Method for improving performance uniformity of medium-low chromium ferritic stainless steel

Abstract

The application discloses a method for improving the performance uniformity of medium-low chromium ferrite stainless steel, which comprises the following steps: controlling the carbon and nitrogen content of the medium-low chromium ferrite stainless steel as C<=0.045% and N<=0.045%; controlling the temperature of the heating furnace soaking section as 1150-1250 DEG C, the rough rolling inlet temperature as 1050-1200 DEG C, the finish rolling final rolling temperature as 900-1000 DEG C, not using the laminar cooling after the finish rolling, and the coiling temperature as >=650 DEG C; setting the heat preservation facilities beside the coiling unit, making the steel coil enter the heat preservation facilities after coiling, and controlling the time from the completion of coiling to the entering of the heat preservation facilities as <=10 min; and controlling the heat preservation time of the steel coil after entering the heat preservation facilities as 24-96 h. The application reduces the yield strength of the medium-low chromium ferrite stainless steel, improves the elongation after fracture of the medium-low chromium ferrite stainless steel, controls the hardness difference between the head and tail of the steel coil and the middle part of the steel coil as <=2, and thus improves the performance of the hot-rolled steel coil of the medium-low chromium ferrite stainless steel and improves the performance uniformity.

Description

Technical Field

[0001] This invention belongs to the field of ferritic stainless steel production technology, and more specifically relates to a method for improving the uniformity of properties of medium and low chromium ferritic stainless steel. Background Technology

[0002] Medium-low chromium ferritic stainless steel generally refers to ferritic stainless steel with a chromium content of 11% to 18%. This type of stainless steel, due to the absence of precious metal elements such as Ni and Mo, and a relatively low chromium content, while possessing good mechanical and corrosion resistance, is widely used in home appliances, hardware, products, and decoration. In recent years, market competition has intensified, and some companies have developed a process for directly cold-rolling hot-rolled coils without annealing, significantly reducing production costs and capturing some market share. Although the "direct rolling of hot-rolled coils" process has a significant cost advantage, the lack of annealing results in poor coil performance, characterized by high strength, high hardness, and low elongation, making it prone to edge cracking and strip breakage during cold rolling. Meanwhile, since the temperature of the steel coil is still relatively high after coiling, the inner and outer parts of the steel coil are in contact with the air and cool quickly during the subsequent natural cooling process. However, the cooling rate of the middle part of the steel coil is slow. The effect of cooling rate causes the performance difference between the head, tail and middle of the steel coil. This leads to drastic fluctuations in the rolling load of the rolling mill during cold rolling production, which necessitates a reduction in the rolling speed and has a significant impact on production efficiency. Moreover, due to the heritability of performance, the fluctuation of the raw material coil performance will also cause the inhomogeneity of the final product performance.

[0003] Therefore, how to further improve the performance and uniformity of hot-rolled steel coils, thereby enhancing the performance uniformity of low-chromium ferritic stainless steel products, and making them more suitable for the high-quality application and development of the "black skin coil direct rolling" process, is an urgent problem that stainless steel enterprises need to solve. Summary of the Invention

[0004] To address the technical problems existing in the prior art, the present invention provides a method for improving the uniformity of properties of medium- and low-chromium ferritic stainless steel, comprising:

[0005] (1) Alloy composition optimization

[0006] The mass percentage content of carbon and nitrogen in the medium-low chromium ferritic stainless steel is controlled to be C≤0.045% and N≤0.045%;

[0007] (2) Temperature control of hot rolling line

[0008] The temperature of the soaking zone of the heating furnace is controlled at 1150℃~1250℃, the entry temperature of the roughing mill is controlled at 1050℃~1200℃, the finishing mill temperature is controlled at 900℃~1000℃, laminar flow cooling is not used after finishing milling, and the coiling temperature is controlled at ≥650℃.

