A method of improving the properties of high strength steel

Abstract

The application relates to a method for improving the performance of high-strength steel, comprising the following steps: obtaining the indirect temperature of the first in-furnace soaking section of a reference strip steel and the theoretical temperature of the first in-furnace soaking section; annealing a current strip steel to obtain the second in-furnace soaking section indirect temperature of the current strip steel at the corresponding time of the reference strip steel and the second in-furnace soaking section theoretical temperature; judging whether the first difference between the first in-furnace soaking section theoretical temperature and the second in-furnace soaking section theoretical temperature is within a first set interval, wherein the second difference between the first in-furnace soaking section indirect temperature and the second in-furnace soaking section indirect temperature is within a second set interval; if not, adjusting the process parameters of the annealing soaking section of the current strip steel to make the first difference within the first set interval, wherein the process parameters include: coal gas flow; and synchronously monitoring the finish elongation and rolling force. The application improves the yield strength difference of different batches of high-strength steel.

Description

Technical Field

[0001] This application relates to the technical fields of automotive steel and engineering structural steel, and in particular to a method for improving the performance of high-strength steel. Background Technology

[0002] High-strength steel, represented by duplex steel (DP steel), has advantages such as good overall formability, mature production technology, and low cost, and is currently widely used in body-in-white. During the annealing process of high-strength steel, because production is continuous, it is difficult to directly measure the strip temperature using thermocouples. Typically, a radiation pyrometer is used for indirect measurement of the strip surface. However, the strip temperature measured by the pyrometer differs from the indirect temperature measurement, which may affect the accurate measurement of the high-strength steel strip temperature and thus cause performance fluctuations.

[0003] In indirect production processes, it is common for high-strength steel of the same grade and specification to exhibit no significant differences in annealing processes across different batches, yet the measured yield strength fluctuates significantly. This can adversely affect downstream customers. Under the same forming conditions, high-strength steel, due to its higher yield strength, is more prone to springback than traditional mild steel, negatively impacting assembly accuracy and stamping production rhythm. Therefore, the performance stability of high-strength steel has become a key concern in the automotive industry. While die compensation technology can effectively mitigate springback, frequent die adjustments are necessary when yield strength fluctuations are large between different batches. Furthermore, excessive yield strength fluctuations may render die adjustments ineffective in compensating for springback. Therefore, an effective method to improve the stability of high-strength steel yield strength across different batches is urgently needed. Summary of the Invention

[0004] This application provides a method for improving the performance of high-strength steel to solve the following technical problem: how to improve the yield strength difference between different batches of high-strength steel.

[0005] In a first aspect, this application provides a method for improving the properties of high-strength steel, the method comprising:

[0006] Obtain the indirect temperature of the first heat soaking section and the theoretical temperature of the first heat soaking section of the reference strip.

[0007] Anneal the current strip steel and obtain the indirect temperature of the second furnace soaking section and the theoretical temperature of the second furnace soaking section at the corresponding time of the current strip steel and the reference strip steel.

[0008] Determine whether the first difference between the theoretical temperature of the first heat-soaking section in the furnace and the theoretical temperature of the second heat-soaking section in the furnace is within a first set range, wherein the second difference between the indirect temperature of the first heat-soaking section in the furnace and the indirect temperature of the second heat-soaking section in the furnace is within a second set range.

[0009] If not, the process parameters of the annealing and soaking section of the current strip are adjusted so that the first difference is within the first set range, wherein the process parameters include: gas flow rate.

[0010] Optionally, the first set range is -10℃ to 10℃.

[0011] Optionally, the second set range is -10℃ to 10℃.

[0012] Optionally, the process parameters may also include: furnace atmosphere temperature, homogenization temperature, homogenization time, and strip running speed.

[0013] Optionally, the method further includes:

[0014] Obtain the finishing elongation of the reference strip and the first rolling force corresponding to the finishing elongation;

[0015] The current strip is finished, and the second rolling force of the current strip corresponding to the finished elongation is obtained;

[0016] Based on the first rolling force, determine whether the fluctuation range of the second rolling force is within the third preset range;

[0017] If not, adjust the current smoothing elongation rate until the fluctuation range is within the third set range.

