A production method for reducing surface cracks of high alloy heat-resistant steel plate

Abstract

The present invention relates to a production method for reducing surface cracks of high-alloy heat-resistant steel plates, including: electroslag remelting annealing: taking electroslag remelting steel ingots for annealing to eliminate thermal stress and organizational stress of the steel ingots; intermediate billet milling: electroslag remelting steel ingots are subjected to billeting to obtain intermediate billets, and the upper and lower surfaces of the intermediate billets are respectively milled to remove surface defects such as cracks and pits; intermediate billet heating: controlling the heating rate, insulation temperature and insulation time to avoid heating cracks; hot rolling: after the intermediate billet of step three is taken out of the furnace, descaling is performed to remove the surface oxide layer, and then rolling is started, and after rough rolling, 10-20mm thickness margin is directly reserved without waiting for temperature to perform finish rolling, and the temperature of the steel plate after rolling is cooled at 450-550℃, and then slowly cooled in a slow cooling cover pile. The method can significantly reduce the generation of surface cracks in the production process, avoid scrapping of steel plates due to surface cracks, and greatly improve the surface quality qualification rate.

Description

Technical Field

[0001] The invention relates to a production method of an iron-based alloy, and in particular to a production method for reducing surface cracks of a high-alloy steel plate. Background Art

[0002] With the continuous improvement of scientific and technological levels, the scale of electrical equipment in thermal power plants has also been expanding year by year in recent years, and higher requirements have been put forward for the high-temperature endurance strength, creep limit and other properties of the materials used. P91 steel is characterized by good high-temperature resistance, excellent high-temperature endurance strength and creep resistance, so it is widely used in the manufacture of boiler superheater tubes, headers and superheated main steam pipes. At present, electric arc furnace smelting, ingot casting, forging or hot rolling methods are mainly used to produce this type of heat-resistant steel plate, but because the steel has a high content of alloy elements, especially Cr, Mo, V, and N, the aforementioned traditional production methods are prone to cause serious cracks on the surface of the billet or steel plate, resulting in product scrapping. It is very necessary to develop a method that can reduce the cracks generated in this type of steel during the manufacturing process. Summary of the invention

[0003] The purpose of the present invention is to provide a production method for reducing surface cracks of high alloy heat resistant steel plates in view of the above-mentioned prior art, so as to reduce the scrapping of high alloy heat resistant steel due to cracks during the production process.

[0004] The technical solution adopted by the present invention to solve the above problems is: a production method for reducing surface cracks of high-alloy heat-resistant steel plates, comprising:

[0005] Step 1, electroslag remelting annealing: take electroslag remelting ingot for annealing, the charging temperature is 250-350℃, and heat to T1℃ at a heating rate of 30±(0~10)℃ / h. After keeping warm for more than 8h, the furnace is cooled to T2℃. T1 is close to AC1 but not more than AC1 to avoid structural transformation, T2 is lower than T1, and the heat preservation is carried out for more than 7h. After the heat preservation is completed, the furnace temperature is reduced to the furnace gas temperature ≤300℃, the furnace door is opened, and the steel is taken out of the furnace at 200-250℃. T1 is very close to AC1, which is considered as high-temperature stress relief annealing. If the heat source is turned off and the temperature is directly reduced to the bottom after the T1 heat preservation is completed, stress cracking may occur, so it is first reduced to T2 and kept warm for a period of time to prevent the steel from cooling too fast in the high temperature section.

[0006] Step 2: Milling the intermediate billet: Take the electroslag remelting steel ingot processed in step 1 and open it to obtain the intermediate billet. Mill the upper and lower surfaces of the intermediate billet to remove surface defects, including cracks and pits. The total milling thickness is 10-20 mm.

