Surface treatment method for pre-rolling crack resistance of large-section titanium alloy square billet

Abstract

The invention discloses a pre-rolling anti-cracking surface treatment method for a large-section titanium alloy square billet, and belongs to the technical field of titanium alloy surface treatment. The method comprises the following steps: S1, sequentially carrying out surface grinding and oil removal treatment on a titanium alloy square billet; s2, the pretreated square billet is subjected to nickel electroplating, and an initial nickel plating layer is formed; s3, the nickel-plated square billet is heated and subjected to heat preservation under the vacuum condition; s4, secondary nickel electroplating is conducted on the square billet after stress relief, and a thickened nickel plating layer is formed; and S5, the secondary nickel-plated square billet is rolled after being subjected to uniform-temperature heating and heat preservation. A uniform and compact nickel plating layer is formed on the surface of the large-section titanium alloy square billet through electroplating, the temperature reduction rate of the surface layer of the square billet after the square billet is discharged from a furnace can be reduced, the temperature gradient between the surface and the interior of the billet is remarkably reduced, and the rolling plasticity of the billet in the rolling starting link is guaranteed; the technical problem that the large-section titanium alloy square billet is prone to cracking in the rolling process can be effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of titanium alloy surface treatment, relates to a method for reducing rolling cracking of large-section titanium alloy billets, and specifically relates to a surface treatment method for pre-rolling crack resistance of large-section titanium alloy billets. Background Art

[0002] In the field of hot rolling processing of titanium alloy bars and wires, the use of large-section square billets through high-speed continuous rolling equipment for production is an important direction for improving material utilization and production efficiency. However, compared with small-size billets, the risk of cracking when rolling large-section titanium alloy billets is higher, which will seriously reduce the surface quality of the finished wire rod and even cause steel piling accidents. The main reasons are as follows: First, compared with pure titanium with high ductility, titanium alloy billets have high strength and poor plasticity. During the rolling deformation process, the deformation resistance is large and the risk of billet cracking is high; second, after the large-section titanium alloy billet is heated out of the furnace, due to the large surface contact area with the air, the surface temperature will drop rapidly (causing the surface plasticity to drop), thus falling below the ideal rolling temperature, and surface cracking is prone to occur in the rough and medium rolling links with large deformation.

[0003] At present, in response to the above problems, three main solutions are adopted in the existing technology: the first is to improve the rolling performance by increasing the heating temperature and extending the holding time, using the plasticity of the material at high temperature, but this method is prone to cause structural defects such as grain coarsening; the second is to use smaller-sized and small-section titanium alloy square billets to reduce the temperature gradient between the core and the surface and thus reduce the risk of subsequent rolling cracking, but this leads to a decrease in the yield of a single rolling, which makes it difficult to meet the production needs of large-size coil products; the third is to improve the plasticity of the material through surface treatment or rolling process optimization. For example, CN119634457A uses multi-pass rolling pretreatment to enhance the surface plasticity of the slab, but its process route is complicated and difficult to match the high-speed continuous rolling rhythm; CN117772792A controls the cracking of the thin plate by coating a high-temperature anti-oxidation layer combined with multi-stage rolling, but this method requires repeated heating and welding operations, which significantly increases the cost.

[0004] Existing surface treatment technologies mostly focus on plate rolling scenarios, and the surface strengthening or coating protection schemes they adopt have obvious limitations in the continuous rolling of large-section square billets: on the one hand, the local rolling treatment used in plate rolling (such as CN119634457A) cannot effectively cover the edge area of the square billet, and the treatment efficiency is difficult to adapt to the mass production needs of long square billets; on the other hand, the special protective layer for thin plates (such as CN117772792A) is prone to peeling during high-temperature and large deformation rolling, which in turn increases the risk of surface defects.

[0005] Therefore, how to fundamentally solve the risk of surface cracking of large-section titanium alloy billets due to the rapid drop in surface temperature after being taken out of the furnace has become a technical problem that urgently needs to be broken through in this field. Summary of the Invention

[0006] The technical problem addressed by this invention is that existing titanium alloy billets with cross-sectional dimensions of 200-300 mm in thickness are prone to cracking during the rolling process. This invention aims to provide a surface treatment method for large-section titanium alloy billets, using a multi-stage electroplating and heating process. This method significantly reduces the risk of billet cracking during high-speed hot rolling and significantly improves the yield rate.

