A heat treatment method to improve the grain size of 200 series stainless steel

Abstract

This invention discloses a heat treatment method for improving the grain size of 200 series stainless steel, comprising: a solution treatment step, wherein the rolled stainless steel strip is solution treated in a horizontal annealing furnace, wherein the stainless steel strip is continuously fed into the horizontal annealing furnace and heated to 1130-1170°C, and held at this temperature; and a cooling step, wherein the solution-treated stainless steel strip is cooled. This invention employs a combination of limited thermal stress (TV) and high-temperature solution treatment, and uses a combination of air cooling, three-stage air mist cooling, and one-stage water cooling during cooling to prevent brittle fracture of the workpiece due to excessive thermal stress. According to this invention, the grain size of the 200 series stainless steel product is guaranteed to be 6.5-7.0 grade, and the product qualification rate is over 95%, making it easy to roll and exhibiting good rolling performance in cold rolling production.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and relates to a heat treatment method for improving the grain size of 200 series stainless steel, and particularly to a heat treatment process for improving the grain size of white-coated 200 series stainless steel. Background Technology

[0002] 200 series stainless steel white coils (NO.1) are generally used for cold-rolled materials, industrial tanks, chemical industrial equipment, etc.

[0003] In existing technologies, 200 series stainless steel white coils (NO.1) are obtained by treating 200 series stainless steel black coils using a high-temperature solution treatment followed by aging. The resulting 200 series NO.1 typically has a grain size of 7.5–8.5, which does not meet the requirement of a grain size of 6.5–7.0, thus increasing the processing difficulty of subsequent cold rolling. When used in the rolling of cold-rolled materials, the small grain size of 200 series stainless steel white coils makes the crystals difficult to break during rolling, requiring greater rolling force to achieve the target thickness. This results in a low rolling reduction rate and a tendency for brittle fracture, leading to increased costs.

[0004] Therefore, a heat treatment method is needed to improve the grain size of stainless steel, so that the grain size of 200 series NO.1 is controlled at grade 6.5 to 7.0, and the product qualification rate reaches more than 95%, so as to ensure the needs of subsequent cold rolling processing of 200 series NO.1 products. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a heat treatment method for improving the grain size of 200 series stainless steel. By optimizing the solution treatment process, the grain size grade of 200 series stainless steel white coils is reduced, ensuring that the grain size is between 6.5 and 7.0. This heat treatment method can improve the difficulty of subsequent cold rolling processes and the performance of the rolled stainless steel.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] A heat treatment method for improving the grain size of 200 series stainless steel, the heat treatment method comprising:

[0008] The solution treatment step involves using a horizontal annealing furnace to perform solution treatment (i.e., annealing) on ​​the rolled stainless steel strip (i.e., black coil). The stainless steel strip is continuously fed into the horizontal annealing furnace and heated to 1130-1170℃ (i.e., 1150±20℃, for example, 1135℃, 1140℃, 1150℃, 1160℃, 1165℃) and held at this temperature.

[0009] The cooling step involves cooling the solution-treated stainless steel strip.

[0010] The present invention performs a solution treatment on 200 series stainless steel at 1150±20℃ to allow the grains to grow sufficiently until a relatively stable grain size is achieved.

[0011] In the above heat treatment method, as a preferred embodiment, the annealing furnace includes a preheating section and a soaking section, wherein the soaking section has four chambers, namely chamber 1, chamber 2, chamber 3, and chamber 4; wherein,

[0012] The air-fuel ratio parameters for each chamber in the soaking tank are controlled as follows: Chamber 1: 10.3%–10.7% (e.g., 10.4%, 10.5%, 10.6%); Chamber 2: 10.8%–11.2% (e.g., 10.9%, 11.0%, 11.1%); Chamber 3: 10.8%–11.2% (e.g., 10.9%, 11.0%, 11.1%); Chamber 4: 11.3%–11.7% (e.g., 11.4%, 11.5%, 11.6%).

