tc is determined. The hot-rolled tempered sheet obtained by tempering under the conditions determined using the method provided in this application exhibits good mechanical properties and cold bending properties.

Description

Technical Field

[0001] This application relates to the field of hot-rolled tempered steel plate production technology, specifically to methods for determining the tempering temperature and tempering time of hot-rolled tempered steel plates and their applications. Background Technology

[0002] Hot-rolled tempered steel sheets, produced from hot-rolled steel plates, are a type of product developed and manufactured in the last decade. These sheets are made by tempering hot-rolled steel plates to remove residual internal stress. Compared to hot-rolled high-strength steel plates of the same strength grade, tempered steel sheets have lower internal stress levels, resulting in significantly improved shape stability after forming. Currently, these products are mainly used in fields such as engineering machinery. To remove residual internal stress quickly, hot-rolled tempered steel sheets typically employ a high-temperature tempering process.

[0003] Traditional high-temperature tempering refers to a heat treatment method where steel parts are heated to austenitize, then rapidly cooled to obtain quenched martensite, and subsequently tempered at a temperature range of 500℃ to 650℃ to obtain tempered sorbite with good comprehensive properties. The specific tempering temperature depends on the required hardness after tempering, and the specific tempering time is determined by the time required for the quenched martensite to transform into tempered sorbite and the time required to reach the required hardness. The inventors discovered that hot-rolled tempered plates obtained by using the traditional high-temperature tempering process cannot simultaneously meet the requirements for mechanical properties and cold bending properties. Summary of the Invention

[0004] This application provides a method for determining the tempering temperature and tempering time of hot-rolled tempered plates and its application, in order to solve the problem that hot-rolled tempered plates obtained by traditional high-temperature tempering processes cannot simultaneously meet the requirements of mechanical properties and cold bending properties.

[0005] In a first aspect, this application provides a method for determining the tempering temperature and tempering time of hot-rolled tempered steel plates, characterized by comprising the following steps:

[0006] S10: Measure Ac1 of the hot-rolled plate to determine the tempering temperature T of the hot-rolled tempered plate. c =Ac1-20~50℃;

[0007] Among them, T c Tempering temperature, in °C;

[0008] Ac1 is the temperature at which ferrite begins to transform into austenite when the hot-rolled plate is heated, in °C.

[0009] S20: At a tempering temperature of T cUnder the condition of [condition], the tempering time required for two or more groups of hot-rolled plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering treatment was determined, and the obtained data from the two or more groups were fitted according to formula (1) to determine the K value and A value.

[0010] t i =K+Ad i (1)

[0011] Among them, t i The required tempering time for a given thickness of hot-rolled sheet, in minutes;

[0012] K is the tempering time base, in minutes;

[0013] A is the tempering time coefficient, in min / mm;

[0014] d i The thickness of the hot-rolled plate is in mm.

[0015] S30: Based on the determined K and A values ​​and the thickness of the hot-rolled plate to be tempered, determine the tempering temperature T for the hot-rolled plate to be tempered according to the above formula (1). c The required tempering time under the given conditions.

[0016] In the technical solution of this application, the temperature Ac1 at which ferrite begins to transform into austenite during the heating of hot-rolled plate is measured, according to formula T. c =Ac1-20~50℃ to determine the tempering temperature of the hot-rolled plate. Under this tempering temperature, the time required for two or more groups of hot-rolled plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering is measured. The data is fitted to obtain the relationship between the tempering time and the thickness of the hot-rolled plate, thereby determining the tempering temperature and tempering time required for the hot-rolled tempered plate. The hot-rolled tempered plate obtained under this tempering condition has good mechanical properties and cold bending properties.

[0017] In some embodiments of this application, step S10, specifically measuring Ac1 of the hot-rolled plate, includes:

[0018] Ac1 of hot-rolled steel plates was determined using the thermal expansion method.

[0019] Secondly, this application provides a method for preparing a hot-rolled tempered sheet, comprising the following steps:

[0020] S100: Determine the tempering temperature and tempering time of the hot-rolled tempered plate according to the method described in any of the above embodiments;

[0021] S200: The hot-rolled plate is tempered at the tempering temperature and tempering time determined in step S100 to obtain the hot-rolled tempered plate.

