Cr13 type ferritic stainless steel and method for improving its strength and toughness and uniformity of properties

Abstract

The application belongs to the technical field of rolling steel, and particularly relates to a Cr13 type ferrite stainless steel and a method for improving the strength and toughness and the uniformity of performance of the Cr13 type ferrite stainless steel, which comprises the following steps: smelting, continuous casting, hot rolling, coiling, cover furnace annealing and hot line pickling, wherein the heating furnace temperature of the hot rolling is 1140+ / -20 DEG C; and the final rolling temperature of the finishing rolling is 890+ / -20 DEG C. The Cr13 type ferrite stainless steel prepared by the method has a yield strength range of a steel coil head, middle and tail reduced from 150 MPa to 60 MPa or less.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology, specifically relating to a Cr13 type ferritic stainless steel and a method for improving its strength, toughness and performance uniformity. Background Technology

[0002] Cr13 ferritic stainless steel, as an economical and low-cost stainless steel, can be used in construction, highway guardrails, municipal engineering facilities, and railway carriages, and is expected to gradually replace carbon steel. While maintaining a certain level of corrosion resistance, the key to Cr13 ferritic stainless steel is to possess excellent comprehensive mechanical properties, such as high strength, high elongation, and good impact toughness, in order to be processed into qualified structural components and other parts.

[0003] Cr13 type ferritic stainless steel is typically delivered as hot-rolled and pickled stainless steel strip. The production process includes smelting, continuous casting, hot rolling, coiling, bell-type furnace annealing, and hot-line pickling. During bell-type furnace annealing, due to forced gas convection, the inner and outer rings of the coil heat up rapidly, while the core needs to be heated slowly through heat conduction. Therefore, the ferrite structure at the head and tail of the coil recrystallizes more fully, resulting in larger grain sizes than the core. This ultimately leads to a strength at the head and tail of the coil that is approximately 150 MPa lower than the center (as shown in Table 1). To ensure the finished steel coil meets factory standards, the lower-strength section needs to be removed after pickling. This not only results in significant resource waste and low production efficiency but also greatly impacts the customer's control over part dimensional accuracy and the machining process.

[0004] Table 1 Typical performance characteristics of hot-rolled Cr13 ferritic stainless steel coils (beginning, middle, and end).

[0005] Yield / MPa 318 452 302 Tensile strength / MPa 518 607 500 Elongation / % 30 24 36 Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a Cr13 type ferritic stainless steel and a method for improving its strength, toughness, and performance uniformity.

[0007] Specifically, the method for improving the strength, toughness, and uniformity of properties of Cr13 type ferritic stainless steel provided by the present invention includes: smelting, continuous casting, hot rolling, coiling, bell-type furnace annealing, and hot wire pickling, wherein the heating furnace temperature for hot rolling is 1140±20℃; and the finishing rolling temperature is 890±20℃.

[0008] The above-mentioned method for improving the strength, toughness, and uniformity of properties of Cr13 type ferritic stainless steel involves laminar flow cooling at the head and tail of the coil during the coiling process, while laminar flow cooling is not performed on the middle part of the coil.

[0009] The above-mentioned method for improving the strength, toughness, and uniformity of properties of Cr13 ferritic stainless steel involves controlling the temperature at the head and tail of the coil to 640±20℃ and the temperature in the middle of the coil to 720±20℃ during the coiling process.

[0010] The above-mentioned method for improving the strength, toughness, and uniformity of properties of Cr13 type ferritic stainless steel involves controlling the following during the smelting process: C: 0.010–0.02 wt%, Mn: 1.6–1.9 wt%, N: 0.008–0.02 wt% in the molten steel at the smelting endpoint.

[0011] The above-mentioned method for improving the strength, toughness, and uniformity of properties of Cr13 ferritic stainless steel, the bell-type furnace annealing process includes: rapidly heating to 500-600℃, then heating to 650-710℃ at a heating rate of 7-15℃ / hour, holding for 8-15 hours, turning off the heating system after holding, allowing natural slow cooling for 3-5 hours, then replacing the cooling hood and rapidly cooling to 200-300℃ before unloading from the furnace.

