Preparation method of highly-strengthened Cr13 series stainless steel cold plate

Abstract

The invention relates to the field of metallurgy and stainless steel production, in particular to a preparation method of a high-strengthening Cr13 series stainless steel cold plate, which realizes Cr13 series stainless steel structure regulation and control, completes strengthening and toughening control of the performance of the Cr13 series stainless steel cold plate and prepares the high-strengthening Cr13 series stainless steel cold plate with the yield strength of more than 450MPa and the tensile strength of more than 450MPa by means of steel-making component control, hot rolling and cover furnace annealing processes, cold plate rolling and heat treatment process optimization and the like. And the percentage elongation after fracture of the Cr13 stainless steel cold plate is more than 15%.

Description

Technical Field

[0001] The present invention relates to the field of metallurgy and stainless steel production, and particularly to a method for preparing a highly strengthened Cr13 series stainless steel cold plate. Background Art

[0002] Cr13 series stainless steel is a material with good corrosion resistance and oxidation resistance, having advantages such as high strength, good workability, and weldability. At the same time, due to its low alloy composition, it has significant economic advantages and is widely used in fields such as building panels, rail transit, household appliances, and kitchen and bathroom.

[0003] With the development of society and the increasing requirements for raw materials in various fields, customers' requirements for the strength, plasticity, and toughness of low-Cr ferritic stainless steel are also getting higher and higher. For customers in some mechanical and construction fields, in order to ensure the structural stability of their products, it is urgent to develop a low-Cr ferritic stainless steel with a yield strength exceeding 450 MPa, an elongation after fracture exceeding 15%, and excellent welding performance. Since the alloy content of Cr element in Cr13 series stainless steel is relatively low and there is a complex phase change process during production, the high-strength control of Cr13 series stainless steel in the actual production process has always been a difficult point.

[0004] Because the Cr content of Cr13 series stainless steel is relatively low, and in order to improve the welding performance, austenite-forming elements such as Ni are usually added. Taking 1.4003 in the European standard as an example, there is a γ-austenite phase region at high temperatures. Thus, in the process of composition design, rolling, and heat treatment, the phase change process of the material can be controlled by adjusting the temperature, thereby realizing the strengthening and toughening of the product.

[0005] The present invention realizes the microstructure regulation of Cr13 series stainless steel by controlling the steelmaking composition, hot rolling, and bell-type furnace annealing processes, optimizing the cold plate rolling and heat treatment processes, etc., completes the strengthening and toughening control of the performance of Cr13 series stainless steel cold plates, and manufactures Cr13 series stainless steel cold plates with a yield strength above 450 MPa and an elongation after fracture above 15%. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a highly strengthened Cr13 series stainless steel cold plate in view of the above problems.

[0007] The purpose of the present invention is achieved as follows: A method for preparing a highly strengthened Cr13 series stainless steel cold plate includes the following steps:

[0008] Step 1: Optimization of steelmaking composition: On the basis of Cr13 series stainless steel, two austenite-forming elements, Mn and Ni, are additionally added, and the control range of Nieq is required to be 1.7 - 3.0. The calculation formula of Nieq is shown in Formula 1:

[0009] Nieq = Ni + (30×C) + (30×N) + (0.5×Mn), Equation 1;

[0010] Step 2: Control of the finishing rolling and coiling temperatures in hot continuous rolling: The finishing rolling temperature should be controlled at (A3 + 150°C) ± 10°C, where the A3 temperature is the end temperature of the transformation from ferrite α phase to austenite γ phase, and the coiling temperature should be controlled at 630°C to 650°C; Step 3: Control of the bell-type furnace annealing process: The bell-type furnace annealing process is to rapidly heat up to A1 - 10°C in 8 ± 0.5 hours, hold for t1 ± 0.5 hours and then cool slowly for 3 ± 0.5 hours, and then replace the cooling hood and rapidly cool to 245 ± 10°C and then take out of the furnace. Here, A1 is the start temperature of the transformation from ferrite α phase to austenite γ phase, and the holding time t1 is related to the wall thickness D of the stainless steel coil as shown in Equation 2:

[0011] t1(min) = 1.2(min / mm) × D(min), Equation 2;

[0012] Step 4: Control of the cold rolling deformation rate: The cold rolling deformation rate of the steel strip should be controlled at 50 - 60%; Step 5: Control of the cold line annealing and pickling processes: The annealing process is to control the temperature at (A3 + 20)°C ± 10°C, the unit heat treatment time at 3 ± 0.5 min / mm, the cooling mode is air cooling without opening the water mist, and the pickling process is NaSO 4 The electrolytic cell current is controlled at 8000A to 9500A, the HF concentration is controlled at 10 - 15 g / L, and HNO 3 is controlled at a concentration of 100 ± 20 g / L, and the Fe3+ concentration is controlled within 50 g / L.