[0007] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0008] In a first aspect, the present invention provides a surface treatment method for pre-rolling and cracking resistance of large-section titanium alloy billets, comprising the following steps:

[0009] S1. Surface pretreatment: Surface grinding and degreasing of titanium alloy billets are performed in sequence;

[0010] S2. The first stage of electroplating: nickel plating is performed on the pretreated billet to form an initial nickel coating;

[0011] S3. Stress relief treatment: The nickel-plated billet is heated and kept in a vacuum;

[0012] S4. Second stage electroplating: The stress-relieved billet is subjected to secondary nickel electroplating to form a thickened nickel coating;

[0013] S5. Heating before rolling: Heating the secondary nickel-plated billet to a uniform temperature and then rolling.

[0014] In the above step S1, the cross-sectional side length of the titanium alloy billet is 200-300 mm, and the length is 5000-6000 mm.

[0015] In the above step S1, after surface grinding, the following conditions are met: surface roughness Ra is 3.3-7.5 μm, side length difference is ≤20 mm, and diagonal difference is ≤10 mm.

[0016] In the above step S1, the degreasing treatment includes: placing the titanium alloy billet in an alkaline degreasing solution and soaking it at 50-90°C for 20-30 minutes; after soaking, washing it in clean water at 45-55°C and 40-70kHz to remove the alkaline solution residue.

[0017] Furthermore, the components of the alkaline degreasing solution include: NaOH 20-30 g / L, Na2CO3 20-35 g / L, Na3PO4 15-25 g / L, and Na2SiO3 5-8 g / L.

[0018] In the above step S2, nickel plating solution is used for electroplating, and its components include: 25-35 g / L nickel sulfate, 20-30 g / L sodium hypophosphite, 10-20 g / L trisodium citrate, 8-12 g / L ammonium chloride, and 8-12 g / L sodium acetate.

[0019] In the above step S2, the first stage electroplating parameters are: pH = 4.5-5.5, electroplating temperature 40-60°C, current density 2-4A / dm 2 , electroplating time is 45 to 60 minutes, and the coating thickness is 15 to 20 μm.

[0020] In the above step S3, the stress relief treatment is performed in a vacuum heating treatment furnace, the heating temperature is 200-350° C., and the holding time is 60-90 minutes.

[0021] In the above step S4, nickel plating solution is used for electroplating, and its components include: 25-35 g / L nickel sulfate, 20-30 g / L sodium hypophosphite, 10-20 g / L trisodium citrate, 8-12 g / L ammonium chloride, and 8-12 g / L sodium acetate.

[0022] In the above step S4, the second stage electroplating parameters are: pH = 4.5-5.5, electroplating temperature 55-65°C, current density 3-5A / dm 2 , electroplating time 3h, total coating thickness 45~50μm.

[0023] In the above step S5, the temperature of the pre-rolling heating is (T β -50)~(T β -20)℃, insulation time 60~90min.

[0024] The beneficial effects of the present invention are:

[0025] (1) The present invention forms a uniform and dense nickel coating on the surface of a large-section titanium alloy square billet by electroplating, which can effectively reduce the temperature drop rate of the surface layer of the square billet after it is taken out of the furnace, significantly reduce the temperature gradient between the surface and the interior of the billet, and ensure the plastic deformation ability of the billet during the rolling stage; at the same time, the nickel coating itself has good thermal conductivity and ductility, which can suppress stress concentration during heating and rolling, thereby greatly reducing the risk of billet cracking during rolling and improving the quality and yield rate of titanium alloy products.

[0026] (2) The present invention adopts a collaborative process of staged electroplating and heat treatment: stress relief treatment is carried out after the first electroplating, which can enhance the diffusion effect between the atoms of the initial coating and the surface of the titanium alloy billet, forming a more solid metallurgical bonding layer; on this basis, the second electroplating is carried out to continue increasing the thickness of the coating. This process method can not only improve the bonding force between the coating and the substrate, but also improve the uniformity of the coating, reduce local stress concentration during the rolling process, and reduce the risk of surface cracking.