[0018] Optionally, the third set interval is ≤10%.

[0019] Optionally, the tensile strength of the high-strength steel is ≥780 MPa.

[0020] The technical solutions provided in this application have the following advantages compared with the prior art:

[0021] The method for improving the performance of high-strength steel provided in this application includes: obtaining the indirect temperature of the first in-furnace soaking section and the theoretical temperature of the first in-furnace soaking section of a reference strip; annealing the current strip and obtaining the indirect temperature of the second in-furnace soaking section and the theoretical temperature of the second in-furnace soaking section of the current strip at the corresponding time as the reference strip; determining whether a first difference between the theoretical temperature of the first in-furnace soaking section and the theoretical temperature of the second in-furnace soaking section is within a first set range, wherein a second difference between the indirect temperature of the first in-furnace soaking section and the indirect temperature of the second in-furnace soaking section is within a second set range; if not, adjusting the process parameters of the annealing soaking section of the current strip to make the first difference within the first set range, wherein the process parameters include: gas flow rate. In the continuous annealing process of high-strength steel strip, the indirect temperature and theoretical temperature of the first soaking zone in the furnace of the reference strip are obtained. These are used as the criteria for judging the indirect temperature and theoretical temperature of the second soaking zone in the furnace of subsequent high-strength steel strips of the same grade, specification, and process. The theoretical temperature of the soaking zone in different furnace areas is kept at a certain fixed value, thereby ensuring the temperature stability of high-strength steel strip in different areas of the furnace and improving the yield strength difference between different batches of high-strength steel. Attached Figure Description

[0022] 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.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart illustrating a method for improving the properties of high-strength steel according to some embodiments of this application;

[0025] Figure 2 A bar chart comparing the yield strength fluctuations between batches of hot-dip galvanized DP980 before and after improvement. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0028] In this application, the terms "comprising," "including," etc., mean "including but not limited to." 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 indirect relationship or order between these entities or operations.

[0029] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0030] In the annealing process of high-strength steel, due to the continuous production, it is difficult to directly measure the strip temperature using thermocouples. Therefore, a radiation pyrometer is typically used for indirect measurement of the strip surface temperature. A significant drawback of radiation pyrometers is that infrared radiation is highly dependent on the surface emissivity of the steel plate, and accurately measuring the surface emissivity of the steel plate is an industry-wide challenge. Consequently, the strip temperature measured by the pyrometer differs from the indirect temperature of the strip. This difference can vary with changes in the surface condition of the strip, leading to significant fluctuations in the properties of strips produced in different batches. For example, the surface condition of cold-rolled high-strength steel coils is affected by hot rolling, pickling, cold rolling, furnace cleaning, and even the atmosphere of the annealing furnace. Changes in process parameters at any stage can alter the surface emissivity of the steel plate, affecting the accuracy of temperature measurement and ultimately impacting the mechanical properties of the finished product. Furthermore, changes in the condition of the pyrometer itself, as well as the pyrometer's measuring angle and the cleanliness of the lens, also affect the accuracy of temperature measurements. Because the annealing furnace is a black box, angles cannot be calibrated or lenses cleaned during normal steel plate production; and any abnormalities detected by the pyrometer often go unnoticed. Therefore, changes in the surface condition of the steel plate and the condition of the pyrometer itself can affect the accurate measurement of the temperature of high-strength steel strip, resulting in performance fluctuations.

[0031] Therefore, this application provides a method for improving the properties of high-strength steel. Figure 1 For a flowchart illustrating a method for improving the properties of high-strength steel according to some embodiments of this application, please refer to [link / reference]. Figure 1 The method includes:

[0032] S1. Obtain the indirect temperature of the first heat soaking section and the theoretical temperature of the first heat soaking section of the reference strip.

[0033] Anneal the current strip steel and obtain the indirect temperature of the second furnace soaking section and the theoretical temperature of the second furnace soaking section at the corresponding time of the current strip steel and the reference strip steel.

