A method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod

Abstract

This invention specifically relates to a method for controlling the iron oxide scale of hot-rolled high-carbon steel wire rod in a salt bath. The continuous casting billet heating process requires thorough heating, and the initial rolling temperature is controlled at 950-1050℃. The high-pressure water descaling step controls the high-pressure water pressure to ≥15 MPa. The rolling process involves roughing, pre-finishing, first cooling, finishing, second cooling, and sizing, adjusting the rolling speed so that the time from the wire rod entering the roughing mill to exiting the sizing mill is no more than 70 seconds. The first and second cooling steps maintain the inlet temperatures of the finishing mill and sizing mill at 850-900℃ and 840-890℃, respectively. The third cooling step involves wire drawing and salt bath, controlling the wire drawing temperature at 820-870℃. After wire drawing, the wire rod is exposed to air on the closely spaced roller conveyor for ≤10 seconds and immersed in the salt bath for 30-90 seconds. Based on the formation and evolution of iron oxide scale in hot-rolled salt bath high-carbon steel wire rod, the thickness and phase composition of the iron oxide scale can be controlled by optimizing the rolling and salt bath processes to meet transportation and customer requirements.

Description

Technical Field

[0001] This invention relates to the field of steel rolling technology in the metallurgical industry, specifically to a method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod. Background Technology

[0002] Hot-rolled high-carbon steel wire rod has a wide range of applications. Its products, such as prestressed steel wire, radial tire skeleton reinforcing wire, and steel wire rope, are widely used in the road, automotive, and construction industries, making it an important raw material for national economic production. The surface iron oxide scale inevitably generated during the production of hot-rolled high-carbon steel wire rod has a significant impact on subsequent transportation and use. On the one hand, steel products generally require long-term, long-distance transportation, especially those involving port stops and sea transport. The humidity of the air and contact with seawater necessitate a denser and thicker iron oxide scale to provide crucial corrosion protection for the wire rod substrate. On the other hand, before being used by customers, the steel needs to undergo different descaling methods to remove the iron oxide scale. Some customers use recyclable pickling media or shorter pickling times, requiring a thinner iron oxide scale to avoid incomplete pickling and ensure the surface quality of the finished product. In recent years, with increasing environmental awareness, more and more customers are using mechanical descaling to remove iron oxide scale, requiring a certain thickness and reasonable structure of the iron oxide scale on the wire rod surface.

[0003] To address the various applications of iron oxide scale, manufacturers have researched multiple methods for controlling iron oxide scale on hot-rolled steel, such as the authorized publication number CN103028611B, a method for flexible control of iron oxide scale on the surface of hot-rolled wire rod, and application publication number CN113695387A, a method for controlling the grain size of iron oxide scale on high-carbon steel wire rod. Since the cooling of the wire rod after rolling is primarily completed in air, research on surface iron oxide scale control methods is limited to the control principles and methods in an air-based medium.

[0004] Currently, an increasing number of high-quality wire rods are using salt bath treatment for controlled cooling. This involves isothermal cooling in a nitrate bath after the wire rod is wound to improve its microstructure and mechanical properties. Based on the production equipment and processes of hot-rolled wire rod, the formation of its iron oxide scale is divided into three stages: primary iron oxide scale (grown in the heating furnace), secondary iron oxide scale (generated during rolling), and tertiary iron oxide scale (formed during cooling after final rolling). Compared to traditional processes, the formation and changes of primary and secondary iron oxide scale in hot-rolled salt bath wire rods follow the same patterns, but the formation and changes of tertiary iron oxide scale are completely different. Furthermore, secondary iron oxide scale provides the basis for the formation and changes of tertiary iron oxide scale. Because the nitrate bath is an oxidizing medium, the chemical reaction between the surface and oxygen ions during the phase transformation process of the red-hot wire rod is completely different from that in air, resulting in significantly different phase transformation kinetics for the iron oxide scale. Practice has shown that when salt bath cooling is performed using traditional air cooling process parameters, the iron oxide scale on the surface of high carbon steel wire falls off in large quantities before it leaves the production line, thus losing its protective function on the substrate. Customers are also unable to mechanically remove the remaining iron oxide scale. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a method for controlling the iron oxide scale of hot-rolled salt bath high-carbon steel wire rod. Based on the generation and evolution law of iron oxide scale in hot-rolled salt bath high-carbon steel wire rod, the thickness and phase composition of the iron oxide scale of the product are controlled by optimizing the rolling and salt bath processes to meet the requirements of transportation and customer use.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod, the method comprising the following steps in sequence: converter steelmaking - LF furnace refining - continuous casting - continuous casting billet heating - high-pressure water descaling - rough rolling - pre-finishing rolling - primary cooling - finishing rolling - secondary cooling - sizing - tertiary cooling - wire drawing - salt bath - water washing - collection and finishing. The continuous casting billet heating step requires thorough heating of the billet and controlling the initial rolling temperature to 950-1050℃; the high-pressure water descaling step controls the high-pressure water pressure to ≥15 MPa; the rough rolling... The process involves pre-finishing, first cooling, finishing, second cooling, and sizing. The rolling speed is adjusted so that the time from the wire rod entering the roughing mill to exiting the sizing mill is no more than 65 seconds. The first and second cooling cycles are used to ensure that the inlet temperature of the finishing mill and the inlet temperature of the sizing mill are 850-900℃ and 840-890℃, respectively. In the third cooling, wire drawing, and salt bath process, the wire drawing temperature is controlled at 820-870℃. After wire drawing, the wire rod is exposed to air on the closely spaced roller conveyor for ≤10 seconds, and the immersion time in the salt bath is 30-90 seconds.

