High hole expansion performance pickled steel sheet and method for producing the same

Abstract

The application discloses a high-hole-expanding-performance pickling steel plate and a production method thereof, and belongs to the technical field of automobile part manufacturing. The production method of the pickling steel plate comprises the following steps: A, pickling the steel plate; B, placing the pickled steel plate in an oxygen-free environment and heat treating at 650-750 DEG C, wherein a continuous bundle-shaped force is applied during the heat treating process. The application promotes the closure of the pickling surface micro-concave holes through high-temperature plastic deformation, effectively compensates for the surface defects caused by the pickling process, and improves the hole-expanding rate of the steel plate.

Description

Technical Field

[0001] This invention belongs to the field of automotive parts manufacturing technology, and relates to a pickled steel sheet with high hole expansion performance and its production method. Background Technology

[0002] With the trend towards lightweighting in the automotive industry, using pickled steel sheets to manufacture structural components such as chassis suspension parts and frame connectors has become the mainstream technical approach. The traditional process for producing pickled steel sheets involves hot rolling and pickling. While the pickling process effectively removes iron oxide scale, the acid corrosion can create micro-pore defects on the steel sheet surface. These micro-defects can easily cause stress concentration during subsequent hole enlargement and forming, leading to edge cracks during flanging and hole enlargement, severely affecting the structural integrity and service safety of the parts.

[0003] Currently, the industry mainly uses two post-treatment processes: chemical passivation and shot peening. Chemical passivation forms a passivation film on the steel plate surface using a chromate solution. While this can cover some shallow pits and improve corrosion resistance, it cannot cover deep pit structures, and the chromium-containing treatment poses environmental risks. Shot peening, on the other hand, creates a compressive stress layer on the surface through shot impact, which can improve the surface condition, but may introduce work hardening, leading to a decrease in material ductility. These processes all provide limited repair of pits through external intervention, and the repair effect is significantly affected by process parameters, making it difficult to achieve stable and reliable performance improvements. Summary of the Invention

[0004] The purpose of this invention is to provide a pickled steel plate with high hole expansion performance and its production method, which effectively improves the hole expansion performance of the pickled steel plate.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] In a first aspect, the present invention proposes a method for producing pickled steel plates with high hole-expanding performance, employing the following technical solution:

[0007] This application provides a method for producing pickled steel plates with high hole expansion performance, including the following steps:

[0008] A. Pickling the hot-rolled steel sheet;

[0009] B. Place the pickled steel plate in an oxygen-free environment and heat treat it at 650-750℃, during which a continuous shaping force is applied.

[0010] As used in this text, "beaming force" refers to a force applied to essentially all surfaces of a steel sheet, rather than to a localized area, with the aim of causing microscopic changes such as displacement or deformation of the internal particles (e.g., metal grains) of the steel sheet, without expecting significant deformation of the sheet's appearance. The magnitude of this beaming force and the duration of a single application of the force significantly distinguish it from metal rolling processes (e.g., hot rolling, cold rolling, or pressing).

[0011] The apparatus used to apply the beam force described in this paper employs dies such as those used in conventional metal extrusion, with the extrusion pressure parameters adjusted accordingly. Since dies used in conventional metal extrusion are well-known to those skilled in the art, they will not be described in detail here.

[0012] Preferably, the oxygen-free environment described in step B is achieved by introducing one of nitrogen, argon, or a mixture of nitrogen and argon as a protective gas.

[0013] Preferably, the protective gas is nitrogen.

[0014] In this way, a nitrogen atmosphere can provide nitrogen to the steel plate, and the nitrogen can penetrate into the interior of the steel plate to form nitrides with the metal, thereby increasing its strength.

[0015] Preferably, the flow rate of the protective gas is 1.5-2 m / s. 3 / h,

[0016] Preferably, the pressure of the anaerobic environment is 10-50 Pa.

[0017] Preferably, the pressure of the beam-forming force and the bidirectional pressure are both 1.5-4.0 MPa.

[0018] Therefore, this specific pressure level can ensure the effective adjustment of the internal microstructure of the steel plate while avoiding macroscopic deformation of the steel plate due to excessive pressure. It ensures that the steel plate maintains its original dimensional accuracy while achieving reasonable displacement and reorganization of metal grains, optimizing the internal structure, and thus improving the hole expansion performance.

