A heat treatment method for a low-carbon high-chromium heat-resistant steel large-size continuous casting billet

Abstract

The application discloses a heat treatment method of low-carbon high-chromium heat-resistant steel large-size continuous casting billets, which comprises the following steps: slow cooling process, the low-carbon high-chromium heat-resistant steel large-size continuous casting billets are slowly cooled and pit-cooled or furnace-cooled to 100-150 DEG C; tempering process, the low-carbon high-chromium heat-resistant steel large-size continuous casting billets slowly cooled to 100-150 DEG C are put into a furnace, heated to 765+ / -10 DEG C at a heating speed less than 100 DEG C / h, kept for 14-16 hours, then furnace-cooled to <=500 DEG C, and out of the furnace for air cooling to room temperature. The application directly carries out tempering after slow cooling of the casting billets instead of conventional annealing, not only significantly shortens the furnace time, but also greatly saves energy and effectively reduces production cost, thus breaking through the annealing+tempering stress relieving heat treatment process mechanism in the field, and effectively solving the technical problems of high production cost and long annealing furnace time in the prior art.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of heat treatment of heat-resistant steel, and particularly relates to a heat treatment method for large-size continuous casting billets of low-carbon high-chromium heat-resistant steel. BACKGROUND

[0002] The low-carbon high-chromium heat-resistant steel material has the characteristics of high-temperature resistance and high-pressure resistance. Since the carbon content is low, it belongs to hypoeutectic steel, the solidification process is complex, and the alloy content is high, it is a crack-sensitive steel grade. In the solidification process and subsequent cooling process of the large-size continuous casting billet, internal stress caused by the superposition of organizational stress and thermal stress is generated. If the internal stress is not eliminated in time, stress concentration will be further formed in the subsequent process, which is easy to cause the cracking of the continuous casting billet.

[0003] The conventional red annealing treatment process has a long incubation time, and the red annealing time can reach more than 90 hours. If the annealing time is short, the phenomenon of supercooled austenite is easy to occur, and there is a problem of incomplete organizational transformation. The supercooled austenite is transformed into martensite at room temperature, which causes large organizational stress of the continuous casting billet. If the subsequent process is not in time, cracking caused by stress concentration will occur, resulting in loss, which will seriously affect the qualified rate of the product and reduce the production stability.

[0004] In view of the long incubation time in the red annealing process and the phenomenon of supercooled austenite after annealing, the existing technology has adopted a new heat treatment process of “annealing + tempering”. This heat treatment process can completely eliminate the internal stress, especially the organizational stress, and the material will not crack again. However, although this heat treatment process can improve the yield rate, it occupies a long annealing furnace time, affects the production rhythm, and causes high production cost due to two times of firing. In today's competitive environment, the cost is also a serious problem.

[0005] Therefore, it is a technical problem to be solved by the technical personnel in the field to develop a heat treatment method for large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, so as to shorten the furnace occupation time, save energy and reduce the production cost. SUMMARY

[0006] In order to solve the above technical problems in the prior art, the present application provides a heat treatment method for large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, comprising:

[0007] The slow cooling process is to slowly cool the large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, and to cool to 100-150 DEG C by furnace cooling or pit cooling.

[0008] The tempering process is to put the low-carbon high-chromium heat-resistant steel large-size continuous casting billets slowly cooled to 100-150 DEG C into the furnace, heat to 765±10 DEG C at a temperature rising speed less than 100 DEG C / h, keep for 14-16 hours, then cool to ≤500 DEG C by furnace cooling, and air cool to room temperature after taking out of the furnace.

[0009] In the heat treatment method of the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel, the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is slowly cooled, furnace-cooled or pit-cooled to 120℃ in the slow cooling process, and the slowly cooled large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is heated to 770℃ at a heating rate of 80℃ / h, kept for 15 hours, then furnace-cooled to 500℃, and air-cooled to room temperature after discharging.

