4.2 K yield strength 1350 MPa grade steel for fusion reactor superconducting magnet structure and manufacturing method of 4.2 K yield strength 1350 MPa grade steel

Abstract

The invention belongs to the technical field of stainless steel, and discloses 4.2 K yield strength 1350 MPa grade steel for a fusion reactor superconducting magnet structure and a manufacturing method of the 4.2 K yield strength 1350 MPa grade steel for the fusion reactor superconducting magnet structure, and the steel comprises the following chemical components: 0.04-0.06% of C, 0.5-1.0% of Si, 6.5-8.0% of Mn, 19.5-21.0% of Cr, 10.0-12.0% of Ni, 1.5-2.0% of Mo, 0.25-0.30% of N and 0.15-0.25% of V. The deformation temperature interval of the high deformation rate and the low deformation rate during ply rolling is controlled by adopting continuous casting blank ply rolling, high-temperature-stage high deformation rate, low-temperature-stage low deformation rate and low-temperature solid solution; the steel plate has the advantages that the rolling temperature is lower than or equal to 1050 DEG C, the finish rolling temperature is higher than 800 DEG C, the thickness of the steel plate reaches 80 mm, the 4.2 K yield strength is 1350-1450 MPa, the tensile strength is 1750-1850 MPa, the ductility is 30%-40%, the fracture toughness is 155-170 MP.m < 1 / 2 >, and the room-temperature magnetic conductivity is smaller than 1.02. The problem of edge crack of the high-nitrogen austenitic stainless steel plate is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of stainless steel and relates to a stainless steel plate for a superconducting magnet structure of a controlled nuclear fusion reactor, and more particularly to a steel for a superconducting magnet structure of a fusion reactor with a 4.2K yield strength of 1350 MPa and a manufacturing method thereof. Background Art

[0002] Compared with the International Thermonuclear Experimental Reactor (ITER), China's new generation fusion engineering test reactor has a superconducting magnet with a maximum magnetic field of 20T and an electromagnetic force of 1400kN / m, which are 54% and 75% higher than those of the ITER experimental reactor respectively. The yield strength of ITER's general low-temperature structural materials such as 304LN or 316LN is only about 350MPa, which can no longer meet the strength requirements. At low temperatures, the residual δ ferrite in 304LN or 316LN will induce martensitic phase transformation accompanied by stress and strain slip and twinning, resulting in a significant increase in magnetic permeability. Therefore, it is urgent to develop a new high-strength, non-magnetic austenitic stainless steel at 4.2K to meet the demand for ultra-low temperature structural steel in future nuclear fusion projects.

[0003] The nitrogen atoms in high-nitrogen austenitic stainless steel generate significant lattice distortion energy, resulting in a short-range ordered arrangement. This promotes planar slip of dislocations during hot rolling, causing dislocations from different slip systems to entangle and form dense pileups. This results in a higher work hardening coefficient than conventional austenitic stainless steel, but defects such as edge cracking often occur during rolling. Cold or warm rolling, however, uses dislocation slip and deformation twinning as the deformation mechanism, resulting in a good balance of strength and plasticity, but is limited to the production of thin-gauge steel and cannot be applied to large structural components. Furthermore, high-reduction cold rolling using the phase reversal principle causes the alloy to undergo martensitic transformation, resulting in an increase in magnetic permeability. Therefore, it is urgent to develop a low-temperature steel composition design and plastic deformation process that can meet the ultra-high yield strength and non-magnetic properties required for nuclear fusion engineering structural steel.

