An antibacterial carbon steel and a preparation method thereof

Abstract

The present invention discloses a low-alloy antibacterial carbon steel and a method for preparing the same. In the low-alloy antibacterial carbon steel of the present invention, by controlling an appropriate Cu / C ratio to incorporate the structural changes of the material during heat treatment, Cu is added to the low-alloy carbon steel containing Cr and Mn. This prevents the precipitation of copper in the form of carbides during heat treatment, thereby effectively adding heavy metals to the matrix. The continuously released heavy metal ions can inhibit the activity of microorganisms and bacteria, significantly improving the antibacterial properties of the material. While maintaining the antibacterial effect of carbon steel, the antibacterial carbon steel of the present invention also improves its strength, wear resistance, and other properties.

Description

Technical Field

[0001] The invention relates to antibacterial steel, in particular to carbon steel capable of preventing and treating microbial corrosion and a preparation method thereof. Background Art

[0002] At present, the development of domestic oil and gas fields still faces a serious threat of microbial corrosion. Every year, perforation of gathering and transportation pipelines caused by the life activities of microorganisms occurs frequently, interfering with the normal development of oil and gas fields and significantly increasing their production costs. Most domestic oil and gas fields still use biocides to inhibit microbial corrosion. However, with the long-term use of biocides, bacteria such as sulfate-reducing bacteria (SRB) and saprophytic bacteria (TGB) have gradually developed resistance, resulting in the continuous increase in the dosage of biocides, but the effect is not significant. In addition, in order to prevent and control microbial corrosion, in addition to the use of biocides, other methods are constantly being adopted in oil and gas field production, such as cleaning of water injection systems, optimization of water injection processes, and cathodic protection. However, due to production costs and poor actual prevention and control effects, corrosion perforation of gathering and transportation pipelines still occurs frequently. Therefore, it is particularly important to find a new method to inhibit microbial corrosion.

[0003] At present, the most common method of utilizing the antibacterial properties of heavy metal elements is to add heavy metal elements to stainless steel to give it certain antibacterial properties. For example, publication number CN102876989B - "A corrosion-resistant and antibacterial stainless steel and its production method" reduces the Cr content and increases Mo, Zr, Al, etc., which can quickly kill bacteria and also has a good bactericidal effect on Vibrio and anaerobic bacteria. CN108728765B - "A strong antibacterial austenitic stainless steel used in chemical production" reduces the risk of bacterial microbial corrosion caused by austenitic stainless steel during use by adding the antibacterial element Ga. CN108728755B - "A highly antibacterial ferritic stainless steel and its preparation method" enables stainless steel to effectively resist high-concentration bacteria under solid solution and aging heat treatment conditions. CN102179672B - "A processing technology for stainless steel antibacterial pipes" produces copper-containing ferrite welded pipes with antibacterial properties by improving the existing processing technology of welded pipes. CN112589093A, "Nanosilver Antibacterial Agent, Preparation Method, and Method for Preparing Antibacterial Stainless Steel," describes the preparation of silver-containing antibacterial steel dispersed with a large number of nano-silver-containing particles, exhibiting excellent antibacterial properties. Others, such as CN103629449A, "Antibacterial Steel Tube," utilize composite tube designs to impart certain antibacterial properties to materials. The tube body is composed of a stainless steel layer and a nano-antibacterial layer. The stainless steel layer is located on the outer layer of the tube, while the nano-antibacterial layer is located on the inner layer of the tube.

[0004] Due to reasons such as cost and actual on-site working conditions, the application of stainless steel pipes in oil and gas extraction and gathering and transportation is limited. Ordinary carbon steel is still the main material in many on-site scenarios. However, there is little research on adding heavy metal elements to ordinary carbon steel. The development of low-cost antibacterial carbon steel is of great significance for inhibiting microbial corrosion and perforation of pipelines. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide an antibacterial carbon steel and a preparation method thereof. Compared with ordinary carbon steel pipes, the antibacterial carbon steel provided by the present invention can significantly improve its resistance to microbial corrosion.

[0006] According to a first aspect of the present invention, the present invention provides a low-alloy antibacterial carbon steel.

