A duplex stainless steel plate with strong corrosion resistance and low anisotropy and a manufacturing method thereof

Abstract

The present application relates to the technical field of special steel material production, and in particular to a duplex stainless steel plate with strong corrosion resistance and low anisotropy and a manufacturing method. The duplex stainless steel plate with strong corrosion resistance and low anisotropy comprises: C: 0.02~0.04%; N: 0.1~0.15%; Cr: 19.5~21.5%; Mn: 4.0~5.0%; Cu: 0.1~0.3%; Ni: 1.0~1.4%; Si: 0.3~0.6%, the remainder being Fe and unavoidable trace elements; the yield strength ratio YS of the duplex stainless steel plate along the rolling direction and the transverse direction is RD : YS TD The ratio is 1.05 to 1.20. The embodiment of the present application provides a duplex stainless steel plate with strong corrosion resistance and small anisotropy and a manufacturing method to solve the problem of insufficient corrosion resistance and strong anisotropy of duplex stainless steel plates in the related art.

Description

Technical Field

[0001] The present application relates to the technical field of special steel material production, and in particular to a duplex stainless steel plate with strong corrosion resistance and low anisotropy and a manufacturing method thereof. Background Art

[0002] Duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel. With its high strength, excellent corrosion resistance and good welding performance, its application in oil and gas transportation, chemical ship manufacturing, nuclear power and other fields is also expanding.

[0003] Duplex stainless steel is rich in ferrite-forming elements. During the solidification process, it is first completely transformed into a ferrite structure. As the temperature further decreases, the austenite phase nucleates and grows in the ferrite matrix to form a duplex structure. In the subsequent thermal deformation process, especially the large deformation hot rolling process, the austenite phase and the ferrite phase are extended and arranged along the rolling direction, and finally a lamellar duplex structure is formed. This lamellar duplex structure is difficult to change significantly in the subsequent traditional heat treatment process. Therefore, the duplex stainless steel sheet prepared by rolling and other methods generally has obvious anisotropy. For example, the plasticity such as the transverse room temperature elongation at break or uniform elongation is significantly lower than that in the rolling direction, while the transverse strength performance is significantly higher than that in the rolling direction. For example, when the duplex stainless steel sheet is stamped, obvious "earing" defects will be generated. In addition, pitting resistance is also an issue that duplex stainless steel needs to focus on. Therefore, while improving the pitting resistance of duplex stainless steel, how to improve the anisotropy of duplex stainless steel sheet is a technical problem that needs to be solved in this field. Summary of the invention

[0004] The embodiments of the present application provide a duplex stainless steel plate with strong corrosion resistance and low anisotropy and a manufacturing method thereof, so as to solve the problem that the duplex stainless steel plate in the related art has insufficient corrosion resistance and strong anisotropy.

[0005] In a first aspect, a duplex stainless steel plate with strong corrosion resistance and low anisotropy is provided, which comprises, by mass percentage:

[0006] C: 0.02~0.04%; N: 0.1~0.15%; Cr: 19.5~21.5%; Mn: 4.0~5.0%; Cu: 0.1~0.3%; Ni: 1.0~1.4%; Si: 0.3~0.6%, the balance is Fe and unavoidable trace elements;

[0007] The yield strength ratio YS of the bidirectional stainless steel plate along the rolling direction and the transverse direction RD : YS TD The ratio is 1.05~1.20.

[0008] In a second aspect, a method for manufacturing a duplex stainless steel plate having strong corrosion resistance and low anisotropy is provided, which comprises the following steps:

[0009] S1. Melting: Put the raw materials into the smelting equipment and heat them to 1500~1700℃ under a protective gas atmosphere to obtain a steel billet;

[0010] S2, hot rolling: heating the steel billet to 1100-1300° C. to obtain a duplex stainless steel hot-rolled steel plate;

[0011] S3, solid solution:

[0012] S4, cold rolling: the duplex stainless steel hot-rolled steel plate after solid solution treatment in S3 is cold rolled for 4 to 5 times to obtain a cold-rolled steel plate, the reduction amount of a single cold rolling is 20 to 30%, and the total reduction amount is 70 to 80%;

[0013] S5, two-phase zone quenching:

[0014] Using thermodynamic calculation software, the temperature T corresponding to the α+γ two-phase ratio of 50% was calculated. D The cold rolled steel sheet after S4 treatment is subjected to two-phase solid solution treatment at a solid solution temperature of T D ±30℃, solution treatment for 20~60min, then water-cooling to below 300℃ to obtain duplex stainless steel plate;

[0015] S6. Post-treatment: Rinse the duplex stainless steel plate obtained in S5 with clean water, then put the rinsed duplex stainless steel plate into 50% nitric acid at 60-80°C, keep it for 10-30 minutes, and then wash the duplex stainless steel plate with clean water.

