In-situ film-forming quenching treatment method for steel strip

Abstract

The invention discloses an in-situ film-forming quenching treatment method of a steel strip, which belongs to the technical field of steel manufacturing, and is characterized in that the steel strip to be treated is heated to convert the structure of the steel strip into austenite, then the steel strip is immersed into a quenching medium, and a film-forming additive in the quenching medium is used for in-situ reaction on the surface of the steel strip to form a layer of protective film while quenching, so that the steel strip is subjected to in-situ film-forming quenching treatment. And cooling to obtain the finished steel strip. And a layer of compact and uniform protective film can be formed on the surface of the steel strip in situ during quenching cooling, so that the production process is simplified, the cost is reduced, and the corrosion resistance of the steel strip is improved. Through a modified molten salt formula and process parameter design, quenching strengthening and surface protection functions are integrated in a single treatment step, the mechanical property and corrosion resistance of the steel strip are optimized, the production process is simplified, and remarkable technical progress and economic benefits are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel manufacturing, and particularly relates to an in-situ film-forming quenching treatment method for steel strips. Background Art

[0002] Steel strips are basic materials widely used in industrial production, and their properties directly affect the quality and service life of the final products. Heat treatment is a key process for optimizing the structure and properties of steel strips, and quenching is a common method for obtaining high-strength and high-hardness steel strips.

[0003] Traditional steel strip quenching processes usually include heating the steel strip to the austenitizing temperature and then rapidly cooling it below the martensite transformation temperature. Common quenching media include water, oil, or conventional salt baths. However, these methods have some inherent limitations: after the steel strip is austenitized at high temperature, if it is cooled in air or non-protective media, its surface is prone to oxidation, forming scale, which affects the surface finish and subsequent processing performance. Although cooling can be carried out under a protective atmosphere, this increases the complexity and cost of the equipment; when using cooling media with a relatively fast cooling rate such as water or oil, the steel strip is prone to generate large internal stresses due to uneven cooling, resulting in warping, deformation, or even cracking; oil-based quenching media may produce oil fumes, polluting the environment and posing a certain fire risk. The treatment of waste oil also increases the environmental burden; conventionally quenched steel strips usually require additional anti-rust treatments such as oil coating, phosphating, passivation, etc. to prevent rusting during storage and transportation. This not only increases the production process but also raises the production cost.

[0004] Although salt bath quenching can provide more uniform and controllable cooling and reduce deformation, conventional salt baths generally do not have the function of directly forming an effective protective film during the quenching process. Therefore, the quenched steel strip still faces the risk of rusting and requires subsequent surface protection treatment. Summary of the Invention

[0005] The present invention discloses an in-situ film-forming quenching treatment method for steel strips, which can simultaneously form a dense and uniform protective film on the surface of the steel strip during quenching cooling, simplifies the production process, reduces the cost, and improves the corrosion resistance of the steel strip.

[0006] The present invention discloses an in-situ film-forming quenching treatment method for steel strips. The method is to heat the steel strip to be treated to transform its structure into austenite, then immerse it in a quenching medium. During quenching, a protective film is formed by in-situ reaction of a film-forming additive in the quenching medium on the surface of the steel strip, and then it is cooled to obtain the finished steel strip.

[0007] As a further improvement of the present invention, it includes the following steps:

[0008] Step 1. Austenitizing treatment: Heat the steel strip to be treated, hold it at a certain temperature, and then quickly immerse it in a quenching medium for quenching treatment.

[0009] Step 2. Cool down the quenched steel strip under a protective atmosphere to obtain the finished steel strip.

[0010] As a further improvement of the present invention, in Step 1, the heating temperature is 870°C to 890°C, the holding time is 150 to 210 s, the temperature of the quenching medium is 210° to 230°C, and the quenching treatment time is 20 to 30 s.

[0011] As a further improvement of the present invention, the quenching medium is a modified molten salt.

[0012] As a further improvement of the present invention, the modified molten salt includes a basic molten salt and a film-forming additive.