[0007] A low-alloy antibacterial carbon steel, whose chemical composition by weight percentage is: carbon: 0.05%-0.65%, manganese: 1%-10%, silicon: 0.1%-0.5%, copper: 1.5%-6.5%, chromium: 1%-5.5%, molybdenum: 0.1%-1.5%, nickel: 0.05%-0.25%, titanium: 0.05%-0.15%, vanadium: 0.05%-0.15%, tungsten: 0.05%-0.15%, the balance being Fe and unavoidable impurities; wherein the Cu / C weight ratio is 14.5-100, preferably 30-60.

[0008] The inventors of this application have discovered that by adding Cu and Cr in an appropriate Cu / C ratio to carbon steel, the matrix continuously releases heavy metal ions, thereby inhibiting the activity of microbial bacteria, and significantly improving the antibacterial properties of carbon steel materials.

[0009] The low-alloy antibacterial carbon steel of the present invention can be prepared by conventional methods in the art, or by the method recommended by the present invention.

[0010] According to a second aspect of the present invention, the present invention provides a method for preparing low-alloy antibacterial carbon steel.

[0011] A method for preparing low-alloy antibacterial carbon steel comprises the following steps:

[0012] (1) Melting: Melt the designed components in proportion and cast them into ingots;

[0013] (2) Forging: Forging the ingot;

[0014] (3) Homogenization treatment;

[0015] (4) Hot rolling: hot rolling the forgings after homogenization treatment;

[0016] (5) Cold rolling: The hot-rolled steel plate is air-cooled to room temperature and then cold-rolled;

[0017] (6) Heat treatment: Preheat the cold-rolled steel plate at 350℃-450℃ for 15-25 minutes, then heat it to 750℃-850℃ for 5-15 minutes and keep it at that temperature for 10-15 minutes, then cool it to 550℃-650℃ and keep it at that temperature for 60-180 minutes, quickly cool it to about 200℃ and then quench it in water.

[0018] Furthermore, the designed components described in step (1) include low-alloy carbon steel containing Cr and Mn, and the designed component proportions, calculated in terms of weight percentage of their chemical compositions, are as follows: carbon: 0.05%-0.65%, manganese: 1%-10%, silicon: 0.1%-0.5%, copper: 1.5%-6.5%, chromium: 1%-5.5%, molybdenum: 0.1%-1.5%, nickel: 0.05%-0.25%, titanium: 0.05%-0.15%, vanadium: 0.05%-0.15%, tungsten: 0.05%-0.15%, and the balance being Fe and unavoidable impurities.

[0019] Furthermore, the smelting in step (1) is preferably carried out in a vacuum induction furnace, and more preferably, protective gas argon is introduced during the smelting process.

[0020] Furthermore, in the forging of step (2), the forging ratio is between 1 and 3.

[0021] Furthermore, the homogenization treatment in step (3) is: homogenization treatment is carried out in an air furnace at 950°C-1250°C, preferably 1100°C-1200°C, keeping the temperature for 0.5-3.5h, preferably 1.5h-2h, and then water cooling.

[0022] Furthermore, the hot rolling starting temperature in step (4) is 1050°C-1250°C, preferably 1050°C-1150°C, and optimally 1080°C-1130°C; the ending temperature is 850°C-950°C, preferably 850°C-900°C, and optimally 850°C-880°C; the total reduction is 10%-50%, preferably 20%-30%, and optimally 25%-28%. The hot rolled steel plate has a thickness of 15 mm-30 mm, preferably 25 mm-28 mm.

[0023] Furthermore, in step (5), the cold rolling has a cold rolling reduction of 10%-30%, preferably 25%-28%.

[0024] Furthermore, the heat treatment conditions in step (6) are as follows: preheating at 400°C-420°C for 15-20 minutes, heating to 750°C-800°C for 10-15 minutes and keeping at that temperature for 10-15 minutes, cooling to 550°C-600°C and keeping at that temperature for 90-120 minutes, and then rapidly cooling to 200±20°C and quenching in water.

