A segmented dew point controlled annealing process for DP780 dual phase steel

Abstract

The application discloses a kind of DP780 dual-phase steel segmented dew point regulation annealing process, including heating section, isothermal section and cooling section;The heating section: the dual-phase steel steel strip is heated to 710~730 first, in this process, the dew point of annealing atmosphere is at-20~-15, while maintaining hydrogen volume fraction at 10~15%;Heated to 810~830 again, in this process, the dew point of annealing atmosphere is reduced to-60~-55, while keeping hydrogen volume fraction in the range of 10~15%;The isothermal section: keep warm at 820 DEG C temperature, during maintaining the dew point of annealing atmosphere at-50~-45;The cooling section: cooling to 450~470, in this process, the dew point of annealing atmosphere is kept at-30~-25.The process will significantly reduce the surface coverage of outer oxide to 5% or less, the plating rate is reduced to 3% or less, while ensuring that the tensile strength of dual-phase steel is not less than 780MPa and elongation is not less than 19%, mechanical properties and coating quality of high level are realized.

Description

Technical Field

[0001] This invention relates to heat treatment processes for metallic materials, and in particular to a segmented dew point controlled annealing process for DP780 duplex steel. Background Technology

[0002] With the increasing demands for lightweight and safety performance in automobiles, high-strength hot-dip galvanized duplex steel is being used more and more widely in vehicle body structural components. DP780 duplex steel, due to its excellent strength-ductility matching characteristics, has become an important representative of advanced high-strength steel. However, during continuous hot-dip galvanizing, alloying elements (such as Mn and Si) on the steel strip surface are prone to selective oxidation in the annealing atmosphere, forming external oxides (such as MnO and SiO₂) that are difficult to wet with molten zinc. x This severely restricts its plating applicability. This problem is particularly prominent in ultra-high strength steel, where the higher alloy content significantly enhances the oxidation tendency, leading to frequent plating defects and affecting the surface quality and subsequent coating performance of the product.

[0003] Existing technologies generally employ a constant low dew point reducing atmosphere to suppress surface oxidation. However, this strategy is difficult to effectively prevent elements such as Mn and Si from segregating to the surface and forming stable external oxides, resulting in a persistently high plating failure rate. If a higher dew point is used to promote internal oxidation and reduce surface enrichment, it is easy to cause excessive oxidation of the matrix ferrite, which will damage the uniformity of the structure and cause a decrease in tensile strength or elongation, making it difficult to balance mechanical properties and coating quality.

[0004] Current technologies cannot dynamically control oxidation behavior according to the different requirements of heating, homogenization, and cooling stages. On the one hand, the initial heating stage lacks a suitable weak oxidizing environment to guide the internal oxidation of alloying elements, leading to increased surface enrichment. On the other hand, the high-temperature isothermal and cooling stages fail to switch to strong reducing conditions in time to remove the surface oxide film and prevent secondary oxidation, resulting in a constant "oxidation-reduction" imbalance on the surface. Therefore, there is an urgent need for an annealing process that can precisely control the dew point in stages, significantly reducing the surface oxide coverage and plating failure rate while ensuring the mechanical properties of DP780 duplex steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a segmented dew point controlled annealing process for DP780 duplex steel to reduce the surface oxide coverage and plating omission rate.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: including a heating section, an isothermal section, and a cooling section; The heating section: The duplex steel strip is first heated to 710℃~730℃, during which the dew point of the annealing atmosphere is controlled at -20℃~-15℃, while maintaining the hydrogen gas fraction at 10%~15%; then the temperature is raised to 820℃, during which the dew point of the annealing atmosphere is lowered to -60℃~-55℃, while maintaining the hydrogen gas fraction at 10%~15%. The isothermal zone is maintained at a temperature of 810℃~830℃, during which the dew point of the annealing atmosphere is maintained at -50℃~-45℃. The cooling section cools the temperature to 450℃~470℃, and during this process, the dew point of the annealing atmosphere is maintained at -30℃~-25℃.

[0007] Furthermore, the heating section has a heating rate controlled between 3°C / s and 10°C / s.

[0008] Furthermore, in the heating section, the rate of change of the dew point is 1.5℃ / s to 3℃ / s.

[0009] Furthermore, the isothermal section is maintained at 820℃ for 100s to 120s.