[0016] Preferably, S3 comprises the following steps:

[0017] S301, pre-solidification: the duplex stainless steel hot-rolled steel plate is pre-solidified at 1100~1200℃ for 30~60min;

[0018] S302, Secondary solid solution: According to the calculation results of Thermo-Calc software, the ferrite transformation temperature T is obtained. α ;

[0019] The hot-rolled duplex stainless steel plate after pre-solidification in S301 is heated to (T α +T0) solution treatment for 10 minutes, then out of the furnace and water-cooled to below 300°C, wherein the T0 value is 10~30°C.

[0020] Preferably, in S4, the secondary cold rolling is performed in 5 passes.

[0021] Preferably, the cold rolling of S4 uses a cold rolling mill, and the cold rolling mill is equipped with an ultrasonic vibration device, and the ultrasonic frequency of the ultrasonic vibration device is 20-50kHz.

[0022] Preferably, in S1, the protective gas is nitrogen or argon.

[0023] The beneficial effects of the technical solution provided by this application include:

[0024] The embodiment of the present application provides a duplex stainless steel plate with strong corrosion resistance and low anisotropy and a manufacturing method. By improving the mass fraction of the stainless steel plate raw material, the anisotropy of the mechanical properties of the manufactured duplex stainless steel measured along different directions of the plane is significantly weakened, such as the yield strength ratio along the rolling direction and the transverse direction, i.e., YS RD : YS TD The value is between 1.05 and 1.20, which effectively reduces the internal anisotropy and improves the corrosion resistance. It can meet the needs of some economical duplex stainless steel cold-rolled strips with high forming quality and certain corrosion resistance for components, thereby meeting the performance requirements. Therefore, it can solve the problems of insufficient corrosion resistance and strong anisotropy of duplex stainless steel plates in related technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 A schematic diagram of the calculation results of the Thermo-Calc software provided in Examples 1 and 2 of the present application;

[0027] Figure 2 A process flow chart of a method for manufacturing a duplex stainless steel plate with strong corrosion resistance and low anisotropy provided in this application;

[0028] Figure 3 Schematic diagram of the metallographic structure of the duplex stainless steel material manufactured in Example 1 of the present application and the uniaxial tensile curves along the rolling direction and the transverse direction;

[0029] Figure 4 Schematic diagram of the metallographic structure of the duplex stainless steel material manufactured in Example 2 of the present application and the uniaxial tensile curves along the rolling direction and the transverse direction;

[0030] Figure 5Schematic diagram of the metallographic structure of the duplex stainless steel material manufactured in the comparative example of the present application, and the uniaxial tensile curves along the rolling direction and the transverse direction;

[0031] Figure 6 The potentiodynamic polarization curves (current density 10 -4 A / cm 2 The corresponding voltage is the critical pitting potential). DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0033] See also Figures 1 to 6 As shown, the present application provides a method for manufacturing a duplex stainless steel plate with strong corrosion resistance and small anisotropy, which can solve the problems of insufficient corrosion resistance and strong anisotropy of the duplex stainless steel plate in the related art. Specifically, the chemical composition of the duplex stainless steel plate includes, by mass percentage, C: 0.02-0.04%; N: 0.1-0.15%; Cr: 19.5-21.5%; Mn: 4.0-5.0%; Cu: 0.1-0.3%; Ni: 1.0-1.4%; Si: 0.3-0.6%, and the balance is Fe and unavoidable trace elements, which includes the following steps:

[0034] S1. Melting: Put the material into the melting equipment and heat it to 1500~1700℃ under protective gas atmosphere to obtain steel billet;

[0035] In some embodiments, the protective gas is nitrogen or argon.