[0034] Determine whether the first difference between the theoretical temperature of the first heat-soaking section in the furnace and the theoretical temperature of the second heat-soaking section in the furnace is within a first set range, wherein the second difference between the indirect temperature of the first heat-soaking section in the furnace and the indirect temperature of the second heat-soaking section in the furnace is within a second set range.

[0035] If not, the process parameters of the annealing and soaking section of the current strip are adjusted so that the first difference is within the first set range, wherein the process parameters include: gas flow rate.

[0036] In some implementations, the first set range is -10°C to 10°C.

[0037] In some implementations, the second set range is -10°C to 10°C.

[0038] In some embodiments, the process parameters also include: furnace atmosphere temperature, homogenization temperature, homogenization time, and strip running speed.

[0039] In this embodiment, a representative high-strength steel (duplex steel) is used. Because the emissivity of the duplex steel surface is affected by various factors such as hot rolling, pickling, cold rolling, furnace cleaning, and the annealing furnace atmosphere, the temperature measured by the pyrometer (indirect temperature in the soaking zone of the furnace) T... D The indirect temperature between high-strength steel and the furnace strip varies to some extent; even for the same grade and specification of high-strength steel produced in different batches, slight differences in the overall process can cause fluctuations in the indirect strip temperature, leading to variations in yield strength and other performance indicators. To avoid significant fluctuations in the mechanical properties of high-strength steel produced in different batches, in addition to monitoring the temperature displayed by the pyrometer, it is also necessary to monitor the calculated temperature of the strip in the furnace area (theoretical temperature of the soaking zone in the furnace) T. C Monitor the situation. C The calculation model is built into the production line's secondary computer and can be controlled by factors such as gas flow rate, furnace atmosphere temperature, homogenization temperature, homogenization time, and strip running speed. Because the strip absorbs heat from the annealing furnace through radiation and heat conduction, theoretically, strips of the same grade, specification, and process produced in different batches will have similar heat transfer rates (T). C They are the same. If the surface condition of the two batches of strip steel changes significantly (i.e., the surface emissivity changes significantly), the T value displayed under the pyrometer control mode will be the same. D Same, but the T-shaped strip C There will be noticeable changes. At this point, the temperature control mode should be switched from the pyrometer control mode to the calculation temperature control mode, that is, to maintain the temperature of different furnace zones. C This ensures the temperature stability of the strip steel in different regions of the furnace by maintaining a fixed value. For example, for a specific grade and specification of strip steel, a reference strip steel with normal performance is selected, and its temperature value T is calculated. C For subsequent production of high-strength steel of the same grade and specification, it is necessary to control the T (temperature). D The difference between the value and the reference strip is not significant (≤±10℃), T C Value and fluctuation of benchmark strip steel T C The fluctuation of the value is controlled within ±10℃.

[0040] In some embodiments, the method further includes:

[0041] S2. Obtain the finishing elongation of the reference strip and the first rolling force corresponding to the finishing elongation;

[0042] The current strip is finished, and the second rolling force of the current strip corresponding to the finished elongation is obtained;

[0043] Based on the first rolling force, determine whether the fluctuation range of the second rolling force is within the third preset range;

[0044] If not, adjust the current smoothing elongation rate until the fluctuation range is within the third set range.

[0045] In some implementations, the third set interval is ≤10%.

[0046] In some embodiments, the high-strength steel has a tensile strength of ≥780 MPa.

[0047] In this embodiment, to further improve the performance stability of high-strength steel from different batches, the elongation and rolling force of the finishing mill should also be monitored simultaneously. Typically, the finishing mill uses a constant elongation mode for control. However, when there are significant differences in the strip temperature between different batches of a certain high-strength steel, using the constant elongation mode will exacerbate fluctuations in yield strength. Therefore, the finishing rolling force should also be monitored during production. For example, for a certain grade and specification of strip steel, a benchmark strip steel with normal performance is selected, and the finishing rolling force at the corresponding finishing elongation is recorded. For subsequent production of the same grade and specification of high-strength steel, under the condition of using the same finishing elongation, the fluctuation of the finishing rolling force needs to be controlled within 10%. The current finishing elongation can be manually adjusted until the fluctuation range falls within a third set range.