[0012] The above-mentioned method for improving the strength, toughness, and uniformity of properties of Cr13 ferritic stainless steel includes the following annealing process in a bell-type annealing furnace: rapidly heating to 550°C, then heating to 680°C at a heating rate of 8°C / hour, holding for 8 hours, shutting off the heating system after holding, allowing natural slow cooling for 3 hours, and then rapidly cooling to 240°C before removing from the furnace using a cooling hood.

[0013] On the other hand, the present invention also provides a Cr13 type ferritic stainless steel, which adopts the above-mentioned method for improving the strength, toughness and performance uniformity of Cr13 type ferritic stainless steel during the preparation process.

[0014] The technical solution of the present invention has the following beneficial effects:

[0015] (1) This invention can increase the first-pass yield of hot-rolled Cr13 type ferritic stainless steel strip by about 6%.

[0016] (2) The Cr13 type ferritic stainless steel prepared according to the method of the present invention has a yield strength difference of less than 60 MPa from the beginning, middle and end of the steel coil. Attached Figure Description

[0017] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention.

[0018] Figure 1 This is a process curve diagram of the bell-type furnace annealing process in this invention. Detailed Implementation

[0019] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0020] When a numerical range is disclosed in this invention, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Further, when the range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Moreover, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed in this invention should be understood to include any and all subranges to which they are incorporated.

[0021] This invention aims to improve the uniformity of recrystallization grain size at the beginning, middle, and end of hot-rolled Cr13 ferritic stainless steel coils and enhance the uniformity of coil strength and continuous coiling performance by optimizing chemical composition, hot rolling line, and bell-type furnace annealing process.

[0022] Specifically, the method for improving the strength, toughness, and property uniformity of Cr13 ferritic stainless steel provided by this invention includes: smelting, continuous casting, hot rolling, coiling, bell-type furnace annealing, and hot wire pickling.

[0023] To improve the strength of finished steel coils, this invention starts with chemical composition optimization, controlling the carbon, manganese, and nitrogen content of Cr13 stainless steel within a reasonable range, specifically: C: 0.010~0.020wt%, Mn: 1.6~1.9wt%, N: 0.008~0.02wt%.

[0024] Practice has shown that when the content of C and N is too low, the production cost of the product increases significantly and is not conducive to improving strength; when the content of C and N is too high, the material's ductility, toughness, corrosion resistance, and weldability are all easily and significantly deteriorated. When the content of Mn is too low, it is not conducive to the stability of the material's strength properties; when the content of Mn is too high, it is easy to cause a sharp deterioration in the material's ductility and toughness.

[0025] The strength of Cr13 ferritic stainless steel can be improved through grain refinement strengthening, solid solution strengthening, second-phase strengthening, and work hardening. Among these, grain refinement strengthening refers to increasing the number of grains and grain boundary density within the material, thereby comprehensively improving its strength, ductility, toughness, and other mechanical properties; it is the most cost-effective method overall. Since the microstructure of the hot-rolled state has a significant impact on the recrystallization morphology of the annealed coil, optimized hot-rolling process parameters are used to control the gradient distribution of grain size throughout the coil. The main improvement techniques are as follows:

[0026] (1) The temperature of the hot rolling furnace is controlled at 1140±20℃, and the finishing rolling temperature is controlled at 890±20℃, so as to refine the grain structure of the hot rolled steel coil.

[0027] (2) In order to suppress recrystallization at the head and tail of the steel coil and promote recrystallization of the grains in the middle, laminar flow cooling is performed on the head and tail of the steel coil during the coiling process, controlled at 640±20℃, while laminar flow cooling is not performed on the middle of the steel coil, controlled at 720±20℃.

[0028] The head of the steel coil refers to all parts from the front end of the steel coil along the reverse rolling direction to 40±10m during the layer cooling process; the tail of the steel coil refers to all parts from the tail end of the steel coil along the rolling direction to 40±10m during the layer cooling process.