[0013] As a further improvement of the present invention, the basic molten salt is at least one of potassium nitrate and sodium nitrite, and the film-forming additive is at least one of sodium dihydrogen phosphate and anhydrous borax.

[0014] As a further improvement of the present invention, the addition amount of sodium dihydrogen phosphate accounts for 2.0 wt% to 3.0 wt% of the total weight of the modified molten salt.

[0015] As a further improvement of the present invention, the addition amount of anhydrous borax accounts for 0.5 wt% to 1.5 wt% of the total weight of the modified molten salt.

[0016] As a further improvement of the present invention, in Step 2, cool down to ≤100°C.

[0017] As a further improvement of the present invention, it specifically includes the following steps:

[0018] Step 1. Austenitizing treatment: Heat the steel strip to be treated to 870°C to 890°C, hold it for 150 seconds to 210 seconds, and then quickly immerse it in a quenching medium at 210° to 230°C for 20 to 30 s of quenching treatment.

[0019] Step 2. Cool down the quenched steel strip to ≤100°C under a nitrogen atmosphere to obtain the finished steel strip.

[0020] The present invention has the following beneficial effects:

[0021] (1) The present invention uses a modified molten salt of a specific composition as a quenching medium. While the steel strip is being quenched, the heat and active surface of the steel strip are used to in-situ induce the reaction of phosphates and borates with the steel matrix in a molten salt environment to generate a protective composite film layer. This avoids the multiple independent processes of cleaning, phosphating, passivation, etc. that must be performed after quenching in traditional processes, greatly shortens the production cycle, reduces equipment footprint and investment, and reduces energy consumption and production costs.

[0022] (2) The present invention can achieve precise control of the cooling process by immersing the steel strip in a modified molten salt at a specific temperature for quenching after austenitization, thereby obtaining excellent strong and tough structures such as refined bainite or martensite, avoiding the risk of cracking caused by too fast water quenching and the problem of insufficient cooling capacity of oil quenching.

[0023] (3) The phosphate-borate composite film layer formed in situ on the surface of the steel strip by the method of the present invention is dense, uniform and firmly bonded to the substrate. Sodium dihydrogen phosphate provides the main phosphating film-forming component to form an iron phosphate passivation layer; the synergistic addition of anhydrous borax, on the one hand, its fluxing effect helps to clean the surface of the steel strip and promote the uniform phosphating reaction, and on the other hand, the participation of the boron element can further improve the density and chemical stability of the film layer, forming a borophosphate composite structure with better performance. The composite film layer can effectively block the corrosive medium and significantly improve the corrosion resistance of the steel strip.

[0024] (4) The present invention integrates quenching strengthening and surface protection functions into a single processing step through modified molten salt formula and process parameter design, thereby optimizing the mechanical properties and corrosion resistance of the steel strip, simplifying the production process, and having significant technological progress and economic benefits. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1

[0027] An in-situ film-forming quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the 65Mn steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 880 °C under a protective atmosphere, and hold it at this temperature for 180 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a modified molten salt quenching medium maintained at 220 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 25 seconds. During this process, the steel strip is rapidly cooled and at the same time a phosphate-based protective film is formed by in-situ reaction on its surface. After the quenching treatment is completed, take out the steel strip from the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 90 °C to obtain a finished steel strip with an in-situ generated protective film on its surface.

[0028] Among them, the modified molten salt is composed of the following components by mass percentage: 50 wt% potassium nitrate, 47.5 wt% sodium nitrite, and 2.5 wt% sodium dihydrogen phosphate as a film-forming additive.

[0029] Example 2

[0030] An in-situ film-forming quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the T8 steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 870 °C under a protective atmosphere, and hold it at this temperature for 150 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a modified molten salt quenching medium maintained at 210 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 20 seconds. During this process, the steel strip is rapidly cooled and at the same time a phosphate-based protective film is formed by in-situ reaction on its surface. After the quenching treatment is completed, take out the steel strip from the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 100 °C to obtain a finished steel strip with an in-situ generated protective film on its surface.