[0027] (3) The nickel coating surface has low friction characteristics. During the high-speed rolling process, it can significantly reduce the sliding friction force between the surface of the titanium alloy and equipment such as rolling rolls and guides, effectively reducing the incidence of defects such as surface scratches during the rolling process, thereby reducing the subsequent peeling treatment amount of the finished coil and improving the material utilization rate. Description of the Drawings

[0028] Figure 1 Pictures of the surface of TC4 before (left) and after (right) electroplating treatment for the example;

[0029] Figure 2 Pictures of the surface quality of the billet after rough rolling (left) and the finished coil (right) of TC4 for the example;

[0030] Figure 3 Pictures of the surface quality of the billet after rough rolling (left) and the finished coil (right) of TC4 for Comparative Example 1;

[0031] Figure 4 Pictures of the surface quality of the billet after rough rolling (left) and the finished coil (right) of TC4 for Comparative Example 2. Detailed Embodiments

[0032] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application clearer, the following further details this application in conjunction with the embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those of ordinary skill in the art.

[0033] The present invention discloses a surface treatment method for preventing cracking during pre-rolling of large-section titanium alloy billets, including the following steps:

[0034] (1) Surface grinding: Select a rolling mill billet with dimensions of 200 - 300×200 - 300×5000 - 6000 mm, perform surface mechanical grinding and edge chamfering on the titanium alloy billet, remove the surface oxide scale and defects such as cracks and double skins, control the surface roughness at Ra 3.3 - 7.5 μm, control the side length dimension difference at ≤20 mm, and the diagonal difference at ≤10 mm.

[0035] (2) Degreasing treatment: Put the ground titanium alloy billet into an alkaline degreasing solution composed of 20 - 30 g / L sodium hydroxide, 20 - 35 g / L sodium carbonate, 15 - 25 g / L sodium phosphate, and 5 - 8 g / L sodium silicate. Heat the alkaline degreasing solution to 50 - 90 °C, soak for 20 - 30 minutes, and stir appropriately during this period to enhance the degreasing effect. Then, put the billet into clean water at 45 - 55 °C for ultrasonic cleaning with a frequency of 40 - 70 kHz to remove the residual alkali solution on the surface.

[0036] (3) First-stage electroplating: Electroplate nickel on the titanium alloy billet processed in step (2). The electroplating solution formula is: nickel sulfate 25 - 35 g / L, sodium hypophosphite 20 - 30 g / L, trisodium citrate 10 - 20 g / L, ammonium chloride 8 - 12 g / L, sodium acetate 8 - 12 g / L, and an appropriate amount of water. Use the titanium alloy billet as the cathode, put it into the electroplating tank, control the pH value to 4.5 - 5.5, the electroplating temperature to 40 - 60 °C, and the current density to 2 - 4 A / dm 2 , and the electroplating time is 45 - 60 minutes to form an initial nickel coating with a thickness of 15 - 20 μm on the surface of the titanium alloy billet. After electroplating, rinse the billet with flowing water to remove the residual electroplating solution on the surface, and then let it dry.

[0037] (4) Stress relief treatment: Send the titanium alloy billet processed in step (3) into a vacuum heat treatment furnace, with the furnace temperature at 200 - 350 °C and the holding time of 60 - 90 min to eliminate the residual stress of the billet and the coating, making the electroplated layer firmly bonded to the surface of the titanium alloy billet.

[0038] (5) Second-stage electroplating: Use the same electroplating solution formula as in step (3) to electroplate the titanium alloy billet processed in step (4) again to further thicken the nickel coating. Control the pH value to 4.5 - 5.5, the electroplating temperature to 55 - 65 °C, and the current density to 3 - 5 A / dm 2 , and the electroplating time is 3 h, and the thickness of the nickel plating layer is 45 - 50 μm. After electroplating, rinse the surface of the billet with flowing water to remove the residual electroplating solution, and then let it dry.

[0039] (6) Pre-rolling heating: Send the billet processed in step (5) into an electric heating furnace for heating, with the temperature set to (T β - 50) - (T β - 20) °C, and hold for 60 - 90 min to ensure that the temperature of each part of the billet is uniform, and then take it out of the furnace for rolling.