[0019] Preferably, the pressure area of ​​the beam force covers more than 90% of the surface area of ​​the steel plate.

[0020] Preferably, the continuous beam-forming force is applied periodically.

[0021] Therefore, periodically applying beam force allows the internal microstructure of the steel plate to undergo intermittent adjustments during heat treatment. During the beam force application phase, the particles inside the steel plate are displaced or deformed by the force; while during the interval phase, the steel plate undergoes a certain degree of "adaptation" and "relaxation" to the previous stress state, which is conducive to further optimization of the microstructure and avoids excessive deformation or stress concentration that may be caused by continuous stress, thereby improving the hole-expanding performance of the steel plate more stably and efficiently.

[0022] Preferably, the application cycle of the beam-forming force is: the duration of each beam-forming force application is 5-10 seconds, and the interval between two adjacent beam-forming force applications is 3-8 seconds.

[0023] Secondly, the present invention provides a pickling steel sheet with high hole expansion performance, which adopts the following technical solution:

[0024] A pickled steel sheet with high hole expansion performance is prepared using the production method described in the first aspect.

[0025] The beneficial effects of this invention are:

[0026] This invention utilizes the enhanced plasticity of pickled steel plates under high temperature by subjecting them to heat treatment in an oxygen-free environment and applying continuous beam force. The periodic vertical beam force induces the plastic closure of micro-pores on the surface of the steel plate, effectively compensating for surface defects caused by the pickling process and improving the porosity of the steel plate. Detailed Implementation

[0027] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0028] In traditional pickling steel sheet manufacturing processes, while the pickling process effectively removes iron oxide scale, it also creates micro-hole defects on the steel sheet surface due to acid corrosion. These micro-holes can easily cause stress concentration during subsequent hole enlargement and forming, leading to edge cracks during flanging and seriously affecting the service safety of automotive structural components.

[0029] To address this problem, those skilled in the art typically focus on surface treatment processes, including but not limited to optimizing pickling solution formulations, chemical passivation, or shot peening. However, optimizing the pickling solution composition aims to remove oxides from the steel plate, but it cannot truly repair the surface defects of the steel plate after pickling. Chemical passivation relies on a chromate passivation film covering shallow recesses, which cannot repair deep recesses and poses environmental risks. While shot peening can create a compressive stress layer on the surface through shot impact, improving the surface condition, it introduces work hardening, leading to a decrease in material ductility.

[0030] Through systematic research, the inventors discovered that the bottleneck in the hole-expanding performance of steel plates lies in the localized stress concentration caused by micro-cavities. Traditional processes only provide limited repair through external interventions (such as passivation film coverage or compressive stress compensation), failing to address the root cause of micro-cavities. To overcome this limitation, the inventors innovatively proposed a synergistic mechanism of high-temperature plastic deformation coupled with dynamic pressure application, fundamentally compensating for the micro-cavities in the steel plate, thereby improving its hole-expanding performance. This invention thus completes the creation of this invention.

[0031] All hot-rolled steel plates used in the embodiments and comparative examples of this invention were purchased from Shanghai Baoli Industrial Co., Ltd., and were B510L hot-rolled steel plates produced by Baosteel. The iron oxide thickness on the surface of the hot-rolled steel plates was 12±3μm.

[0032] Hot-rolled steel plate pretreatment: Cut the hot-rolled steel plate into samples with dimensions of 300mm×200mm×4mm.

[0033] The heat treatment in this embodiment of the invention uses a vacuum hot press furnace, which is equipped with a 50T servo hydraulic system and a customized silicon carbide coated graphite mold (size 290×190mm×3mm, surface roughness Ra≤0.8μm).

[0034] Example 1

[0035] A method for producing pickled steel sheets with high hole-expanding performance includes the following steps:

[0036] A. Place the pretreated hot-rolled steel plate sample in an acid bath with a volume of 5m³. 3 The solution is placed in an acid bath and immersed in a 15wt% hydrochloric acid solution at 75±2℃ for 5 minutes, while simultaneously applying ultrasonic vibration at a frequency of 10Hz. Then it is rinsed with a 0.8MPa high-pressure water gun and finally dried with hot air at 120℃.

