A method for improving the surface quality of pickled steel sheets used in automotive exterior panels; pickled steel sheets

Abstract

This invention provides a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, and the pickled steel sheet itself, belonging to the field of steel preparation. The method includes: obtaining a slab with a predetermined chemical composition; heating the slab and then rolling it to obtain a hot-rolled sheet; coiling the hot-rolled sheet at a predetermined coiling temperature and then rapidly cooling it to obtain a hot-rolled coil; and pickling the hot-rolled coil at a predetermined pickling speed to obtain a pickled steel sheet; the pickling solution contains a predetermined concentration of pickling accelerator. By rationally designing the chemical composition of the pickled steel sheet and controlling the heating process parameters, the influence of sulfides and phosphides of the microalloying element Ti in the steel on the surface quality can be reduced; by controlling the coiling temperature, the influence of excessive iron oxide scale and intergranular oxidation on the surface quality under high-temperature coiling processes can be reduced. This improves the surface quality of pickled steel sheets used in automotive exterior panels.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a method for improving the surface quality of pickled steel sheets used for automotive exterior panels, and the pickled steel sheets themselves. Background Technology

[0002] High-grade automotive exterior panels typically incorporate titanium (Ti) to fix the carbon and nitrogen (C) elements in the steel. The resulting TiC can also improve strength to some extent. Ti is a relatively economical element with good strengthening effects. However, during the production of ultra-low carbon automotive exterior panels containing Ti, poor surface quality after pickling is common, mainly manifested as residual iron and color difference defects, leading to incomplete galvanizing and slag spots on the subsequent galvanized surface.

[0003] Currently, controlling galvanized incomplete coating and slag spot defects is limited to the galvanizing process, at most involving the surface cleanliness of the raw materials, and does not involve coordinated control throughout the entire hot-rolling-cold-rolling process. For example, controlling annealing temperature and dew point temperature to avoid bright spot defects; controlling acid concentration and acid temperature and other pickling process adjustments to improve surface quality; however, the surface quality controlled by the above methods is not ideal. Therefore, there is an urgent need to develop a method to improve the surface quality of pickled steel sheets used for automotive exterior panels. Summary of the Invention

[0004] This application provides a method and a pickled plate for improving the surface quality of pickled plates used in automotive exterior panels, in order to solve the following technical problem: how to improve the surface quality of pickled plates used in automotive exterior panels.

[0005] In a first aspect, this application provides a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, the method comprising:

[0006] A slab with a specified chemical composition is obtained;

[0007] The slab is heated, and the following parameters of the heating are controlled: heating temperature, holding time and excess air coefficient of the heating atmosphere, and then rolled to obtain a hot-rolled plate.

[0008] The hot-rolled sheet is wound at a set winding temperature and then rapidly cooled to obtain a hot-rolled coil.

[0009] The hot-rolled coil is pickled at a set pickling speed to obtain a pickled plate; the pickling solution contains a set concentration of pickling accelerator.

[0010] Optionally, the specified chemical composition, by mass fraction, includes: C: 0.0010%–0.0025%, Mn: 0.10%–0.15%, Si: 0.005%–0.01%, P ≤ 0.02%, Ti: 0.03%–0.07%, with the balance being Fe and unavoidable impurities.

[0011] Optionally, the heating temperature is 1180℃~1200℃, the holding time is 170min~200min, and the excess air coefficient of the heating atmosphere is 1.05~1.20.

[0012] Optionally, the finishing mill stands F1 to F7 used for rolling are made of high-speed steel rolls.

[0013] Optionally, the set winding temperature is 680℃~710℃.

[0014] Optionally, the rapid cooling method includes water cooling tank cooling.

[0015] Optionally, the time interval between the winding and the rapid cooling is ≤60 min.

[0016] Optionally, the pickling speed is set to 100 m / min to 140 m / min.

[0017] Optionally, the concentration of the pickling accelerator is set at 0.15% to 0.25%.

[0018] Secondly, this application provides an acid-washed plate prepared by the method described in any one of the embodiments of the first aspect.