[0022] In the technical solution of this application, the hot-rolled tempered plate obtained by tempering the hot-rolled plate using the tempering temperature and tempering time determined by the method provided in the first aspect of this application has good mechanical properties and cold bending properties, and meets the relevant delivery standards and usage requirements.

[0023] In some embodiments of this application, in step S200, the hot-rolled plate contains Mo.

[0024] In some embodiments of this application, in step S200, the metallographic structure of the hot-rolled plate is bainite + ferrite + pearlite.

[0025] In some embodiments of this application, in step S200, the hot-rolled plate has the following chemical composition by mass percentage:

[0026] C: 0.03–0.09%;

[0027] Si: 0.1-0.5%;

[0028] Mn: 1.0–1.9%;

[0029] Ti: 0.11–0.18%;

[0030] Nb: 0.01–0.06%;

[0031] Mo: 0.1–0.7%;

[0032] Al: 0.02–0.06%;

[0033] P: 0–0.015%;

[0034] S: 0~0.003%;

[0035] N: 0–0.006%;

[0036] The remainder consists of iron and unavoidable impurity elements.

[0037] In some embodiments of this application, step S200 specifically includes:

[0038] The hot-rolled plate is tempered in a continuous tempering furnace at the tempering temperature and time determined in step S100 to obtain the hot-rolled tempered plate, wherein the continuous tempering furnace uses a gas combustion flame to heat the hot-rolled plate from both the top and bottom.

[0039] In some embodiments of this application, in step S200, the tempering temperature during the tempering process is 620–650°C;

[0040] Optionally, the tempering time base K is 6 mm, and the tempering time coefficient A is 1.5 min / mm.

[0041] In some embodiments of this application, the yield strength of the hot-rolled tempered plate is ≥750MPa;

[0042] Optionally, the tensile strength of the hot-rolled tempered plate is 815–940 MPa;

[0043] Optionally, the elongation after fracture of the hot-rolled tempered plate is ≥15%;

[0044] Optionally, the longitudinal impact energy of the hot-rolled tempered plate at -40℃ is ≥47J;

[0045] Optionally, the hot-rolled tempered sheet has a cold bending performance D = a - no cracks after a 180° cold bend.

[0046] Thirdly, this application provides a hot-rolled tempered sheet, which is prepared according to the preparation method described in any of the above embodiments.

[0047] In the technical solution of this application, the hot-rolled tempered plate prepared by the preparation method of the hot-rolled tempered plate provided in the second aspect of this application has good mechanical properties and cold bending properties. Attached Figure Description

[0048] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0049] Figure 1 This is a metallographic diagram of the hot-rolled plate before tempering.

[0050] Figure 2 Metallographic structure of a 10mm thick 700MPa hot-rolled tempered plate obtained by tempering at 610℃ for 21min.

[0051] Figure 3 Metallographic structure of a 10mm thick, 700MPa hot-rolled tempered sheet obtained by tempering at 650℃ for 21 minutes.

[0052] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0053] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0054] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0055] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0056] The inventors discovered that there are currently no effective standards for the tempering process of hot-rolled tempered steel sheets. In existing technology, the tempering process for hot-rolled tempered steel sheets mainly refers to the traditional high-temperature tempering process, i.e., a tempering temperature of 500℃~650℃, while the tempering time is calculated in tons (t). Ι =K Ι +A Ι *D(2) is determined. In equation (2): t Ι Tempering time, in minutes; K Ι This is the base value for tempering time, in minutes, ranging from 30 to 60 minutes; A Ι , where is the tempering time coefficient (min / mm), and the value range for tempering in air is 1.0 to 1.5, depending on the chemical composition of the steel plate; D is the thickness of the steel plate (mm). The inventors discovered that the mechanical properties and cold bending properties of the hot-rolled plate after the above tempering process cannot simultaneously meet the requirements of relevant technical standards.

