Method for eliminating internal cracks in 304L stainless steel plate for nuclear power

Abstract

This invention discloses a method for eliminating internal cracks in 304L stainless steel medium-thick plates used in nuclear power plants, comprising: controlling the chemical composition of molten steel to C: 0.01-0.03%, Si: 0.3-0.7%, Mn: 1.5-1.9%, Cr: 18.05-19.5%, Ni: 9.05-11%, P≤0.03%, S≤0.005%, N: 0.02-0.08%, and controlling the ferrite content in the steel to 2.8-7.7%; and continuously casting in the continuous casting process. The cooling water volume in the secondary cooling section is controlled at 0.55–0.65 L / kg. In the hot rolling process, the slab heating temperature is controlled at 1248℃–1258℃, and the holding time is controlled at 1.8–2.5 min / mm based on the slab thickness. The final rolling temperature is controlled above 1040℃. In the heat treatment process, the hot-rolled steel plate undergoes solution treatment, with the solution treatment temperature controlled at 1065–1075℃ and the holding time controlled at 1.5–2.0 min / mm based on the steel plate thickness. This invention controls the ferrite content within a reasonable range, reducing local ferrite aggregation, thereby effectively solving the cracking defects caused by local ferrite aggregation in the hot rolling process of 304L stainless steel medium-thick plates for nuclear power applications.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel product manufacturing technology, and more specifically relates to a method for eliminating internal cracks in 304L stainless steel medium-thick plates used in nuclear power. Background Technology

[0002] 304L stainless steel medium-thick plates with a thickness of 40mm to 65mm are generally used in nuclear power plant reactor internals, safety injection tanks, and pressure relief tanks. Due to the special service environment, their composition is higher than that of ordinary 304L stainless steel, with higher alloying contents such as Cr and Ni, and the internal quality requirements of the steel plate are extremely high. However, due to the higher alloying contents such as Cr and Ni, the ferrite in the 304L stainless steel medium-thick plates for nuclear power plants is not easy to transform into austenite. During the rolling process of the steel plate, ferrite is prone to local aggregation, which can lead to cracks.

[0003] Ultrasonic testing was conducted on samples of 304L stainless steel medium-thick plates for nuclear power plants produced using existing technology. Both direct and oblique testing methods were used to assess the internal quality of the steel plates. The results showed that approximately 25% of the plates had internal crack defects. Microstructural analysis of the steel plates with internal crack defects revealed a large number of large-sized ferrite particles distributed at an angle to the rolling direction (see...). Figure 1 There is a significant accumulation of ferrite near the crack (see...). Figure 2 It can be seen that the formation of internal cracks in the 304L stainless steel medium and thick plates used in nuclear power is closely related to the accumulation of ferrite.

[0004] Internal cracks in 304L stainless steel medium-thick plates used in nuclear power plants severely impact delivery schedules and affect the safe operation and service life of nuclear power equipment. Therefore, developing a method to eliminate internal cracks in 304L stainless steel medium-thick plates used in nuclear power plants, specifically addressing the cracking defects caused by localized ferrite aggregation during hot rolling, has become a pressing technical problem for those skilled in the art. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, this invention provides a method for eliminating internal cracks in 304L stainless steel medium-thick plates used in nuclear power plants, comprising:

[0006] In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.01~0.03%, Si: 0.3~0.7%, Mn: 1.5~1.9%, Cr: 18.05~19.5%, Ni: 9.05~11%, P≤0.03%, S≤0.005%, N: 0.02~0.08%. The ferrite content δ in the steel is controlled as 2.8~7.7, where δ=3.2×(Cr%+1.5×Si%)-2.5×(Ni%+0.5×Mn%+30×C%+30×N%)-24.7;

[0007] In the continuous casting process, the cooling water volume of the secondary cooling section is controlled at 0.55 to 0.65 L / kg, and the billet is cut into slabs that meet the unit weight requirements by flame cutting.

[0008] In the hot rolling process, the slab is fed into a heating furnace for heating, and the heating temperature is controlled at 1248℃~1258℃. The holding time is controlled at 1.8~2.5min / mm based on the slab thickness. After the slab exits the furnace, it is quickly rolled to maintain the temperature, and the final rolling temperature is controlled above 1040℃.