[0025] In the present invention, by adding Cu to a low-alloy carbon steel containing Cr and Mn at a certain Cu / C ratio, heavy metals can be effectively added to the matrix. The continuously released heavy metal ions can inhibit the activity of microorganisms and bacteria, significantly improving the antibacterial properties of the material. More preferably, the present invention adds alloying elements such as W, Mo, Ni, Ti, and V to the carbon steel to improve the overall mechanical properties of the material.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The low-alloy antibacterial carbon steel provided by the present invention, taking into account the structural changes of the material during heat treatment, adds Cu to the low-alloy carbon steel containing Cr and Mn by controlling the appropriate Cu / C ratio. This can prevent the precipitation of copper in the form of carbides during heat treatment, thereby effectively adding heavy metals to the matrix. The continuously released heavy metal ions can inhibit the activity of microorganisms and bacteria, significantly improving the antibacterial properties of the material. At the same time, the addition of elements such as Cr and Mn also helps stabilize Cu in the matrix, ensuring the antibacterial properties of the carbon steel.

[0028] 2. The processing and treatment of the materials in the method of the present invention play an important role in the antibacterial properties of the materials, especially the heat treatment of the materials in step (6). The cold-rolled steel plate is repeatedly treated under specific heating and cooling times, specific holding temperatures, and specific holding times to ensure that the copper can be stably present in the carbon steel matrix, while regulating the formation of certain precipitation phases of other alloying elements, thereby ensuring the antibacterial effect of the carbon steel while improving the strength, wear resistance, and other properties of the carbon steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The metallographic microstructure photos of the samples obtained in Comparative Example 1, Comparative Example 2, Comparative Example 3, Example 1 and Example 2;

[0030] Among them: (a) ordinary carbon steel; (b) antibacterial carbon steel (ordinary heat treatment); (c) antibacterial carbon steel-1; (d) antibacterial carbon steel-2; (e) antibacterial carbon steel-3.

[0031] Figure 2 The microscopic morphology of the samples obtained in Comparative Example 1, Comparative Example 2, Comparative Example 3, Example 1 and Example 2 after being corroded by sulfate-reducing bacteria (SRB) for one month;

[0032] Among them: (a) ordinary carbon steel; (b) antibacterial carbon steel (ordinary heat treatment); (c) antibacterial carbon steel-1; (d) antibacterial carbon steel-2; (e) antibacterial carbon steel-3.

[0033] Figure 3 The pitting morphology of the samples obtained in Comparative Example 1, Comparative Example 2, Comparative Example 3, Example 1 and Example 2 after being corroded by sulfate-reducing bacteria (SRB) for one month;

[0034] Among them: (a) ordinary carbon steel; (b) antibacterial carbon steel (ordinary heat treatment); (c) antibacterial carbon steel-1; (d) antibacterial carbon steel-2; (e) antibacterial carbon steel-3. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and features of the present invention more clearly defined, the present invention is now further described in detail with reference to the accompanying drawings and implementation methods. It should be noted that the embodiments described are only some of the embodiments of the present invention. Based on the embodiments of the present invention, other embodiments obtained by ordinary technicians in this field without making any creative work are all within the scope of protection of the present invention.

[0036] The antibacterial performance test methods of the samples prepared in the Examples and Comparative Examples are as follows:

[0037] The experimental medium was API culture medium, sterilized and deoxygenated with nitrogen. Before the experiment, the cleaned corrosion weight loss coupons were placed in a sealed container filled with culture medium. The container was then inoculated with the appropriate bacteria (SRB) according to the experimental conditions and sealed. This process was performed in an anaerobic glove box. During the experiment, the sealed container was placed in a 38°C constant-temperature biochemical incubator to maintain temperature and protect from light.

[0038] After the experiment, the corroded coupons need to be processed in time. The processing process is as follows:

[0039] ① Fix the corrosion products. Fix the corroded coupons in glutaraldehyde, phosphate buffer, 25%, 50%, 75% and 100% ethanol in sequence. Then, place the samples in a nitrogen environment and air dry them naturally.

[0040] ② The specimens used for SEM observation should be gold-sprayed on the observation surface to better protect the biofilm and corrosion product film on the surface;

[0041] ③ Place the corrosion test piece in the prepared rust remover and ultrasonically clean it. Soak the cleaned test piece in acetone, remove it, blow dry it, and weigh it. The weighed sample is used for subsequent pitting size testing.

[0042] Example 1

[0043] The Cu-alloyed antibacterial carbon steel-2 provided in this embodiment has the following chemical composition by weight percentage: C: 0.1%, Mn: 4%, Si: 0.2%, Cu: 5%, Cr: 3%, W: 0.15%, Mo: 0.25%, Ni: 0.15%, Ti: 0.15%, V: 0.15%, and the balance is Fe and unavoidable impurities; wherein the Cu / C weight ratio is 50.