[0048] The above-described methods for improving the properties of high-strength steel are applicable to high-strength steels of all composition systems and strength grades, especially those with a tensile strength ≥780 MPa. For example, after adopting these methods, the batch-to-batch yield strength fluctuation of hot-dip galvanized DP980 was reduced from approximately 130 MPa to around 80 MPa, demonstrating a significant improvement effect.

[0049] For example, a complete process for preparing duplex steel includes: smelting, continuous casting, heating, hot rolling, pickling, cold rolling, annealing, galvanizing, and finishing. The heating temperature is 1150-1250℃, the final hot rolling temperature is 900℃, and the coiling temperature is 250℃. The hot-rolled plate is pickled, annealed, and galvanized. The slab heating temperature of 1150-1250℃ is for homogenization of the microstructure and solid solution of microalloying elements. Too high a temperature may lead to abnormal grain growth, while too low a temperature may result in uneven composition and insufficient solid solution of microalloying elements. The final rolling temperature of 900℃ is mainly to ensure a good hot-rolled microstructure; too high a final rolling temperature may lead to coarse grains, while too low a final rolling temperature may result in mixed grains. The coiling temperature of 250℃ is set to obtain a fully martensitic microstructure. Coiling temperatures below 200℃ will make coiling difficult, while coiling temperatures above 200℃ will make it difficult to obtain a fully martensitic microstructure.

[0050] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0051] This application provides a method for improving the properties of high-strength steel, the method comprising:

[0052] S11. Obtain the indirect temperature of the first heat soaking section and the theoretical temperature of the first heat soaking section of the reference strip.

[0053] Anneal the current strip steel and obtain the indirect temperature of the second furnace soaking section and the theoretical temperature of the second furnace soaking section at the corresponding time of the current strip steel and the reference strip steel.

[0054] Determine whether the first difference between the theoretical temperature of the first heat-soaking section in the furnace and the theoretical temperature of the second heat-soaking section in the furnace is within a first set range, wherein the second difference between the indirect temperature of the first heat-soaking section in the furnace and the indirect temperature of the second heat-soaking section in the furnace is within a second set range.

[0055] If not, the process parameters of the annealing and soaking section of the current strip are adjusted so that the first difference is within the first set range, wherein the process parameters include: gas flow rate.

[0056] S21. Obtain the finishing elongation of the reference strip and the first rolling force corresponding to the finishing elongation;

[0057] The current strip is finished, and the second rolling force of the current strip corresponding to the finished elongation is obtained;

[0058] Based on the first rolling force, determine whether the fluctuation range of the second rolling force is within the third preset range;

[0059] If not, adjust the current smoothing elongation rate until the fluctuation range is within the third set range.

[0060] In this embodiment of the application, the indirect temperature T of the soaking zone inside the furnace is... D The theoretical temperature (calculated temperature) T in the soaking zone of the furnace, measured by a pyrometer. C The second difference between the indirect temperatures of the first and second heat-dip galvanizing sections is measured by the computer-integrated calculation model of the continuous hot-dip galvanizing production line and controlled by factors such as gas flow rate, furnace atmosphere temperature, soaking temperature, soaking time, and strip running speed. Specifically, please refer to Table 1 for the chemical composition (wt%) of the high-strength steel, Table 2 for the annealing process parameters of the high-strength steel, Table 3 for the control mode results in the methods for improving the performance of high-strength steel, and Table 4 for the mechanical properties of the high-strength steel.

[0061] Table 1 shows the chemical composition (wt%) of high-strength steel, with the remainder being Fe and unavoidable impurities.