[0031] The modified molten salt is composed of the following components by mass percentage: 97.8 wt% potassium nitrate, and 2.2 wt% sodium dihydrogen phosphate as a film-forming additive.

[0032] Example 3

[0033] An in-situ film-forming quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the SK5 steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 890 °C under a protective atmosphere, and hold it at this temperature for 210 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a modified molten salt quenching medium maintained at 230 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 30 seconds. During this process, the steel strip is rapidly cooled and at the same time a phosphate-borate composite protective film is formed by in-situ reaction on its surface. After the quenching treatment is completed, take out the steel strip from the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 80 °C to obtain a finished steel strip with a protective film formed in-situ on its surface.

[0034] The modified molten salt is composed of the following components by mass percentage: 96.5 wt% of sodium nitrite, 3.0 wt% of sodium dihydrogen phosphate, and 0.5 wt% of anhydrous borax as a film-forming additive.

[0035] Example 4

[0036] An in-situ film-forming quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the 50CrV steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 875 °C under a protective atmosphere, and hold it at this temperature for 200 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a modified molten salt quenching medium maintained at 225 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 28 seconds. During this process, the steel strip is rapidly cooled and at the same time a phosphate-borate composite protective film is formed by in-situ reaction on its surface. After the quenching treatment is completed, take out the steel strip from the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 95 °C to obtain a finished steel strip with a protective film formed in-situ on its surface.

[0037] The modified molten salt is composed of the following components by mass percentage: 48 wt% of potassium nitrate, 49 wt% of sodium nitrite, 2.0 wt% of sodium dihydrogen phosphate, and 1.0 wt% of anhydrous borax as a film-forming additive.

[0038] Example 5

[0039] An in-situ film-forming quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the 60Si2Mn steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 885 °C under a protective atmosphere, and hold it at this temperature for 160 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a modified molten salt quenching medium maintained at 215 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 22 seconds. During this process, the steel strip is rapidly cooled and at the same time a phosphate-based protective film is formed by in-situ reaction on its surface. After the quenching treatment is completed, take the steel strip out of the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 85 °C to obtain a finished steel strip with an in-situ generated protective film on its surface.

[0040] The modified molten salt consists of the following components by mass percentage: 60 wt% potassium nitrate, 37.2 wt% sodium nitrite, and 2.8 wt% sodium dihydrogen phosphate as a film-forming additive.

[0041] Comparative Example 1

[0042] A molten salt quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the 65Mn steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 880 °C under a protective atmosphere, and hold it at this temperature for 180 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into a basic molten salt quenching medium maintained at 220 °C for quenching treatment. The immersion time of the steel strip in the molten salt is 25 seconds. During this process, the steel strip only undergoes rapid cooling. After the quenching treatment is completed, take the steel strip out of the molten salt and cool it under a nitrogen protective atmosphere until the temperature of the steel strip drops to 90 °C to obtain a quenched steel strip with no in-situ generated protective film on its surface.

[0043] Among them, the basic molten salt consists of the following components by mass percentage: 51.28 wt% potassium nitrate and 48.72 wt% sodium nitrite, and no film-forming additives such as sodium dihydrogen phosphate or anhydrous borax are added to this molten salt;

[0044] Comparative Example 2

[0045] A traditional oil quenching treatment method for steel strips. First, perform conventional degreasing and pickling pretreatment on the 65Mn steel strip to be treated to ensure a clean surface. Then, feed the steel strip into a continuous heating furnace, heat it to 880 °C under a protective atmosphere, and hold it at this temperature for 180 seconds to fully austenitize the steel strip structure. Next, quickly immerse the heated steel strip into ISO VG 32 fast quenching oil maintained at 60 °C for quenching treatment. After the quenching treatment is completed, take the steel strip out of the oil, usually need to be cleaned to remove surface oil, and then cooled in the air to room temperature to obtain a quenched steel strip.