[0019] The technical solutions provided in this application have the following advantages compared with the prior art:

[0020] This application provides a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, and the pickled steel sheet itself, comprising: obtaining a slab with a set chemical composition; heating the slab while controlling the following parameters of the heating: heating temperature, holding time, and excess air coefficient of the heating atmosphere, followed by rolling to obtain a hot-rolled sheet; coiling the hot-rolled sheet at a set coiling temperature, followed by rapid cooling to obtain a hot-rolled coil; and pickling the hot-rolled coil at a set pickling speed to obtain a pickled steel sheet; wherein the pickling solution contains a pickling accelerator of a set concentration. By rationally designing the chemical composition of the pickled steel sheet and controlling the heating process parameters, the influence of sulfides and phosphides of the microalloying element Ti in the steel on the surface quality can be reduced; by controlling the coiling temperature, the influence of excessive iron oxide scale and intergranular oxidation on the surface quality under high-temperature coiling processes can be reduced; and by controlling the pickling process parameters, iron oxide scale and intergranular oxidation defects on the surface can be effectively removed. This improves the surface quality of the pickled steel sheets used in automotive exterior panels. Attached Figure Description

[0021] 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.

[0022] 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.

[0023] Figure 1 A flowchart illustrating a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, provided in an embodiment of this application;

[0024] Figure 2 This is a surface morphology diagram of the pickling plate provided in Embodiment 1 of this application;

[0025] Figure 3 A surface morphology diagram of the pickled plate provided in Comparative Example 1 of this application;

[0026] Figure 4 This is a surface morphology diagram of the pickling plate provided in Embodiment 2 of this application;

[0027] Figure 5 This is a surface morphology diagram of the pickled plate provided in Comparative Example 2 of this application;

[0028] Figure 6 This is a surface morphology diagram of the pickling plate provided in Embodiment 3 of this application;

[0029] Figure 7 This is a surface morphology diagram of the pickled plate provided in Comparative Example 3 of this application;

[0030] Figure 8 This is a surface morphology diagram of the pickling plate provided in Embodiment 4 of this application;

[0031] Figure 9 This is a surface morphology diagram of the pickled plate provided in Comparative Example 4 of this application. Detailed Implementation

[0032] 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.

[0033] 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.

[0034] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0035] 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.

[0036] High-grade automotive outer panels typically incorporate titanium (Ti) to fix the carbon (C) and nitrogen (N) elements in the steel. The resulting TiC can also improve strength to some extent. Ti is a relatively economical element with good strengthening effects. However, compared to the iron (Fe) element in the steel matrix, Ti is more prone to oxidation and combines with sulfur (S) to form TiS, weakening grain boundaries. The combustion gas in hot rolling mills, originating from blast furnaces and coking furnaces, inevitably contains sulfur. When the sulfur content in the furnace combustion gas is abnormal, TiS accumulates on the surface of the billet within the furnace, leading to iron oxide scale formation on the substrate during rolling, which is then exposed after pickling. Therefore, appropriately controlling the strong oxidizing atmosphere in the furnace can burn off the TiS accumulated on the surface of the billet, reducing defects. Additionally, phosphorus (P) also tends to accumulate at grain boundaries. When Ti is present in the steel, the formed Ti phosphides may cause grain boundary cracking during hot rolling.

[0037] Therefore, this application comprehensively considers the influence of sulfides and phosphides of the microalloying element Ti in steel on surface quality, and also considers the impact of excessive iron oxide scale and intergranular oxidation on surface quality under high-temperature coiling processes. It provides a method to improve the surface quality of ultra-low carbon automotive outer panels through process adjustments during hot rolling and pickling. This method solves the problem of poor surface quality in the pickling of ultra-low carbon automotive outer panels without increasing costs; it is simple and highly effective.

[0038] Figure 1 This is a flowchart illustrating a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, as provided in an embodiment of this application.

[0039] Please see Figure 1 This application provides a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, the method comprising:

[0040] S1. Obtain a slab with a set chemical composition;

[0041] In some embodiments, the specified chemical composition, by mass fraction, includes: C: 0.0010% to 0.0025%, Mn: 0.10% to 0.15%, Si: 0.005% to 0.01%, P ≤ 0.02%, Ti: 0.03% to 0.07%, with the balance being Fe and unavoidable impurities.