[0004] For the fusion engineering structural steel, only medium plate products can be used, and the current conventional forming process for producing medium plates is hot rolling. Compared with cold rolling forming process, hot rolling enhances the thermal activation effect of the rolled piece, reduces the dislocation slip resistance, is beneficial to promote cross slip and diffusion climb, thereby reducing the dislocation density and reducing the deformation resistance, but the strength of the steel plate is not as good as the work-hardened cold-rolled product. The present application designs a higher content of Cr, Mo, N and Mn, uses the above-mentioned drag effect of solid solution atoms to block dislocation slip, so that a large number of two-phase particles are precipitated after rolling, strain-induced precipitation consumes deformation energy, causes "Zener effect", hinders the movement of grain boundaries and dislocations, and suppresses the occurrence of dynamic recrystallization, thus showing certain work hardening characteristics. However, the high manganese and nitrogen austenitic stainless steel medium plate formed by hot rolling with the above design idea is prone to edge cracking, and the dislocation strengthening effect cannot be achieved at too high rolling temperature, which limits the application of high manganese and nitrogen austenitic stainless steel in fusion engineering. SUMMARY

[0005] The present application aims to provide a 4.2K yield strength 1350MPa grade fusion reactor superconducting magnet structure steel and its manufacturing method, which ensures that the performance indicators of the plate meet the design requirements of the latest generation of fusion pilot reactor, and can also be applied to the fields of superconducting, hydrogen storage and other fields with special requirements for ultra-low temperature toughness and ultra-low temperature strength.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: One of the technical schemes of the present application is to provide a 4.2K yield strength 1350MPa grade fusion reactor superconducting magnet structure steel, the chemical composition of the steel is as follows: C: 0.04%~0.06%, Si: 0.5%~1.0%, Mn: 6.5%~8.0%, Cr: 19.5%~21.0%, Ni: 10.0%~12.0%, Mo: 1.5%~2.0%, N: 0.25%~0.30%, V: 0.15%~0.25%, S≤0.002%, P≤0.02%, the balance being Fe and unavoidable impurities. The thickness of the steel plate is 20~80mm, and the width is ≥2000mm.

[0007] The above composition design and process parameters are mainly because: N: 0.25%~0.30% of nitrogen is designed, which can greatly improve the yield strength of austenitic stainless steel by using the characteristics of large lattice distortion energy of nitrogen atoms occupying the octahedral interstitial position of face-centered cubic crystal in austenitic stainless steel, and basically has no adverse effect on plasticity. However, its toughness will decrease significantly; by using the short-range ordered arrangement characteristics of nitrogen atoms, low strain rolling can promote dislocation plane slip, rapidly proliferate and form pile-ups, ensure a higher work hardening rate at the stage of rolling close to the finished thickness, and effectively improve the strength of the steel plate.

[0008] V: 0.15%~0.25% vanadium is designed. Through vanadium microalloying and controlled rolling, the precipitation of MX phase and Laves phase can be promoted. It has good stability and does not dissolve at high temperature. It can pin grain boundaries and dislocations during thermoplastic deformation, playing the role of fine grain strengthening and mass strengthening. In addition, it can prolong the incubation period of high temperature precipitation phase Cr2N, thereby inhibiting the precipitation of Cr2N at grain boundaries and effectively improving hot workability.

[0009] C: The carbon content is designed to be 0.04%~0.06%. The role of carbon is to ensure the strength of high manganese nitrogen austenitic stainless steel with low stacking fault energy, and the composite solid solution strengthening by replacing part of the nitrogen with C can reduce the ductile-brittle transition temperature.

[0010] The remaining elements include Si, Mn, Cr, Ni, and Mo. The main consideration is to meet the low SFE design and control the stacking fault energy (SFE) at: 20.5mJ / m 2 <SFE<30mJ / m 2 , has a lower ductile-brittle transition temperature, when SFE < 20mJ / m 2 During tensile deformation, martensitic transformation will be induced in the local stress concentration area. The specific stacking fault energy calculation formula is: SFE=-25.7+2[Ni]+41[C]+0.9[Cr]-77[N]-13[Si]+3.2[Mn]+9.3[Mo], where [w] is the mass percentage of the element, %. It satisfies 20.5mJ / m 2 <SFE<30mJ / m 2 The Si content is 0.5%~1.0%; Mn: 6.5%~8%; Cr: 19.5%~21%; Ni: 10.0%~12.0%; Mo: 1.5%~2.0%.