[0007] Preferably, the composition and content of the high-carbon steel are as follows: C: 0.70-1.0 wt.%; Si: ≤0.50 wt.%; Mn: ≤0.90 wt.%; Cr: ≤0.35 wt.%; P: ≤0.025 wt.%; S: ≤0.025 wt.%; Ni: ≤0.20 wt.%; Cu: ≤0.20 wt.%; the remainder being Fe and unavoidable impurities.

[0008] Preferably, the size of the continuously cast billet is no greater than 180mm*240mm. 2 .

[0009] Preferably, in the water washing-collection finishing step, the high-temperature wire rods after exiting the salt bath are immediately cleaned and dried using hot water at 90°C, and then cooled to room temperature by water cooling and air cooling.

[0010] Preferably, in the continuous casting billet heating step, the heating furnace used for the first heating stage, the second heating stage, and the soaking stage has a temperature not higher than 1050℃, 1100℃, and 1100℃, respectively, and a time not greater than 40min, 60min, and 60min, respectively, and the total heating time is 110-150min.

[0011] Compared with the prior art, the beneficial effects of the present invention are:

[0012] The thickness of the iron oxide scale on the wire rod when it enters the salt bath is controlled by controlling the rolling process temperature, the wire drawing temperature, and the exposure time to air. The phase transformation degree and final thickness of the iron oxide scale are controlled by controlling the salt bath temperature and time, while taking into account the internal microstructure and mechanical properties of the wire rod. Since the FeO in the iron oxide scale is relatively loose due to the high formation temperature and high production speed, it is easy to fall off at low temperatures. On the other hand, Fe3O4 is relatively dense and is not easy to fall off at low temperatures. By forming Fe3O4 of appropriate size and proportion in the FeO layer, the bonding force between the iron oxide scale and the matrix can be improved. This invention controls the initial iron oxide scale thickness and the degree of FeO to Fe3O4 transformation, so that the final iron oxide scale thickness is 8-16μm, of which the FeO layer thickness is 5-13μm, and the size of Fe3O4 formed in the FeO layer is ≤1μm. This makes the iron oxide scale on the surface of hot-rolled salt bath high-carbon steel wire rod less prone to peeling and easier to remove mechanically. Attached Figure Description

[0013] Figure 1 Metallographic microscope images of iron oxide scale obtained in an embodiment of the present invention;

[0014] Figure 2 This is a metallographic microscope image of iron oxide scale obtained as a comparative example of the present invention. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] This embodiment provides a method for controlling the iron oxide scale of hot-rolled salt bath high-carbon steel wire rod. The high-carbon steel with grade C82D2 has the following chemical composition: C: 0.82 wt.%; Si: 0.2 wt.%; Mn: 0.62 wt.%; Cr: 0.01 wt.%; P: 0.015 wt.%; S: 0.011 wt.%; Ni: 0.007 wt.%; Cu: 0.014; the remainder being Fe and unavoidable impurities.