[0044] The preparation method of the Cu alloyed antibacterial carbon steel-2 comprises the following steps:

[0045] 1) Melting: Add antibacterial carbon steel-2 into a vacuum induction furnace according to the designed components and proportions for melting, and then cast into ingots;

[0046] 2) Forging: The ingot is forged with a forging ratio of 2.8;

[0047] 3) Homogenization treatment: homogenize the forging in an air furnace at 1150°C, keep it warm for 1.5 hours, and then cool it in water;

[0048] 4) Hot rolling: The homogenized forgings were hot rolled at a starting temperature of 1150°C, an ending temperature of 850°C, a total reduction of 25%, and a 20 mm thick steel plate.

[0049] 5) Cold rolling: The hot-rolled steel plate is air-cooled to room temperature and then cold-rolled with a cold rolling reduction of 25%;

[0050] 6) Heat treatment: Further, the cold-rolled steel plate is preheated at 420°C for 15 minutes, heated to 750°C for 10 minutes and kept at that temperature for 15 minutes, cooled to 550°C and kept at that temperature for 90 minutes, and then rapidly cooled to about 200°C and water quenched.

[0051] The metallographic photograph of the sample under the metallographic microscope, the microscopic morphology after sulfate-reducing bacteria corrosion for one month and the pitting morphology are shown in Figure 1 (d) Figure 2 (d) and Figure 3 (d).

[0052] Example 2

[0053] The Cu-alloyed low-alloy antibacterial carbon steel-3 of this embodiment has the following chemical composition by weight percentage: C: 0.08%, Mn: 3%, Si: 0.15%, Cu: 6.4%, Cr: 2%, W: 0.1%, Mo: 0.2%, Ni: 0.1%, Ti: 0.1%, V: 0.1%, and the balance is Fe and unavoidable impurities; the Cu / C weight ratio is 80.

[0054] The preparation method of the low-alloy antibacterial carbon steel specifically comprises the following steps:

[0055] 1) Melting: Add antibacterial carbon steel-3 into a vacuum induction furnace according to the designed components and proportions for melting, and then cast into ingots;

[0056] 2) Forging: The ingot is forged with a forging ratio of 2.5;

[0057] 3) Homogenization treatment: homogenize the forging in an air furnace at 1150°C, keep it warm for 1 hour, and then cool it in water;

[0058] 4) Hot rolling: The homogenized forgings were hot rolled at a starting temperature of 1100°C, an ending temperature of 850°C, a total reduction of 30%, and a 20 mm thick steel plate.

[0059] 5) Cold rolling: The hot-rolled steel plate is air-cooled to room temperature and then cold-rolled with a cold rolling reduction of 15%;

[0060] 6) Heat treatment: Further, the cold-rolled steel plate is preheated at 400°C for 15 minutes, heated to 750°C for 15 minutes and kept at that temperature for 10 minutes, cooled to 550°C and kept at that temperature for 60 minutes, and then rapidly cooled to about 200°C and water quenched.

[0061] The metallographic photographs of the obtained samples under the microscope, the microscopic morphology and the pitting morphology after sulfate-reducing bacteria corrosion for one month are shown in Figure 1 (e) Figure 2 (e) and Figure 3 (e).

[0062] Comparative Example 1

[0063] Comparative Example 1 provides a common low-alloy carbon steel without copper, whose chemical composition by weight percentage is: C: 0.3%, Mn: 3%, Si: 0.1%, W: 0.1%, Cr: 1.5%, Mo: 0.1%, Ni: 0.05%, Ti: 0.05%, and the balance is Fe and unavoidable impurities.

[0064] The specific steps of the preparation method include:

[0065] 1) Melting: Add ordinary carbon steel into a vacuum induction furnace according to the designed components and proportions for melting, and then cast into ingots;

[0066] 2) Forging: The ingot is forged with a forging ratio of 2.8;

[0067] 3) Homogenization treatment: homogenize the forging in an air furnace at 1150°C, keep it warm for 1 hour, and then cool it in water;

[0068] 4) Hot rolling: The homogenized forgings were hot rolled at a starting temperature of 1050°C, an ending temperature of 850°C, a total reduction of 25%, and a 20 mm thick steel plate.