[0011] Furthermore, the metallographic structure of the finished steel plate is equiaxed austenite grains with a size of 16-21 μm, wherein the volume fraction of delta ferrite is less than 0.01%.

[0012] Furthermore, the mechanical properties of the finished steel plate are: yield strength at 4.2K temperature of 1350~1450MPa, tensile strength of 1750~1850MPa, elongation of 30%~40%, fracture toughness of 155~170MP·m 1 / 2 , room temperature magnetic permeability is less than 1.02.

[0013] The second technical solution of the present invention is to provide a method for manufacturing steel for fusion reactor superconducting magnet structure with a yield strength of 4.2K and a grade of 1350MPa. The method adopts the method of "continuous casting billet calendering + high deformation rate in high temperature stage + low deformation rate in low temperature stage + low temperature solid solution" to control the deformation temperature range of high strain rate and low strain rate during calendering. The method specifically includes the following steps: (1) Grouping; (2) The continuous casting billet is stacked and rolled: the stainless steel continuous casting billet after grouping is heated to 1120-1180℃, and is kept for 4-6h, the starting rolling temperature is ≤1050℃, and the final rolling temperature is >800℃; (3) Separation after rolling; (4) Low-temperature solid solution of the steel plate.

[0014] Further, in the step (1), the grouping process is as follows: the stainless steel continuous casting billets with the same geometric size and thickness of 100-300mm are grouped, the contact surface is polished and cleaned to remove the iron oxide scale, the roughness of the contact surface is controlled to be 1.5-3.5μm, the contact surface is coated with a separating agent with a thickness of 20-30μm, and the contact surface is welded around the edge, and the edge is welded by using stainless steel welding rods for bottoming and filling.

[0015] The purpose of controlling the roughness to be 1.5-3.5μm is to ensure that the contact surface has sufficient contact area, promote the mechanical engagement of the rolled piece during plastic deformation, increase the friction, and reduce the relative sliding during rolling; the purpose of using stainless steel welding material is to ensure the strength of the weld, and the contact surface does not open during rolling.

[0016] Coating the contact surface with a separating agent can ensure smooth rolling and easy separation of the steel plate after rolling; controlling the thickness to be 20-30μm can ensure that the separating agent has good thermal stability during rolling, and avoid the atomic mutual diffusion at the upper and lower contact surfaces of the rolled piece to form a firm metallurgical bond.

[0017] Further, in the step (2), according to two-stage rolling, the first stage deformation rate is ≥60%, the deformation temperature is 1050-950℃, and the roller speed is 5-7m / s; the second stage deformation rate is ≥30%, the deformation temperature is 949-800℃, and the roller speed is 3-4m / s.

[0018] Different from the traditional medium plate production method, the purpose of designing the stacking and rolling process is mainly to consider the deformation characteristics of the medium plate rolling mill, the frictional resistance and the deformation resistance cause the deformation degree of the surface layer to one-fourth to be greater than the core, by using this characteristic, through the stacking and rolling method, the loose and segregated serious area of the original billet core becomes the one-fourth position of the double-layer stacked and rolled billet, thereby increasing the deformation permeability of the original billet core and improving the core quality of the steel plate; in addition, the increased strain of the steel plate core can also promote the lattice distortion of the grains in the core of the rolled piece during plastic deformation, increase the dislocation density, and form dislocation pile-up, steps and dislocation walls, etc. between dislocations, which hinder the further movement of dislocations, through low dislocation energy design, even at high temperature deformation, the recovery speed is still relatively slow, and the work hardening effect cannot be completely eliminated, so that the dislocation density of the steel plate surface layer to the core can continuously increase with the increase of the deformation degree.