[0057] Conventional high-temperature tempering processes must complete the microstructure transformation from quenched martensite to tempered sorbite and achieve the required hardness. For low-alloy high-strength hot-rolled tempered steel plates, Mo (Mo) is generally added to ensure strength and improve weldability, thereby increasing the plate strength and improving the toughness of the weld heat-affected zone. However, the addition of Mo promotes the formation of bainite in the hot-rolled microstructure, resulting in a microstructure of bainite + ferrite + a small amount of pearlite. Since the microstructure of the hot-rolled plate before tempering is not quenched martensite, it is difficult to simultaneously meet the user's requirements for the mechanical properties and cold bending performance of the tempered plate using traditional high-temperature tempering processes. Therefore, a method is needed to determine the tempering temperature and tempering time of hot-rolled tempered plates so that they can simultaneously possess good mechanical properties and cold bending performance.

[0058] In a first aspect, this application provides a method for determining the tempering temperature and tempering time of hot-rolled tempered steel plates, characterized by comprising the following steps:

[0059] S10: Measure Ac1 of hot-rolled plate to determine the tempering temperature T of hot-rolled tempered plate. c =Ac1-20~50℃;

[0060] Among them, T c Tempering temperature, in °C;

[0061] Ac1 is the temperature at which ferrite begins to transform into austenite when the hot-rolled plate is heated, in °C.

[0062] S20: At a tempering temperature of T c Under the condition of [condition], the tempering time required for two or more groups of hot-rolled plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering treatment was determined, and the obtained data from the two or more groups were fitted according to formula (1) to determine the K value and A value.

[0063] t i =K+Ad i (1)

[0064] Among them, t i The required tempering time for a given thickness of hot-rolled sheet, in minutes;

[0065] K is the tempering time base, in minutes;

[0066] A is the tempering time coefficient, in min / mm;

[0067] d i The thickness of the hot-rolled plate is in mm.

[0068] S30: Based on the determined K and A values ​​and the thickness of the hot-rolled plate to be tempered, the tempering temperature T for the hot-rolled plate to be tempered is determined according to the above formula (1).c The required tempering time under the given conditions.

[0069] In the technical solution of this application, the tempering temperature of the hot-rolled plate is determined by measuring the temperature Ac1 at which ferrite begins to transform into austenite when the hot-rolled plate is heated. Under this tempering temperature, the time required for two groups of hot-rolled plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering is measured. The data is fitted to obtain the relationship between the tempering time and the thickness of the hot-rolled plate, thereby determining the tempering temperature and tempering time of the hot-rolled tempered plate. The hot-rolled tempered plate obtained under this tempering condition has good mechanical properties and cold bending properties.

[0070] In the technical solution of this application, in step S10, the tempering temperature T of the hot-rolled tempered plate is determined. c =Ac1 -20~50℃. Generally speaking, the main purpose of tempering hot-rolled plates is to reduce the level of residual internal stress. The higher the temperature, the more beneficial it is to eliminate residual internal stress. However, Ac1 is the temperature at which ferrite begins to transform into austenite when the hot-rolled plate is heated. When the tempering temperature is higher than Ac1, the hot-rolled plate will undergo austenite transformation, and during the cooling process after tempering, it will transform into ferrite + pearlite, resulting in a significant reduction in plasticity and toughness. Therefore, the tempering temperature should be controlled below Ac1. At the same time, considering that the furnace temperature fluctuation range in the tempering furnace is generally ±15℃, a certain safety factor is considered to ensure that the actual tempering temperature of the steel plate does not exceed the AC1 point, and considering the efficiency of tempering in removing internal stress, as well as the influence of tempering temperature on mechanical properties, the tempering temperature of hot-rolled tempered plates is set to T. c =Ac1-20~50℃.

[0071] In some embodiments of this application, step S10, measuring the Ac1 of the hot-rolled plate, specifically includes:

[0072] Ac1 of hot-rolled steel plates was determined using the thermal expansion method.

[0073] In some of the above embodiments, the Ac1 of hot-rolled plates is determined by the thermal expansion method. The thermal expansion method is relatively simple to operate and the measurement results are relatively accurate.