[0037] B. Transfer the dried pickled steel plate to a vacuum autoclave, and introduce nitrogen gas as a protective gas at a flow rate of 1.8 m / s. 3 The vacuum hot press furnace has a furnace pressure of 30Pa and uses a segmented heating method. The heating rate is 20℃ / min in the 0-600℃ range and adjusted to 8℃ / min in the 600-700℃ range. The final temperature is 700℃ and held for 45 minutes. During the holding period, a periodic vertical beam force is applied through a 50T servo hydraulic system and a silicon carbide coated graphite mold. After the holding period, the steel plate is cooled to ≤60℃ with the furnace to complete the processing.

[0038] The forming force is a bidirectional pressure applied to the steel plate, the direction of which is perpendicular to the surface of the steel plate, and the bidirectional pressure is achieved through a hydraulic system and a silicon carbide coated graphite mold.

[0039] The specific settings for the periodic vertical beam force are as follows:

[0040] The hydraulic system is set to a pressure of 100kN (corresponding to a pressure of 1.8MPa), meaning the hydraulic system simultaneously applies a force of 100kN to the upper and lower molds of the silicon carbide-coated graphite mold, with a pressure coverage of 91.8%. The periodicity is as follows: each application of a shaping force to the steel plate lasts for 10 seconds, with a 5-second interval between adjacent applications, repeated 20 times. Stroke control: the initial mold closing gap is 0.5mm, and the actual pressure stroke is 0.3mm (adapting to fluctuations in steel plate thickness).

[0041] In this embodiment 1, a pickled steel plate with high hole expansion performance is prepared using the method described above.

[0042] Example 2

[0043] A method for producing pickled steel sheets with high hole-expanding performance includes the following steps:

[0044] A. Place the pretreated hot-rolled steel plate sample in an acid bath with a volume of 5m³. 3 The solution is placed in an acid bath and immersed in a 15wt% hydrochloric acid solution at 75±2℃ for 5 minutes, while simultaneously applying ultrasonic vibration at a frequency of 10Hz. Then it is rinsed with a 0.8MPa high-pressure water gun and finally dried with hot air at 120℃.

[0045] B. Transfer the dried pickled steel plate to a vacuum autoclave, and introduce a mixture of nitrogen and argon gas (nitrogen:argon = 3:1) at a flow rate of 1.5 m / s. 3 The vacuum hot press furnace operates at a pressure of 10 Pa per hour, using a segmented heating method. The heating rate is 20℃ / min in the 0-600℃ range and adjusted to 8℃ / min in the 600-700℃ range. The final temperature is raised to 650℃ and held for 45 minutes. During the holding period, a periodic vertical beam force is applied through a 50T servo hydraulic system and a silicon carbide coated graphite mold. After the holding period, the steel plate is cooled with the furnace to ≤60℃, completing the process.

[0046] The forming force is a bidirectional pressure applied to the steel plate, the direction of which is perpendicular to the surface of the steel plate, and the bidirectional pressure is achieved through a hydraulic system and a silicon carbide coated graphite mold.

[0047] The specific settings for the periodic vertical beam force are as follows:

[0048] The hydraulic system is set to a pressure of 83 kN (corresponding to a pressure of 1.5 MPa), meaning the hydraulic system simultaneously applies a force of 83 kN to the upper and lower molds of the silicon carbide-coated graphite mold, with a pressure coverage of 91.8%. The periodicity is as follows: each application of a shaping force to the steel plate lasts for 10 seconds, with a 3-second interval between adjacent applications, repeated 20 times. Stroke control: the initial mold closing gap is 0.5 mm, and the actual pressure stroke is 0.3 mm (adapting to fluctuations in steel plate thickness).

[0049] In this embodiment 2, a pickled steel plate with high hole expansion performance is prepared using the method described above.

[0050] Example 3

[0051] A method for producing pickled steel sheets with high hole-expanding performance includes the following steps:

[0052] A. Place the pretreated hot-rolled steel plate sample in an acid bath with a volume of 5m³. 3 The solution is placed in an acid bath and immersed in a 15wt% hydrochloric acid solution at 75±2℃ for 5 minutes, while simultaneously applying ultrasonic vibration at a frequency of 10Hz. Then it is rinsed with a 0.8MPa high-pressure water gun and finally dried with hot air at 120℃.