[0010] In the heat treatment method of the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel, the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is prepared by the following steps:

[0011] (I) Electric furnace smelting

[0012] In the electric furnace smelting process, strong oxidation operation is adopted to reduce the content of gas and inclusions in the steel, the end point C and P content is controlled to be C≤0.05% and P≤0.008%, the electric furnace tapping temperature is controlled to be ≥1750℃, and tapping is strictly prohibited to be slagged; the synthetic slag of slagging material and the alloy roasted for more than 30 minutes are added into the ladle online; aluminum deoxidizer is added into the steel stream during the electric furnace tapping process;

[0013] (II) LF furnace refining

[0014] When the temperature of the molten steel is ≥1560℃, silicon-calcium powder is added to the slag to form a reducing slag, the composition is adjusted and controlled according to the target product requirements, and (FeO) is controlled to be <0.40%; after the last batch of alloy is added, it is kept for more than 15 minutes; when all the element compositions meet the requirements, the refining slag has good fluidity, and the temperature of the molten steel is 1660-1680℃, the molten steel is tapped into the VD;

[0015] (III) VD degassing

[0016] During the VD degassing treatment, it is kept for more than 18 minutes under high vacuum, and hydrogen is determined immediately after the vacuum is broken, and [H] is controlled to be ≤1.5ppm; silicon-calcium wire is fed, the argon soft stirring time is controlled to be ≥20 minutes, and the temperature meets the requirements after the covering agent is added for tapping;

[0017] (IV) Continuous casting

[0018] During the continuous casting process, the tundish temperature is controlled to be ≤1530℃, and the cooling water parameters are controlled as follows: the cooling water amount of the crystallizer is 3800±100L / min, and the specific water amount of the secondary cooling water is 0.11±0.01L / kg steel; during the process of drawing the cylinder, the drawing speed is adjusted in time according to the tundish temperature.

[0019] As a specific embodiment, in the heat treatment method of the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel, the large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is a low-carbon high-chromium heat-resistant steel continuous large round billet with a specification of Φ690mm.

[0020] The heat treatment method of the low-carbon high-chromium heat-resistant steel large-size continuous casting billet has the following advantages and beneficial effects:

[0021] The present application uses slow cooling instead of conventional annealing, and directly tempering after slow cooling of the billet. Not only the furnace time is significantly shortened, but also the energy is greatly saved, and the production cost is effectively reduced. According to the statistics and analysis of actual production data, compared with the "annealing + tempering" heat treatment process of the prior art, the "slow cooling + tempering" heat treatment method of the present application shortens the furnace time by about 50 hours per batch, and reduces the cost by about 600 yuan / ton of steel. The present application breaks through the "annealing + tempering" stress relief heat treatment process mechanism in the field, and effectively solves the technical problems of high production cost and long annealing furnace time in the prior art. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present application clearer, the technical scheme of the present application will be described clearly and completely below in combination with specific embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0023] The heat treatment method of the low-carbon high-chromium heat-resistant steel large-size continuous casting billet of the present application comprises:

[0024] (1) Slow cooling process: slow cooling the low-carbon high-chromium heat-resistant steel large-size continuous casting billet, and furnace cooling or pit cooling to 100-150℃;

[0025] (2) Tempering process: putting the low-carbon high-chromium heat-resistant steel large-size continuous casting billet cooled to 100-150℃ into the furnace, heating to 765±10℃ at a temperature rising speed of less than 100℃ / h, keeping for 14-16 hours, then furnace cooling to ≤500℃, and air cooling to room temperature after taking out of the furnace.

[0026] Preferably, in the slow cooling process, the low-carbon high-chromium heat-resistant steel large-size continuous casting billet is slow cooled, and furnace cooled or pit cooled to 120℃. In the tempering process, the slow cooled low-carbon high-chromium heat-resistant steel large-size continuous casting billet is heated to 770℃ at a temperature rising speed of 80℃ / h, kept for 15 hours, then furnace cooled to 500℃, and air cooled to room temperature after taking out of the furnace. In the above heat treatment method of the low-carbon high-chromium heat-resistant steel large-size continuous casting billet, the low-carbon high-chromium heat-resistant steel large-size continuous casting billet is prepared by the following steps:

[0027] (I) Electric furnace smelting

[0028] In the process of electric furnace smelting, strong oxidation operation is adopted to reduce the content of gas and inclusion in the steel, the end point C, P content is controlled to be C≤0.05%, P≤0.008%, the electric furnace tapping temperature is controlled to be ≥1750℃, and the tapping is strictly prohibited to be slagged; the synthetic slag of the slagging material and the alloy roasted for more than 30 minutes are added into the ladle on line; the aluminum deoxidizer is added into the steel stream during the electric furnace tapping.