[0009] In the heat treatment process, a normalizing furnace is used to perform solution treatment on the hot-rolled steel plate. The solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 1.5-2.0 min / mm based on the thickness of the steel plate.

[0010] Furthermore, in the above-mentioned method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power, the thickness of the 304L stainless steel medium-thick plates for nuclear power is 40mm to 65mm.

[0011] As one specific implementation method, the above-mentioned method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power is used to produce 50mm thick 304L stainless steel plates for nuclear power, wherein:

[0012] In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.021%, Si: 0.41%, Mn: 1.63%, Cr: 18.31%, Ni: 9.12%, P: 0.015%, S: 0.002%, N: 0.067%, and the ferrite content δ in the steel is controlled at 4.42.

[0013] In the continuous casting process, billets are produced on a continuous casting machine with a cross-section of 180mm thick * 1250mm wide. The cooling water ratio in the secondary cooling section of the continuous casting is controlled at 0.6L / kg. The billets are cut into slabs that meet the unit weight requirements by flame cutting and then ground.

[0014] In the hot rolling process, the heating temperature is controlled at 1248℃~1258℃, the holding time is controlled at 360min, and the final rolling temperature is controlled at 1055℃.

[0015] In the heat treatment process, the solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 75min.

[0016] As one specific implementation method, the above-mentioned method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power is used to produce 40mm thick 304L stainless steel plates for nuclear power, wherein:

[0017] In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.018%, Si: 0.32%, Mn: 1.64%, Cr: 18.62%, Ni: 9.15%, P: 0.028%, S: 0.002%, N: 0.072%, thereby controlling the ferrite content δ in the steel to 4.74.

[0018] In the continuous casting process, billets are produced on a continuous casting machine with a cross-section of 200mm thick * 1500mm wide. The cooling water ratio in the secondary cooling section of the continuous casting is controlled at 0.65L / kg. The billets are cut into slabs that meet the unit weight requirements by flame cutting and then ground.

[0019] In the hot rolling process, the heating temperature is controlled at 1248℃~1258℃, the holding time is controlled at 400min, and the final rolling temperature is controlled at 1048℃.

[0020] In the heat treatment process, the solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 60min.

[0021] The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants of the present invention controls the ferrite content within a reasonable range through composition design, control of cooling water intensity in the secondary cooling section of continuous casting, control of hot rolling heating temperature and holding time, and control of final rolling temperature. This reduces local ferrite aggregation and effectively solves the crack defects caused by local ferrite aggregation in 304L stainless steel medium-thick plates for nuclear power plants during hot rolling. Specifically, it has the following advantages and beneficial effects:

[0022] (1) This invention controls the ferrite content within a reasonable range through composition design, avoiding the local accumulation of ferrite during rolling due to high ferrite content and the adverse effects on the welding performance of steel plates due to low ferrite content. Thus, it effectively eliminates internal crack defects in 304L stainless steel medium and thick plates for nuclear power while ensuring the performance of steel plates.

[0023] (2) The present invention reduces the cooling water ratio of the secondary cooling section of continuous casting, thereby reducing the cooling intensity of the secondary cooling section. During the continuous casting cooling process, ferrite has sufficient time to transform into austenite, and the ferrite content on the entire cross section of the continuous casting billet decreases by about 1% to 2%. The relatively low ferrite content of the billet makes it difficult for ferrite to accumulate during the hot rolling process, thereby effectively eliminating internal crack defects in 304L stainless steel medium and thick plates for nuclear power.

[0024] (3) By optimizing the hot rolling heating temperature and holding time, this invention can reduce the ferrite content of the billet as much as possible in a relatively short time and reduce the local aggregation of ferrite, thereby effectively eliminating the internal crack defects of 304L stainless steel medium and thick plates for nuclear power.

[0025] (4) This invention reduces the number of descaling passes and minimizes temperature drop by rapidly rolling at high temperature, thereby increasing the final rolling temperature. This makes it easier for ferrite to transform into austenite under the action of rolling and high temperature, avoiding local accumulation of ferrite. This effectively eliminates internal crack defects in 304L stainless steel medium and thick plates for nuclear power. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 The image shows the metallographic structure of a medium-thick 304L stainless steel plate for nuclear power plants with internal crack defects produced using existing technology.