[0009] (3) Slow cooling and heat preservation treatment of steel coils after coiling

[0010] Above-ground or underground insulation facilities are installed next to the coiling unit. The walls around the insulation facilities are covered with insulation and heat insulation materials. After the steel coil is coiled, it enters the insulation facilities. The time from the completion of coiling to entering the insulation facilities is controlled to be ≤10 minutes. The insulation time after the steel coil enters the insulation facilities is controlled to be 24h to 96h.

[0011] Furthermore, in the above-mentioned method for improving the uniformity of properties of medium and low chromium ferritic stainless steel, a temperature compensation device is added inside the insulation facility, and the compensation temperature of the temperature compensation device is controlled to not exceed 200°C.

[0012] Furthermore, in the above-mentioned method for improving the uniformity of properties of medium and low chromium ferritic stainless steel, the heat preservation facility is a ground-level heat preservation cover installed next to the coiling unit. The heat preservation cover is made of steel plate, and the surrounding steel plates of the heat preservation cover are covered with heat preservation and heat insulation materials.

[0013] Furthermore, in the above-mentioned method for improving the uniformity of properties of medium and low chromium ferritic stainless steel, the insulation facility is an underground insulation pit installed next to the coiling unit, and the walls of the insulation pit are equipped with insulation and heat insulation materials.

[0014] Furthermore, in the above-mentioned method for improving the uniformity of properties of low-chromium ferritic stainless steel, the chemical composition of the low-chromium ferritic stainless steel by mass percentage is: C≤0.045%, Si≤0.75%, Mn≤0.75%, P≤0.040%, S≤0.040%, Cr 11.0%~18.0%, N≤0.045%, with the remainder being Fe and unavoidable impurities.

[0015] Furthermore, in the above-mentioned method for improving the uniformity of properties of medium-low chromium ferritic stainless steel, the difference between the Rockwell hardness of the beginning and end of the medium-low chromium ferritic stainless steel coil and the Rockwell hardness of the middle of the coil is ≤2.

[0016] The method of the present invention for improving the uniformity of properties of medium and low chromium ferritic stainless steel has the following advantages and beneficial effects:

[0017] Through techniques such as alloy composition optimization, hot rolling line temperature control, and slow cooling and heat preservation treatment of coils after coiling, the yield strength of medium-low chromium ferritic stainless steel has been reduced by 10MPa to 30MPa, while the elongation after fracture has been increased by 1% to 4%. Furthermore, the difference between the Rockwell hardness HRB at the beginning and end of the coil and the Rockwell hardness HRB in the middle of the coil is controlled to ≤2. This effectively reduces the material strength and hardness, improves the performance of hot-rolled coils of medium-low chromium ferritic stainless steel, and enhances its performance uniformity, making it more suitable for the "black skin coil direct rolling" process, thereby improving the performance uniformity of the final product of medium-low chromium ferritic stainless steel.

[0018] When the medium-low chromium ferritic stainless steel treated by this invention is applied to the "black coil direct rolling" process, the rolling load of the rolling mill is uniform during the cold rolling process, and there are no rolling fluctuations or abnormal speed reduction problems in the production process. No production problems such as edge cracks or strip breakage occur, which can effectively improve production efficiency, reduce product quality defects, and further reduce production and manufacturing costs. It is suitable for and promotes the high-quality application and development of the "black coil direct rolling" process. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0020] The method of improving the performance uniformity of low-chromium ferritic stainless steel of the present invention reduces the strength and hardness of the material through technical measures such as alloy composition optimization, hot rolling line temperature control, and slow cooling and heat preservation treatment of the coil after coiling. This improves the performance of hot-rolled steel coils of low-chromium ferritic stainless steel and enhances the performance uniformity, making them more suitable for the "black skin coil direct rolling" process. This improves the performance uniformity of low-chromium ferritic stainless steel products and effectively increases production efficiency.