[0036] S2, hot rolling: heating the steel billet to 1100-1300° C. to obtain a duplex stainless steel hot-rolled steel plate;

[0037] S3, solution treatment: heat the duplex stainless steel hot-rolled steel plate to 1100~1200℃ and keep it at the solution treatment temperature for 1~2h;

[0038] S301, pre-solidification: The duplex stainless steel hot-rolled steel plate is pre-solidified at 1100-1200℃ for 30-60min, the purpose of which is to promote the solute atoms to dissolve in the matrix at a lower temperature while avoiding excessive coarsening of the grains;

[0039] S302, secondary solid solution: see Figure 1 According to the calculation results of Thermo-Calc software, the ferrite transformation temperature T α ;

[0040] The hot-rolled duplex stainless steel plate after pre-solidification in S301 is heated to (T α +T0) (i.e., ferrite single-phase region) solution treatment for 10 to 30 minutes, then out of the furnace and water-cooled to below 300°C, wherein the T0 value is 10 to 30°C, in order to form a ferrite single-phase structure.

[0041] S4, cold rolling: the duplex stainless steel hot-rolled steel plate after solid solution treatment in S3 is cold rolled for 4 to 5 times to obtain a cold-rolled steel plate, the reduction amount of a single cold rolling is 20 to 30%, and the total reduction amount is 70 to 80%;

[0042] In some embodiments, the cold rolling in S4 uses a cold rolling mill, and the cold rolling mill is equipped with an ultrasonic vibration device, and the ultrasonic frequency of the ultrasonic vibration device is 20~50kHz; further, the ultrasonic vibration device is arranged on the roller, and when the cold rolling mill is used to cold roll the duplex stainless steel hot-rolled steel plate, the ultrasonic vibration device is synchronously turned on, so that the roller generates high-frequency vibration during the rolling process. This vibration is transmitted to the duplex stainless steel hot-rolled steel plate through the roller, so as to reduce friction, reduce stress concentration, and promote plastic deformation of the material; through high-frequency micro-vibration acting on the surface of the duplex stainless steel hot-rolled steel plate, the molecules and grains inside the duplex stainless steel hot-rolled steel plate undergo tiny displacement and deformation, thereby improving the performance of the material.

[0043] S5, two-phase zone quenching:

[0044] Use thermodynamic calculation software to calculate the temperature T corresponding to the α+γ two-phase ratio of 50% D The cold rolled steel sheet after S4 treatment is subjected to two-phase solid solution treatment at a solid solution temperature of T D ±30℃, solution treatment for 20~60min, then water-cooling to below 300℃ to obtain duplex stainless steel plate;

[0045] S6. Post-treatment: Rinse the duplex stainless steel plate obtained in S5 with clean water, then put the rinsed duplex stainless steel plate into 50% nitric acid at 60-80°C, keep it for 10-30 minutes, and then wash the duplex stainless steel plate with clean water.

[0046] Example 1

[0047] See also Figure 2 As shown, the method for manufacturing a duplex stainless steel plate with strong corrosion resistance and low anisotropy in this embodiment includes the following steps:

[0048] S1. Melting: Put the raw materials into a smelting device and heat them to 1500° C. in a nitrogen atmosphere to obtain a steel billet;

[0049] S2, hot rolling: heating the steel billet to 1100° C. to obtain a duplex stainless steel hot-rolled steel plate;

[0050] S3, solid solution: the duplex stainless steel plate with a thickness of 5mm is subjected to solid solution treatment:

[0051] S301, pre-solidification: the duplex stainless steel hot-rolled steel plate is pre-solidified at 1100℃ for 30min;

[0052] S302, Secondary solid solution: According to the calculation results of Thermo-Calc software, the ferrite transformation temperature T α 1295℃;

[0053] The duplex stainless steel hot-rolled steel plate after pre-solidification in S301 was heated to 1305°C, kept at this temperature for 10 minutes, and then water-cooled to room temperature.

[0054] The chemical composition of this duplex steel is C: 0.025%; N: 0.12%; Cr: 19.8%; Mn: 4.5%; Cu: 0.15%; Ni: 1.2%; Si: 0.5%.

[0055] S4, cold rolling: the duplex stainless steel hot-rolled steel plate after the solid solution treatment in S3 is rolled 4 times at room temperature using a cold rolling mill to obtain a rolled stainless steel plate; the cold rolling mill includes a rolling roller, on which a piezoelectric ceramic ultrasonic transducer fixed by a mechanical coupling device is arranged, and the ultrasonic frequency of the piezoelectric ceramic ultrasonic transducer is 20kHz; the pressing amount per single pass is 20-30%, and after four passes of rolling, the rolling pressing amount is 70%, and the final thickness of the stainless steel plate after rolling is 1.5mm.