[0017] The method sequentially includes the following steps: converter steelmaking, LF furnace refining, continuous casting, continuous casting billet heating, high-pressure water descaling, rough rolling, pre-finishing rolling, primary cooling, finishing rolling, secondary cooling, sizing reduction, tertiary cooling, wire drawing, salt bath, water washing, and collection and finishing. The converter steelmaking-LF furnace refining-continuous casting steps utilize conventional process parameters and methods to complete steel smelting and continuous casting billet manufacturing. The manufactured continuous casting billet has a size of 180*240mm. 2 .

[0018] The continuous casting billet heating step requires thoroughly heating the billet and controlling the initial rolling temperature to 1000℃. The heating furnace used has heating temperatures of 1020℃, 1060℃, and 1060℃ for the first heating stage, the second heating stage, and the soaking stage, respectively, with heating times of 30min, 45min, and 45min, respectively, for a total heating time of 120min.

[0019] The high-pressure water descaling step requires careful control of the high-pressure water pressure to remove all primary iron oxide scale. Adjusting the high-pressure water pressure ensures complete removal of the iron oxide scale formed during the heating of the continuously cast billet, preventing primary iron oxide scale residue from remaining on the billet and being pressed into the steel matrix during subsequent rolling, where it further oxidizes into red rust. In this embodiment, the high-pressure water descaling step controls the high-pressure water pressure at 18 MPa.

[0020] The steps of roughing, pre-finishing, primary cooling, finishing, secondary cooling, and sizing require reducing the thickness of the secondary iron oxide scale and ensuring that the proportion of FeO in its phase composition is not less than 90%. Simultaneously, the workpiece temperature is maintained within a reasonable range during the rolling process to ensure that the deformation of the iron oxide scale is coordinated with that of the matrix during rolling. Furthermore, lowering the initial rolling temperature and increasing the rolling speed shortens the rate and time of secondary iron oxide scale formation. In this embodiment, the rolling speed is adjusted to 112 m / s, so that the rolling process time from the wire rod entering the roughing mill to exiting the sizing unit is 50 s. Primary and secondary cooling are used to achieve inlet temperatures of 880°C for the finishing mill and 880°C for the sizing unit.

[0021] The described three-stage cooling-coiling-salt bath process requires the salt bath temperature to be maintained within the range of 480-550℃ due to the requirements of the internal microstructure and mechanical properties of the wire rod. This temperature range coincides with the most active temperature range for the eutectoid reaction of FeO (FeO→α-Fe+Fe3O4). The only adjustable parameters are the initial thickness of the iron oxide scale when the wire rod enters the salt bath and the reaction time in the salt bath. The initial thickness of the tertiary iron oxide scale is the final thickness of the secondary iron oxide scale. By cooling the wire rod at a lower coiling temperature and ensuring it enters the salt bath as soon as possible after coiling, a significant increase in the initial thickness of the tertiary iron oxide scale can be avoided. In this embodiment, the coiling temperature is controlled at 840℃, the wire rod is exposed to air on the closely spaced roller conveyor for 5 seconds after coiling, and the immersion time in the salt bath is 35 seconds.

[0022] The water washing-collection finishing step involves using 90°C hot water to immediately clean and dry the high-temperature wire rod after it has exited the salt bath, and then cooling it to room temperature with water and air to prevent further changes in the phase ratio of the iron oxide scale.

[0023] Comparative Example

[0024] Except for the salt bath treatment after wire casting, all other parameters in this comparative example, including the continuous casting billet heating process, initial rolling temperature, rolling process temperature, and rolling speed, adopted the traditional air-cooled process parameters without any changes to the salt bath process. The high-carbon steel of the same grade, C82D2, has the following chemical composition: C: 0.83 wt.%; Si: 0.2 wt.%; Mn: 0.60 wt.%; Cr: 0.01 wt.%; P: 0.011 wt.%; S: 0.011 wt.%; Ni: 0.010 wt.%; Cu: 0.012; the remainder being Fe and unavoidable impurities.

[0025] The smelting method also sequentially includes the following steps: converter steelmaking, LF furnace refining, continuous casting, continuous casting billet heating, high-pressure water descaling, rough rolling, pre-finishing rolling, primary cooling, finishing rolling, secondary cooling, sizing reduction, tertiary cooling, wire drawing, salt bath, water washing, and collection and finishing. The converter steelmaking-LF furnace refining-continuous casting steps use conventional process parameters and methods to complete the steelmaking and continuous casting billet manufacturing. The manufactured continuous casting billet has a size of 180*240mm. 2 .