[0021] Specifically, the method of the present invention for improving the uniformity of properties of low-chromium ferritic stainless steel includes:

[0022] (1) Alloy composition optimization

[0023] Carbon and nitrogen are austenite-forming elements that can significantly increase the proportion of the austenite phase region, reduce the ferrite-austenite phase transformation temperature, and improve the hardenability of steel. However, as the carbon and nitrogen content in steel increases, the strength and hardness increase, while the plasticity and toughness decrease. Therefore, in this invention, the carbon and nitrogen content is further reduced, and the mass percentage content of carbon and nitrogen is controlled to be ≤0.045%.

[0024] Specifically, in the method for improving the uniformity of properties of low-chromium ferritic stainless steel of the present invention, the chemical composition of the low-chromium ferritic stainless steel by mass percentage is: C≤0.045%, Si≤0.75%, Mn≤0.75%, P≤0.040%, S≤0.040%, Cr 11.0%~18.0%, N≤0.045%, with the remainder being Fe and unavoidable impurities.

[0025] (2) Temperature control of hot rolling line

[0026] Since hot-rolled steel coils are directly coiled and cooled to room temperature without annealing, the properties of the resulting coil are its final properties. To improve the properties of the steel coil, it is necessary to optimize the coiling temperature. The cooling process after coiling no longer involves deformation and can be considered a static recovery and recrystallization process. Increasing the temperature and extending the holding time can effectively promote recovery and recrystallization. Therefore, increasing the coiling temperature and extending the time the steel coil spends in the high-temperature range can improve the material's properties, namely, reducing strength and hardness while increasing elongation after fracture.

[0027] Based on the above analysis, the present invention optimizes the temperature control of the hot rolling line, specifically: the temperature of the soaking zone of the heating furnace is controlled at 1150℃~1250℃, the roughing mill inlet temperature is controlled at 1050℃~1200℃, the finishing mill final rolling temperature is controlled at 900℃~1000℃, laminar flow cooling is not used after finishing milling, and the coiling temperature is controlled at ≥650℃.

[0028] (3) Slow cooling and heat preservation treatment of steel coils after coiling

[0029] Increasing the temperature of a steel coil can improve its overall performance to some extent. However, if the coil continues to cool naturally after winding, the problem of faster cooling at the coil's head and tail, and slower cooling in the middle, remains unresolved. This means the coil will still exhibit higher strength and hardness at the head and tail compared to the middle. To address this issue, the cooling rate at the coil's head and tail needs to be reduced to minimize the temperature difference between the inside and outside of the coil.

[0030] In view of this, the present invention adopts a measure of slow cooling and heat preservation treatment of steel coils after winding. Specifically, above-ground or underground heat preservation facilities are set up next to the winding unit. The walls around the heat preservation facilities are equipped with heat preservation and heat insulation materials. After the steel coil is wound, it enters the heat preservation facilities, and the time from the completion of winding to entering the heat preservation facilities is controlled to be ≤10 minutes to avoid excessive temperature drop of the steel coil due to heat loss. The heat preservation time of the steel coil after entering the heat preservation facilities is controlled to be 24h to 96h.

[0031] As an optional implementation, a temperature compensation device can be added to the insulation facility according to actual production conditions. To avoid a significant increase in costs, the compensation temperature of the temperature compensation device is controlled to not exceed 200°C.

[0032] In one specific implementation, the aforementioned insulation facility is a ground-level insulation cover installed next to the winding unit. The insulation cover is made of steel plate, and the steel plate around the insulation cover is covered with insulation and heat-insulating materials.

[0033] In one specific implementation, the aforementioned insulation facility is an underground insulation pit installed next to the winding unit, with insulation and heat-insulating materials installed on the surrounding walls of the insulation pit.

[0034] The following describes in detail the method of improving the uniformity of properties of medium- and low-chromium ferritic stainless steel, with reference to specific embodiments of the present invention and comparative examples of the prior art.