[0029] (II) LF furnace refining

[0030] When the temperature of the molten steel is ≥1560℃, the silicon-calcium powder is added into the slag to form the reducing slag, the composition is adjusted and controlled according to the target product requirements, and (FeO) is controlled to be <0.40%; after the last batch of alloy is added, it is kept for more than 15 minutes; when all the element compositions are up to the standard, the refining slag has good fluidity, and the temperature of the molten steel is 1660-1680℃, the tapping is carried out to enter the VD.

[0031] (III) VD degassing

[0032] During the VD degassing treatment, it is kept for more than 18 minutes under high vacuum degree, the hydrogen is determined immediately after the breaking of the vacuum, [H] is controlled to be ≤1.5ppm; the silicon-calcium wire is fed, the argon soft stirring time is controlled to be ≥20 minutes, the covering agent is added after the temperature reaches the standard, and the tapping is carried out.

[0033] (IV) Continuous casting

[0034] During the continuous casting process, the tundish temperature is controlled to be ≤1530℃, and the cooling water parameters are controlled as follows: the cooling water amount of the crystallizer is 3800±100L / min, and the specific water amount of the secondary cooling water is 0.11±0.01L / kg steel; during the process of drawing the cylinder, the drawing speed is adjusted in time according to the tundish temperature.

[0035] The following will be described in detail in combination with specific embodiments.

[0036] The following embodiments of the present application are used for smelting and continuously casting three furnace specifications of Φ690mm low-carbon high-chromium heat-resistant steel continuous casting round billets, and then carrying out heat treatment, and the specific steps include:

[0037] (I) Electric furnace smelting

[0038] During the process of electric furnace smelting, strong oxidation operation is adopted to reduce the content of gas and inclusion in the steel, the end point C, P content is controlled to be C≤0.05%, P≤0.008%, the electric furnace tapping temperature is controlled to be ≥1750℃, and the tapping is strictly prohibited to be slagged; the synthetic slag of the slagging material and the alloy roasted for more than 30 minutes are added into the ladle on line; the aluminum deoxidizer is added into the steel stream during the electric furnace tapping.

[0039] (II) LF furnace refining

[0040] When the temperature of the molten steel is ≥ 1560℃, 180kg of silicon-calcium powder is added to the slag to form a reducing slag, the composition is adjusted and controlled according to the requirements of the target product, and (FeO) is controlled to be < 0.40%; after the last batch of alloy is added, it is maintained for more than 15 minutes; when all the element compositions meet the requirements, the fluidity of the refining slag is good, and the temperature of the molten steel is 1660-1680℃, the molten steel is tapped into a VD.

[0041] (III) VD degassing

[0042] During the VD degassing treatment, the vacuum degree is controlled to be ≤ 67Pa, the holding time is more than 18 minutes, the hydrogen is determined immediately after the vacuum is broken, and [H] is controlled to be ≤ 1.5ppm; 250-280m of silicon-calcium wire is fed, the argon soft stirring time is controlled to be ≥ 20 minutes, the temperature meets the requirements, and ≥ 200K of covering agent is added before the molten steel is tapped.

[0043] (IV) continuous casting

[0044] During the continuous casting process, the temperature of the tundish is controlled to be ≤ 1530℃, the cooling water parameters are controlled as follows: the cooling water amount of the crystallizer is 3800±100L / min, the specific water amount of the secondary cooling water is 0.11±0.01L / kg of steel, and the casting speed is controlled to be 0.23m / min, so that a low-carbon high-chromium heat-resistant steel continuous casting large round billet with a specification of Φ690mm is prepared.

[0045] (V) heat treatment

[0046] (1) slow cooling process: the continuous casting large round billet is slowly cooled, and is furnace-cooled or pit-cooled to 120℃, and then is tempered in a furnace;

[0047] (2) tempering process: the slowly cooled continuous casting large round billet is heated to 770℃ at a heating speed of 80℃ / h, is kept for 15 hours, then is furnace-cooled to 500℃, and is air-cooled to room temperature after being taken out of the furnace.