[0013] The beneficial effects of the present invention are: Through the implementation of the above processes, a Cr13 series stainless steel cold plate with a yield strength of more than 450 MPa and an elongation after fracture of more than 15% is manufactured, meeting the usage requirements of relevant customers in the construction and machinery fields. Description of the Drawings

[0014] The present invention will be further described below with reference to the drawings.

[0015] Figure 1 is the metallographic structure diagram of the high-strengthened Cr13 series stainless steel of the present invention. Specific Embodiments

[0016] Cr13 series stainless steel is a material with good corrosion resistance and oxidation resistance properties, having advantages such as high strength, good workability, weldability, etc. At the same time, due to the low alloy content, it has significant economic advantages and is widely used in fields such as building panels, rail transit, household appliances, kitchen and bathroom, etc. With the development of society and the increasing requirements for raw materials in various fields, customers' requirements for the strength, plasticity and toughness of low-Cr ferritic stainless steel are also getting higher and higher. It is urgent to develop a low-Cr ferritic stainless steel with a yield strength exceeding 450 MPa, an elongation after fracture exceeding 15%, and excellent welding performance at the same time.

[0017] By means of controlling steelmaking composition, hot rolling and bell annealing processes, optimizing cold plate rolling and heat treatment processes, etc., the microstructure regulation of Cr13 series stainless steel is realized, the strengthening and toughening control of the properties of Cr13 series stainless steel cold plates is completed, and Cr13 series stainless steel cold plates with a yield strength above 450 MPa and an elongation after fracture above 15% are manufactured.

[0018] 1. Optimization of steelmaking composition: Since the Cr content of Cr13 series stainless steel is relatively low, there is a situation of ferrite-austenite two-phase transformation without adding additional austenite-forming elements, but the finished product strength still cannot reach the level of 450 MPa grade. In order to improve the finished product strength, austenite-forming elements such as Mn and Ni need to be added additionally on the basis of Cr13 series stainless steel, and the control range of Nieq should be 1.7 - 3.0. The calculation formula of Nieq is shown in Equation 1.

[0019] Nieq = Ni + (30×C) + (30×N) + (0.5×Mn) Equation 1

[0020] 2. Control of finishing rolling and coiling temperatures in hot strip continuous rolling: To ensure the uniformity of the overall metallographic structure and mechanical properties of hot-rolled steel strips, it is necessary to control the finishing rolling temperature and coiling temperature during the hot strip continuous rolling process. The finishing rolling temperature should be controlled above A3 + 150 °C, where the A3 temperature is the end temperature of the transformation from ferrite α phase to austenite γ phase, and the coiling temperature should be controlled within the range of 630 °C - 650 °C. At this temperature, the diffusion rates of large-sized elements such as Mn and Ni will decrease significantly, thereby reducing or even avoiding the transformation of undercooled austenite structure into ferrite and carbides. By controlling the finishing rolling and coiling temperatures, it is ensured that the product structure at the end of rolling is a uniform martensite structure.

[0021] 3. Bell-type furnace annealing process control: The bell-type furnace annealing process should rapidly heat up to A1 - 10°C in 8 hours, hold for t1 ± 0.5 hours and then cool slowly for 3 hours, and then replace the cooling hood to rapidly cool to 245°C and then take out of the furnace. Here, A1 is the starting temperature of the transformation from ferrite α phase to austenite γ phase. By controlling the heat treatment temperature, it is ensured that the banded structure in the hot-rolled steel strip is fully eliminated without martensite phase transformation. The holding time t1 is related to the wall thickness D of the stainless steel coil, specifically as shown in Formula 2. By controlling the holding and slow cooling times, the uniformity of the metallographic structure and mechanical properties of the hot-rolled steel strip is ensured.