[0040] Specific embodiments will be enumerated below to explain the solution of the present invention. Those skilled in the art will understand that the following embodiments are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the embodiments regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in this field or according to the product specifications. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0041] Embodiment

[0042] I. Surface treatment for preventing cracking during pre-rolling of large-section titanium alloy billets

[0043] Embodiment: Perform surface treatment on a TC4 titanium alloy billet before rolling. Its phase transformation point is 980 °C. The specific steps are as follows:

[0044] (1) Surface grinding: Select a large-section TC4 titanium alloy billet with dimensions of 200×200×5500 mm. Conduct surface mechanical grinding and edge chamfering treatment on the titanium alloy billet to remove surface scale, cracks, double skins and other defects. Control the surface roughness to Ra 3.5 μm, the side length dimension difference to 17 mm, and the diagonal difference to 8 mm.

[0045] (2) Degreasing treatment: Place the ground titanium alloy billet into an alkaline degreasing solution composed of 25 g / L sodium hydroxide, 25 g / L sodium carbonate, 15 g / L sodium phosphate and 6 g / L sodium silicate, and soak it at 70 °C for 30 min. Stir appropriately during this period to enhance the degreasing effect; subsequently, place the billet into clean water at 50 °C for ultrasonic cleaning with a frequency of 60 kHz to remove the residual alkali solution on the surface.

[0046] (3) First-stage electroplating: Perform nickel electroplating on the titanium alloy billet treated in step (2). The nickel plating solution formula is: nickel sulfate 25 g / L, sodium hypophosphite 20 g / L, trisodium citrate 10 g / L, ammonium chloride 10 g / L, sodium acetate 8 g / L, with appropriate amount of water; the titanium alloy billet serves as the cathode and is placed into the electroplating tank. Control the pH value to 5.0, the electroplating temperature to 50 °C, and the current density to 2.0 A / dm 2 , and the electroplating time is 50 minutes to form an initial nickel coating with a thickness of 15 μm on the surface of the titanium alloy billet; after electroplating, rinse the billet with running water to remove the residual plating solution on the surface, and then place it to dry.

[0047] (4) Stress relief treatment: Send the titanium alloy billet treated in step (3) into a vacuum heat treatment furnace. The furnace temperature is 300 °C, and the holding time is 80 min to eliminate the residual stress of the billet and the coating, so that the electroplated layer is firmly bonded to the surface of the titanium alloy billet.

[0048] (5) Second-stage electroplating: Using the same electroplating solution formula as in step (3), electroplate the titanium alloy billet after the treatment in step (4) again to further thicken the nickel coating. Control the pH value at 5.5, the electroplating temperature at 55 °C, and the current density at 5.0 dm 2 The electroplating time is 3 h, and the total thickness of the nickel coating is 47 μm. After electroplating, wash the residual plating solution on the surface of the billet with running water, and then place it to dry.

[0049] (6) Pre-rolling heating: Feed the billet after the treatment in step (5) into an electric heating furnace for heating. Set the temperature at 950 °C and keep it warm for 90 min to ensure that the temperature of each part of the billet is uniform. Then take it out of the furnace for rolling to obtain coiled bars.

[0050] Comparative example 1: Carry out pre-rolling surface treatment on the TC4 titanium alloy billet according to the procedures shown in the example. The difference from the example is that no two-stage electroplating treatment is carried out (that is, the procedures in steps 1-5 in the example are cancelled), and the remaining procedures and process parameters are the same as those in the example.

[0051] Comparative example 2: Carry out pre-rolling surface treatment on the TC4 titanium alloy billet according to the procedures shown in the example. The difference from the example is that no second-stage electroplating treatment is carried out (that is, the procedure in step 5 in the example is cancelled), and the remaining procedures and process parameters are the same as those in the example.