[0007] Step 3, heating the intermediate billet: before loading the intermediate billet into the furnace, open the furnace door to cool down, the loading temperature is ≤300℃, and the furnace is stewed for 5-6h after loading; after stewing, heat up to 600-700℃ at a rate of 20-30℃ / h, keep warm for more than 2h, then heat up to 800℃ at a rate of 30-50℃ / h, keep warm for more than 3h; finally, heat up to 1250℃ at a rate of 80-100℃ / h, keep warm for 10-12h;

[0008] Step 4, hot rolling: after the intermediate billet in step 3 is discharged from the furnace, the surface oxide layer is removed by descaling, and then rolling is started. The starting temperature of rough rolling is 1060-1120°C, and the final rolling temperature of rough rolling is 930-990°C. After rough rolling, a 10-20mm thickness margin is reserved directly for finishing rolling without waiting for temperature to control the plate shape and the final steel plate thickness. After rolling, the steel plate temperature is 450-550°C and cooled in a cooling bed, and then slowly cooled in a slow cooling hood pile for more than 96 hours.

[0009] Step 5: Heat treatment: Temper the steel plate to meet the mechanical properties.

[0010] As one of the embodiments of the present application, the elements of the high alloy heat-resistant steel plate are C: 0.08-0.12%, Mn: 0.30-0.50%, Si: 0.20-0.40%, P≤0.012%, S≤0.005%, Cr: 8.0-9.5%, Mo: 0.85-1.05%, V: 0.18-0.25%, Nb: 0.06-0.10%, Al≤0.02%, N: 0.035-0.070%, and the remainder is Fe and unavoidable impurity elements, and the electroslag remelting ingot is smelted with this elemental composition.

[0011] Preferably, in step 1, T1 is 800°C and T2 is 700°C.

[0012] Preferably, in step 2, the milling of the surface is divided into rough milling and fine milling, the milling depth is more than 3 mm / time during rough milling, and the milling depth is more than 0.5 mm / time during fine milling.

[0013] As one of the implementation modes of the present application, in step 2, the thickness of the intermediate blank is 200 mm, and the thickness of the intermediate blank after milling is 180-190 mm.

[0014] As one of the implementation methods of the present application, in step five, the heat treatment includes normalizing and tempering to obtain a tempered bainite structure, and the mechanical properties meet the following requirements: yield strength ≥465MPa, tensile strength ≥635MPa, and elongation ≥18%.

[0015] Compared with the prior art, the advantages of the present invention are:

[0016] The principle of reducing surface cracks of high-alloy heat-resistant steel plates by the method of the present invention is as follows: first, the electroslag remelted steel ingot is heated to a higher temperature below the Ac1 phase transition temperature at a slower heating rate for insulation, then furnace cooled to a lower temperature below the Ac1 phase transition temperature for further insulation and then furnace cooled to room temperature to achieve full annealing and stress elimination of the steel ingot, and prevent cracks in the steel ingot due to stress. Then, the intermediate billet obtained by blanking the electroslag remelted steel ingot is subjected to rough milling and fine milling to remove surface defects and ensure that there are no defects on the surface of the intermediate billet that cause cracks on the surface of the steel plate before rolling the steel plate. Next, the intermediate billet is slowly heated to avoid stress cracks. Finally, the high-pressure water descaling pressure and number are accurately controlled during the process of rolling the intermediate billet into steel plate, and the rolled steel plate is placed in a slow cooling hood for slow cooling to prevent surface cracks due to excessive cooling speed. Through the above means, cracks on the surface of hot-rolled steel plates can be effectively reduced or even avoided.

[0017] The surface crack area of ​​the steel plate produced by the conventional method accounts for more than 5% of the area of ​​the whole steel plate, and even the whole steel plate may be scrapped due to the surface cracks. However, the surface crack area of ​​the high-alloy heat-resistant steel plate produced by the method of the present invention accounts for less than 0.5% of the area of ​​the whole steel plate, which is much lower than that of the steel plate produced by the conventional method, and the whole steel plate is not scrapped due to surface cracks (avoided), which significantly improves the surface quality pass rate of the steel plate; compared with the existing method, the number of blank grinding times is reduced, which can effectively reduce the production cost; at the same time, the steel plate has uniform structure, fine grains, excellent mechanical properties, wherein the yield strength is ≥465MPa, the tensile strength is ≥635MPa, the elongation is ≥18%, and it has good performance. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0019] Example 1