[0035] Example 1

[0036] The chemical composition of the medium chromium ferritic stainless steel used in Example 1, by weight percentage, is C: 0.028%, Si: 0.28%, Mn: 0.26%, P: 0.032%, S: 0.001%, Cr: 18.0%, N: 0.032%, with the remainder being Fe and unavoidable impurities.

[0037] The temperature control of the hot rolling line in Example 1 is as follows: the temperature of the soaking zone of the heating furnace is 1250℃, the entry temperature of the roughing mill is 1200℃, the finishing mill finishing temperature is 1000℃, laminar flow cooling is not used after finishing milling, and the coiling temperature is 795℃.

[0038] The slow cooling and heat preservation treatment of the steel coil after winding in Example 1 is controlled as follows: a ground-based heat preservation facility is set up next to the winding unit. The heat preservation facility is a ground-based heat preservation cover made of steel plate. The steel plates around the heat preservation cover are covered with heat preservation and heat insulation materials. No temperature compensation device is added inside the heat preservation cover. After the steel coil is wound, it enters the heat preservation cover. A total of 20 steel coils are loaded into the heat preservation cover in one batch. The time from the completion of winding to the entry of each steel coil into the heat preservation cover is 4 min to 5 min, with an average of 4.2 min. The heat preservation time after the steel coil enters the heat preservation cover is 96 h. After 96 h of slow cooling and heat preservation, the temperature of the steel coil is 282℃ to 325℃, with an average of 306℃.

[0039] Comparative Example 1

[0040] The medium-chromium ferritic stainless steel used in Comparative Example 1 is the same as that used in Example 1, and the hot rolling line temperature of Comparative Example 1 is controlled in the same way as that of Example 1. However, the slow cooling and heat preservation treatment of the coiled steel was not performed in Comparative Example 1.

[0041] Actual testing showed that the average yield strength of the low-chromium ferritic stainless steel treated in Comparative Example 1 was 362 MPa, and the average elongation after fracture was 17%. The average yield strength of the low-chromium ferritic stainless steel treated in Example 1 was 345 MPa, and the average elongation after fracture was 19%. It can be seen that through Example 1 of this invention, the yield strength of the low-chromium ferritic stainless steel decreased by 17 MPa, while the elongation after fracture increased by 2%. Furthermore, testing showed that the average Rockwell hardness (HRB) of the low-chromium ferritic stainless steel coil treated in Example 1 was 82 at both ends, and 81 in the middle, indicating good uniformity in hardness.

[0042] Example 2

[0043] The chemical composition of the medium chromium ferritic stainless steel used in Example 2, by weight percentage, is C: 0.022%, Si: 0.25%, Mn: 0.25%, P: 0.028%, S: 0.001%, Cr: 11.0%, N: 0.018%, with the remainder being Fe and unavoidable impurities.

[0044] The hot rolling line temperature control in Example 2 is as follows: the temperature of the heating furnace soaking section is 1210℃, the roughing mill inlet temperature is 1170℃, the finishing mill final rolling temperature is 985℃, laminar flow cooling is not used after finishing milling, and the coiling temperature is 752℃.

[0045] The slow cooling and heat preservation treatment of the steel coil after winding in Example 2 is controlled as follows: an underground heat preservation facility is set up next to the winding unit. The heat preservation facility is a rectangular underground heat preservation pit. The walls of the heat preservation pit are covered with heat preservation and heat insulation materials. No temperature compensation device is added inside the heat preservation pit. After the steel coil is wound, it enters the heat preservation pit. A total of 25 steel coils are loaded into the heat preservation pit in one batch. The time from the completion of winding to the entry of each steel coil into the heat preservation pit is 3 min to 5 min, with an average of 3.5 min. The heat preservation time after the steel coil enters the heat preservation pit is 96 h. After 96 h of slow cooling and heat preservation, the temperature of the steel coil is 263℃ to 287℃, with an average of 274℃.