[0069] 5) Cold rolling: The hot-rolled steel plate is air-cooled to room temperature and then cold-rolled with a cold rolling reduction of 30%.

[0070] 6) Heat treatment: Further, the cold-rolled steel plate is preheated at 400°C for 15 minutes, heated to 750°C for 15 minutes and kept at that temperature for 10 minutes, cooled to 550°C and kept at that temperature for 60 minutes, and then rapidly cooled to about 200°C and water quenched.

[0071] The metallographic photograph of the sample under the metallographic microscope, the microscopic morphology after sulfate-reducing bacteria corrosion for one month and the pitting morphology are shown in Figure 1 (a) Figure 2 (a) and Figure 3 (a).

[0072] Comparative Example 2

[0073] Compared with Example 1, the heat treatment step of the antibacterial carbon steel (conventional heat treatment) was changed to holding the cold-rolled steel plate at 700°C for 40 minutes and then water quenching. All other parts were the same as in Example 1.

[0074] The metallographic photograph of the sample under the metallographic microscope, the microscopic morphology after sulfate-reducing bacteria corrosion for one month and the pitting morphology are shown in Figure 1 (b) Figure 2 (b) and Figure 3 (b).

[0075] Comparative Example 3

[0076] Comparative Example 3 provides a Cu-alloyed low-alloy antibacterial carbon steel-1, whose chemical composition by weight percentage is: C: 0.1%, Mn: 3%, Si: 0.15%, Cu: 1.2%, Cr: 2%, W: 0.1%, Mo: 0.2%, Ni: 0.1%, Ti: 0.1%, V: 0.1%, and the balance is Fe and unavoidable impurities; wherein the Cu / C weight ratio is 12.

[0077] Preparation process of antibacterial carbon steel-1: Compared with Example 2, except for the Cu / C ratio, all other parts are the same as Example 2.

[0078] The metallographic photograph of the sample under the metallographic microscope, the microscopic morphology after sulfate-reducing bacteria corrosion for one month and the pitting morphology are shown in Figure 1 (c) Figure 2 (c) and Figure 3 (c).

[0079] The research results are analyzed with reference to the accompanying drawings and examples:

[0080] Depend on Figure 1 From the metallographic organization diagrams of the five materials, it can be seen that the organizations of ordinary carbon steel, antibacterial carbon steel (ordinary heat treatment), antibacterial carbon steel-1, antibacterial carbon steel-2 and antibacterial carbon steel-3 are all relatively uniform, and no obvious metal element precipitation phase was found in the organizations of antibacterial carbon steel (ordinary heat treatment), antibacterial carbon steel-1, antibacterial carbon steel-2 and antibacterial carbon steel-3.

[0081] Depend on Figure 2 As can be seen, in the presence of SRB, the surface of ordinary carbon steel formed a thicker layer of corrosion products and spherical FeS, but no obvious SRB attachment was observed. On the surface of antibacterial carbon steel (conventional heat treatment), a larger amount of SRB attached and aggregated, forming a distinct biofilm that completely covered the sample surface. On the surface of antibacterial carbon steel-1, a small amount of SRB aggregated to form a biofilm, while the majority of the remaining SRB adsorbed on the surface without forming a biofilm, and pretreatment scratches were still visible on the sample surface. On the surface of antibacterial carbon steel-2, a small amount of SRB attached, but no obvious biofilm was observed, and pretreatment scratches were evident on the sample substrate surface. On the surface of antibacterial carbon steel-3, only a small amount of SRB attached, and no obvious biofilm was observed, but pretreatment scratches were evident on the sample substrate surface. In other words, in the presence of SRB, the five materials preferentially corroded ordinary carbon steel, forming a thicker layer of corrosion products. Among the four antibacterial carbon steels, the number of SRB on the surface of antibacterial carbon steel (ordinary heat treatment) was the largest, and an obvious biofilm was formed; the number of SRB attached to the surface of antibacterial carbon steel-1 and the biofilm on the surface were significantly reduced; the number of SRB attached to the surfaces of antibacterial carbon steel-2 and antibacterial carbon steel-3 was further reduced, and pretreatment scratches on the sample surface could be clearly observed, and the number of SRB on the surface of antibacterial carbon steel-2 was the least.