[0028] Figure 2 The image shows the metallographic structure of the cracked area in a medium-thick plate of 304L stainless steel for nuclear power plants produced using existing technology and containing internal crack defects.

[0029] Figure 3 The image shows the metallographic structure of a steel plate produced using the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to Embodiment 1 of this invention.

[0030] Figure 4 The image shows the metallographic structure of a steel plate produced using the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to Embodiment 2 of the present invention. Detailed Implementation

[0031] 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.

[0032] In the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants of the present invention, the term "medium-thick plate" refers to a steel plate with a thickness specification of 40mm to 65mm.

[0033] The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to the present invention includes:

[0034] In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.01–0.03%, Si: 0.3–0.7%, Mn: 1.5–1.9%, Cr: 18.05–19.5%, Ni: 9.05–11%, P≤0.03%, S≤0.005%, N: 0.02–0.08%. Through this steel chemical composition design, the ferrite content δ in the steel can be controlled to 2.8–7.7, where δ = 3.2 × (Cr% + 1.5 × Si%) - 2.5 × (Ni% + 0.5 × Mn% + 30 × C% + 30 × N%) - 24.7. This avoids both the localized accumulation of ferrite during rolling due to high ferrite content and the negative impact on the weldability of the steel plate due to excessively low ferrite content.

[0035] In the continuous casting process, the cooling water volume in the secondary cooling section is controlled at 0.55–0.65 L / kg (0.55–0.65 liters of water per kilogram of steel). Flame cutting is used to cut the billet into slabs that meet the unit weight requirements. When the cooling intensity of the secondary cooling section decreases, ferrite has sufficient time to transform into austenite during the continuous casting cooling process, resulting in a decrease in ferrite content. Experimental tests show that when the cooling water volume in the secondary cooling section is reduced from the conventional 0.75–0.85 L / kg to 0.55–0.65 L / kg, the ferrite content across the entire cross-section of the billet decreases by approximately 1%–2%. The relatively low ferrite content in the billet makes it less prone to ferrite aggregation during hot rolling.

[0036] In the hot rolling process, (1) the slab is sent into the heating furnace for heating, and the heating temperature is controlled at 1248℃~1258℃. The holding time is controlled at 1.8~2.5min / mm based on the slab thickness; (2) after the slab exits the furnace, it is rolled quickly to reduce the number of descaling passes, minimize the temperature drop, and control the final rolling temperature above 1040℃. Under the condition of heating in the single-phase region of austenite, the ferrite formed under non-equilibrium solidification will gradually disappear with the extension of the holding time, and the higher the temperature, the more favorable it is for the diffusion of alloying elements, and the more obvious the reduction. When the heating temperature is in the two-phase region of austenite and ferrite, the reduction of ferrite content will be significantly reduced with the extension of the holding time. It has been determined by experiments that the ferrite content can be reduced in a short time when the holding temperature is 1248~1258℃. In addition, by increasing the final rolling temperature, the higher deformation temperature during the hot rolling process is conducive to the diffusion of elements, and the ferrite is more likely to transform into austenite under the action of rolling and high temperature. Experiments have shown that when the final rolling temperature is above 1040℃, the ferrite content can be effectively reduced, and local ferrite aggregation can be avoided.

[0037] In the heat treatment process, a normalizing furnace is used to perform solution treatment on the hot-rolled steel plate. The solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 1.5-2.0 min / mm based on the thickness of the steel plate.

[0038] The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants of the present invention controls the ferrite content within a reasonable range through composition design, control of cooling water intensity in the continuous casting secondary cooling section, control of hot rolling heating temperature and holding time, and control of final rolling temperature. This reduces the local aggregation of ferrite and effectively solves the crack defects caused by local aggregation of ferrite in 304L stainless steel medium-thick plates for nuclear power plants during hot rolling.

[0039] The following describes the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants, with reference to specific embodiments.