[0026] The continuous casting billet heating step requires thoroughly heating the billet and controlling the initial rolling temperature to 1120℃. The heating furnace used has heating temperatures of 1040℃, 1150℃, and 1180℃ in the first heating stage, the second heating stage, and the soaking stage, respectively, with heating times of 30min, 60min, and 60min, respectively, for a total heating time of 150min.

[0027] In this comparative example, the high-pressure water dephosphorization step is controlled at a pressure of 18 MPa.

[0028] The steps of roughing-pre-finishing-first-cooling-finishing-second-cooling-reducing sizing are described. The rolling speed is adjusted to 100m / s, so that the rolling process time from the wire rod entering the roughing mill to exiting the reducing sizing unit is 70s. The first and second cooling are used to make the inlet temperature of the finishing mill and the inlet temperature of the reducing sizing unit 920℃ and 920℃, respectively.

[0029] The three-step cooling-spinning-salt bath process controls the spinning temperature at 920℃, the time the coil is exposed to air on the closely spaced roller conveyor after spinning is 15s, and the soaking time in the salt bath is 120s.

[0030] The water washing-collection finishing step involves using 90°C hot water to immediately clean and dry the high-temperature wire rod after it has exited the salt bath, and then cooling it to room temperature with water and air to prevent further changes in the phase ratio of the iron oxide scale.

[0031] Please refer to the metallographic microscope images of the iron oxide scale obtained in the examples. Figure 1 For comparative metallographic micrographs of iron oxide scale, please refer to [link / reference]. Figure 2 The total thickness of the iron oxide scale in the examples and the comparative examples is similar, but the outer Fe3O4 layer thickness in the examples is less than that in the comparative examples, and the Fe3O4 particle size in the inner FeO layer is approximately 0.6-1 μm, while the Fe3O4 particle size in the FeO layer of the comparative examples is 3-5 μm. It is clearly shown in the figures that the iron oxide scale layer in the examples is tightly bonded to the substrate, while the iron oxide scale in the comparative examples has detached from the substrate.

[0032] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod, characterized in that: The method sequentially includes the following steps: converter steelmaking, LF furnace refining, continuous casting, continuous casting billet heating, high-pressure water descaling, rough rolling, pre-finishing rolling, primary cooling, finishing rolling, secondary cooling, sizing reduction, tertiary cooling, wire drawing, salt bath, water washing, and collection and finishing. The continuous casting billet heating step requires thorough heating of the billet and controlling the initial rolling temperature to 950-1050℃; the high-pressure water descaling step controls the high-pressure water pressure to ≥15MPa; the rough rolling-pre-finishing-primary cooling-finishing-secondary cooling-sizing reduction step... In the sizing process, the rolling speed is adjusted so that the rolling time from the entry of the wire rod into the roughing mill to the exit of the reducing and sizing mill is no more than 70 seconds. The inlet temperature of the finishing mill and the inlet temperature of the reducing and sizing mill are 850-900℃ and 840-890℃ respectively, using one and two cooling cycles. In the three-cooling-firing-salt bath process, the firing temperature is controlled at 820-870℃, the time the wire rod is exposed to air on the closely spaced roller conveyor after firing is ≤10 seconds, and the immersion time in the salt bath is 30-90 seconds.

2. The method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod according to claim 1, characterized in that, The composition and content of each component of the high-carbon steel are as follows: C: 0.70-1.0 wt.%; Si: ≤0.50 wt.%; Mn: ≤0.90 wt.%; Cr: ≤0.35 wt.%; P: ≤0.025 wt.%; S: ≤0.025 wt.%; Ni: ≤0.20 wt.%; Cu: ≤0.20 wt.%; the remainder is Fe and unavoidable impurities.

3. The method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod according to claim 1, characterized in that: The dimensions of the continuously cast billet are no greater than 180mm × 240mm.

4. The method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod according to claim 1, characterized in that: The water washing-collection finishing step involves immediately cleaning and drying the high-temperature wire rods after they have exited the salt bath using 90°C hot water, and then cooling them to room temperature using water and air.

5. The method for controlling iron oxide scale in hot-rolled salt bath high-carbon steel wire rod according to claim 1, characterized in that: The heating steps for the continuously cast billet involve heating a heating furnace with temperatures in the first heating stage, the second heating stage, and the soaking stage not exceeding 1050℃, 1100℃, and 1100℃ respectively, for times not exceeding 40min, 60min, and 60min respectively, and a total heating time of 110-150min.

Images