[0046] Comparative Example 2

[0047] The medium-chromium ferritic stainless steel used in Comparative Example 2 is the same as that used in Example 2, and the hot rolling line temperature of Comparative Example 2 is controlled in the same way as that of Example 2. However, the slow cooling and heat preservation treatment of the coiled steel was not performed in Comparative Example 2.

[0048] Actual testing showed that the average yield strength of the low-chromium ferritic stainless steel treated in Comparative Example 2 was 354 MPa, and the average elongation after fracture was 18%. The average yield strength of the low-chromium ferritic stainless steel treated in Example 2 was 332 MPa, and the average elongation after fracture was 21%. It can be seen that through Example 2 of this invention, the yield strength of the low-chromium ferritic stainless steel decreased by 22 MPa, while the elongation after fracture increased by 3%. Furthermore, testing revealed that the average Rockwell hardness (HRB) of the low-chromium ferritic stainless steel coil treated in Example 2 was 79 at both ends, and 78 in the middle, indicating good uniformity in hardness.

[0049] Example 3

[0050] The chemical composition of the medium chromium ferritic stainless steel used in Example 3, by weight percentage, is C: 0.022%, Si: 0.25%, Mn: 0.25%, P: 0.028%, S: 0.001%, Cr: 11.0%, N: 0.018%, with the remainder being Fe and unavoidable impurities.

[0051] The hot rolling line temperature control in Example 3 is as follows: heating furnace soaking zone temperature 1150℃, roughing mill inlet temperature 1050℃, finishing mill final rolling temperature 900℃, laminar flow cooling is not used after finishing mill, and coiling temperature 692℃.

[0052] The slow cooling and heat preservation treatment of the steel coil after winding in Example 3 is controlled as follows: an underground heat preservation facility is set up next to the winding unit. The heat preservation facility is a rectangular underground heat preservation pit. The walls of the heat preservation pit are equipped with heat preservation and heat insulation materials. No temperature compensation device is added inside the heat preservation pit. After the steel coil is wound, it enters the heat preservation pit. A total of 20 steel coils are loaded into the heat preservation pit in one batch. The time from the completion of winding to the entry of each steel coil into the heat preservation pit is 3 min to 5 min, with an average of 3.3 min. The heat preservation time after the steel coil enters the heat preservation pit is 24 h. After 24 h of slow cooling and heat preservation, the temperature of the steel coil is 465℃ to 506℃, with an average of 483℃.

[0053] Comparative Example 3

[0054] The medium-chromium ferritic stainless steel used in Comparative Example 3 is the same as that used in Example 3, and the hot rolling line temperature of Comparative Example 3 is controlled in the same way as that of Example 3. However, the slow cooling and heat preservation treatment of the coiled steel was not performed in Comparative Example 3.

[0055] Actual testing showed that the average yield strength of the low-chromium ferritic stainless steel treated in Comparative Example 3 was 365 MPa, and the average elongation after fracture was 18%. The average yield strength of the low-chromium ferritic stainless steel treated in Example 3 was 348 MPa, and the average elongation after fracture was 20%. It can be seen that through Example 3 of this invention, the yield strength of the low-chromium ferritic stainless steel decreased by 17 MPa, and the elongation after fracture increased by 2%. Furthermore, testing showed that the average Rockwell hardness (HRB) of the low-chromium ferritic stainless steel coil treated in Example 3 was 80 at both ends, and 78 in the middle of the coil, indicating good uniformity in hardness.

[0056] In summary, the main process parameters of Examples 1-3 of the present invention and Comparative Examples 1-3 of the prior art are summarized in Table 1 below, and the performance test results of the medium-low chromium ferritic stainless steel after treatment by Examples 1-3 of the present invention and Comparative Examples 1-3 of the prior art are summarized in Table 2 below.

[0057] Table 1. Process parameters for examples and comparative examples

[0058]

[0059] Table 2. Performance test results of the examples and comparative examples.