[0040] Example 1

[0041] The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to Embodiment 1 of the present invention is used to produce 50mm thick 304L stainless steel plates for nuclear power plants, and specifically includes:

[0042] (1) In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.021%, Si: 0.41%, Mn: 1.63%, Cr: 18.31%, Ni: 9.12%, P: 0.015%, S: 0.002%, N: 0.067%, thereby controlling the ferrite content δ in the steel to 4.42;

[0043] (2) In the continuous casting process, the billet is produced on a continuous casting machine with a cross section of 180mm thickness * 1250mm width. The cooling water ratio of the secondary cooling section of the continuous casting is controlled at 0.6L / kg. The billet is cut into slabs that meet the single weight requirements by flame cutting and then ground.

[0044] (3) In the hot rolling process, the slab is sent into the heating furnace for heating. The heating temperature is controlled at 1248℃~1258℃ and the holding time is controlled at 360min. After the slab is taken out of the furnace, it is rolled quickly to reduce the number of descaling passes and the final rolling temperature is controlled at 1055℃.

[0045] (4) In the heat treatment process, the hot-rolled steel plate is solution treated in a normalizing furnace. The solution treatment temperature is controlled at 1065-1075℃ and the holding time is controlled at 75min.

[0046] Microstructure analysis was performed on the 304L stainless steel plate for nuclear power produced using the internal crack elimination method for medium-thick plates of 304L stainless steel for nuclear power plants according to Example 1 of the present invention. For example... Figure 3 As shown, the ferrite content in the 304L stainless steel plate used in nuclear power plants is significantly reduced, no local ferrite aggregation is found, and there are no internal crack defects.

[0047] Example 2

[0048] The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to Embodiment 2 of the present invention is used to produce 40mm thick 304L stainless steel plates for nuclear power plants, and specifically includes:

[0049] (1) In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.018%, Si: 0.32%, Mn: 1.64%, Cr: 18.62%, Ni: 9.15%, P: 0.028%, S: 0.002%, N: 0.072%, thereby controlling the ferrite content δ in the steel to 4.74;

[0050] (2) In the continuous casting process, the billet is produced on a continuous casting machine with a cross section of 200mm thickness * 1500mm width. The cooling water ratio of the secondary cooling section of the continuous casting is controlled at 0.65L / kg. The billet is cut into slabs that meet the single weight requirements by flame cutting and then ground.

[0051] (3) In the hot rolling process, the slab is sent into the heating furnace for heating. The heating temperature is controlled at 1248℃~1258℃ and the holding time is controlled at 400min. After the slab is taken out of the furnace, it is rolled quickly to reduce the number of descaling passes and the final rolling temperature is controlled at 1048℃.

[0052] (4) In the heat treatment process, the hot-rolled steel plate is solution treated in a normalizing furnace. The solution treatment temperature is controlled at 1065-1075℃ and the holding time is controlled at 60min.

[0053] Microstructure analysis was performed on the 304L stainless steel plate for nuclear power produced using the internal crack elimination method for medium-thick plates of 304L stainless steel for nuclear power plants according to Example 2 of the present invention. For example... Figure 4 As shown, the ferrite content in the 304L stainless steel plate used in nuclear power plants is significantly reduced, no local ferrite aggregation is found, and there are no internal crack defects.

[0054] In summary, the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants of the present invention controls the ferrite content within a reasonable range through composition design, control of cooling water intensity in the continuous casting secondary cooling section, control of hot rolling heating temperature and holding time, and control of final rolling temperature. This reduces the local aggregation of ferrite and effectively solves the crack defects caused by local aggregation of ferrite in 304L stainless steel medium-thick plates for nuclear power plants during hot rolling.

[0055] Specifically, compared with the prior art, the method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants of the present invention has the following advantages and beneficial effects:

[0056] (1) This invention controls the ferrite content within a reasonable range through composition design, avoiding the local accumulation of ferrite during rolling due to high ferrite content and the adverse effects on the welding performance of steel plates due to low ferrite content. Thus, it effectively eliminates internal crack defects in 304L stainless steel medium and thick plates for nuclear power while ensuring the performance of steel plates.

[0057] (2) The present invention reduces the cooling water ratio of the secondary cooling section of continuous casting, thereby reducing the cooling intensity of the secondary cooling section. During the continuous casting cooling process, ferrite has sufficient time to transform into austenite, and the ferrite content on the entire cross section of the continuous casting billet decreases by about 1% to 2%. The relatively low ferrite content of the billet makes it difficult for ferrite to accumulate during the hot rolling process, thereby effectively eliminating internal crack defects in 304L stainless steel medium and thick plates for nuclear power.