[0010] Furthermore, the cooling section includes a slow cooling section and a fast cooling section; the slow cooling section cools the temperature to 640℃ to 660℃ at a cooling rate of 7℃ / s to 12℃ / s; the fast cooling section cools the temperature to 450℃ to 470℃ at a cooling rate of 18℃ / s to 28℃ / s.

[0011] The beneficial effects of adopting the above technical solution are as follows: the present invention effectively promotes the initial diffusion of alloying elements and inhibits the excessive growth of early external oxides by controlling the heating section; it ensures sufficient austenitization and reduction reaction of internal oxides by controlling the soaking section; and it ensures the stability of ferrite precipitation and martensite transformation initiation conditions by controlling the slow cooling section, thereby obtaining the expected dual-phase microstructure ratio and mechanical properties.

[0012] This invention is particularly suitable for continuous hot-dip galvanizing processes. By precisely controlling the dew point parameters during the annealing process in stages, the weak oxidizing properties are first used in the heating stage to promote the internal oxidation of alloying elements to reduce surface enrichment, and then the dew point is rapidly reduced to avoid excessive internal oxidation. In the isothermal stage, a strong reducing atmosphere is maintained to remove surface oxides and expose the substrate. In the cooling stage, the dew point is controlled to prevent secondary oxidation. As a result, the surface oxide coverage is significantly reduced to below 5%, and the uncoated rate is greatly reduced from the original 15% to 20% to below 3%, while ensuring that the tensile strength of duplex steel is not less than 780 MPa and the elongation is not less than 19%.

[0013] The steel strip treated by this invention exhibits a significantly reduced external oxide coverage and controllable internal oxide layer depth. It is suitable not only for subsequent hot-dip galvanizing processes but also as a high-quality substrate for other surface treatment processes such as electroplating, organic coating (color coating), or can be directly delivered as a high-strength steel product. Especially when used in continuous hot-dip galvanizing, it achieves a high level of balance between mechanical properties and coating quality. Attached Figure Description

[0014] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0015] Figure 1 This is a cross-sectional photograph of the hot-dip galvanized DP780 coating obtained in Example 1; Figure 2 This is a cross-sectional photograph of the hot-dip galvanized DP780 obtained in Comparative Example 1; Figure 3 This is the elemental distribution diagram obtained by glow discharge spectrum (GDOES) measurement of the hot-dip galvanized DP780 duplex steel obtained in Example 1; Figure 4 This is the elemental distribution diagram obtained by glow discharge spectrum (GDOES) measurement of the hot-dip galvanized DP780 duplex steel obtained in Comparative Example 4. Detailed Implementation

[0016] The segmented dew point controlled annealing process for this DP780 duplex steel includes the following steps: S1. Heating section: The duplex steel is heated in an annealing furnace, first in heating section 1, then in heating section 2.

[0017] The first heating stage involves heating the duplex steel strip to 710℃~730℃ at a rate controlled between 3℃ / s and 10℃ / s. During this process, the dew point of the annealing atmosphere is maintained between -20℃ and -15℃, while the hydrogen component of the annealing atmosphere is kept between 10% and 15%. The annealing atmosphere is established by introducing a mixture of nitrogen, hydrogen, and water in a specific ratio into the annealing furnace. The partial pressure of water vapor is precisely calculated and controlled to achieve the target dew point range. This weakly oxidizing condition selectively oxidizes Mn and Si elements without causing significant oxidation of the ferrite matrix. In this process, the weakly oxidizing atmosphere promotes the initial internal oxidation of alloying elements Mn and Si in the steel, effectively reducing the enrichment of these elements on the steel strip surface. In engineering implementation, the introduction of water vapor involves adjusting the deionized water evaporation rate using a high-precision mass flow controller, and then mixing it with preheated nitrogen via a saturated steam generator before entering the furnace, ensuring that the water vapor partial pressure fluctuation does not exceed ±0.5 kPa. Meanwhile, by controlling the heating rate between 3℃ / s and 10℃ / s, a uniform temperature field distribution within the steel strip is ensured, while providing sufficient time for the internal oxidation kinetics of the alloying elements. Under these conditions, Mn and Si elements preferentially combine with oxygen at the austenite grain boundaries to form dispersed internal oxide particles with a size controlled within the range of 50nm to 200nm, effectively suppressing the driving force for element diffusion to the surface.