[0029] The bell-type furnace annealing process inherently suffers from uneven heating, with the inner and outer rings of the steel coil experiencing rapid temperature increases, leading to excessive grain growth at the coil's head and tail. The core of the coil, however, requires slow heating via heat conduction, resulting in smaller grains in the central portion. To achieve a more uniform grain size throughout the hot-rolled steel strip after annealing, this invention optimizes the bell-type furnace annealing process.

[0030] Specifically, the bell-type furnace annealing process of the present invention includes: rapidly heating to 500-600℃, then heating to 650-710℃ at a heating rate of 7-15℃ / hour, holding for 8-15 hours, turning off the heating system after holding, allowing natural slow cooling for 3-5 hours, then replacing the cooling hood and rapidly cooling to 200-300℃ before unloading from the furnace.

[0031] When the annealing process is not within the scope of protection of this invention, two typical performance defects will appear: one is that the strength of the strip head and tail is low, the strength of the middle part is high, and the elongation does not meet the factory standard; the other is that the whole coil is not fully annealed, the overall strength is too high, the plasticity is deteriorated, and it does not meet the factory standard requirements.

[0032] Preferably, in the bell-type furnace annealing process, the temperature is rapidly increased to 550°C, and then heated to 680°C at a rate of 8°C / hour. The holding time must be guaranteed for 8 hours. After the holding time is completed, the heating system is turned off, and the furnace is allowed to cool slowly and naturally for 3 hours before the cooling hood is replaced and the furnace is rapidly cooled to 240°C before being removed from the furnace.

[0033] On the other hand, the present invention also provides a Cr13 type ferritic stainless steel, which is prepared by using the method described above for improving the strength, toughness and performance uniformity of Cr13 type ferritic stainless steel.

[0034] Example

[0035] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments, unless otherwise specified, are performed according to conventional methods and conditions.

[0036] Example 1

[0037] 1. Chemical composition (wt%)

[0038] The composition of the experimental steel was controlled as follows: C: 0.0133%, Mn: 1.653%, Si: 0.35%, Cr: 11.3%, N: 0.012%, P: ≤0.004%, S: ≤0.015%.

[0039] 2. Hot rolling process

[0040] (1) The temperature of the hot rolling mill heating furnace is controlled at 1140±20℃;

[0041] (2) The finishing rolling temperature shall be controlled at 890±20℃;

[0042] (3) The winding process is controlled at 640±20℃ for the head and tail layers and 720±20℃ for the middle layer without layer cooling.

[0043] 3. Bell-type furnace annealing process

[0044] The bell-type furnace annealing process is as follows: rapidly heat to 550℃, then heat to 680℃ at a heating rate of 8℃ / hour, and hold for 8 hours. After holding, turn off the heating system, allow natural slow cooling for 3 hours, then replace the cooling hood and rapidly cool to 240℃ before removing from the furnace.

[0045] 4. Mechanical properties

[0046] After production using the specific process parameters described above, the mechanical properties of the product are shown in Table 2. The yield strength difference between the beginning, middle, and end of the steel coil decreased from 150 MPa to approximately 60 MPa, while the tensile strength and elongation remained basically consistent.

[0047] Table 2 Performance of the steel coil head, middle, and tail in Example 1

[0048] Yield / MPa 376 430 362 Tensile strength / MPa 524 519 500 Elongation / % 28 29 30

[0049] Example 2

[0050] 1. Chemical composition (wt%)

[0051] The composition of the experimental steel was controlled as follows: C: 0.0123%, Mn: 1.645%, Si: 0.41%, Cr: 11.2%, N: 0.013%, P: ≤0.004%, S: ≤0.015%.

[0052] 2. Hot rolling process

[0053] (1) The temperature of the hot rolling mill heating furnace is controlled at 1140±20℃;

[0054] (2) The finishing rolling temperature shall be controlled at 890±20℃;

[0055] (3) The winding process is controlled at 640±20℃ for the head and tail layers and 720±20℃ for the middle layer without layer cooling.