[0058] (3) By optimizing the hot rolling heating temperature and holding time, this invention can reduce the ferrite content of the billet as much as possible in a relatively short time and reduce the local aggregation of ferrite, thereby effectively eliminating the internal crack defects of 304L stainless steel medium and thick plates for nuclear power.

[0059] (4) This invention reduces the number of descaling passes and minimizes temperature drop by rapidly rolling at high temperature, thereby increasing the final rolling temperature. This makes it easier for ferrite to transform into austenite under the action of rolling and high temperature, avoiding local accumulation of ferrite. This effectively eliminates internal crack defects in 304L stainless steel medium and thick plates for nuclear power.

[0060] It should 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 eliminating internal cracks in 304L stainless steel medium-thick plates used in nuclear power plants, characterized in that, include: In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.01~0.03%, Si: 0.3~0.7%, Mn: 1.5~1.9%, Cr: 18.05~19.5%, Ni: 9.05~11%, P≤0.03%, S≤0.005%, N: 0.02~0.08%. The ferrite content δ in the steel is controlled as 2.8~7.7, where δ=3.2×(Cr%+1.5×Si%)-2.5×(Ni%+0.5×Mn%+30×C%+30×N%)-24.7; In the continuous casting process, the cooling water volume of the secondary cooling section is controlled at 0.55 to 0.65 L / kg, and the billet is cut into slabs that meet the unit weight requirements by flame cutting. In the hot rolling process, the slab is fed into a heating furnace for heating, and the heating temperature is controlled at 1248℃~1258℃. The holding time is controlled at 1.8~2.5min / mm based on the slab thickness. After the slab exits the furnace, it is quickly rolled to maintain the temperature, and the final rolling temperature is controlled above 1040℃. In the heat treatment process, a normalizing furnace is used to perform solution treatment on the hot-rolled steel plate. The solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 1.5-2.0 min / mm based on the thickness of the steel plate.

2. The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to claim 1, characterized in that, The thickness specifications of the 304L stainless steel medium-thick plate for nuclear power are 40mm to 65mm.

3. The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to claim 1, used to produce 50mm thick 304L stainless steel plates for nuclear power plants, wherein: In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.021%, Si: 0.41%, Mn: 1.63%, Cr: 18.31%, Ni: 9.12%, P: 0.015%, S: 0.002%, N: 0.067%, and the ferrite content δ in the steel is controlled at 4.

42. In the continuous casting process, billets are produced on a continuous casting machine with a cross-section of 180mm thick * 1250mm wide. The cooling water ratio in the secondary cooling section of the continuous casting is controlled at 0.6L / kg. The billets are cut into slabs that meet the unit weight requirements by flame cutting and then ground. In the hot rolling process, the heating temperature is controlled at 1248℃~1258℃, the holding time is controlled at 360min, and the final rolling temperature is controlled at 1055℃. In the heat treatment process, the solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 75min.

4. The method for eliminating internal cracks in 304L stainless steel medium-thick plates for nuclear power plants according to claim 1, used to produce 40mm thick 304L stainless steel plates for nuclear power plants, wherein: In the steelmaking process, the chemical composition of molten steel is controlled by weight percentage as follows: C: 0.018%, Si: 0.32%, Mn: 1.64%, Cr: 18.62%, Ni: 9.15%, P: 0.028%, S: 0.002%, N: 0.072%, thereby controlling the ferrite content δ in the steel to 4.

74. In the continuous casting process, billets are produced on a continuous casting machine with a cross-section of 200mm thick * 1500mm wide. The cooling water ratio in the secondary cooling section of the continuous casting is controlled at 0.65L / kg. The billets are cut into slabs that meet the unit weight requirements by flame cutting and then ground. In the hot rolling process, the heating temperature is controlled at 1248℃~1258℃, the holding time is controlled at 400min, and the final rolling temperature is controlled at 1048℃. In the heat treatment process, the solution treatment temperature is controlled at 1065-1075℃, and the holding time is controlled at 60min.

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