[0018] The second stage of heating involves further heating of the duplex steel strip to 810℃~830℃, with the heating rate controlled at 3℃ / s~10℃ / s. During this process, the dew point of the annealing atmosphere is lowered to -60℃~-55℃, with a dew point decrease rate of 1.5℃ / s~3℃ / s, while maintaining the hydrogen component of the annealing atmosphere within the range of 10%~15%. This rapid dew point reduction is achieved by switching to a dry nitrogen-hydrogen mixture and supplementing it with a dehydration device, such as a molecular sieve adsorption dehydration device. The switching action is preferably controlled by a high-precision dew point sensor and a fast-response solenoid valve to ensure a dew point change rate of 1.5℃ / s to 3℃ / s. During this process, within the time window of heating from 720℃ to 820℃, the dew point of the annealing atmosphere is rapidly reduced to the range of -60℃ to -55℃, while maintaining the hydrogen component within the range of 10% to 15%, to avoid excessive development of the internal oxidation process. Furthermore, the temperature range was selected based on a comprehensive consideration of the austenitization initiation temperature and the diffusion rate of alloying elements in DP780 duplex steel. At 720℃, the diffusion capacity of Mn and Si is significantly enhanced, but stable external oxides have not yet formed. At this point, if a high dew point is maintained, the internal oxidation reaction will continue, leading to coarsening of oxide particles and potential penetration of grain boundaries, which would weaken the mechanical properties of the matrix. However, a rapid transition to a strong reducing environment can promptly terminate the internal oxidation process, preserving a fine and dispersed oxide distribution and providing favorable nucleation sites for subsequent phase transformations.

[0019] S2, Isothermal Section: The duplex steel strip is held at an isothermal temperature of 810℃~830℃ for 100s~120s, during which the dew point of the annealing atmosphere is maintained at -50℃~-45℃. The atmosphere in this step is a strongly reducing atmosphere, maintained by continuously introducing a dry nitrogen-hydrogen mixture with a dew point of -50℃ to -45℃. Hydrogen acts as the main reducing agent, reacting with surface oxides to generate water vapor, which is promptly discharged from the furnace. Ideally, the nitrogen-hydrogen mixture and water vapor are discharged at a gas exchange rate of no less than twice the furnace volume per minute. During this process, the small amount of iron oxide that may form on the steel strip surface is reduced to an active sponge iron structure, thus fully exposing the clean steel substrate surface. This reduction reaction follows the principle of chemical equilibrium, with the reaction formula: FeO + H2 → Fe + H2O. The generated water vapor is discharged through the vacuum system. The isothermal holding time is 100s–120s. Experiments have verified that this duration simultaneously satisfies the requirements for sufficient austenite homogenization, phase transformation completion, and surface oxide reduction, achieving an optimized balance between mechanical properties and surface quality. During this holding time, the austenite grain size is controlled between 8μm and 12μm, and carbon elements diffuse fully within the austenite, laying the compositional foundation for the martensitic phase transformation during subsequent cooling. Simultaneously, surface iron oxide is completely reduced, resulting in excellent zinc liquid wetting ability.

[0020] S3. Cooling Section: The duplex steel strip is cooled to 450℃~470℃. During this process, the dew point of the annealing atmosphere is maintained at -30℃~-25℃ to prevent secondary oxidation of the steel strip surface during cooling. The final cooling temperature of 450℃~470℃ is set based on the zinc bath temperature requirements of the hot-dip galvanizing process, ensuring that the steel strip surface is in an optimal active state before entering the zinc bath to achieve good zinc layer adhesion. At this temperature, a dense and uniform Fe-Al inhibitory layer can be formed, increasing zinc layer adhesion.

[0021] The cooling section preferably consists of a slow cooling section and a fast cooling section; in the slow cooling section, the duplex steel strip is cooled to 640℃~660℃ at a cooling rate of 7℃ / s~12℃ / s; in the fast cooling section, the temperature is further reduced to 450℃~470℃ at a cooling rate of 18℃ / s~28℃ / s.

[0022] Throughout the entire process, all dew point controls are monitored and adjusted in real time using a high-precision dew point meter. Furthermore, this process is applied to a continuous hot-dip galvanizing production line, where annealed DP780 duplex steel undergoes hot-dip galvanizing, with the steel strip running speed adjusted between 80m / min and 120m / min depending on the thickness.