[0052] II. Quality inspection of the billet after surface treatment and the coiled bars after rolling

[0053] 1. Appearance quality inspection of the billet and the coiled bars

[0054] For the billet after two-stage electroplating treatment in the example, its surface friction roughness is 2.5 - 3.0 μm, the friction coefficient is 0.15, and the scratch critical load Lc (coating adhesion) is 25.8 N. Figure 1 Pictures of the TC4 in the example before (left) and after (right) surface electroplating treatment, Figure 2 Pictures of the surface quality of the billet (left) and the finished coiled bars (right) of the TC4 billet in the example after rough rolling. As Figure 1 can be seen, after two-stage electroplating treatment, there is a uniform and dense nickel coating on the surface of the billet, and the surface is smooth. As Figure 2 can be seen, after the billet is rough rolled, no cracks appear on the surface of the billet, no visible coating peeling occurs, and the surface quality of the finished coiled bars is good.

[0055] In comparative example 1, without surface treatment, its surface friction roughness is 8.5 - 12 μm, the surface roughness is relatively high, and the friction coefficient is 0.45. Figure 3 Pictures of the surface quality of the billet (left) and the finished coiled bars (right) of the TC4 billet in comparative example 1 after rough rolling. As Figure 3It can be seen that after rough rolling of the bloom, obvious cracks appear locally on the surface of the billet, and large-area cracks extending along the rolling direction appear on the surface of the finished coil, with serious surface scratches.

[0056] For the coating only subjected to the first-stage electroplating treatment in Comparative Example 2, the friction coefficient is 0.3, and the scratch critical load Lc (coating adhesion) is 10.2 N. Figure 4 The surface quality pictures of the billet (left) and the finished coil (right) of the TC4 bloom after rough rolling in Comparative Example 2 are shown by Figure 3 It can be seen that after rough rolling of the bloom, microcracks appear locally on the surface of the billet, and crack patches extending along the rolling direction appear locally on the surface of the finished coil.

[0057] 2. Surface properties and process temperature detection of the bloom and the coil

[0058] (1) An infrared thermal imager was used to record the surface temperature changes of the billet after being taken out of the furnace and before rough rolling respectively. The results are shown in Table 1.

[0059] Table 1 Surface temperature results of the TC4 bloom after being taken out of the furnace and before rough rolling

[0060]

[0061] As can be seen from Table 1, when the billet reaches the rough rolling mill after being taken out of the furnace, the surface temperature drop rate of the example (nickel-plated bloom) decreases significantly. Within 0 - 30 seconds, the average temperature drop rate is 1.7 °C / s, while that of Comparative Example 1 is 3.6 °C / s and that of Comparative Example 2 is 3.0 °C / s. Although Comparative Example 2 has only one layer of coating and the effect of reducing the surface temperature drop rate is not obvious, the overall temperature gradient of the billet is not much different. Thus, it can be seen that after being treated by the method of the present invention, the surface temperature drop rate can be reduced by about 52%, and the temperatures at the head, middle, and tail sections of the billet are more uniform.

[0062] (2) The surface quality of the bloom and the coil was detected, and the surface defects of the finished coil were counted. The data results are shown in Table 2.

[0063] Table 2 Statistical results of the surface quality data of the TC4 finished coil

[0064] Index Example Comparative Example 1 Comparative Example 2 Number of surface cracks (pieces / m) Not detected 12～15 5～10 Maximum crack depth (mm) Not detected 1.2～2.5 0.5～1.5 Surface scratch rate (%) 5％ 35％ 8％ Yield rate (%) 96％ 84％ 88％ Peeling treatment amount (mm) Remove 0.15 - 0.3mm Remove 0.7mm Remove 0.5mm

[0065] As can be seen from Table 2, the number of surface defects of the finished discs in the embodiment is relatively small, cracks are basically eliminated, the scratch rate is reduced by 85%, the peeling amount is reduced from 0.7mm to 0.15~0.3mm, and the yield rate is increased by 12%. This shows that the nickel plating layer in the embodiment suppresses radiative heat dissipation and air convection, homogenizes the temperature distribution, effectively maintains the plasticity of the surface layer, and basically eliminates cracks; at the same time, by reducing friction, it reduces scratches on the titanium alloy surface and equipment such as rollers and guides, and significantly improves the surface quality of the finished discs. In contrast, the surface of the finished disc in Comparative Example 2 still has crack defects, but the degree of surface scratches is comparable, the peeling amount is reduced from 0.5mm to 0.15~0.3mm, and the yield rate is increased by 8%. This shows that the dual-stage electroplating + stress relief treatment process in the embodiment increases the bonding strength of the coating by 150% (Lc increases from 10.2N to 25.8N), making it less likely to peel off. Although the first-stage electroplating process in Comparative Example 2 can reduce surface scratch defects, it cannot effectively alleviate the surface temperature drop after the billet is taken out of the furnace, resulting in a decrease in surface plasticity and cracks, resulting in an increase in the amount of peeling of the finished coil and a decrease in the yield rate.