[0060] project Yield strength / MPa Elongation after fracture / % HRB at both ends of the steel coil HRB in the middle of the steel coil Example 1 345 19 82 81 Example 2 332 21 79 78 Example 3 348 20 80 78 Comparative Example 1 362 17 / / Comparative Example 2 354 18 / / Comparative Example 3 365 18 / /

[0061] In summary, the method of the present invention for improving the performance uniformity of low-chromium ferritic stainless steel, through alloy composition optimization, hot rolling line temperature control, and slow cooling and heat preservation treatment of the coil after coiling, reduces the yield strength of low-chromium ferritic stainless steel by 10MPa to 30MPa, increases the elongation after fracture by 1% to 4%, and controls the difference between the Rockwell hardness HRB at the beginning and end of the coil and the Rockwell hardness HRB in the middle of the coil to ≤2. This effectively reduces the material strength and hardness, improves the performance of hot-rolled coils of low-chromium ferritic stainless steel, and enhances its performance uniformity, making it more suitable for the "black skin coil direct rolling" process, thereby improving the performance uniformity of the final product of low-chromium ferritic stainless steel.

[0062] Furthermore, when the medium-low chromium ferritic stainless steel treated by this invention is applied to the "black coil direct rolling" process, the rolling load of the rolling mill is uniform during the cold rolling process, and there are no rolling fluctuations or abnormal speed reduction problems in the production process. No production problems such as edge cracks or strip breakage occur, which can effectively improve production efficiency, reduce product quality defects, and further reduce production and manufacturing costs. It is suitable for and promotes the high-quality application and development of the "black coil direct rolling" process.

[0063] It should be noted that, in this document, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device.

[0064] It should also be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for improving the uniformity of properties in low- and medium-chromium ferritic stainless steel, characterized in that, include: (1) Alloy composition optimization The chemical composition of the medium-low chromium ferritic stainless steel by mass percentage is as follows: C: 0.022%, Si: 0.25%, Mn: 0.25%, P: 0.028%, S: 0.001%, Cr: 11.0%, N: 0.018%, with the remainder being Fe and unavoidable impurities; (2) Temperature control of hot rolling line The temperature of the soaking zone of the heating furnace is controlled at 1150℃, the entry temperature of the roughing mill is controlled at 1050℃, the final rolling temperature of the finishing mill is controlled at 900℃, laminar flow cooling is not used after finishing milling, and the coiling temperature is controlled at 692℃. (3) Slow cooling and heat preservation treatment of steel coils after coiling Above-ground or underground insulation facilities are installed next to the coiling unit. The walls around the insulation facilities are covered with insulation and heat insulation materials. After the steel coil is coiled, it enters the insulation facilities. The time from the completion of coiling to the entry of the steel coil into the insulation facilities is controlled to be 3 to 5 minutes. After the steel coil enters the insulation facilities, the insulation time is controlled to be 24 hours. After 24 hours of slow cooling and insulation, the temperature of the steel coil is 465℃ to 506℃. The difference in Rockwell hardness between the beginning and end of the low-chromium ferritic stainless steel coil and the middle of the coil is ≤2.

2. The method for improving the uniformity of properties of low-chromium ferritic stainless steel as described in claim 1, characterized in that, A temperature compensation device is added to the insulation facility, and the compensation temperature of the temperature compensation device is controlled to not exceed 200℃.

3. The method for improving the uniformity of properties of medium- and low-chromium ferritic stainless steel as described in claim 1, characterized in that, The insulation facility is a ground-level insulation cover installed next to the winding unit. The insulation cover is made of steel plate, and the steel plate around the insulation cover is covered with insulation and heat insulation materials.

4. The method for improving the uniformity of properties of low-chromium ferritic stainless steel as described in claim 1, characterized in that, The insulation facility is an underground insulation pit located next to the winding unit, and the walls of the insulation pit are covered with insulation and heat insulation materials.

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