[0042] In actual production, it was found that because the steel contains a certain amount of Ti, it combines with S in the furnace atmosphere during the heating process of the billet, and the resulting sulfides have an adverse effect on the surface quality of the slab. Therefore, the Ti content in the slab is limited to 0.03%–0.07%. When the Ti content is below 0.03%, it is difficult to completely remove dissolved C and N atoms, causing aging of the strip steel. When the Ti content is above 0.07%, the sulfides penetrate the surface of the billet, and the defects of scale indentation after rolling increase significantly. In addition, the P content is controlled to be below 0.02% to reduce the harm of Ti phosphide. Requirements are also set for the Mn content during the smelting process. The lower limit of Mn is 0.1%. While ensuring strength, the Mn content should not exceed 0.15% to avoid Mn diffusion to the grain boundaries and causing intergranular oxidation defects during the high-temperature slow cooling process after coiling. For example, the content of C can be 0.0010%, 0.0012%, 0.0015%, 0.0018%, 0.0020%, 0.0025%, etc.; the content of Mn can be 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, etc.; the content of Si can be 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.01%, etc.; the content of P can be 0.005%, 0.010%, 0.012%, 0.016%, 0.02%, etc.; and the content of Ti can be 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, etc.

[0043] S2. The slab is heated, and the following parameters of the heating are controlled: heating temperature, holding time and excess air coefficient of the heating atmosphere, and then rolled to obtain a hot-rolled plate.

[0044] In some embodiments, the heating temperature is 1180℃~1200℃, the holding time is 170min~200min, and the excess air coefficient of the heating atmosphere is 1.05~1.20.

[0045] Furnace parameters have a significant impact on surface quality. The heating temperature affects the enrichment of Ti, S, and P elements on the slab surface and the morphology of Ti phosphides and sulfides. When the heating temperature exceeds 1200℃ and the time exceeds 200min, Ti sulfides change from solid to liquid and extend into the slab along the grain boundaries. Ti phosphides also adhere to the liquid sulfides and enter the slab, resulting in incomplete descaling and exacerbating the intrusion of iron oxide scale. When the temperature is below 1180℃ and the time is below 170min, it is not easy to remove the slag inherited from steelmaking from the slab surface. Therefore, a slab heating furnace temperature of 1180-1200℃ and a holding time of 170-200min, while controlling the furnace air coefficient at 1.05-1.20, can effectively remove slag and is also beneficial for the removal of some Ti phosphides and sulfides. For example, the heating temperature can be 1180℃, 1185℃, 1190℃, 1195℃, 1200℃, etc., the holding time can be 170min, 180min, 190min, 195min, 200min, etc., and the excess air coefficient of the heating atmosphere can be 1.05, 1.10, 1.12, 1.15, 1.18, 1.20, etc.

[0046] In some embodiments, the finishing mill stands F1 to F7 used for rolling are made of high-speed steel rolls.

[0047] Studies of the hot rolling process have revealed that when using ordinary infinitely cold-hardened rolls, pickling color difference defects frequently occur, and the probability of scale residue increases significantly. High-speed steel rolls have better surface wear resistance, and are less prone to groove formation during rolling, reducing stress in contact with the strip and lowering the risk of scale indentation. Actual production has shown that using high-speed steel rolls on all stands (F1-F7) of the finishing mill for automotive outer panels, and arranging them in the early roll stage, yields the best surface quality.

[0048] S3. The hot-rolled plate is wound at a set winding temperature and then rapidly cooled to obtain a hot-rolled coil;

[0049] In some embodiments, the set winding temperature is 680°C to 710°C.

[0050] In some implementations, the rapid cooling method includes water cooling tank cooling.

[0051] In some implementations, the time interval between the winding and the rapid cooling is ≤60 min.