[0022] t1 (min) = 1.2 (min / mm) × D (min) Formula 2

[0023] 4. Cold rolling reduction rate control: The cold rolling reduction rate of the steel strip should be controlled at 50 - 60%. By controlling the cold rolling reduction rate to be greater than 50%, it can provide energy and austenite nucleation sites for the phase transformation during the subsequent annealing process of the steel strip, improving the efficiency and uniformity of the tissue transformation. By controlling the cold rolling reduction rate to be less than 60%, the problem of strip breakage caused by too low elongation during the annealing process can be avoided.

[0024] 5. Cold wire annealing and pickling process control: The annealing process is controlled at a temperature of (A3 + 20)°C ± 10°C, and the unit heat treatment time is controlled at 3 min / mm, and the cooling mode adopts slow cooling. The pickling process is that the current of the electrolytic cell is controlled in the range of 8000A - 9500A, and the HF concentration is controlled at 10 - 15 g / L. By controlling the annealing temperature and time, it is ensured that the material structure fully undergoes austenite transformation. Since the Ms point of Cr13 series stainless steel is around 300°C, during the cooling process, as the temperature decreases, the austenite structure in the steel strip will undergo martensite transformation, thus greatly increasing the strength of the steel strip. By the way of slow cooling, part of the austenite structure can be transformed into ferrite, so that the strip retains a certain amount of elongation performance. Since the Cr13 series belongs to low-Cr stainless steel and has weak surface oxidation resistance, it is necessary to appropriately increase the current and acid concentration to avoid the phenomenon of blackening on the surface of the steel strip. 4 The electrolytic cell current is controlled in the range of 8000A - 9500A, and the HF concentration is controlled at 10 - 15 g / L. By controlling the annealing temperature and time, it is ensured that the material structure fully undergoes austenite transformation. Since the Ms point of Cr13 series stainless steel is around 300°C, during the cooling process, as the temperature decreases, the austenite structure in the steel strip will undergo martensite transformation, thus greatly increasing the strength of the steel strip. By the way of slow cooling, part of the austenite structure can be transformed into ferrite, so that the strip retains a certain amount of elongation performance. Since the Cr13 series belongs to low-Cr stainless steel and has weak surface oxidation resistance, it is necessary to appropriately increase the current and acid concentration to avoid the phenomenon of blackening on the surface of the steel strip.

[0025] Example 1

[0026] The components are shown in Table 1. It is calculated that Nieq of this material is 2.21, meeting the requirements of 1.7 - 3.0. From the phase diagram, the A3 temperature of this material is 800 °C, and the finishing rolling temperature is calculated to be 950 °C. Therefore, the finishing rolling temperature is controlled at 950 °C ± 10 °C, and the coiling temperature is controlled at 640 °C. From the phase diagram, the A1 temperature of this material is 697 °C, and the annealing temperature of the bell-type furnace is calculated to be 700 °C. The wall thickness of the stainless steel coil is measured to be 550 mm, and the annealing time is calculated to be 11 h. The actual annealing process is to rapidly heat up to 690 ± 10 °C in 8 hours, hold for 11 hours, then slowly cool for 3 hours, and then replace the cooling hood and quickly cool to 245 °C before discharging. The thickness of the hot plate is 3.0 mm, the thickness of the cold plate is 1.5 mm, and the deformation rate is 50%. The annealing temperature of the cold line is calculated to be 820 °C ± 10 °C, the actual temperature is 820 °C, the unit heat treatment time is 3 min / mm, and the cooling mode adopts the slow cooling mode. During the pickling of the cold line, NaSO 4 The electrolytic cell current is 9000 A, and the HF concentration is controlled at 13 g / L.

[0027] A total of 7 stainless steel hot-rolled coils are produced, with an average yield strength of 497 MPa, a tensile strength of 587 MPa, and an elongation after fracture of 19.5%.

[0028] Table 1 Specific components (wt%) of Example 1

[0029]

[0030] Example 2

[0031] The components are shown in Table 2. It is calculated that Nieq of this material is 2.46, meeting the requirements of 1.7 - 3.0. From the phase diagram, the A3 temperature of this material is 800 °C, and the finishing rolling temperature is calculated to be 950 °C. Therefore, the finishing rolling temperature is controlled at 950 °C ± 10 °C, and the coiling temperature is controlled at 640 °C. From the phase diagram, the A1 temperature of this material is 690 °C, and the annealing temperature of the bell-type furnace is calculated to be 700 °C. The wall thickness of the stainless steel coil is measured to be 500 mm, and the annealing time is calculated to be 10 h. The actual annealing process is to rapidly heat up to 690 °C in 8 hours, hold for 10 hours, then slowly cool for 3 hours, and then replace the cooling hood and quickly cool to 245 °C before discharging. The thickness of the hot plate is 5.5 mm, the thickness of the cold plate is 2.5 mm, and the deformation rate is 54.5%. The annealing temperature of the cold line is calculated to be 820 °C ± 10 °C, the actual temperature is 822 °C, the unit heat treatment time is 3 min / mm, and the cooling mode adopts the slow cooling mode. During the pickling of the cold line, NaSO 4 The electrolytic cell current is 9300 A, and the HF concentration is controlled at 14 g / L.