[0062] steel grades C Si Mn Cr Al P S Nb Ti DP780 0.10 0.3 2.0 0.15 0.03 0.010 0.002 0.015 0.012 DP980 0.11 0.35 2.3 0.45 0.04 0.011 0.003 0.02 0.018

[0063] Table 2 Annealing process parameters for high-strength steel

[0064]

[0065] Table 3. Results of control modes in methods for improving the properties of high-strength steel

[0066]

[0067]

[0068] Table 4 Mechanical properties of high-strength steel

[0069]

[0070] In summary, Comparative Example 1 illustrates the yield strength fluctuations of two batches produced under the conventional pyrometer + finishing elongation control mode. The total number of rolls in both batches was 90. The pyrometer measurements of the homogenization temperature T for both batches are as follows: D The difference is not significant, but the calculated temperature T C There are significant differences, and the T of each batch C Fluctuations exceeding 20°C indicate significant differences in the indirect temperature of strips from different coils; furthermore, the finishing rolling force fluctuations within each batch are also above 10%, ultimately resulting in a batch-to-batch yield strength fluctuation as high as 121 MPa. Comparative Example 2 uses a pyrometer + finishing elongation / rolling force control mode, TD and T C The fluctuation was similar to that of Comparative Example 1, but because both finishing elongation and rolling force were monitored simultaneously, the rolling force fluctuation per batch was reduced to less than 10%, resulting in improved yield strength fluctuation between batches, reduced to 102 MPa. Example 1 used a pyrometer / calculated temperature + finishing elongation / rolling force control mode, because T was monitored simultaneously... D and T C This results in each batch of T D and T C The fluctuations were significantly improved. In addition, the improvement effect of simultaneously monitoring the finishing elongation and rolling force was further reduced to 75 MPa between batches, and the improvement effect was even more obvious.

[0071] Comparative Example 3 shows the yield strength fluctuation under the conventional pyrometer + finishing elongation control mode. D and T C The rolling force fluctuation was similar to that of Comparative Example 1, with a batch-to-batch yield fluctuation of 134 MPa. Comparative Example 4 shows the improvement after using a pyrometer + finishing elongation / rolling force, reducing the batch-to-batch yield fluctuation to 112 MPa. Example 2 shows the results after using a pyrometer / calculated temperature + finishing elongation / rolling force control mode, further reducing the batch-to-batch yield fluctuation to 79 MPa, demonstrating a significant improvement. Figure 2 This is a bar chart comparing the batch-to-batch yield strength fluctuations of hot-dip galvanized DP980 before and after improvement; please refer to [link / reference]. Figure 2 This indicates that the method for improving the performance of high-strength steel provided in the embodiments of this application significantly reduces the batch-to-batch yield strength fluctuation of hot-dip galvanized DP980.

[0072] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for improving the properties of high-strength steel, characterized in that, The method includes: Obtain the indirect temperature of the first heat soaking section and the theoretical temperature of the first heat soaking section of the reference strip. Anneal the current strip steel and obtain the indirect temperature of the second furnace soaking section and the theoretical temperature of the second furnace soaking section at the corresponding time of the current strip steel and the reference strip steel. Determine whether the first difference between the theoretical temperature of the first heat-soaking section in the furnace and the theoretical temperature of the second heat-soaking section in the furnace is within a first set range, wherein the second difference between the indirect temperature of the first heat-soaking section in the furnace and the indirect temperature of the second heat-soaking section in the furnace is within a second set range. If not, the process parameters of the annealing soaking section of the current strip are adjusted so that the first difference is within the first set range, wherein the process parameters include: gas flow rate; The first set range is -10℃ to 10℃, and the second set range is -10℃ to 10℃; The method further includes: Obtain the finishing elongation of the reference strip and the first rolling force corresponding to the finishing elongation; The current strip is finished, and the second rolling force of the current strip corresponding to the finished elongation is obtained; Based on the first rolling force, determine whether the fluctuation range of the second rolling force is within the third preset range; If not, adjust the current smoothing elongation until the fluctuation range is within the third set range; The third set interval is ≤10%.

2. The method according to claim 1, characterized in that, The process parameters also include: furnace atmosphere temperature, homogenization temperature, homogenization time, and strip running speed.

3. The method according to claim 1, characterized in that, The tensile strength of the high-strength steel is ≥780 MPa.

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