[0052] During the slow, natural cooling process after hot-rolled steel coils are coiled, the iron oxide scale thickens and becomes uneven in thickness, easily causing color differences and defects during pickling. Furthermore, oxygen easily penetrates the grain boundaries of the strip matrix, and Mn elements enriched at the grain boundaries form grain boundary oxides. Therefore, to ensure solid solution strengthening, the coiling temperature needs to be controlled within a certain range. However, in practice, maintaining the mechanical properties of the strip cannot significantly reduce the coiling temperature. Therefore, based on controlling the hot-rolled steel coiling temperature at 680–710℃, the coil is placed in a water-cooling tank for rapid cooling within 60 minutes after coiling. This reduces the high-temperature dwell time and avoids intergranular oxidation defects during the coiling process. For example, the set coiling temperature can be 680℃, 682℃, 685℃, 690℃, 695℃, 700℃, 705℃, 710℃, etc.; the time interval between coiling and rapid cooling can be 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes, etc.

[0053] S4. The hot-rolled coil is pickled at a set pickling speed to obtain a pickled plate; the pickling solution contains a set concentration of pickling accelerator.

[0054] In some embodiments, the set pickling speed is 100 m / min to 140 m / min.

[0055] In some embodiments, the concentration of the pickling accelerator is set at 0.15% to 0.25%.

[0056] Pickling speed has a significant impact on the surface quality of the pickled strip. The optimal pickling speed is controlled between 100 and 140 m / min. While pickling speeds below 100 m / min can remove iron oxide scale and intergranular oxides, acid residue is more likely to remain on the strip surface, potentially affecting the production capacity of the pickling line. Pickling speeds exceeding 140 m / min make it difficult to remove intergranular oxide defects. Therefore, adding an appropriate accelerator can significantly improve the removal of intergranular oxides. A accelerator concentration below 0.15% results in poor removal, while a concentration above 0.25% increases iron loss and negatively impacts yield. Therefore, adding a 0.15%–0.25% accelerator to the acid solution can remove iron oxide scale and surface intergranular oxide defects without causing over-pickling. For example, the pickling speed can be set to 100 m / min, 110 m / min, 120 m / min, 130 m / min, 135 m / min, 140 m / min, etc.; the concentration of the pickling accelerator can be set to 0.15%, 0.18%, 0.20%, 0.22%, 0.25%, etc.

[0057] Based on a general inventive concept, this application provides an acid-washing plate.

[0058] The pickling plate is realized based on the above-described pickling plate preparation method. The specific steps of the pickling plate preparation method can be referred to the above embodiments. Since the pickling plate adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be elaborated here.

[0059] 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 industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0060] In this embodiment of the application, molten iron is smelted to obtain the chemical composition of the slab in Table 1.

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

[0062] Group C Mn Si P Ti Examples 1-4 and Comparative Examples 2-4 0.0019％ 0.13％ 0.01％ 0.01％ 0.05％ Comparative Example 1 0.0015％ 0.11％ 0.008％ 0.03％ 0.08％

[0063] Based on the above-mentioned chemical composition of the slab, this embodiment provides a method for improving the surface quality of pickled steel sheets used in automotive exterior panels, comprising the following steps:

[0064] S11. Obtain a slab with a set chemical composition;

[0065] S21. The slab is heated, and the following parameters of the heating are controlled: heating temperature, holding time and excess air coefficient of the heating atmosphere, and then rolled to obtain a hot-rolled plate.

[0066] S31. The hot-rolled plate is wound at a set winding temperature and then rapidly cooled to obtain a hot-rolled coil.

[0067] S41. The hot-rolled coil is pickled at a set pickling speed to obtain a pickled plate; the pickling solution contains a set concentration of pickling accelerator.

[0068] Table 2. Process parameters for improving the surface quality of pickled steel sheets used in automotive exterior panels.

[0069]

[0070] Appendix Figures 2-9 Detailed explanation:

[0071] Figure 2 This is a surface morphology diagram of the pickling plate provided in Embodiment 1 of this application;

[0072] Figure 3 This is a surface morphology diagram of the pickled plate provided in Comparative Example 1 of this application.

[0073] Depend on Figure 2 and Figure 3 The comparison shows that the higher the content of alloying elements Ti and P (Ti = 0.08% and P = 0.03% in Comparative Example 1), the more severe the iron scale residue and color difference defects on the pickled surface. Reducing the content of Ti and P alleviates the iron scale residue phenomenon to a certain extent and also reduces the color difference. This indicates that controlling the content of alloying elements Ti and P can effectively reduce iron scale residue defects.