[0046] Performance detection

[0047] Surface film evaluation: According to the standard of GB / T 9286-1998 "Cross-Cut Test for Paints and Varnishes - Film, the grid with a spacing of 1 mm is scratched on the surface of the sample using a cross-cut tool, then taped and quickly peeled off. The adhesion grade of the film layer is evaluated according to the peeling situation of the paint film (0 - 5 grades, 0 grade is the best).

[0048] Hardness test: Use an HVS-1000 Vickers hardness tester to select 5 points along the center line on the cross-section of each steel strip sample for hardness measurement. The applied load is HV1 (9.807 N), and the holding time is 15 seconds. Calculate the average hardness value and evaluate the uniformity of the hardness distribution.

[0049] Cooling performance test. Before the steel strip sample enters the quenching medium, K-type thermocouples are welded at its center and 5 mm away from the edge. Record the temperature-time curve of the steel strip cooling from the austenitizing temperature to the end temperature of the martensitic transformation, calculate the average cooling rate in the range of 800 °C to 500 °C, compare the temperature difference between the center and the edge at the same moment, and evaluate the cooling uniformity.

[0050] Corrosion resistance test: According to the standard of GB / T 10125-2012 "Artificial Atmosphere Corrosion Tests - Salt Spray Tests", the neutral salt spray test (NSS) is carried out. Hang the steel strip sample with a size of 100 mm × 50 mm in the YWX / Q-150 salt spray test chamber. The test temperature is set at 35 °C, the concentration of the sodium chloride solution is 50 g / L, the pH value is 7.0, and continuous spraying is carried out. Take out the sample regularly for observation, record the time (h) when the first rust spot appears, and evaluate the percentage of the rusted area on the sample surface after 72 hours (%), and grade it with reference to the standard atlas of GB / T6461-2002.

[0051] Test data

[0052] Table 1. Data table of surface film performance test

[0053] Sample Number Adhesion Grade of Cross-Cut Test Example 1 Grade 1 Example 2 Grade 1 Example 3 Grade 0 Example 4 Grade 1 Example 5 Grade 1 Comparative Example 1 Not applicable (no film layer) Comparative Example 2 Not applicable (no film layer)

[0054] Table 2. Data table of hardness test

[0055]

[0056]

[0057] Table 3. Data table of cooling performance test

[0058] Sample Number Average Cooling Rate (800 - 500°C, °C / s) Maximum Temperature Difference (center - edge, °C) Example 1 105 12 Example 2 95 15 Example 3 115 10 Example 4 110 11 Example 5 100 13 Comparative Example 1 108 18 Comparative Example 2 55 30

[0059] Table 4. Data table of corrosion resistance test

[0060]

[0061]

[0062] Data analysis

[0063] As can be seen from Table 1, the adhesion of the steel strip samples in Examples 1 to 5 all reached Grade 1 or Grade 0 through the cross-cut test, which formed a sharp contrast with Comparative Example 1 and Comparative Example 2. The data shows that the film-forming additive in the modified molten salt quenching medium used in the present invention can successfully induce in-situ chemical reactions by utilizing the heat and active surface of the steel strip during quenching, generating a protective film firmly bonded to the substrate.

[0064] As can be seen from Table 2, the average Vickers hardness of the steel strip samples in Examples 1 to 5 all reached above 670 HV1, and the standard deviation of hardness was between 7 - 10 HV1, showing high hardness and good hardness uniformity; the average hardness of Comparative Example 1 was close to that of the examples, but the standard deviation of its hardness increased compared with the example samples, indicating a decrease in uniformity; the average hardness of Comparative Example 2 was only 620 HV1, about 8% - 12% lower than the example samples, and the standard deviation was as high as 25 HV1, with the worst uniformity. This shows that the modified molten salt medium used in the present invention not only has sufficient cooling capacity to obtain high quenching hardness, but also realizes a more uniform hardness distribution than pure molten salt quenching and oil quenching through the characteristics of the molten salt and the possible fine-tuning effect of the film layer on the cooling process.