[0020] The present invention provides a method for reducing surface cracks of a high-alloy heat-resistant steel plate. The mass percentages of the steel plate are as follows: C: 0.10%, Mn: 0.45%, Si: 0.25%, P: 0.010%, S: 0.003%, Cr: 8.7%, Mo: 0.98%, V: 0.22%, Nb: 0.075%, Alt: 0.01%, N: 0.048%, and the remainder is Fe and unavoidable impurity elements. The process steps are as follows:

[0021] (1) The annealing of the electroslag remelting ingot is that the ingot enters the furnace at a temperature of 280°C, is heated to 800°C at a rate of 30°C / h, is kept at this temperature for 8 hours, is cooled to 700°C, is kept at this temperature for 7 hours, and after the end of the holding period, the furnace door is opened at a temperature of 290°C and the ingot is taken out of the furnace at 220°C.

[0022] (2) The intermediate billet milling process is to mill the upper and lower surfaces of the electroslag remelted ingot after it is opened. The intermediate billet thickness is 200 mm. The defects such as cracks and pits that affect the surface of the rolled steel plate are completely removed. The milling depth is 3 mm / time during rough milling and 0.5 mm / time during fine milling. The thickness of the intermediate billet after milling is 189.5 mm.

[0023] (3) The intermediate billet heating process is to open the furnace door to cool down the intermediate billet before loading it into the furnace. The loading temperature is 270°C and the furnace is stewed for 5.3 hours. After stewing, the temperature is increased to 650°C at a rate of 22°C / h and kept for 2 hours. Then, the temperature is increased to 800°C at a rate of 38°C / h and kept for 3 hours. Finally, the temperature is increased to 1250°C at a rate of 90°C / h and kept for 11 hours.

[0024] (4) Hot rolling means that after the steel billet is taken out of the furnace, a rough descaling is carried out. The descaling high-pressure water pressure is 21MPa to remove the primary oxidized iron scale; the starting temperature of the rough rolling is 1070℃, and the final temperature of the rough rolling is 970℃; after the rough rolling, a 14mm thickness margin is reserved for finishing rolling without waiting for the temperature to control the plate shape and final size of the steel plate. The thickness of the finished steel plate is 40mm. After rolling, the steel plate temperature is 460℃ and the cooling bed is cooled in the slow cooling hood for 96 hours.

[0025] After the slow cooling of the steel plate, the crack area on the steel plate surface accounts for 0.2% of the area of ​​the entire steel plate.

[0026] In order to meet the use requirements, the steel plate must be normalized and tempered to obtain tempered troostite structure and excellent mechanical properties. The steel plate is normalized at 1050℃, with a furnace time coefficient of 2min / mm and cooled in still air; tempered at 755℃, with a furnace time coefficient of 3min / mm and cooled in still air. The steel plate has a yield strength of 480Mpa, a tensile strength of 655Mpa, and an elongation of 19.0%, and has good performance.

[0027] Example 2

[0028] The present invention provides a method for reducing surface cracks of a high-alloy heat-resistant steel plate. The mass percentages of the steel plate are as follows: C: 0.09%, Mn: 0.46%, Si: 0.23%, P: 0.011%, S: 0.004%, Cr: 8.8%, Mo: 0.95%, V: 0.21%, Nb: 0.073%, Alt: 0.01%, N: 0.047%, and the remainder is Fe and unavoidable impurity elements. The process steps are as follows:

[0029] (1) The annealing of the electroslag remelting ingot is that the ingot enters the furnace at a temperature of 270°C, is heated to 800°C at a rate of 30°C / h, and is kept at this temperature for 8 hours. The furnace is then cooled to 700°C and kept at this temperature for 7 hours. After the end of the holding period, the furnace door is opened at a temperature of 260°C and the ingot is taken out of the furnace at 210°C.