[0074] Figure 4 This is a surface morphology diagram of the pickling plate provided in Embodiment 2 of this application;

[0075] Figure 5 This is a surface morphology diagram of the pickled plate provided in Comparative Example 2 of this application.

[0076] Depend on Figure 4 and Figure 5 The comparison shows that appropriate heating temperature and time, and increasing the excess air coefficient in the heating furnace can reduce the adverse effects of Ti and P elements, and alleviate iron scale residue defects.

[0077] Figure 6 This is a surface morphology diagram of the pickling plate provided in Embodiment 3 of this application;

[0078] Figure 7 This is a surface morphology diagram of the pickled plate provided in Comparative Example 3 of this application.

[0079] Depend on Figure 6 and Figure 7 The comparison shows that using high-speed steel rolls, reducing the winding temperature, and reducing the high-temperature dwell time after winding can significantly reduce the severity of surface color difference and defects.

[0080] Figure 8 This is a surface morphology diagram of the pickling plate provided in Embodiment 4 of this application;

[0081] Figure 9 This is a surface morphology diagram of the pickled plate provided in Comparative Example 4 of this application.

[0082] Depend on Figure 8 and Figure 9 The comparison shows that using a higher pickling rate and a suitable accelerator concentration can significantly reduce the severity of surface color difference and defects. Furthermore, the surface quality of the pickled plate provided in Example 4 is superior to that of the pickled plates in Examples 1-3 and Comparative Examples 1-4.

[0083] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0084] (1) In the embodiments of the present invention, the manufacturing method for improving the surface quality of ultra-low carbon automotive outer panels after pickling can effectively reduce the occurrence rate of surface quality defects after pickling of ultra-low carbon high-grade automotive outer panels, and can also provide a reference for improving the surface quality of other steels with similar composition.

[0085] (2) In this embodiment of the invention, the problem of galvanized slag accumulation caused by hot rolling is solved, and the degradation rate of the outer plate caused by slag accumulation is reduced from 2.7% to less than 0.1%, which effectively improves the overall level of the outer plate and the normal operation of the production organization.

[0086] (3) In the embodiments of the present invention, the manufacturing method for improving the surface quality of pickled ultra-low carbon automotive outer panels is provided, and the content of elements in steel, especially P, is controlled. The hot rolling process adjusts the heating furnace atmosphere, especially the cooling process after coiling. The pickling process adjusts the pickling speed and adds accelerators. Through the above control of composition and key process points, the problem of poor quality such as iron scale residue and intergranular oxidation on the pickled surface caused by unreasonable control of composition and hot rolling and pickling processes is solved.

[0087] (4) In the embodiments of the present invention, the manufacturing method for improving the pickling surface quality of ultra-low carbon automotive outer panels is simple, economical and efficient. Using the present invention, the pickling quality can be improved relatively easily without increasing the production difficulty of the production line or affecting the performance of the finished product.

[0088] 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 surface quality of pickled steel sheets used in automotive exterior panels, characterized in that, The method includes: A slab with a set chemical composition is obtained, wherein the set chemical composition includes: C: 0.0010%~0.0025%, Mn: 0.10%~0.15%, Si: 0.005%~0.01%, P≤0.02%, Ti: 0.03%~0.07%, and the balance is Fe and unavoidable impurities; The slab is heated, and the excess air coefficient of the heating atmosphere is controlled to be 1.10~1.20 to form a strong oxidizing atmosphere to burn off TiS. The heating temperature is controlled to be 1180℃~1200℃, and the holding time is 170min~200min. Then it is rolled to obtain a hot-rolled plate. The hot-rolled sheet is coiled and then rapidly cooled to obtain a hot-rolled coil at a coiling temperature of 680℃~710℃. The hot-rolled coil is pickled at a pickling speed of 100m / min to 140m / min to obtain a pickled plate; the pickling solution contains a pickling accelerator at a concentration of 0.15% to 0.25%. The time interval between the winding and the rapid cooling is ≤60 min.

2. The method according to claim 1, characterized in that, The finishing mill stands F1 to F7 used in the rolling process employ high-speed steel rolls.

3. The method according to claim 1, characterized in that, The rapid cooling method includes water cooling tank cooling.

4. A pickling plate prepared by the method according to any one of claims 1 to 3.

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