[0065] As can be seen from Table 3, the average cooling rate of the steel strip samples in Examples 1 to 5 in the key temperature range of 800 - 500 °C was in the range of 95 - 115 °C / s, which was at a medium to fast level, conducive to obtaining refined quenched microstructures. At the same time, the maximum temperature difference between the center and the edge during quenching was controlled between 10 - 15 °C, showing good cooling uniformity; the cooling rate of Comparative Example 1 was close to that of the examples, but the temperature difference was slightly larger; the cooling rate of Comparative Example 2 was significantly slower, only about half of that of the examples, which explained its lower quenching hardness, and the temperature difference between its center and edge was as high as 30 °C, with extremely uneven cooling, easily leading to non-uniform microstructure and deformation. The data confirms that the modified molten salt quenching process of the present invention can provide faster cooling conditions than oil quenching and more uniform cooling conditions than pure molten salt quenching.

[0066] As can be seen from Table 4, the steel strip samples of Examples 1 to 5 showed excellent corrosion resistance in the neutral salt spray test. The rusted area of all the samples of the examples was less than 2% after 72 hours of testing. Among them, no obvious rust spots appeared in Examples 1, 3, 4, and 5 even within 72 hours, and Example 3 even exceeded 96 hours. In contrast, rust appeared in Comparative Example 1 after 8 hours, and the rusted area exceeded 50% after 72 hours; rust appeared in Comparative Example 2 after 12 hours, and the rusted area exceeded 40% after 72 hours. The corrosion resistance time of the samples of the examples was increased by at least 7.5 times compared with that of the comparative examples, and the rusted area was reduced by dozens of times, which proved that the phosphate or phosphate-borate composite film layer formed in situ on the surface of the steel strip by the method of the present invention has excellent protection ability, can effectively block the corrosion medium, and greatly improves the corrosion resistance of the steel strip.

[0067] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. All equivalent changes and improvements made within the scope of the application of the present invention shall still fall within the scope covered by the patent of the present invention.

Claims

1. An in-situ film-forming quenching method for a steel strip, characterized in that: The process is to heat the steel strip to be treated, transform its structure into austenite, and then immerse it in a quenching medium. During quenching, the film-forming additives in the quenching medium react in situ to form a protective film on the surface of the steel strip, and then cool it to obtain a finished steel strip.

2. The processing method according to claim 1, characterized in that: The following steps are involved: Step 1: Austenitizing treatment: The steel strip to be treated is heated, kept warm, and then quickly immersed in a quenching medium for quenching treatment; Step 2: Cool the quenched steel strip in a protective atmosphere to obtain a finished steel strip.

3. The processing method according to claim 2, characterized in that: In the step 1, the heating temperature is 870°C to 890°C, the holding time is 150 to 210 seconds, the temperature of the quenching medium is 210° to 230°C, and the quenching time is 20 to 30 seconds.

4. The processing method according to claim 3, characterized in that: The quenching medium is modified molten salt.

5. The processing method according to claim 4, characterized in that: The modified molten salt comprises a basic molten salt and a film-forming additive.

6. The processing method according to claim 5, characterized in that: The basic molten salt is at least one of potassium nitrate and sodium nitrite, and the film-forming additive is at least one of sodium dihydrogen phosphate and anhydrous borax.

7. The processing method according to claim 6, characterized in that: The added amount of sodium dihydrogen phosphate accounts for 2.0wt% to 3.0wt% of the total weight of the modified molten salt.

8. The processing method according to claim 6, characterized in that: The amount of anhydrous borax added is 0.5wt% to 1.5wt% of the total weight of the modified molten salt.

9. The processing method according to claim 2, characterized in that: In the step 2, the temperature is cooled to ≤100°C.

10. The processing method according to claim 2, characterized in that: The specific steps include: Step 1, austenitizing treatment: heat the steel strip to be treated to 870°C to 890°C, keep it warm for 150 seconds to 210 seconds, and then quickly immerse it in a quenching medium at 210° to 230°C for 20 to 30 seconds of quenching treatment; Step 2: Cool the quenched steel strip to ≤100° C. in a nitrogen atmosphere to obtain a finished steel strip.

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