[0074] In the technical solution of this application, in step S20, since the tempering time of the hot-rolled tempered plate is affected not only by the chemical composition of the raw steel plate and the hot rolling process, but also by the thickness of the hot-rolled tempered plate, the tempering time is linearly positively correlated with the thickness of the hot-rolled tempered plate. The thicker the plate, the longer the tempering time required. Therefore, in this step, it is mainly necessary to determine the relationship between the tempering time and the thickness of the hot-rolled tempered plate. Therefore, determining the relationship between tempering time and thickness requires at least two sets of data. Thus, it is necessary to use hot-rolled plates of the same type with different thicknesses for debugging to determine the above relationship. Generally speaking, the longer the tempering time, the lower the impact toughness, and the lower the production efficiency. However, if the tempering time is too short, it will result in insufficient tempering, insufficient reduction of residual internal stress, and a decrease in mechanical properties and cold bending performance. Therefore, in the technical solution of this application, at the tempering temperature determined in step S10, the tempering time is determined according to the time required for the hot-rolled tempered plate to reach the maximum yield strength or maximum tensile strength. This yields two or more sets of data on the required tempering times for hot-rolled plates with different thicknesses. These data are then linearly fitted to obtain the relationship between the tempering time and thickness of the hot-rolled tempered plate, i.e., the K value and A value are determined. Then, the required tempering time for the hot-rolled tempered plate can be calculated using the known or detected thickness of the hot-rolled plate.

[0075] In the technical solution of this application, step S30 determines the required tempering time for the hot-rolled plate to be tempered at the tempering temperature Tc based on the determined K and A values ​​and the known or detected thickness of the hot-rolled plate to be tempered, according to the above formula (1).

[0076] Secondly, this application provides a method for preparing a hot-rolled tempered sheet, comprising the following steps:

[0077] S100: Determine the tempering temperature and tempering time of the hot-rolled tempered plate according to the method of any of the above embodiments;

[0078] S200: Temper the hot-rolled plate at the tempering temperature and time determined in step S100 to obtain a hot-rolled tempered plate.

[0079] In the technical solution of this application, the hot-rolled tempered plate obtained by tempering the hot-rolled plate using the tempering temperature and tempering time determined by the method provided in the first aspect of this application has good mechanical properties and cold bending properties, and meets the relevant delivery standards and usage requirements.

[0080] In the technical solution of this application, in step S100, the tempering temperature and time determined by the method provided in the first aspect, compared with the traditional high-temperature tempering process used in the prior art, can determine different tempering processes for hot-rolled plates with different chemical compositions and different hot rolling processes, thereby optimizing the tempering process, obtaining hot-rolled tempered plates with better performance, while taking into account the mechanical properties and cold bending properties of hot-rolled tempered plates, improving production efficiency and yield, and reducing production costs.

[0081] In step S200, conventional tempering treatment is performed according to the tempering temperature and time determined in step S100 to obtain a hot-rolled tempered plate with good mechanical properties and cold bending properties.

[0082] In some embodiments of this application, in step S200, the hot-rolled plate contains the element Mo.

[0083] In some of the above embodiments, as mentioned in the prior art, in order to improve the strength and weldability of hot-rolled tempered plates, Mo is generally added to the hot-rolled plates. Mo is conducive to the formation of bainite in the hot-rolled structure, making the hot-rolled structure bainite + ferrite + a small amount of pearlite, rather than quenched martensite. Hot-rolled tempered plates containing Mo produced by traditional high-temperature tempering processes cannot simultaneously meet the user's requirements for mechanical properties and cold bending properties. However, the preparation method provided in this application can effectively solve the above problems.

[0084] In some embodiments of this application, in step S200, the metallographic structure of the hot-rolled plate is bainite + ferrite + pearlite.

[0085] In some of the above embodiments, the microstructure of the hot-rolled plate is bainite + ferrite + pearlite. As is known from the prior art, only when the microstructure of the hot-rolled plate is quenched martensite (quenched martensite can be obtained by online quenching) can the heat-treated plate with good comprehensive performance be obtained by tempering the hot-rolled plate using the traditional high-temperature tempering process. However, it is not well applicable to hot-rolled plates with other microstructures. For hot-rolled plates with bainite + ferrite + pearlite, the preparation method provided in this application can produce a hot-rolled tempered plate with excellent performance.

[0086] In some embodiments of this application, in step S200, the hot-rolled plate has the following chemical composition by mass percentage:

[0087] C: 0.03–0.09%;

[0088] Si: 0.1-0.5%;

[0089] Mn: 1.0–1.9%;

[0090] Ti: 0.11–0.18%;

[0091] Nb: 0.01–0.06%;

[0092] Mo: 0.1–0.7%;

[0093] Al: 0.02–0.06%;

[0094] P: 0–0.015%;

[0095] S: 0~0.003%;

[0096] N: 0–0.006%;

[0097] The remainder consists of iron and unavoidable impurity elements.