[0013] And / or, optimize the furnace pressure (i.e., furnace internal pressure) of each furnace chamber. The furnace pressure parameters of each furnace chamber in the soaking section are controlled as follows: control the furnace pressure of chamber 1 to 2-4 Pa (e.g., 2.5 Pa, 3.0 Pa, 3.5 Pa), control the furnace pressure of chambers 2-3 to +10-12 Pa (e.g., 10.5 Pa, 11.0 Pa, 11.5 Pa) respectively; control the furnace pressure of chamber 4 to 6-8 Pa (e.g., 6.5 Pa, 7.0 Pa, 7.5 Pa);

[0014] And / or, optimize the combustion air pressure of each furnace chamber. The combustion air pressure parameters of each furnace chamber in the soaking section are controlled as follows: the combustion air pressure of chambers 1-4 is controlled at 7-8 kPa (e.g., 7.2 Pa, 7.5 Pa, 7.8 Pa).

[0015] In the above heat treatment method, as a preferred embodiment, the horizontal annealing furnace is set in the solution treatment section of the continuous rolling annealing pickling solution production line (hereinafter referred to as: RAPL production line). For 200 series stainless steel strips with a thickness of less than 2.4 mm, the process section speed is controlled at ≤100 m / min (e.g., 95 m / min, 90 m / min, 80 m / min, 70 m / min) so that the TV value of the solution treatment section of the production line is controlled at ≤120 m / min*mm. The TV value of the heat treatment section refers to the product of the strip thickness and the strip speed in the process section. The process section speed is the running speed of the strip during heat treatment.

[0016] In the above heat treatment method, as a preferred embodiment, the 200 series stainless steel includes CJ1L and CJ5L, wherein, by mass percentage,

[0017] The chemical composition of CJ1L is as follows: C: 0.07%–0.10%, Si: 0.3%–0.7%, Mn: 9.0%–10.5%, P≤0.05%, S≤0.006%, Cr: 13.0%–15.0%, Ni: 1.0%–1.8%, Cu: 0.75%–1.0%, N: 0.10%–0.20%, with the balance being Fe and unavoidable impurities.

[0018] The chemical composition of CJ5L is as follows: C: 0.11%–0.17%, Si: 0.27%–0.70%, Mn: 9.0%–10.5%, P≤0.05%, S≤0.006%, Cr: 13.0%–14.0%, Ni: 1.0%–1.5%, Cu: 0.1%–0.5%, N: 0.10%–0.17%, with the balance being Fe and unavoidable impurities.

[0019] In this invention, the heat treatment method is applicable to the production of 200 series stainless steel white coils with steel grades CJ5L and CJ1L.

[0020] In the above heat treatment method, for 200 series stainless steel strip with steel grade CJ1L, the process speed is controlled at ≤100m / min; for 200 series stainless steel strip with steel grade CJ5L, the process speed is controlled at ≤90m / min.

[0021] In the above heat treatment method, the temperature of each zone of the annealing furnace is based on the heating curve of 200 series stainless steel (e.g., Figure 1 The furnace temperatures of the preheating section and the soaking section are preferably set according to the thickness of the 200 series stainless steel.

[0022] In the above heat treatment method, preferably, for 200 series stainless steel strip with CJ1L grade and a thickness of less than 2.4 mm, the furnace temperature of the preheating section is set to 525-645℃ (e.g., 530℃, 550℃, 580℃, 600℃, 620℃, 640℃); the furnace temperature of furnace chamber 1 is set to 950-985℃ (e.g., 955℃, 960℃, 970℃, 980℃). The furnace temperature in furnace zone 2 is set at 1050–1095℃ (e.g., 1055℃, 1060℃, 1070℃, 1080℃, 1090℃), the furnace temperature in furnace zone 3 is set at 1150–1170℃ (e.g., 1155℃, 1160℃, 1165℃), and the furnace temperature in furnace zone 4 is set at 1125–1140℃ (e.g., 1130℃, 1135℃).