[0019] By controlling the roll speed and the cumulative reduction ratios during recrystallization and non-recrystallization rolling, and adopting a high deformation rate rolling strategy, the onset and progression of the softening process is shortened, the softening process is inhibited, the dislocation movement speed is accelerated, and the dynamic softening of the rolled piece is limited, with work hardening being the dominant factor. As the thickness approaches the finished product, the distortion energy caused by the dislocation stress field accumulates to a certain level, where it is released, recrystallization occurs, and a large number of dislocations disappear, resulting in a softening effect that is better than recovery. This significantly reduces deformation resistance, which facilitates the control of plate shape and thickness accuracy.

[0020] When rolling to 950℃, low strain rate deformation is adopted, and strain-induced precipitation promotes the precipitation of two-phase particles. The precipitation process consumes strain storage energy, and the precipitated phase pins the large-angle grain boundaries, limiting the migration of strain grain boundaries and local bowing, reducing nucleation, increasing the thermal deformation activation energy, and ultimately inhibiting dynamic recrystallization, ensuring that continuous and discontinuous dynamic recrystallization does not occur below 950℃, which is beneficial to inhibiting the occurrence of mixed crystals.

[0021] When the rolling temperature is too high, it is difficult for the precipitated phase to undergo strain-induced precipitation, and the microalloyed two-phase particles cannot pin the large-angle grain boundaries and dislocations. In addition, too high a deformation temperature will also accelerate the diffusion rate of defects such as vacancies, strengthen the cross-slip and climb of dislocations, and make recovery more easily activated, making it easier for hot deformation dynamic recrystallization to occur. Therefore, in order to ensure the hardening effect of rolling deformation, the deformation start temperature is limited to below 1050℃.

[0022] Furthermore, in step (3), plasma is used to cut the periphery of the steel plate, and a vacuum suction cup crane is used to lift the upper steel plate to separate the steel plates.

[0023] Furthermore, in step (4), the steel plate is heated with the furnace, the initial furnace temperature is below 200°C, the heating rate below 900°C is controlled at 0.8~1.2°C / min; the heating rate between 900~1000°C is controlled at 4~6°C / min; the holding temperature is controlled at 1000~1020°C, and the holding time is controlled at 0.08~0.12min / mm.

[0024] Considering that there are a large number of dislocations in the rolled structure after large deformation by stacking, the plasticity and toughness are significantly reduced; the residual stress of the deformed structure poses a great hidden danger to service safety. Therefore, a slow heating strategy is adopted when heating from room temperature to 900°C to eliminate dislocations and residual stress by recovery. Since the present invention controls the amount of precipitated phases such as Cr2N and σ that seriously affect the mechanical properties through reasonable composition design, and considering the high sensitivity of grain growth rate to solution temperature, the solution temperature is set at 1000~1020°C and the holding time is 0.08~0.12min / mm, which can enable recrystallization to be completed rapidly in a short time without causing grain growth, and finally obtain a uniform equiaxed solid solution grain structure with a size of 16~21μm.

[0025] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts large tandem rolling deformation for the ingot, and the rolled piece can continuously accumulate dislocation density during the high-temperature plastic deformation. The distortion energy caused by the dislocation stress field will be released after accumulating to a certain extent, and the grain core will be formed at the most severely distorted position, thereby causing recrystallization to occur, so that a large number of dislocations disappear, which is better than the recovery softening effect. Through the tandem rolling process, the nominal thickness of the steel plate in the rolling process is increased, the cooling rate in the later stage of rolling is reduced, and the plastic deformation is always maintained at a high temperature, thereby significantly reducing the deformation resistance; through V micro-alloying controlled rolling, the precipitation of MX phase and Laves phase is promoted, so that the grain boundaries and dislocations can be pinned during the plastic deformation of the rolled piece, the grains are refined and the precipitation of Cr2N at the grain boundaries is suppressed, effectively solving the edge cracking problem of the high-nitrogen austenitic stainless steel plate; finally, by designing a low-temperature solid solution process, it is ensured that the recrystallization is completed quickly in a short time without causing the grains to grow, and finally a uniform equiaxed solid solution grain structure with a size of 16-21 μm is obtained. The maximum thickness of the steel plate is 80mm, the 4.2K yield strength exceeds 1350MPa, and the fracture toughness exceeds 150MP·m 1 / 2 Non-magnetic austenitic stainless steel.