[0056] S5, two-phase zone quenching: The stainless steel plate after cold rolling in S4 is subjected to three solid solution treatments, that is, it is kept at 1100℃ for 20 minutes, and then taken out of the furnace and water-cooled to below 300℃ to obtain a duplex stainless steel plate.

[0057] Specifically, Thermo-Calc was used to calculate the temperature T corresponding to the α+γ two-phase region when the two-phase ratio was 50%. D , T D It is 1080℃.

[0058] S6. Post-treatment: The duplex stainless steel plate obtained in S5 is rinsed with clean water, and then the rinsed duplex stainless steel plate is placed in 50% nitric acid at 60°C, kept for 20 minutes, and then the duplex stainless steel plate is washed with clean water.

[0059] The metallographic structure of the duplex stainless steel plate produced in this embodiment is as follows: Figure 3 (a) shows the uniaxial tensile curves along the rolling direction (RD) and transverse direction (TD). Figure 3 (b) shown.

[0060] The method used in this embodiment basically eliminates the original banded structure. In addition, the anisotropy of mechanical properties along different directions is weak, such as the yield strength ratio along the rolling direction and the transverse direction, i.e., YS RD : YS TD The value of is only 1.13, see Figure 6 As shown in (a), the critical pitting potential tested by the standard method is 0.2 V (vs. SCE).

[0061] Example 2

[0062] See also Figure 2 As shown, the method for manufacturing a duplex stainless steel plate with strong corrosion resistance and low anisotropy in this embodiment includes the following steps:

[0063] S1. Melting: Put the raw materials into a melting device and heat them to 1700° C. in an argon atmosphere to obtain a steel billet;

[0064] S2, hot rolling: heating the steel billet to 1300° C. to obtain a duplex stainless steel hot-rolled steel plate;

[0065] S3, solid solution: the duplex stainless steel plate with a thickness of 5mm is subjected to solid solution treatment:

[0066] S301, pre-solidification: the duplex stainless steel hot-rolled steel plate is pre-solidified at 1150°C for 60 minutes;

[0067] S302, Secondary solid solution: According to the calculation results of Thermo-Calc software, the ferrite transformation temperature T α 1295℃;

[0068] The duplex stainless steel hot-rolled steel plate after pre-solidification in S301 was heated to 1320°C, kept at this temperature for 10 minutes, and then water-cooled to room temperature.

[0069] The chemical composition of this dual-phase steel is C: 0.025%; N: 0.12%; Cr: 19.8%; Mn: 4.5%; Cu: 0.15%; Ni: 1.2%; Si: 0.5%. Its metallographic structure is as follows Figure 2 As shown in a in .

[0070] S4, cold rolling: the duplex stainless steel hot-rolled steel plate after the solid solution treatment in S3 is rolled 5 times at room temperature using a cold rolling mill to obtain a rolled stainless steel plate; the cold rolling mill includes a rolling roller, on which a piezoelectric ceramic ultrasonic transducer fixed by a mechanical coupling device is arranged, and the ultrasonic frequency of the piezoelectric ceramic ultrasonic transducer is 50kHz; the pressing amount per single pass is 20-30%, and after five rolling passes, the total pressing amount reaches 80%, and the final thickness of the stainless steel plate after rolling is 1mm.

[0071] In this embodiment, the single pressing amount in the cold rolling step is not completely equal, and the final total pressing amount is 80%.

[0072] S5, two-phase zone quenching: the stainless steel plate after cold rolling in S4 is subjected to two-phase zone solution treatment, that is, it is kept at 1080℃ for 30min, and then taken out of the furnace and water-cooled to 200℃ to obtain a duplex stainless steel plate.

[0073] Specifically, Thermo-Calc was used to calculate the temperature T corresponding to the α+γ two-phase region when the two-phase ratio was 50%. D , T D It is 1080℃.

[0074] S6. Post-treatment: The duplex stainless steel plate obtained in S5 is rinsed with clean water, and then the rinsed duplex stainless steel plate is placed in 50% nitric acid at 80°C, kept for 30 minutes, and then the duplex stainless steel plate is washed with clean water.