[0053] B. Transfer the dried pickled steel plate to a vacuum autoclave, and introduce nitrogen gas as a protective gas at a flow rate of 2 m / s. 3 The vacuum hot press furnace has a furnace pressure of 50 Pa and uses a segmented heating method. The heating rate is 20℃ / min in the 0-600℃ range and adjusted to 8℃ / min in the 600-700℃ range. The final temperature is 750℃ and held for 45 minutes. During the holding period, a periodic vertical beam force is applied through a 50T servo hydraulic system and a silicon carbide coated graphite mold. After the holding period, the steel plate is cooled to ≤60℃ with the furnace to complete the processing.

[0054] The forming force is a bidirectional pressure applied to the steel plate, the direction of which is perpendicular to the surface of the steel plate, and the bidirectional pressure is achieved through a hydraulic system and a silicon carbide coated graphite mold.

[0055] The specific settings for the periodic vertical beam force are as follows:

[0056] The hydraulic system is set to a pressure of 220 kN (corresponding to a pressure of 4 MPa), meaning that the hydraulic system simultaneously applies a force of 220 kN to the upper and lower molds of the silicon carbide-coated graphite mold, with a pressure coverage of 91.8%. The periodicity is as follows: each application of a shaping force to the steel plate lasts for 5 seconds, with an 8-second interval between adjacent applications, repeated 20 times. Stroke control: the initial mold closing gap is 0.5 mm, and the actual pressure stroke is 0.3 mm (adapting to fluctuations in steel plate thickness).

[0057] In this embodiment 3, a pickled steel plate with high hole expansion performance is prepared using the above-described method.

[0058] Example 4

[0059] A method for producing pickled steel sheets with high hole-expanding performance includes the following steps:

[0060] A. Place the pretreated hot-rolled steel plate sample in an acid bath with a volume of 5m³. 3 The solution is placed in an acid bath and immersed in a 15wt% hydrochloric acid solution at 75±2℃ for 5 minutes, while simultaneously applying ultrasonic vibration at a frequency of 10Hz. Then it is rinsed with a 0.8MPa high-pressure water gun and finally dried with hot air at 120℃.

[0061] B. Transfer the dried pickled steel plate to a vacuum autoclave, and introduce nitrogen gas as a protective gas at a flow rate of 1.8 m / s. 3 The vacuum hot press furnace has a furnace pressure of 30Pa and uses a segmented heating method. The heating rate is 20℃ / min in the 0-600℃ range and adjusted to 8℃ / min in the 600-700℃ range. The final temperature is 700℃ and held for 45 minutes. During the holding period, a periodic vertical beam force is applied through a 50T servo hydraulic system and a silicon carbide coated graphite mold. After the holding period, the steel plate is cooled to ≤60℃ with the furnace to complete the processing.

[0062] The forming force is a bidirectional pressure applied to the steel plate, the direction of which is perpendicular to the surface of the steel plate, and the bidirectional pressure is achieved through a hydraulic system and a silicon carbide coated graphite mold.

[0063] The specific settings for the periodic vertical beam force are as follows:

[0064] The hydraulic system is set to a pressure of 150 kN (corresponding to a pressure of 2.72 MPa), meaning the hydraulic system simultaneously applies a force of 150 kN to the upper and lower molds of the silicon carbide-coated graphite mold, with a pressure coverage of 91.8%. The periodicity is as follows: each application of a shaping force to the steel plate lasts for 8 seconds, with a 6-second interval between adjacent applications, repeated 18 times. Stroke control: the initial mold closing gap is 0.5 mm, and the actual pressure stroke is 0.3 mm (adapting to fluctuations in steel plate thickness).

[0065] In this embodiment 4, a pickled steel plate with high hole expansion performance is prepared using the method described above.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that the pickled steel plate was not subjected to step B.