[0056] 3. Bell-type furnace annealing process

[0057] The bell-type furnace annealing process is as follows: rapidly heat to 550℃, then heat to 680℃ at a heating rate of 8℃ / hour, and hold for 8 hours. After holding, turn off the heating system, allow natural slow cooling for 3 hours, then replace the cooling hood and rapidly cool to 240℃ before removing from the furnace.

[0058] 4. Mechanical properties

[0059] After production using the specific process parameters described above, the mechanical properties of the product are shown in Table 3 below. The range of yield strength between the beginning, middle, and end of the steel coil decreases from approximately 150 MPa to approximately 60 MPa, and the range of tensile strength decreases from approximately 100 MPa to approximately 20 MPa.

[0060] Table 3 Performance of Stainless Steel Coil Beginning, Middle, and End Parts in Example 2

[0061] Yield / MPa 384 447 369 Tensile strength / MPa 532 530 510 Elongation / % 26.5 28.5 33

[0062] Comparative Example 1

[0063] When the hot rolling process and the bell-type furnace annealing process are within the protection scope of this invention, but the composition is not within the scope of this invention.

[0064] 1. Chemical composition (wt%)

[0065] The composition of the experimental steel was controlled as follows: C: 0.008%, Mn: 1.49%, Si: 0.38%, Cr: 11.3%, N: 0.009%, P: ≤0.004%, S: ≤0.015%.

[0066] 2. Hot rolling process

[0067] (1) The temperature of the hot rolling mill heating furnace is controlled at 1140±20℃;

[0068] (2) The finishing rolling temperature shall be controlled at 890±20℃;

[0069] (3) The winding process is controlled at 640±20℃ for the head and tail layers and 720±20℃ for the middle layer without layer cooling.

[0070] 3. Bell-type furnace annealing process

[0071] The bell-type furnace annealing process is as follows: rapidly heat to 550℃, then heat to 680℃ at a heating rate of 8℃ / hour, and hold for 8 hours. After holding, turn off the heating system, allow natural slow cooling for 3 hours, then replace the cooling hood and rapidly cool to 240℃ before removing from the furnace.

[0072] 4. Mechanical properties

[0073] After production using the specific process parameters described above, the mechanical properties of the product are shown in Table 4 below. The range of yield strength between the beginning, middle, and end of the steel coil decreases from approximately 150 MPa to approximately 100 MPa, and the range of tensile strength decreases from approximately 100 MPa to approximately 50 MPa.

[0074] Table 4. Performance of stainless steel coil heads, middle sections, and tail sections (Comparative Example 1)

[0075] Yield / MPa 350 452 344 Tensile strength / MPa 515 547 495 Elongation / % 28 25 30

[0076] Comparative Example 2

[0077] When the chemical composition and bell-type furnace annealing process are within the scope of protection of this invention, but the hot rolling process is not.

[0078] 1. Chemical composition (wt%)

[0079] The composition of the experimental steel was controlled as follows: C: 0.011%, Mn: 1.7%, Si: 0.45%, Cr: 11.2%, N: 0.012%, P: ≤0.004%, S: ≤0.015%.

[0080] 2. Hot rolling process

[0081] (1) The temperature of the hot rolling mill heating furnace is controlled at 1160±20℃;

[0082] (2) The finishing rolling temperature shall be controlled at 920±20℃;

[0083] (3) The winding temperature is controlled at 720±20℃, and no layer cooling is applied;

[0084] 3. Bell-type furnace annealing process

[0085] The bell-type furnace annealing process is as follows: rapidly heat to 550℃, then heat to 680℃ at a heating rate of 8℃ / hour, and hold for 8 hours. After holding, turn off the heating system, allow natural slow cooling for 3 hours, then replace the cooling hood and rapidly cool to 240℃ before removing from the furnace.

[0086] 4. Mechanical properties

[0087] After production using the specific process parameters described above, the mechanical properties of the product are shown in Table 5 below. The range of yield strength between the beginning, middle, and end of the steel coil decreased from approximately 150 MPa to approximately 120 MPa, and the range of tensile strength decreased from approximately 100 MPa to approximately 50 MPa, but the tensile strength of all three components showed a significant decrease.