[0098] In some of the above embodiments, the above chemical composition is the main component of the hot-rolled tempered plate with a yield strength of 700MPa. The applicant used a conventional high-temperature tempering process to prepare the above hot-rolled tempered plate and found that the tensile strength and longitudinal impact energy at -40℃ of the obtained hot-rolled tempered plate could not meet the delivery requirements. Therefore, the applicant conducted a systematic study and improvement on its tempering process. The hot-rolled tempered plate with a yield strength of 700MPa obtained by using the preparation method provided in this application can meet the relevant performance standards required by the customer.

[0099] In some embodiments of this application, step S200 specifically includes:

[0100] The hot-rolled plate is tempered in a continuous tempering furnace at the tempering temperature and time determined in step S100 to obtain a hot-rolled tempered plate. The continuous tempering furnace uses a gas combustion flame to heat the hot-rolled plate from both the top and bottom.

[0101] In some of the above embodiments, a continuous tempering furnace is used to temper the hot-rolled plate. The advantage of this is that it has higher production efficiency than a box-type resistance furnace. At the same time, the continuous tempering furnace uses gas combustion flames to heat the hot-rolled plate from both the top and bottom, which can make the tempering temperature of the steel plate more uniform, which is conducive to the control of plate shape and performance, thereby improving the yield.

[0102] In some embodiments of this application, in step S200, the tempering temperature during the tempering process is 620–650°C;

[0103] Optionally, the tempering time base K is 6 min, and the tempering time coefficient A is 1.5 min / mm.

[0104] In some of the above embodiments, the Ac1 of the hot-rolled plate was measured to be 670°C. Therefore, the tempering temperature for the hot-rolled tempered plate is 620-650°C, which can ensure production efficiency and avoid the austenitic transformation of the original structure during tempering, thus preventing the product performance from deteriorating.

[0105] Furthermore, under the condition of tempering temperature of 650℃, the relationship between the thickness of the hot-rolled plate and the tempering time is obtained by calculation using the method provided in this application: t=6+1.5d, that is, the tempering time base K is 6min, the tempering time coefficient A is 1.5min / mm, and d is the thickness of the hot-rolled plate in mm.

[0106] In some embodiments of this application, the yield strength of the hot-rolled tempered sheet is ≥750MPa;

[0107] Optionally, the tensile strength of the hot-rolled tempered plate is 815–940 MPa;

[0108] Optionally, the elongation after fracture of the hot-rolled tempered sheet is ≥15%;

[0109] Optionally, the longitudinal impact energy of the hot-rolled tempered plate at -40℃ is ≥47J;

[0110] Optionally, the cold bending performance of hot-rolled tempered sheet is D = a - no cracks after cold bending 180°.

[0111] In some of the above embodiments, the mechanical properties and cold bending properties of the hot-rolled tempered plates with a yield strength of 700MPa prepared using some embodiments of this application were tested, and they all met the standards set according to customer requirements: yield strength ≥750MPa, tensile strength of 815~940MPa, elongation after fracture ≥15%, longitudinal impact energy at -40℃ ≥47J, and cold bending performance D=a with no cracks after cold bending at 180℃. It can be seen that although customers have high requirements for the mechanical properties and cold bending properties of 700MPa-level hot-rolled tempered plates, the hot-rolled tempered plates prepared according to the preparation method provided in this application can still meet these requirements.

[0112] Optionally, the cold bending performance of hot-rolled tempered sheet is D = a - no cracks after cold bending 180°.

[0113] Thirdly, this application provides a hot-rolled tempered sheet, which is prepared according to the preparation method of any of the above embodiments.

[0114] In the technical solution of this application, the hot-rolled tempered plate prepared by the preparation method of the hot-rolled tempered plate provided in the second aspect of this application has good mechanical properties and cold bending properties.

[0115] The following examples illustrate in more detail the method for determining the tempering temperature and tempering time of hot-rolled tempered plates and its application, but the present application is by no means limited to these examples.