[0030] (2) The intermediate billet milling process is to mill the upper and lower surfaces of the electroslag remelted ingot after it is opened. The intermediate billet thickness is 200 mm. The defects such as cracks and pits that affect the surface of the rolled steel plate are completely removed. The milling depth is 3 mm / time during rough milling and 0.5 mm / time during fine milling. The thickness of the intermediate billet after milling is 188 mm.

[0031] (3) The intermediate billet heating process is to open the furnace door to cool down the intermediate billet before loading it into the furnace. The loading temperature is 290°C and the furnace is kept in the furnace for 5.5 hours. After the furnace is kept in the furnace, the temperature is increased to 650°C at a rate of 24°C / h and kept at this temperature for 2 hours. Then, the temperature is increased to 800°C at a rate of 37°C / h and kept at this temperature for 3 hours. Finally, the temperature is increased to 1250°C at a rate of 95°C / h and kept at this temperature for 11.2 hours.

[0032] (4) Hot rolling means that after the steel billet is taken out of the furnace, a rough descaling is carried out. The descaling high-pressure water pressure is 21MPa to remove the primary oxide scale; the starting temperature of the rough rolling is 1080℃, and the final temperature of the rough rolling is 950℃; after the rough rolling, a 12mm thickness margin is reserved for finishing rolling without waiting for the temperature to control the plate shape and final size of the steel plate. The thickness of the finished steel plate is 35mm. After rolling, the steel plate temperature is 455℃ and the cooling bed is cooled in the slow cooling hood for 96 hours.

[0033] After the slow cooling of the steel plate, the crack area on the steel plate surface accounts for 0.4% of the entire steel plate area.

[0034] The steel plate is normalized at 1050℃, with a furnace time coefficient of 2min / mm and cooled in still air; tempered at 760℃, with a furnace time coefficient of 3min / mm and cooled in still air. The steel plate has a yield strength of 485Mpa, a tensile strength of 665Mpa, and an elongation of 18.5%, and has good performance.

[0035] Example 3

[0036] The present invention provides a method for reducing surface cracks of a high-alloy heat-resistant steel plate. The mass percentages of the steel plate are as follows: C: 0.085%, Mn: 0.43%, Si: 0.25%, P: 0.012%, S: 0.004%, Cr: 8.6%, Mo: 0.94%, V: 0.22%, Nb: 0.076%, Alt: 0.01%, N: 0.046%, and the remainder is Fe and unavoidable impurity elements. The process steps are as follows:

[0037] (1) The annealing of the electroslag remelting ingot is that the ingot enters the furnace at a temperature of 265°C, is heated to 800°C at a rate of 30°C / h, is kept at this temperature for 8 hours, is cooled to 700°C, and is kept at this temperature for 7 hours. After the end of the keeping temperature, the furnace door is opened at a temperature of 250°C, and the ingot is taken out of the furnace at 205°C.

[0038] (2) The intermediate billet milling process is to mill the upper and lower surfaces of the electroslag remelted ingot after it is opened. The intermediate billet thickness is 200 mm. The defects such as cracks and pits that affect the surface of the rolled steel plate are completely removed. The milling depth is 3 mm / time during rough milling and 0.5 mm / time during fine milling. The thickness of the intermediate billet after milling is 186 mm.

[0039] (3) The intermediate billet heating process is to open the furnace door to cool down the intermediate billet before loading it into the furnace. The loading temperature is 295°C and the furnace is kept in the furnace for 5.6 hours. After the furnace is kept in the furnace, the temperature is increased to 660°C at a rate of 26°C / h and kept at this temperature for 2 hours. Then, the temperature is increased to 800°C at a rate of 39°C / h and kept at this temperature for 3 hours. Finally, the temperature is increased to 1250°C at a rate of 98°C / h and kept at this temperature for 11.3 hours.