[0068] Comparative Example 2

[0069] The difference from Example 1 is that a 2050mm hot rolling mill was used to hot roll the pickled steel sheet. The hot rolling temperature was set to 850°C, the rolling speed to 5m / s, and the reduction rate to 20%. The hot rolling process was carried out in air without the introduction of a protective gas or the application of a forming force. After hot rolling, the steel sheet was air-cooled to room temperature.

[0070] Comparative Example 3

[0071] The difference from Example 1 is that when applying the beam-forming force in step B, the hydraulic system is set to a pressure of 278 kN (corresponding to a pressure of 5 MPa).

[0072] Comparative Example 4

[0073] The difference from Example 1 is that when the beam-forming force is applied in step B, the hydraulic system is set to a pressure of 28 kN (corresponding to a pressure of 0.5 MPa).

[0074] Test case

[0075] According to GB / T 15825.4-2008 standard, the porosity of the pickled steel plates in Examples 1-4 and Comparative Examples 1-4 was tested (5 parallel samples per group). The specific test procedures and related instructions are as follows:

[0076] 1. Sample preparation: A circular hole with a diameter of φ10.0±0.1mm is pre-drilled at the center of each sample;

[0077] 2. Testing equipment: INSTRON 5985 universal testing machine, equipped with a 60° conical punch, blank holder force of 50kN, and stamping speed of 0.5mm / s;

[0078] 3. Test: Press the conical punch into the pre-made hole until the crack is through, and measure the average hole diameter after enlargement;

[0079] 4. Calculation formula:

[0080] Where: d0: initial hole diameter before hole expansion; d: average hole diameter when crack penetration occurs after hole expansion (the maximum and minimum values ​​need to be measured and then averaged); λ: hole expansion rate, expressed as a percentage.

[0081] The specific data for the test cases are shown in Table 1.

[0082] Table 1

[0083] Group Hole expansion rate % Example 1 107.7±0.7 Example 2 104.2±0.8 Example 3 112.7±0.6 Example 4 110.2±0.7 Comparative Example 1 83.7±0.9 Comparative Example 2 78.5±1.0 Comparative Example 3 92.3±0.9 Comparative Example 4 87.6±1.1

[0084] As shown in Table 1:

[0085] The porosity of Examples 1-4 was 104.2±0.8% to 112.7±0.6%, significantly higher than that of Comparative Examples 1-4, verifying the repair effect of the oxygen-free high-temperature and periodic pressure process of the present invention on the closure of micro-concave pores. The present invention effectively compensates for surface defects caused by the pickling process and improves the porosity of pickled steel plates by coupling high-temperature plastic deformation with dynamic pressure.

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for producing pickled steel plates with high hole-expanding performance, characterized in that, Includes the following steps: A. Pickling the hot-rolled steel sheet; B. Place the pickled steel plate in an oxygen-free environment and heat treat it at 650-750℃, during which a continuous shaping force is applied. The tool used to apply the binding force is a die used in metal extrusion processing; The forming force is a bidirectional pressure applied to the steel plate, the direction of which is perpendicular to the surface of the steel plate, and the pressure of which is 1.5-4.0 MPa. The continuous beam-forming force is applied periodically, and the application period of the beam-forming force is as follows: the duration of each beam-forming force application is 5-10 seconds, and the interval between two adjacent beam-forming force applications is 3-8 seconds.

2. The method for producing a pickled steel plate with high hole expansion performance according to claim 1, characterized in that, The oxygen-free environment described in step B is achieved by introducing one of the following gases as a protective gas: nitrogen, argon, or a mixture of nitrogen and argon.

3. The method for producing a pickled steel plate with high hole expansion performance according to claim 2, characterized in that, The protective gas is nitrogen.

4. The method for producing a pickled steel plate with high hole expansion performance according to claim 2, characterized in that, The flow rate of the protective gas is 1.5-2 m³ / h.

5. The method for producing a pickled steel plate with high hole expansion performance according to claim 2, characterized in that, The pressure of the anaerobic environment is 10-50 Pa.

6. The method for producing a pickled steel plate with high hole expansion performance according to claim 1, characterized in that, The pressure area of ​​the beam force covers more than 90% of the surface area of ​​the steel plate.

7. A pickling steel sheet with high hole expansion performance, characterized in that, Prepared using any of the production methods described in claims 1-6.