[0082] Depend on Figure 3 As can be seen, after 30 days of SRB corrosion and removal of corrosion products, pretreatment scratches were found on all five materials, indicating that uniform corrosion was not noticeable. In terms of pitting, consistent with the corrosion product results, the plain carbon steel exhibited the most pitting and the greatest depth. The antibacterial carbon steel (conventional heat-treated) also exhibited a high number of pittings, but the depth was shallower. Antibacterial carbon steel-1 exhibited only a few pittings; antibacterial carbon steel-3 exhibited even fewer pittings; and no significant pitting was observed on antibacterial carbon steel-2.

Claims

1. A low alloy antibacterial carbon steel, characterized in that: Its chemical composition by weight percentage is as follows: carbon: 0.05%-0.1%, manganese: 1%-10%, silicon: 0.1%-0.5%, copper: 1.5%-6.5%, chromium: 1%-5.5%, molybdenum: 0.1%-1.5%, nickel: 0.05%-0.25%, titanium: 0.05%-0.15%, vanadium: 0.05%-0.15%, tungsten: 0.05%-0.15%, the balance being Fe and unavoidable impurities; wherein the Cu / C weight ratio is 14.5-100; The method for preparing the low-alloy antibacterial carbon steel comprises the following steps: (1) Melting: Melt the designed components in proportion and cast them into ingots; (2) Forging: Forging the ingot; (3) Homogenization treatment; (4) Hot rolling: hot rolling the forgings after homogenization treatment; (5) Cold rolling: The hot-rolled steel plate is air-cooled to room temperature and then cold-rolled; (6) Heat treatment: Preheat the cold-rolled steel plate at 350℃-450℃ for 15-25 minutes, then heat it to 750℃-850℃ for 5-15 minutes and keep it at that temperature for 10-15 minutes, then cool it to 550℃-650℃ and keep it at that temperature for 60-180 minutes, quickly cool it to 200℃ and then quench it in water.

2. The low-alloy antibacterial carbon steel according to claim 1, characterized in that: The Cu / C weight ratio is 30-60.

3. The low alloy antibacterial carbon steel according to claim 1, characterized in that: During the smelting process in step (1), protective gas argon is introduced.

4. The low alloy antibacterial carbon steel according to claim 1, characterized in that: The forging ratio of the forging in step (2) is between 1 and 3.

5. The low alloy antibacterial carbon steel according to claim 1, characterized in that: The homogenization treatment in step (3) is as follows: homogenization treatment is carried out in an air furnace at 950°C-1250°C, heat preservation for 0.5-3.5 hours, and then water cooling.

6. The low alloy antibacterial carbon steel according to claim 5, characterized in that: The temperature of the homogenization treatment in step (3) is 1100° C.-1200° C., and the holding time is 0.5-3.5 h.

7. The low alloy antibacterial carbon steel according to claim 1, characterized in that: The hot rolling conditions in step (4) are: starting temperature of 1050°C-1250°C, ending temperature of 850°C-950°C, and total pressing amount of 10%-50%.

8. The low alloy antibacterial carbon steel according to claim 7, characterized in that: The hot rolling conditions in step (4) are: starting temperature of 1050°C-1150°C, ending temperature of 850°C-900°C, and total reduction amount of 20%-30%.

9. The low-alloy antibacterial carbon steel according to claim 7 or 8, characterized in that: Step (4) hot rolling to obtain a steel plate with a thickness of 15 mm to 30 mm.

10. The low alloy antibacterial carbon steel according to claim 9, characterized in that: Step (4) hot rolling to obtain a steel plate with a thickness of 25 mm to 28 mm.

11. The low alloy antibacterial carbon steel according to claim 1, characterized in that: The cold rolling pressure in step (5) is 10%-30%.

12. The low alloy antibacterial carbon steel according to claim 11, characterized in that: The cold rolling pressure in step (5) is 25%-28%.

13. The low alloy antibacterial carbon steel according to claim 1, characterized in that: The heat treatment conditions in step (6) are as follows: preheating at 400°C-420°C for 15-20 minutes, heating to 750°C-800°C for 10-15 minutes and keeping warm for 10-15 minutes, cooling to 550°C-600°C and keeping warm for 90-120 minutes, and then rapidly cooling to 200°C and quenching in water.

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