[0023] For 200 series stainless steel strip with CJ5L grade and a thickness of less than 2.4mm, the furnace temperature in the preheating section is set to 595–610℃ (e.g., 598℃, 600℃, 605℃, 608℃); the furnace temperature in furnace zone 1 is set to 1045–1060℃ (e.g., 1048℃, 1050℃, 1055℃, 1058℃); the furnace temperature in furnace zone 2 is set to 1090–1130℃ (e.g., 1095℃, 1100℃, 1120℃, 1125℃); the furnace temperature in furnace zone 3 is set to 1175–1190℃ (e.g., 1180℃, 1185℃); and the furnace temperature in furnace zone 4 is set to 1140–1160℃ (e.g., 1145℃, 1150℃, 1155℃).

[0024] In the above heat treatment method, as a preferred embodiment, the stainless steel strip is heated to a temperature of 1130-1170°C and then held at that temperature for 70-120 seconds.

[0025] In the above heat treatment method, as a preferred embodiment, the thickness of the stainless steel strip is less than 2.4 mm.

[0026] In the above heat treatment method, as a preferred embodiment, the air-fuel ratio is 10.5% for chamber 1, 11.0% for chamber 2, 11.0% for chamber 3, and 11.5% for chamber 4. Here, the air-fuel ratio is the mass ratio between air and fuel in the annealing furnace.

[0027] In this invention, the annealing furnace is divided into a heating chamber (corresponding to the preheating section) and a soaking chamber (corresponding to the soaking section), which can achieve the best annealing effect and better eliminate rolling stress, grain formation and growth to obtain the required microstructure and properties.

[0028] The reason for dividing it into four chambers is that the strip steel is gradually heated from the preheating section to chamber 1 to chamber 4, and the temperature is controlled according to the heating curve; dividing it into several heating chambers makes it easier to control the temperature, furnace pressure, and oxygen content.

[0029] This invention divides the soaking zone into four chambers. The strip steel is gradually heated from the preheating zone to soaking chamber 1 and then to chamber 4. Each chamber is separated by support rollers to ensure the uniformity of annealing, reduce the sag of the strip steel, and enhance the production stability of the annealing furnace. Furthermore, the strip steel moves continuously within the furnace supported by these rollers; this arrangement effectively reduces the nitrogen oxide content in the flue gas after combustion. In addition, dividing the soaking zone into four chambers facilitates control of furnace temperature, furnace pressure, and oxygen content.

[0030] In this invention, the purpose of controlling the furnace pressure is to reduce heat loss, increase annealing efficiency, thereby improving grain size and keeping it within the target grade range. In actual production, the optimal furnace pressure is determined by comparing the relationship between various furnace pressures and combustion efficiencies.

[0031] In the above heat treatment method, as a preferred embodiment, the air-fuel ratio is controlled by increasing the gas flow rate by closing the burner.

[0032] In the above heat treatment method, as a preferred embodiment, the pressure of the combustion air in chambers 1-4 is controlled at 7.25 kPa to 7.35 kPa (e.g., 7.28 kPa, 7.30 kPa, 7.32 kPa).

[0033] In this invention, production is carried out by increasing the gas flow rate by closing the burner, and by reasonably setting the air-fuel ratio, furnace pressure and combustion air pressure of each furnace chamber, the plate temperature (solution temperature of stainless steel strip) is ensured to be stable and the plate temperature fluctuation is reduced.

[0034] In the above heat treatment method, as a preferred embodiment, the flue gas generated by the annealing furnace is extracted from the end of the high-temperature zone of the annealing furnace towards the preheating section to achieve heat reuse and reduce unit consumption.

[0035] This invention, through the design of the temperatures in each of the above-mentioned furnace zones, increases the temperature gradient in each of the three chambers of the furnace zone, enabling the reuse of heat. Adjusting the furnace pressure slows down the emission of flue gas, thereby retaining the heat of the flue gas for reuse. Increasing the combustion partial pressure can improve the flame intensity and increase thermal energy, thereby reducing consumption.

[0036] This invention achieves a 200 series stainless steel white sheet coil (NO.1) with grain size controlled within the range of 6.5 to 7.0 by controlling TV value, annealing temperature and reducing heat loss, while reducing unit consumption.