[0048] Through actual detection, the chemical composition and hardness of the low-carbon high-chromium heat-resistant steel large-specification continuous casting billet produced and heat-treated by using the heat treatment method of the low-carbon high-chromium heat-resistant steel large-specification continuous casting billet of the embodiment of the application are as shown in Tables 1 and 2:

[0049] Table 1: main chemical composition content of the continuous casting billet (mass percentage, %)

[0050] C Cr Mo W Nb V N First furnace 0.09 8.8 0.38 1.56 0.05 0.17 0.045 Second furnace 0.08 8.9 0.36 1.58 0.05 0.18 0.053 Third furnace 0.10 8.65 0.40 1.60 0.06 0.15 0.058

[0051] Table 2: hardness of the continuous casting billet after slow cooling + tempering heat treatment

[0052] Number of furnaces First furnace Second furnace Third furnace Continuous casting billet specification φ690 mm φ690 mm φ690 mm Hardness after slow cooling, HB 390 403 420 Hardness after tempering, HB 183 189 185

[0053] It can be seen that the hardness of the low-carbon high-chromium heat-resistant steel large-size continuous casting blank after being treated by the heat treatment method of the application is obviously reduced, is consistent with the hardness of the tempered martensite, and effectively releases the structure stress generated by the martensite transformation in the slow cooling process, thereby preventing the subsequent casting blank from cracking.

[0054] In summary, compared with the prior art, the heat treatment method of the low-carbon high-chromium heat-resistant steel large-size continuous casting blank has the following advantages and beneficial effects:

[0055] According to the actual production data statistics and analysis, compared with the prior art, the heat treatment method of the application has the following advantages and beneficial effects:

[0056] It should be noted that in this document, the term "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that the object or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or includes elements inherent to such object or device.

[0057] It should be further noted that the above examples are only used to illustrate the technical solutions of the application, and are not intended to limit the application; although the application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can still be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the application.

Claims

1. A heat treatment method for large-size continuously cast billets of low-carbon, high-chromium heat-resistant steel, characterized in that, Instead of conventional annealing, slow cooling is used, followed by direct tempering after slow cooling of the billet. This "slow cooling + tempering" heat treatment method is employed to prepare large-size continuous casting billets of low-carbon, high-chromium heat-resistant steel, including: Slow cooling process: Slowly cool large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, either by furnace cooling or pit cooling to 100℃~150℃; Tempering process: Large-size continuous casting billets of low-carbon high-chromium heat-resistant steel that have been slowly cooled to 100℃~150℃ are put into the furnace and heated to 765±10℃ at a heating rate of less than 100℃ / h. They are held at this temperature for 14~16 hours, then cooled to ≤500℃ in the furnace and air-cooled to room temperature after being taken out of the furnace. In the slow cooling process, the large-size continuous casting billet of low carbon high chromium heat-resistant steel is slowly cooled, either in the furnace or in a pit, to 120°C. In the tempering process, the slow-cooled large-size continuous casting billet of low carbon high chromium heat-resistant steel is heated to 770°C at a heating rate of 80°C / h, held for 15 hours, and then cooled to 500°C in the furnace before being air-cooled to room temperature. The large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is prepared by the following steps: (a) Electric furnace smelting During the electric arc furnace smelting process, strong oxidation is employed to reduce the content of gases and inclusions in the steel. The final C and P content is controlled to be C≤0.05% and P≤0.008%, respectively. The tapping temperature of the electric arc furnace is controlled to be ≥1750℃, and slag is strictly prohibited during tapping. Slag-forming materials and alloys baked for more than 30 minutes are added to the ladle online. Aluminum deoxidizer is added with the steel stream during the tapping process. (ii) LF furnace refining When the molten steel temperature is ≥1560℃, add silicon-calcium powder to the slag to form a reducing slag. Adjust and control the composition according to the target product requirements, and control (FeO) <0.40%. After the last batch of alloy is added, maintain for more than 15 minutes. When all elemental compositions meet the standards, the refining slag has good fluidity, and the molten steel temperature is 1660~1680℃, tap the steel into VD. (iii) Degassing of VD During VD degassing, maintain a high vacuum for more than 18 minutes. After breaking the vacuum, immediately determine the hydrogen concentration and control [H] ≤ 1.5ppm. Feed in silicon-calcium wire, control the argon gas soft stirring time to ≥ 20 minutes, and add a covering agent to tap out the steel after the temperature reaches the standard. (iv) Continuous casting During continuous casting, the tundish temperature is controlled at ≤1530℃, and the cooling water parameters are controlled as follows: the crystallizer cooling water flow rate is 3800±100L / min, and the secondary cooling water flow rate is 0.11±0.01L / kg steel; the casting speed is adjusted at any time according to the tundish temperature during the casting process. The large-size continuous casting billet of low-carbon high-chromium heat-resistant steel is a large round billet of low-carbon high-chromium heat-resistant steel with a specification of Φ690mm.