[0026] Through the design of high C and high N composition, the solid solution strengthening effect of C and N is fully utilized; the low Ni and low Mo composition design meets the medium and low stacking fault energy and has a lower ductile-brittle transition temperature; through V microalloying, combined with controlled rolling, the precipitation of MX phase and Laves phase can be promoted and pinned to grain boundaries and dislocations, playing the role of fine grain strengthening and mass strengthening and inhibiting the precipitation of Cr2N at grain boundaries, ensuring that the rolled piece does not produce edge cracks when deformed above 800℃, effectively improving the hot working performance of high nitrogen austenitic stainless steel. The above composition design ensures that the stacking fault energy is within a reasonable range, avoiding the deformation of the steel plate under the tensile stress of the huge electromagnetic force when serving at ultra-low temperature, and the possibility of inducing martensitic phase transformation in the local stress concentration area; the rationally designed medium and thick plate casting multi-layer rolling deformation process and low-temperature short-time solution process, the cumulative deformation is more than twice the deformation of the conventional single block continuous casting billet, combined with the low stacking fault energy composition design, under the condition of large strain, it can ensure that the rolled piece will fully undergo lattice distortion during the plastic deformation process, and the interaction between dislocations will form dislocation pileups, steps and dislocation walls etc., hindering the further movement of dislocations, inhibiting recovery, retaining the work hardening effect, and causing the dislocation density to increase continuously with the increase of deformation degree. At the same time, the original coarse grains can be fully flattened, a large number of deformation bands divide and refine the grains, and a large number of stacking faults are generated inside the grains. Finally, after low-temperature short-time solid solution, the residual stress of the steel plate is eliminated, the yield strength at 4.2K is 1350~1450MPa, the tensile strength is 1750~1850MPa, the elongation is greater than 30%, the fracture toughness is greater than 150MP·m1 / 2, and the room temperature permeability is less than 1.02. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a metallographic photograph of the finished steel plate obtained in Example 1 of the present invention; Figure 2 This is a transmission microstructure photograph of the finished steel plate obtained in Example 2 of the present invention at a temperature of 4.2K; Figure 3 The tensile stress-strain curve of the finished steel plate at 4.2K is obtained in Example 3 of the present invention. DETAILED DESCRIPTION

[0028] The following examples are only some of the best embodiments of the present invention and do not limit the scope of the aforementioned invention and technical means.

[0029] A 4.2K yield strength, 1350 MPa-class steel for fusion reactor superconducting magnet structures has a chemical composition of C: 0.04%-0.06%; Si: 0.5%-1.0%; Mn: 6.5%-8.0%; S≤0.002%; P≤0.02%; Cr: 19.5%-21.0%; Ni: 10.0%-12.0%; Mo: 1.5%-2.0%; N: 0.25%-0.30%; V: 0.15%-0.25%; the balance being Fe and unavoidable impurities. Table 1 shows the composition of each example.

[0030]

[0031] A method for manufacturing steel for fusion reactor superconducting magnet structures with a 4.2K yield strength and a 1350MPa grade is disclosed. The method employs a process of "continuously cast billet calendering + high deformation rate in a high temperature stage + low deformation rate in a low temperature stage + low temperature solid solution" to control the deformation temperature ranges of high and low strain rates during calendering. The method specifically comprises the following steps: (1) Assembly: Select stainless steel continuous casting billets with the same geometric dimensions and a thickness of 100-300 mm for assembly. The contact surfaces are polished and cleaned to remove iron oxide scale. The contact surface roughness is controlled to 1.5-3.5 μm. Silicon nitride (Si3N4) is applied to the contact surfaces with a thickness of 20-30 μm. Silicon nitride is used as an isolation agent due to its high temperature stability, good heat resistance, and lubricity. The contact surfaces are welded and sealed. The sealing is performed using 317 steel electrodes for bottoming and filling. Table 2 shows the assembly and billet heating process for each embodiment.