[0116] Example 1

[0117] Determination of tempering temperature and time for 700MPa grade hot-rolled tempered steel plates:

[0118] Chemical composition of steel plate:

[0119] C: 0.055%;

[0120] Si: 0.25%;

[0121] Mn: 1.85%;

[0122] Ti: 0.149%;

[0123] Nb: 0.060%;

[0124] Mo: 0.55%;

[0125] Al: 0.035%;

[0126] P: 0–0.011%;

[0127] S: 0.0025%;

[0128] N: 0.0048%;

[0129] The remainder consists of iron and unavoidable impurity elements.

[0130] A thermal expansion sample was prepared from a 700MPa hot-rolled plate. The Ac1 of the 700MPa hot-rolled plate was 670℃ when the heating rate was 1℃ / s, and the tempering temperature was Ac1-20=670-20=650℃. Therefore, the tempering temperature was determined to be 650℃.

[0131] Hot-rolled plates of 700MPa with specifications of 6×1500×5000 and 10×1500×5000 were tempered at 650℃ for different times to obtain hot-rolled tempered plates. Their yield strength and tensile strength were tested. It was found that the time for the 700MPa hot-rolled plate of 700MPa with specifications of 6×1500×5000 to reach the maximum yield strength and tensile strength was 15 min, and the time for the 700MPa hot-rolled plate of 700MPa with specifications of 10×1500×5000 to reach the maximum yield strength and tensile strength was 21 min. Therefore, the relationship between the tempering time and the thickness of the 700MPa hot-rolled tempered plate is: t=6+1.5d.

[0132] The mechanical properties and cold bending properties of 700MPa hot-rolled tempered steel sheets with a specification of 10×1500×5000 mm were determined after tempering at different tempering temperatures for the same time. The results are shown in Table 1.

[0133] Table 1

[0134]

[0135]

[0136] As shown in Table 1, impact toughness decreases with increasing tempering temperature. Tempering temperatures of 530–650℃ have no significant effect on elongation, but tempering at 670℃ reduces elongation. Yield strength and tensile strength are highest at 650℃, and cold bending performance is best between 630 and 650℃. No cracks were observed with a 0.5a mandrel diameter of 180°. Compared to untempered hot-rolled plates, tempering reduces impact toughness, increases elongation, and significantly improves cold bending performance. The reason for this is that… Figures 1-3 As shown, Figure 1 The image shows the metallographic structure of the hot-rolled plate before tempering, consisting of bainite, ferrite, and a small amount of pearlite. Figure 2 Metallographic structure of a 10mm thick, 700MPa hot-rolled tempered plate obtained by tempering at 610℃ for 21 min. Figure 3 Metallographic images of a 10mm thick 700MPa hot-rolled tempered plate obtained by tempering at 650℃ for 21 min were shown. Comparison revealed that after tempering at 650℃ for 21 min, both bainite and pearlite in the hot-rolled plate decomposed into ferrite and fine spherical carbides, forming a tempered microstructure of ferrite and fine spherical carbides. The disappearance of bainite and pearlite and the reduction in internal stress level in the tempered microstructure were the main factors leading to the increased elongation. The decomposition of bainite, resulting in the disappearance of the substructure within the original austenite grains and coarsening of the microstructure, was the main reason for the decrease in impact toughness. Although the impact toughness decreased after tempering, it remained at a relatively high level, thus significantly improving cold bending performance. This is mainly because, once the impact toughness reaches a certain level, cold bending performance depends primarily on the microstructure and internal stress level of the steel plate. A single ferrite microstructure exhibits the best cold bending performance; the higher the uniformity of the ferrite microstructure and the lower the internal stress level, the better the cold bending performance. The increase in strength after tempering of hot-rolled steel plates is due to secondary precipitation strengthening and a decrease in internal stress levels during the tempering process. Currently, high-titanium microalloying is commonly used for strengthening in hot-rolled high-strength structural steels. To improve toughness, the coiling temperature during hot rolling is typically below 620℃, which inhibits precipitation strengthening after coiling to some extent. Therefore, hot-rolled high-strength structural steels usually exhibit a secondary precipitation strengthening effect when tempered at temperatures above 620℃. Consequently, the impact toughness of hot-rolled high-strength structural steel plates decreases after tempering, while tensile strength, yield strength, and cold bending performance increase.