[0037] In the above heat treatment method, as a preferred embodiment, the cooling process sequentially employs a first-stage air cooling, a three-stage air mist cooling, and a first-stage water cooling to prevent the workpiece from becoming brittle due to excessive thermal stress. Specifically, during cooling, the workpiece is first cooled with air to approximately 700°C (690°C–710°C, for example, 695°C, 700°C, 705°C), then cooled with a mixture of air and water mist to approximately 350°C (340°C–360°C, for example, 345°C, 350°C, 355°C), and finally directly cooled with water to below 80°C to complete the cooling process.

[0038] In this invention, to prevent strip deformation during cooling, air cooling and water cooling are used to control the strip shape. At the same time, the cooling parameters of each section are adjusted (e.g., air cooling, air and water mist mixed cooling, and then direct water cooling process parameters) to better extend grain growth under low TV and high plate temperature to ensure that the grain size is between 6.5 and 7.0.

[0039] In the solution treatment process of 200 series stainless steel, if the plate temperature is 1130±20℃, the furnace pressure in chambers 2-3 is +5Pa, and the combustion air pressure in chambers 1-4 is 4.0KPa, the resulting 200 series stainless steel will have a fine grain size. During subsequent cold rolling, the grains are not easily broken, requiring greater rolling force to achieve the target thickness. This leads to a low rolling reduction rate and a tendency for brittle fracture, resulting in increased costs. Furthermore, this technology is energy-intensive, produces a harder material after annealing, and the grain size does not meet the standards.

[0040] The technical solution of this invention can obtain 200 series stainless steel with larger grain size, which is convenient for subsequent cold rolling processing. Therefore, it solves the problem of small grain size of 200 series stainless steel, reduces scrap loss, and ensures the needs of subsequent cold rolling processing of products.

[0041] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0042] 1. For the first time, a method combining limited TV and high-temperature solid solution is adopted. During cooling, a method of one-stage air cooling, three-stage air mist cooling and one-stage water cooling is used to avoid the workpiece from brittle fracture due to excessive thermal stress.

[0043] 2. The heat treatment process of this invention can prevent the material from having underdeveloped grains, resulting in fine grains;

[0044] 3. The 200 series stainless steel products produced according to the heat treatment process of this invention have a grain size guaranteed to be 6.5 to 7.0 grade, and the product qualification rate reaches more than 95%. The 200 series stainless steel products are easy to roll and have good performance in cold rolling production. Attached Figure Description

[0045] Figure 1 This is the heating curve of the 200 series stainless steel material in this invention.

[0046] Figure 2 The metallographic structure of the 200 series stainless steel prepared in Example 1 of the present invention is shown.

[0047] Figure 3 The metallographic structure of 200 series stainless steel prepared in Example 2 of the present invention.

[0048] Figure 4 The metallographic structure of the 200 series stainless steel prepared in Comparative Example 1 of this invention is shown.

[0049] Figure 5 The metallographic structure of the 200 series stainless steel prepared in Comparative Example 2 of this invention is shown. Detailed Implementation

[0050] The present invention will now be described in detail with reference to embodiments thereof. Various examples are provided by way of explanation and not by way of limitation. Indeed, those skilled in the art will recognize that modifications and variations may be made to the invention without departing from its scope or spirit. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the present invention encompass such modifications and variations falling within the scope of the appended claims and their equivalents.

[0051] Example 1

[0052] A heat treatment method for improving the grain size of 200 series stainless steel uses CJ5L black steel coils with a specification of 2.2mm*1245mm (thickness*width) as raw material. The chemical composition by mass percentage includes: C: 0.17%, Si: 0.47%, Mn: 9.74%, P: 0.049%, S: 0.002%, Cr: 13.28%, Ni: 1.14%, Cu: 0.233%, N: 0.14%. The material is processed on a continuous rolling, annealing, pickling, and solution treatment production line (RAPL production line). The rolled raw material has a specification of 1.5mm*1245mm. The rolled black steel coils are then heat-treated. The heat treatment method includes the following steps:

[0053] Solution treatment steps:

[0054] The rolled steel coil is fed into a horizontal annealing furnace at a uniform speed. After preheating and heating treatment, the strip is heated to 1131℃ and then homogenized at this temperature for 80 seconds. The TV value of the production line and the parameters of the horizontal annealing furnace are set as follows:

[0055] The speed of the process section is controlled below 80m / min, so that the production TV is controlled within 120m / min*mm;

[0056] The temperatures of each zone in the annealing furnace are set according to the heating curve of 200 series stainless steel. Specifically, the temperature of the preheating zone is controlled at 602℃, the temperature of zone 1 in the soaking zone is controlled at 1050℃, the temperature of zone 1 in the soaking zone is controlled at 1052℃, the temperature of zone 2 in the soaking zone is controlled at 1090℃, the temperature of zone 2 in the soaking zone is controlled at 1124℃, the temperature of zone 3 in the soaking zone is controlled at 1180℃, the temperature of zone 3 in the soaking zone is controlled at 1183℃, the temperature of zone 4 in the soaking zone is controlled at 1150℃, and the temperature of zone 4 in the soaking zone is controlled at 1145℃.

[0057] The air-fuel ratio settings for the furnace area are as follows: 10.5% for chamber 1, 11.0% for chambers 2 and 3, and 11.5% for chamber 4.

[0058] The furnace pressure settings for the furnace area are as follows: 4.0 Pa for chamber 1, 10 Pa for chambers 2 and 3, and 8.0 Pa for chamber 4.

[0059] The combustion air pressure setting for the furnace area is as follows: the combustion air pressure in chambers 1-4 is 7.3 kPa;

[0060] Cooling steps:

[0061] The cooling process employs a combination of air cooling, three-stage air-mist cooling, and a single-stage water cooling. Specifically, during cooling, the air is first cooled to approximately 700°C, then the air is mixed with water mist and cooled to approximately 350°C, and finally, the water is directly cooled to below 80°C to complete the cooling process.

[0062] The mechanical properties and grain size of the stainless steel produced according to the above heat treatment process are shown in Table 1, and the metallographic structure is as follows: Figure 2 As shown.

[0063] Depend on Figure 2 It can be seen that the grain size of the 200 series stainless steel prepared in this embodiment is grade 6.8.

[0064] Table 1. Mechanical properties and grain size of the 200 series stainless steel after heat treatment in Example 1.

[0065]

[0066] Following the heat treatment process for improving the grain size of 200 series stainless steel in this embodiment, multiple batches of 200 series stainless steel have met the requirements for both performance and grain size testing, and have been successfully delivered. Furthermore, the heat-treated 200 series stainless steel in this embodiment is easy to process in cold rolling production and exhibits good rolling performance.

[0067] Example 2

[0068] A heat treatment method for improving the grain size of 200 series stainless steel uses CJ1L black steel coils with a specification of 2.0mm*1245mm (thickness*width) as raw material. The coils are processed on a continuous rolling, annealing, pickling, and solution treatment production line (RAPL line). The rolled raw material has a specification of 1.3mm*1245mm and a chemical composition by mass percentage: C: 0.08%, Si: 0.56%, Mn: 9.9%, P: 0.049%, S: 0.002%, Cr: 13.56%, Ni: 1.24%, Cu: 0.822%, N: 0.12%. The rolled CJ1L black steel coils are then subjected to heat treatment, which includes the following steps:

[0069] Solution treatment steps:

[0070] The rolled steel coil is fed into a horizontal annealing furnace at a uniform speed. After preheating and heating treatment, the strip is heated to 1130℃ and then homogenized at this temperature for 75 seconds. The TV value of the production line and the parameters of the horizontal annealing furnace are set as follows:

[0071] The speed of the process section is controlled below 90m / min, so that the production TV is controlled within 120m / min*mm;

[0072] The temperatures of each zone in the annealing furnace are set according to the heating curve of 200 series stainless steel. Specifically, the temperature of the preheating section is controlled at 537℃, the temperature of zone 1 is controlled at 954℃, the temperature of zone 1 is controlled at 983℃, the temperature of zone 2 is controlled at 1056℃, the temperature of zone 2 is controlled at 1091℃, the temperature of zone 3 is controlled at 1165℃, the temperature of zone 3 is controlled at 1153℃, the temperature of zone 4 is controlled at 1132℃, and the temperature of zone 4 is controlled at 1128℃.