[0032]

[0033] (2) Roll-bonding: The assembled stainless steel continuous casting slabs were heated to 1120-1180°C and held for 4-6 hours. The starting rolling temperature was ≤1050°C and the final rolling temperature was >800°C. Roll-bonding was performed in two stages. The first stage had a deformation rate ≥60%, a deformation temperature of 1050-950°C, and a roll speed of 5-7 m / s. The second stage had a deformation rate ≥30%, a deformation temperature of 949-800°C, and a roll speed of 3-4 m / s. Table 3 shows the roll-bonding and post-rolling separation processes for various examples.

[0034]

[0035] (3) Separation after rolling: Use plasma to cut the four sides of the steel plate, and use a vacuum suction cup crane to lift the upper steel plate for steel plate separation.

[0036] (4) Low-temperature solution treatment of the steel plate. The steel plate is heated in the furnace. The initial furnace temperature is below 200°C. The heating rate below 900°C is controlled at 0.8-1.2°C / min; the heating rate between 900°C and 1000°C is controlled at 4-6°C / min; the holding temperature is controlled at 1000-1020°C, and the holding time is controlled at 0.08-0.12 min / min. Table 4 shows the solution treatment process parameters for each example.

[0037]

[0038] Table 5 shows the grain size and properties of each embodiment.

[0039]

[0040] As can be seen from Table 5, the metallographic structure of the finished steel plate is equiaxed austenite grains with a size of 16-21 μm, with a delta ferrite volume fraction of less than 0.01%. The mechanical properties of the finished steel plate are: yield strength at 4.2K of 1350-1450 MPa, tensile strength of 1750-1850 MPa, elongation of 30%-40%, and fracture toughness of 155-170 MP·m. 1 / 2 , room temperature magnetic permeability is less than 1.02.

[0041] Figure 1 The metallographic microstructure photographs of the finished steel plate obtained in Example 1 show that the low stacking fault energy component design combined with the double-layer roll-bonding deformation process and the low-temperature short-time solution process ensures that the rolled piece undergoes sufficient lattice distortion during plastic deformation under large strain conditions, forming a large amount of dislocation pile-up that hinders dislocation movement and preserves the work hardening effect. Finally, after the low-temperature short-time solution process, the steel plate has a uniform austenite equiaxed grain structure exceeding level 8. Figure 2The transmissive micrograph of the finished steel plate obtained in Example 2 at 4.2K shows that the stacking fault energy of the alloy is significantly reduced by nitrogen alloying. The stacking fault energy is further reduced under ultra-low temperature conditions, leading to the nucleation and expansion of a large number of nanotwins during deformation, significantly improving the strain hardening ability of the material. Figure 3 From the tensile stress-strain curve of the finished steel plate obtained in Example 3 at a temperature of 4.2K, it can be seen that due to the ultra-low temperature plastic deformation causing dislocation proliferation, nano-twins cause the stress to continue to rise, and the tensile strength reaches more than 1750 MPa. In addition, the fine grains and high density of dislocations inhibit the formation of Lüders bands, resulting in the curve showing continuous yield characteristics without an obvious yield platform. The high density of dislocations and twins hinders dislocation slip, the sample deforms uniformly, and the yield strength reaches more than 1350 MPa.

[0042] It should be noted that the above embodiments are only for illustrating the technical concept and features of the present invention and are not intended to limit the present invention. Any equivalent replacement or modification that does not depart from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A 4.2K yield strength 1350MPa grade fusion reactor superconducting magnet structure steel, characterized in that: The chemical composition of steel in weight percentage is: C: 0.04%~0.06%, Si: 0.5%~1.0%, Mn: 6.5%~8.0%, S≤0.002%, P≤0.02%, Cr: 19.5%~21.0%, Ni: 10.0%~12.0%, Mo: 1.5%~2.0%, N: 0.25%~0.30%, V: 0.15%~0.25%, and the balance is Fe and unavoidable impurities.