Claims

1. Surface treatment method for crack resistance of large-section titanium alloy billet during pre-rolling, characterized in that, It includes the following steps: S1. Surface pretreatment: Grind and degrease the titanium alloy billet in sequence. S2. First-stage electroplating: Electroplate nickel on the pretreated billet to form an initial nickel coating. S3. Stress relief treatment: Heat and hold the nickel-plated billet under vacuum conditions. S4. Second-stage electroplating: Perform secondary electroplating of nickel on the billet after stress relief to form a thickened nickel coating. S5. Pre-rolling heating: Heat the billet with secondary nickel plating evenly and hold it, then roll it.

2. The surface treatment method for crack resistance in preliminary rolling of large-section titanium alloy billets according to claim 1, characterized in that: In step S1, the cross-sectional side length of the titanium alloy billet is 200 - 300 mm, and the length is 5000 - 6000 mm.

3. The surface treatment method for pre-rolling crack resistance of a large-section titanium alloy billet according to claim 1, characterized in that: In step S1, after the surface grinding, it meets the following requirements: the surface roughness Ra is 3.3 - 7.5 μm, the side length dimension difference ≤ 20 mm, and the diagonal difference ≤ 10 mm.

4. The surface treatment method for pre-rolling crack resistance of a large-section titanium alloy billet according to claim 1, characterized in that: In step S1, the degreasing treatment includes: placing the titanium alloy billet in an alkaline degreasing solution, soaking it at 50 - 90 °C for 20 - 30 min; after soaking, cleaning and removing the residual alkali solution in clean water at 45 - 55 °C under the condition of 40 - 70 kHz.

5. The surface treatment method for pre-rolling crack resistance of a large-section titanium alloy billet according to claim 4, characterized in that: The components of the alkaline degreasing solution include: 20 - 30 g / L of NaOH, 20 - 35 g / L of Na2CO3, 15 - 25 g / L of Na3PO4, and 5 - 8 g / L of Na2SiO3.

6. The surface treatment method for crack resistance in pre-rolling of large cross-section titanium alloy billets according to claim 1, characterized in that: In steps S2 and S4, the same-component nickel plating solution is used for electroplating; The components of the nickel plating solution include: 25 - 35 g / L of nickel sulfate, 20 - 30 g / L of sodium hypophosphite, 10 - 20 g / L of trisodium citrate, 8 - 12 g / L of ammonium chloride, and 8 - 12 g / L of sodium acetate.

7. The surface treatment method for pre-rolling crack resistance of a large-section titanium alloy billet according to claim 1, characterized in that: In step S2, the electroplating parameters in the first stage are: pH = 4.5 - 5.5, electroplating temperature 40 - 60°C, current density 2 - 4 A / dm 2 , electroplating time 45 - 60 minutes, coating thickness 15 - 20 μm.

8. The surface treatment method for crack resistance in pre-rolling of large-section titanium alloy billets according to claim 1, characterized in that: In step S3, the stress relief treatment is carried out in a vacuum heating treatment furnace, the heating temperature is 200 - 350 °C, and the holding time is 60 - 90 min.

9. The surface treatment method for pre-rolling crack resistance of a large-section titanium alloy billet according to claim 1, characterized in that: In step S4, the electroplating parameters in the second stage are: pH = 4.5 - 5.5, electroplating temperature 55 - 65 °C, current density 3 - 5 A / dm 2 , electroplating time 3 h, total coating thickness 45 - 50 μm.

10. The surface treatment method for crack resistance in pre-rolling of large cross-section titanium alloy billets according to claim 1, characterized in that: In step S5, the temperature of pre-rolling heating is (T β - 50) to (T β - 20) °C, and the heat preservation time is 60 to 90 min.

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