[0073] The air-fuel ratio settings for the furnace area are as follows: 10.5% for chamber 1, 11.0% for chambers 2 and 3, and 11.5% for chamber 4.

[0074] The furnace pressure settings for the furnace area are as follows: 4.0 Pa for chamber 1, 10 Pa for chambers 2 and 3, and 8.0 Pa for chamber 4.

[0075] The combustion air pressure setting for the furnace area is as follows: the combustion air pressure in chambers 1-4 is 7.3 kPa;

[0076] Cooling steps:

[0077] The cooling process employs a combination of air cooling, three-stage air-mist cooling, and a single-stage water cooling. Specifically, during cooling, the air is first cooled to approximately 700°C, then the air is mixed with water mist and cooled to approximately 350°C, and finally, the water is directly cooled to below 80°C to complete the cooling process.

[0078] After production according to the above heat treatment process, the test results are shown in Table 2. The metallographic structure is as follows: Figure 3 As shown. By Figure 3 It can be seen that the grain size of the 200 series stainless steel prepared in this embodiment is grade 6.9.

[0079] Table 2. Mechanical properties and grain size of the 200 series stainless steel after heat treatment in Example 2.

[0080]

[0081] Following the heat treatment process for improving the grain size of 200 series stainless steel in this embodiment, multiple batches of 200 series stainless steel have met the requirements for performance and grain size testing, and have been successfully delivered. Furthermore, the heat-treated 200 series stainless steel in this embodiment is easy to process in cold rolling production and exhibits good rolling performance.

[0082] Comparative Example 1

[0083] A heat treatment method for 200 series stainless steel uses CJ5L black steel coils with a specification of 2.2mm*1245mm as raw material, and performs heat treatment on the rolled CJ5L black steel coils. The specification of the raw material after rolling is 1.5mm*1245mm. The heat treatment method includes the following steps:

[0084] Solution treatment steps:

[0085] The rolled steel coil is fed into a horizontal annealing furnace at a uniform speed. After preheating and heating treatment, the strip is heated to 1115℃ and then homogenized at this temperature for 70 seconds. The TV value of the production line and the parameters of the horizontal annealing furnace are set as follows:

[0086] The process speed is 85m / min, and the TV production rate is 127m / min*mm.

[0087] The temperature of the preheating section of the annealing furnace is controlled at 578℃; the temperature of the first chamber of the soaking zone is controlled at 1022℃; the temperature of the first chamber of the second zone is controlled at 1023℃; the temperature of the second chamber of the first zone is controlled at 1037℃; the temperature of the second chamber of the second zone is controlled at 1082℃; the temperature of the third chamber of the first zone is controlled at 1137℃; the temperature of the third chamber of the second zone is controlled at 1140℃; the temperature of the fourth chamber of the first zone is controlled at 1122℃; and the temperature of the fourth chamber of the second zone is controlled at 1117℃.

[0088] The air-fuel ratio settings for the furnace area are as follows: 11.0% for chamber 1, 14.0% for chambers 2 and 3, and 11.5% for chamber 4.

[0089] The furnace pressure settings for the furnace area are as follows: the furnace pressure in chamber 1 is -2.0 Pa (the parameter used in actual production), the furnace pressure in chambers 2 and 3 is 9.0 Pa, and the furnace pressure in chamber 4 is 6.0 Pa.

[0090] The combustion air pressure setting for the furnace area is as follows: the combustion air pressure in chambers 1-4 is 4.0 kPa;

[0091] Cooling steps:

[0092] The cooling process employs a two-stage air cooling, a two-stage air mist cooling, and a one-stage water cooling method. Specifically, during cooling, the air is first cooled to about 700°C, then the air is mixed with water mist and cooled to about 350°C, and finally the water is directly cooled to below 80°C to complete the cooling.