2. The steel for fusion reactor superconducting magnet structure with a 4.2K yield strength of 1350 MPa according to claim 1, characterized in that: The contents of Si, Mn, Cr, Ni and Mo satisfy the low stacking fault energy SFE design, and the stacking fault energy is controlled to be 20.5 mJ / m 2 <SFE<30mJ / m 2 The empirical formula for stacking fault energy is: SFE=-25.7+2[Ni]+41[C]+0.9[Cr]-77[N]-13[Si]+3.2[Mn]+9.3[Mo], where [w] is the mass percentage of the element, %.

3. The steel for fusion reactor superconducting magnet structure with a 4.2K yield strength of 1350 MPa according to claim 1, characterized in that: The thickness of the steel plate is 20~80mm and the width is ≥2000mm.

4. The steel for fusion reactor superconducting magnet structure with a 4.2K yield strength of 1350 MPa according to claim 1, characterized in that: The metallographic structure of the steel plate is equiaxed austenite grains with a size of 16~21μm, in which the volume fraction of delta ferrite is less than 0.01%.

5. The steel for fusion reactor superconducting magnet structure with a 4.2K yield strength of 1350 MPa according to claim 1, characterized in that: Yield strength at 4.2K is 1350~1450MPa, tensile strength is 1750~1850MPa, elongation is 30%~40%, and fracture toughness is 155~170MP·m 1 / 2 , room temperature magnetic permeability is less than 1.

02.

6. A method for manufacturing a 4.2K yield strength 1350 MPa grade fusion reactor superconducting magnet structure steel according to any one of claims 1 to 4, characterized in that: The method of continuous casting slab rolling + high deformation rate in high temperature stage + low deformation rate in low temperature stage + low temperature solution is adopted to control the deformation temperature range of high strain rate and low strain rate during rolling; specifically, the steps include: (1) Assembly; (2) Rolling: The stainless steel continuous casting billet after assembly is heated to 1120~1180℃, kept warm for 4~6h, and then rolled in two stages, with the starting rolling temperature ≤1050℃ and the final rolling temperature >800℃; (3) Separation after rolling; (4) Low-temperature solid solution of steel plates.

7. The method for manufacturing a 4.2K yield strength 1350 MPa grade fusion reactor superconducting magnet structure steel according to claim 6, characterized in that: In the step (1), the billet assembly process is as follows: stainless steel continuous casting billets with the same geometric dimensions and a thickness of 100-300 mm are selected for assembly, the contact surfaces are polished and cleaned to remove iron oxide scale, the contact surface roughness is controlled to be 1.5-3.5 μm, an isolation agent is applied to the contact surface with a thickness of 20-30 μm, and the contact surfaces are welded and sealed. The edge sealing is performed by flat welding with a stainless steel electrode for base and filling.

8. The method for manufacturing a 4.2K yield strength 1350 MPa grade fusion reactor superconducting magnet structure steel according to claim 6, characterized in that: In the step (2), the deformation rate in the first stage is ≥60%, the deformation temperature is 1050~950°C, and the roller speed is 5~7m / s; the deformation rate in the second stage is ≥30%, the deformation temperature is 949~800°C, and the roller speed is 3~4m / s.

9. The method for manufacturing a 4.2K yield strength 1350 MPa grade fusion reactor superconducting magnet structure steel according to claim 6, characterized in that: In the step (3), plasma is used to cut the periphery of the steel plate, and a vacuum suction cup crane is used to lift the upper steel plate to separate the steel plates.

10. The method for manufacturing a fusion reactor superconducting magnet structure steel with a 4.2K yield strength of 1350 MPa according to claim 6, characterized in that: In the step (4), the steel plate is heated with the furnace, the initial furnace temperature is below 200°C, the heating rate below 900°C is controlled at 0.8~1.2°C / min; the heating rate between 900~1000°C is controlled at 4~6°C / min; the holding temperature is controlled at 1000~1020°C, and the holding time is controlled at 0.08~0.12min / mm.

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