[0023] Examples and comparative examples: To further verify the technical effect of this process, a typical DP780 duplex steel was selected for comparative testing. The composition of DP780 duplex steel is: 0.12C-2.1Mn-0.6Si-0.3Cr.

[0024] (1) The speed regimes for the DP780 hot-dip galvanized duplex steel produced in each embodiment and comparative example are shown in Table 1; Table 1: Target Speed ​​Values ​​for Hot-Dip Galvanizing DP780 Process End Speed

[0025] (2) The temperature setting and dew point control processes for each step in each embodiment and comparative example are shown in Table 2; Table 2: Temperature settings and dew point control processes for each segment in the examples and comparative examples

[0026] (3) The DP780 duplex steel obtained in each embodiment and comparative example was hot-dip galvanized, and the typical working condition test statistics of the galvanized products are shown in Table 3. Table 3: Statistical data of instrument inspection under typical operating conditions

[0027] The above embodiments employ this segmented dew point controlled annealing process. Comparative Examples 1-3 use dew points outside the range of this process, and Comparative Example 4 uses a constant dew point process at -20℃. The results show that the spot plating failure rate after using this annealing process decreased from 18.7% at a constant dew point to 3.0%. Furthermore, this annealing process significantly improves the spot plating failure rate. In the embodiments and comparative examples, the temperature regime was changed. Mechanical property tests show that the tensile strength of the DP780 duplex steel after annealing in all embodiments is greater than 780 MPa, and the elongation is greater than 18%, both exceeding the standard requirements. Figure 1 and 2 As can be seen from scanning electron microscopy, the DP780 duplex steel in the example, after annealing, showed no continuous oxide film on its surface, with only scattered nano-sized oxide particles embedded in the matrix; the Fe-Al inhibition layer had good flatness and extremely shallow internal oxidation depth; while the DP780 duplex steel in the comparative example, after annealing, had a poorly flat Fe-Al inhibition layer, severe internal oxidation, and coarse oxide particles. Figure 3 and Figure 4 As can be seen, glow discharge tests on the coating surfaces of the examples and comparative examples revealed that no oxygen peak was detected in the examples, indicating a clear inhibition layer. However, in the comparative examples, oxygen and manganese peaks were observed below the inhibition layer at the Fe-Zn interface, with a high degree of agreement between the two. This indicates that selective oxidation of manganese occurred in the comparative examples, and the presence of oxides led to incomplete coating and color difference on the material surface.

Claims

1. A segmented dew point controlled annealing process for DP780 duplex steel, characterized in that: It includes a heating section, an isothermal section, and a cooling section; The heating section: The duplex steel strip is first heated to 710℃~730℃, during which the dew point of the annealing atmosphere is controlled at -20℃~-15℃, while maintaining the hydrogen gas fraction at 10%~15%; then the temperature is raised to 810℃~830℃, during which the dew point of the annealing atmosphere is lowered to -60℃~-55℃, while maintaining the hydrogen gas fraction at 10%~15%; The isothermal zone is maintained at a temperature of 810℃~830℃, during which the dew point of the annealing atmosphere is maintained at -50℃~-45℃. The cooling section cools the temperature to 450℃~470℃, and during this process, the dew point of the annealing atmosphere is maintained at -30℃~-25℃.

2. The segmented dew point controlled annealing process for DP780 duplex steel according to claim 1, characterized in that: The heating section has a heating rate controlled between 3℃ / s and 10℃ / s.

3. The segmented dew point controlled annealing process for DP780 duplex steel according to claim 1, characterized in that: The rate of change of the dew point in the heating section is 1.5℃ / s to 3℃ / s.

4. The segmented dew point controlled annealing process for DP780 duplex steel according to claim 1, characterized in that: The isothermal zone is maintained at 810℃~830℃ for 100s~120s.

5. The segmented dew point controlled annealing process for DP780 duplex steel according to any one of claims 1-4, characterized in that: The cooling section includes a slow cooling section and a fast cooling section; the slow cooling section cools the temperature to 640℃~660℃ at a cooling rate of 7℃ / s~12℃ / s; the fast cooling section cools the temperature to 450℃~470℃ at a cooling rate of 18℃ / s~28℃ / s.

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