[0093] Table 3 shows the mechanical properties and grain size of the heat-treated stainless steel in Comparative Example 1.

[0094]

[0095] After production according to the above heat treatment process, the test results are shown in Table 3, and the metallographic structure is shown in the attached figure. Figure 4 As shown. By Figure 4 It can be seen that the 200 series stainless steel prepared in this embodiment has a grain size of 7.2, which is not easy to roll in cold rolling production and has relatively poor rolling performance.

[0096] Comparative Example 2

[0097] A heat treatment method for 200 series stainless steel (black steel coil of grade CJ5L) differs from Example 1 in that the process parameters for the high-temperature solution treatment are different, while the other steps are the same as in Example 1. Specifically,

[0098] Solution treatment steps:

[0099] The rolled steel coil is fed into a horizontal annealing furnace at a uniform speed. After preheating and heating treatment, the strip is heated to 1115℃ and then homogenized at this temperature for 80 seconds. The TV value of the production line and the parameters of the horizontal annealing furnace are set as follows:

[0100] The speed of the process section is controlled below 80m / min, so that the production TV is controlled within 120m / min*mm;

[0101] The temperature of the preheating section of the annealing furnace is controlled at 602℃; the temperature of Zone 1, Chamber 1 is controlled at 1022℃; the temperature of Zone 2, Chamber 1 is controlled at 1068℃; the temperature of Zone 1, Chamber 2 is controlled at 1037℃; the temperature of Zone 2, Chamber 2 is controlled at 1098℃; the temperature of Zone 1, Chamber 3 is controlled at 1159℃; the temperature of Zone 2, Chamber 3 is controlled at 1156℃; the temperature of Zone 1, Chamber 4 is controlled at 1122℃; and the temperature of Zone 2, Chamber 4 is controlled at 1117℃.

[0102] The air-fuel ratio settings for the furnace area are as follows: 11.0% for chamber 1, 14.0% for chambers 2 and 3, and 11.5% for chamber 4.

[0103] The furnace pressure settings for the furnace area are as follows: Chamber 1 furnace pressure is -2.0 Pa, Chambers 2 and 3 furnace pressure is 9.0 Pa, and Chamber 4 furnace pressure is 6.0 Pa.

[0104] The combustion air pressure setting for the furnace area is as follows: the combustion air pressure in chambers 1-4 is 4.0 kPa.

[0105] The test results of the 200 series stainless steel produced using the heat treatment method of this comparative example are shown in Table 4. The metallographic structure is as follows: Figure 5 As shown. By Figure 5 It is known that the grain size of 200 series stainless steel is grade 7.4, which makes it difficult to roll in cold rolling production and results in relatively poor rolling performance.

[0106] Table 4 shows the mechanical properties and grain size of the heat-treated stainless steel in Comparative Example 2.

[0107]