[0032] A total of 5 stainless steel hot-rolled coils are produced, with an average yield strength of 489 MPa, a tensile strength of 564 MPa, and an elongation after fracture of 19%.

[0033] Table 2 Specific components of Example 2 (wt%)

[0034]

[0035] Example 3

[0036] The components are shown in Table 3. It is calculated that the Nieq of this material is 2.87, meeting the requirements of 1.7 - 3.0. Through the phase diagram, the A3 temperature of this material is 790°C, and the final rolling temperature of finish rolling is calculated to be 940°C. Therefore, the final rolling temperature is controlled at 940°C ± 10°C, and the coiling temperature is controlled at 640°C. Through the phase diagram, the A1 temperature of this material is 690°C, the annealing temperature of the bell-type furnace is calculated to be 700°C, the wall thickness of the stainless steel coil is measured to be 525 mm, the annealing time is calculated to be 10.5 h, and the actual annealing process is to rapidly heat up to 690°C in 8 hours, hold for 10.5 hours and then cool slowly for 3 hours, and then replace the cooling hood to quickly cool to 245°C and then take out of the furnace. The thickness of the hot plate is 5.5 mm, the thickness of the cold plate is 2.5 mm, and the deformation rate is 54.5%. The annealing temperature of the cold line is calculated to be 810°C ± 10°C, the actual temperature is 815°C, the unit heat treatment time is 3 min / mm, and the cooling mode adopts slow cooling mode. During the pickling of the cold line, NaSO 4 The current of the electrolytic cell is 9100 A, and the HF concentration is controlled at 13 g / L.

[0037] A total of 4 stainless steel hot-rolled coils are produced, with an average yield strength of 477 MPa, a tensile strength of 568 MPa, and an elongation after fracture of 16%.

[0038] Table 3 Specific components of Example 3 (wt%)

[0039]

[0040] The above are only specific embodiments of the present invention, but the structural features within the scope of protection of the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the field of the present invention are covered by the patent scope of the present invention.

Claims

1. A method for preparing a high-strengthened Cr13 stainless steel cold plate, characterized in that: The following steps are involved: Step 1: Optimization of steelmaking composition: Add two austenite-forming elements, Mn and Ni, to Cr13 stainless steel, and require that the Nieq control range should be 1.7 to 3.

0. The Nieq calculation formula is shown in Formula 1: Nieq = Ni + (30×C) + (30×N) + (0.5×Mn) Equation 1; Step 2: Hot rolling finishing and coiling temperature control: The finishing rolling temperature should be controlled at (A3+150℃)±10℃, where A3 temperature is the final temperature of the transformation from ferrite α phase to austenite γ phase, and the coiling temperature should be controlled at 630℃~650℃; Step 3: Bell-type furnace annealing process control: Bell-type furnace annealing process is to quickly heat up to A1-10℃ in 8±0.5 hours, keep warm for t1±0.5 hours, then slowly cool for 3±0.5 hours, then replace the cooling hood and quickly cool to 245±10℃ before taking out of the furnace, where A1 is the starting temperature for the transformation from ferrite α phase to austenite γ phase, and the holding time t1 is related to the wall thickness D of the stainless steel coil as shown in Formula 2: t1(min)=1.2(min / mm)×D(min) Formula 2; Step 4: Cold rolling deformation rate control: The cold rolling deformation rate of the steel strip should be controlled to 50-60%; Step 5: Cold wire annealing and pickling process control: The annealing process is as follows: the temperature is controlled at (A3+20)℃±10℃, the unit heat treatment time is controlled at 3±0.5min / mm, the cooling mode is air cooling without water mist, the pickling process is as follows: the NaSO4 electrolytic cell current is controlled at 8000A~9500A, the HF concentration is controlled at 10~15g / L, the HNO3 concentration is controlled at 100±20g / L, and the Fe3+ concentration is controlled within 50g / L.

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