[0137] Overall, the hot-rolled tempered sheet with a tempering temperature of 650℃ has better comprehensive performance, and its mechanical properties and cold bending performance are higher than the customer's delivery standards.

[0138] The mechanical properties and cold bending properties of 700MPa hot-rolled tempered steel sheets with a specification of 6×1500×5000 mm and different tempering times were determined at the same tempering temperature. The results are shown in Table 2.

[0139] Table 2

[0140]

[0141] As shown in Table 2, the 6mm thick 700MPa hot-rolled tempered plate exhibits the highest yield strength and tensile strength, as well as the best cold bending performance, when tempered at 650℃ for 15 minutes. However, the impact toughness decreases with the extension of tempering time. Therefore, considering all factors, the optimal tempering condition for the 6mm thick 700MPa hot-rolled tempered plate is tempering at 650℃ for 15 minutes. The resulting hot-rolled tempered plate meets all the customer's delivery requirements.

[0142] In summary, by using the method provided in this application to determine the tempering temperature and tempering time of hot-rolled tempered plates for tempering treatment, hot-rolled tempered plates with good mechanical properties and cold bending properties can be obtained. Moreover, the method is simple and applicable to different types of hot-rolled tempered plates.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a hot-rolled tempered sheet, characterized in that, Includes the following steps: S100: Determine the tempering temperature and tempering time for hot-rolled tempered plates; S200: The hot-rolled plate is tempered in a continuous tempering furnace at the tempering temperature and time determined in step S100 to obtain the hot-rolled tempered plate. The continuous tempering furnace uses a gas combustion flame to heat the hot-rolled plate from both the top and bottom. The metallographic structure of the hot-rolled plate is bainite + ferrite + pearlite. The tempered structure of the hot-rolled tempered plate is ferrite + fine spherical carbides. In step S200, the chemical composition of the hot-rolled plate, by mass percentage, is as follows: C：0.03～0.09％； Si: 0.1-0.5%; Mn: 1.0–1.9%; Ti: 0.11–0.18%; Nb: 0.01–0.06%; Mo: 0.1–0.7%; Al：0.02～0.06％； P：0～0.015％； S：0～0.003％； N：0～0.006％； The remainder consists of iron and unavoidable impurity elements; The hot-rolled tempered plate has a yield strength ≥750MPa; a tensile strength of 815~940MPa; an elongation after fracture ≥15%; a longitudinal impact energy at -40℃ ≥47J; and a cold bending performance D=a with no cracks after a 180° cold bend. The method for determining the tempering temperature and tempering time of hot-rolled tempered plates includes the following steps: S10: Measure Ac1 of the hot-rolled plate to determine the tempering temperature T of the hot-rolled tempered plate. c =Ac1-20~50℃; Among them, T c Tempering temperature, in °C; Ac1 is the temperature at which ferrite begins to transform into austenite when the hot-rolled plate is heated, in °C. S20: At a tempering temperature of T c Under the condition of [condition], the tempering time required for two or more groups of hot-rolled plates of different thicknesses to reach the maximum yield strength or maximum tensile strength after tempering treatment was determined, and the obtained data from the two or more groups were fitted according to equation (1) to determine the K value and A value. t i =K+Ad i (1) Among them, t i The required tempering time for a given thickness of hot-rolled sheet, in minutes; K is the tempering time base, in minutes; A is the tempering time coefficient, in min / mm; d i The thickness of the hot-rolled plate is in mm. S30: Based on the determined K and A values ​​and the thickness of the hot-rolled plate to be tempered, determine the tempering temperature T for the hot-rolled plate to be tempered according to the above formula (1). c The required tempering time under the specified conditions, wherein the tempering time base K is 6 min and the tempering time coefficient A is 1.5 min / mm.

2. The preparation method according to claim 1, characterized in that, In step S10, the determination of Ac1 of the hot-rolled plate specifically includes: Ac1 of hot-rolled steel plates was determined using the thermal expansion method.

3. The preparation method according to claim 1, characterized in that, In step S200, the tempering temperature during the tempering process is 620–650°C.

4. A hot-rolled tempered sheet, characterized in that, Hot-rolled tempered sheet prepared by the preparation method according to any one of claims 1 to 2.

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