Claims

1. A heat treatment method for improving the grain size of 200 series stainless steel, characterized in that, The heat treatment method includes: The solution treatment step involves using a horizontal annealing furnace to perform solution treatment on the rolled stainless steel strip. The stainless steel strip is continuously fed into the horizontal annealing furnace and heated to 1130~1170℃, and held at this temperature. The cooling step involves cooling the solution-treated stainless steel strip. After heating the stainless steel strip to 1130~1170℃, hold it at that temperature for 70~120 seconds. The thickness of the stainless steel strip is less than 2.4 mm; the temperature of each zone of the annealing furnace is set according to the heating curve of 200 series stainless steel. The annealing furnace includes a preheating section and a soaking section. The soaking section has four chambers: chamber 1, chamber 2, chamber 3, and chamber 4. The furnace pressure of each chamber is optimized, and the furnace pressure parameters of each chamber in the soaking section are controlled as follows: the furnace pressure of chamber 1 is controlled at 2-4 Pa; the furnace pressures of chambers 2 and 3 are controlled at 10-12 Pa; and the furnace pressure of chamber 4 is controlled at 6-8 Pa. The combustion air pressure in each furnace chamber was optimized. The combustion air pressure parameters in each furnace chamber of the soaking section were controlled as follows: the combustion air pressure in chambers 1-4 was controlled at 7~8 kPa respectively. The air-fuel ratio parameters for each chamber in the soaking tank are controlled as follows: Chamber 1: 10.3%~10.7%; Chamber 2: 10.8%~11.2%; Chamber 3: 10.8%~11.2%; Chamber 4: 11.3%~11.7%. In the cooling process, the cooling is carried out in sequence using a first-stage air cooling, a third-stage air mist cooling, and a first-stage water cooling. During the cooling process, the air is first cooled to 680°C~720°C, then the air is mixed with water mist and cooled to 320°C~380°C, and then directly cooled to 80°C to complete the cooling. The horizontal annealing furnace is located in the solution treatment section of the continuous rolling annealing pickling solution production line. For 200 series stainless steel strips with a thickness of 2.4mm or less, the process section speed is controlled at ≤100m / min so that the TV value of the solution treatment section of the production line is controlled at ≤120 m / min*mm. The TV value of the heat treatment section refers to the product of the strip thickness and the strip speed in the process section. The process section speed is the running speed of the strip during heat treatment.

2. The heat treatment method according to claim 1, characterized in that, The 200 series stainless steel includes CJ1L and CJ5L, wherein, by mass percentage, The chemical composition of CJ1L is as follows: C: 0.07%~0.10%, Si: 0.3%~0.7%, Mn: 9.0%~10.5%, P≤0.05%, S≤0.006%, Cr: 13.0%~15.0%, Ni: 1.0%~1.8%, Cu: 0.75%~1.0%, N: 0.10%~0.20%, with the balance being Fe and unavoidable impurities; The chemical composition of CJ5L is as follows: C: 0.11%~0.17%, Si: 0.27%~0.70%, Mn: 9.0%~10.5%, P≤0.05%, S≤0.006%, Cr: 13.0%~14.0%, Ni: 1.0%~1.5%, Cu: 0.1%~0.5%, N: 0.10%~0.17%, with the balance being Fe and unavoidable impurities.

3. The heat treatment method according to claim 1, characterized in that, The furnace temperatures of the preheating and soaking sections are set according to the thickness of the 200 series stainless steel.

4. The heat treatment method according to claim 1, characterized in that, For 200 series stainless steel strip with CJ1L grade and a thickness of less than 2.4mm, the furnace temperature in the preheating section is set to 525~645℃; the furnace temperature in furnace zone 1 is set to 950~985℃, the furnace temperature in furnace zone 2 is set to 1050~1095℃, the furnace temperature in furnace zone 3 is set to 1150~1170℃, and the furnace temperature in furnace zone 4 is set to 1125~1140℃. For 200 series stainless steel strip with CJ5L grade and a thickness of less than 2.4mm, the furnace temperature in the preheating section is set to 595~610℃; the furnace temperature in furnace zone 1 is set to 1045~1060℃, the furnace temperature in furnace zone 2 is set to 1090~1130℃, the furnace temperature in furnace zone 3 is set to 1175~1190℃, and the furnace temperature in furnace zone 4 is set to 1140~1160℃.

5. The heat treatment method according to claim 1, characterized in that, The air-fuel ratio is 10.5% for 1-chamber, 11.0% for 2-chamber, 11.0% for 3-chamber, and 11.5% for 4-chamber.

6. The heat treatment method according to claim 1, characterized in that, The air-fuel ratio is controlled by increasing the gas flow rate by shutting off the burners.

7. The heat treatment method according to claim 1, characterized in that, The pressure of the combustion air in chambers 1-4 is controlled at 7.25 kPa to 7.35 kPa, respectively.

8. The heat treatment method according to claim 1, characterized in that, In the heat treatment method, the flue gas generated by the annealing furnace is extracted from the end of the high-temperature zone of the annealing furnace towards the preheating section.

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