[0040] (4) Hot rolling means that after the steel billet is taken out of the furnace, a rough descaling is carried out. The descaling high-pressure water pressure is 21MPa to remove the primary oxidized iron scale; the starting temperature of the rough rolling is 1090℃, and the final temperature of the rough rolling is 940℃; after the rough rolling, a 11mm thickness margin is reserved for finishing rolling without waiting for the temperature to control the plate shape and final size of the steel plate. The thickness of the finished steel plate is 30mm. After rolling, the steel plate temperature is 450℃ and the cooling bed is cooled in the slow cooling hood for 96 hours.

[0041] After the slow cooling of the steel plate, the crack area on the steel plate surface accounts for 0.5% of the entire steel plate area.

[0042] The steel plate is normalized at 1050℃, with a furnace time coefficient of 2min / mm and cooled in still air; tempered at 760℃, with a furnace time coefficient of 3min / mm and cooled in still air. The steel plate has a yield strength of 495Mpa, a tensile strength of 670Mpa, and an elongation of 18.0%, and has good performance.

[0043] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A production method for reducing surface cracks of high alloy heat-resistant steel plate, characterized in that: include, Step 1, electroslag remelting annealing: take electroslag remelting steel ingot for annealing, the charging temperature is 250-350℃, heat to T1℃ at a heating rate of 30±(0~10)℃ / h, keep warm for more than 8h, then cool to T2℃, T1 is close to AC1 but not more than AC1 to avoid structural transformation, T2 is lower than T1, keep warm for more than 7h, after the insulation is completed, the temperature in the furnace is lowered to the furnace gas temperature ≤300℃, the furnace door is opened, and the furnace is taken out at 200-250℃; Step 2: Milling the intermediate billet: Take the electroslag remelting steel ingot processed in step 1 and open it to obtain the intermediate billet. Mill the upper and lower surfaces of the intermediate billet to remove surface defects, including cracks and pits. The total milling thickness is 10-20 mm. Step 3, heating the intermediate billet: before loading the intermediate billet into the furnace, open the furnace door to cool down, the loading temperature is ≤300℃, and the furnace is stewed for 5-6h after loading; after stewing, heat up to 600-700℃ at a rate of 20-30℃ / h, keep warm for more than 2h, then heat up to 800℃ at a rate of 30-50℃ / h, keep warm for more than 3h; finally, heat up to 1250℃ at a rate of 80-100℃ / h, keep warm for 10-12h; Step 4, hot rolling: after the intermediate billet in step 3 is taken out of the furnace, the surface oxide layer is removed by descaling, and then rolling is started. The starting temperature of rough rolling is 1060-1120℃, and the final rolling temperature of rough rolling is 930-990℃. After rough rolling, a 10-20mm thickness margin is reserved directly for finishing rolling without waiting for temperature to control the plate shape and the final steel plate thickness. After rolling, the steel plate temperature is 450-550℃ and cooled on a cooling bed, and then slowly cooled in a slow cooling hood pile; Step 5: Heat treatment: Temper the steel plate to meet the mechanical properties.

2. The method according to claim 1, characterized in that: The weight percentage of the elements of the steel plate is C: 0.08-0.12%, Mn: 0.30-0.50%, Si: 0.20-0.40%, P≤0.012%, S≤0.005%, Cr: 8.0-9.5%, Mo: 0.85-1.05%, V: 0.18-0.25%, Nb: 0.06-0.10%, Al≤0.02%, N: 0.035-0.070%, and the balance is Fe and inevitable impurity elements, and the electroslag remelting steel ingot is smelted with this element composition.

3. The method according to claim 1, characterized in that: Step 1, T1 is 800°C and T2 is 700°C.

4. The method according to claim 1, characterized in that: Step 2: The milling surface is divided into rough milling and fine milling. The milling depth is more than 3mm / time during rough milling, and more than 0.5mm / time during fine milling.

5. The method according to claim 1, characterized in that: Step 2: The thickness of the intermediate blank is 200mm, and the thickness of the intermediate blank after milling is 180-190mm.

6. The method according to claim 1, characterized in that: Step 5, heat treatment includes normalizing and tempering to obtain a tempered bainite structure, and the mechanical properties meet the following requirements: yield strength ≥465MPa, tensile strength ≥635MPa, and elongation ≥18%.