[0088] Table 5. Performance of stainless steel coil heads, middle sections, and tail sections (Comparative Example 2)

[0089] Yield / MPa 338 462 318 Tensile strength / MPa 478 527 470 Elongation / % 32 23 34

[0090] Comparative Example 3

[0091] When the hot rolling process and chemical composition are within the scope of protection of this invention, but the annealing process is not.

[0092] 1. Chemical composition (wt%)

[0093] The composition of the experimental steel was controlled as follows: C: 0.011%, Mn: 1.7%, Si: 0.45%, Cr: 11.2%, N: 0.012%, P: ≤0.004%, S: ≤0.015%.

[0094] 2. Hot rolling process

[0095] (1) The temperature of the hot rolling mill heating furnace is controlled at 1140±20℃;

[0096] (2) The finishing rolling temperature shall be controlled at 890±20℃;

[0097] (3) The winding process is controlled at 640±20℃ for the head and tail layers and 720±20℃ for the middle layer without layer cooling.

[0098] 3. Bell-type furnace annealing process

[0099] The bell-type furnace annealing process is as follows: rapidly heat to 700℃, hold for 18 hours, shut down the heating system after holding, allow natural slow cooling for 3 hours, then replace the cooling hood and rapidly cool to 240℃ before unloading.

[0100] 4. Mechanical properties

[0101] After production using the specific process parameters described above, the mechanical properties of the product are shown in Table 6 below. The yield strength of the steel coil head, middle, and tail is greatly improved, but the elongation is severely deteriorated.

[0102] Table 6. Performance of stainless steel coil heads, middle sections, and tail sections in Comparative Example 3

[0103] Yield / MPa 496 653 455 Tensile strength / MPa 586 712 585 Elongation / % 18 14 22

[0104] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for improving the strength and toughness and the uniformity of properties of a Cr 13 type ferritic stainless steel, comprising: Smelting, continuous casting, hot rolling, coiling, bell-type furnace annealing, hot wire pickling, characterized in that, The heating furnace temperature for hot rolling is 1140±20℃; the finishing rolling temperature is 890±20℃. During the coiling process, laminar flow cooling is applied to the head and tail of the steel coil, while laminar flow cooling is not applied to the middle of the steel coil. The temperature of the head and tail of the steel coil is controlled at 640±20℃, and the temperature of the middle of the steel coil is controlled at 720±20℃. The head of the steel coil refers to all parts from the front end of the steel coil along the reverse rolling direction to 40±10 m when laminar flow cooling is applied; the tail of the steel coil refers to all parts from the tail end of the steel coil along the rolling direction to 40±10 m when laminar flow cooling is applied. The bell-type furnace annealing process includes: rapidly heating to 500-600℃, then heating to 650-710℃ at a heating rate of 7-15℃ / hour, holding at that temperature for 8-15 hours, turning off the heating system after holding, allowing it to cool naturally and slowly for 3-5 hours, then replacing the cooling hood and rapidly cooling to 200-300℃ before removing it from the furnace.

2. The method according to claim 1, characterized in that, During the smelting process, the C content in the molten steel at the smelting endpoint is controlled as follows: 0.010~0.02 wt%, Mn content as 1.6~1.9 wt%, and N content as 0.008~0.02 wt%.

3. The method according to claim 1, characterized in that, The bell-type furnace annealing process includes: rapidly heating to 550°C, then heating to 680°C at a heating rate of 8°C / hour, holding at that temperature for 8 hours, turning off the heating system after holding, allowing it to cool naturally and slowly for 3 hours, then replacing the cooling hood and rapidly cooling to 240°C before removing it from the furnace.

4. A Cr13 type ferritic stainless steel, characterized in that, It is prepared by the method described in any one of claims 1-3 for improving the strength, toughness and property uniformity of Cr13 type ferritic stainless steel.

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