[0075] The metallographic structure of the duplex stainless steel plate produced in this embodiment is as follows: Figure 4 (a) shows the uniaxial tensile curves along the rolling direction (RD) and transverse direction (TD). Figure 4 (b) shown.

[0076] The method used in this embodiment basically eliminates the original banded structure. In addition, the anisotropy of mechanical properties along different directions is weak, such as the yield strength ratio along the rolling direction and the transverse direction, i.e., YS RD : YS TD The value of is only 1.15, see Figure 6 As shown in (a), the critical pitting potential tested by the standard method is 0.19 V (vs. SCE).

[0077] It should be noted that the present application also provides other embodiments, which are different from Example 1 in that the heating temperature in step S1 is 1600°C, the heating temperature in step S2 is 1200°C, the pre-solidification temperature in S301 is 1200°C, and the treatment time is 40min; the temperature is raised to 1325°C in S302, the solid solution treatment is performed for 60min in S5, and the rinsed duplex stainless steel plate is placed in 50% nitric acid at 70°C in S6. After keeping it for 10min, the duplex stainless steel plate is washed with clean water. The obtained duplex stainless steel plate is close to the physical phase results of Example 1 and Example 2, which will not be repeated here, and only the results of Example 1 and Example 2 are shown and explained in the drawings.

[0078] Comparative Example

[0079] The manufacturing method of the duplex stainless steel plate with strong corrosion resistance and small anisotropy in this comparative example comprises the following steps:

[0080] S1. Melting: Put the raw materials into a smelting device and heat them to 1700°C in a nitrogen atmosphere to obtain a steel billet;

[0081] S2, hot rolling: heating the steel billet to 1100° C. to obtain a duplex stainless steel hot-rolled steel plate;

[0082] S3. Solution treatment: The duplex stainless steel plate with a thickness of 5 mm was solution treated at 1100°C for 30 min and then water cooled to room temperature.

[0083] The chemical composition of this duplex steel is C: 0.025%; N: 0.12%; Cr: 19.8%; Mn: 4.5%; Cu: 0.15%; Ni: 1.2%; Si: 0.5%.

[0084] The metallographic structure of the duplex stainless steel plate prepared in this comparative example is as follows: Figure 5 (a) shows the uniaxial tensile curves along the rolling direction (RD) and transverse direction (TD). Figure 5 (b) shown.

[0085] The mechanical properties of the duplex stainless steel plate obtained in this comparative example are highly anisotropic along different directions. Taking the yield strength ratio along the rolling direction and the transverse direction as an example, that is, YS RD : YS TD The value of is 1.9, see Figure 6 As shown, the critical pitting potential tested by the standard method is 0.14 V (vs. SCE).

[0086] In summary, the duplex stainless steel plate YS RD : YS TDThe difference in yield strength along the rolling direction and the transverse direction is greater, and the anisotropy is stronger. The duplex stainless steel plate YS manufactured in Example 1 and Example 2 RD : YS TD Smaller, that is, smaller anisotropy.

[0087] The critical pitting potential of the duplex stainless steel plate manufactured in the comparative example is lower than that of the duplex stainless steel plates manufactured in Example 1 and Example 2, that is, the duplex stainless steel plates obtained in Example 1 and Example 2 have a stronger ability to resist pitting corrosion, that is, their corrosion resistance is better; that is, the duplex stainless steel plates obtained in Example 1 and Example 2 have stronger corrosion resistance and smaller anisotropy.

[0088] Furthermore, the present application also provides a duplex stainless steel plate with strong corrosion resistance and low anisotropy, which comprises, by mass percentage:

[0089] C: 0.02~0.04%; N: 0.1~0.15%; Cr: 19.5~21.5%; Mn: 4.0~5.0%; Cu: 0.1~0.3%; Ni: 1.0~1.4%; Si: 0.3~0.6%, the remainder is Fe and unavoidable trace elements;

[0090] The yield strength ratio YS of the bidirectional stainless steel plate along the rolling direction and the transverse direction RD : YS TD The ratio is 1.05~1.20.

[0091] Specifically, the preparation process thereof refers to the above-mentioned Examples 1 and 2.

[0092] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A duplex stainless steel plate with strong corrosion resistance and small anisotropy, characterized in that: In terms of mass percentage, it includes: C: 0.02~0.04%; N: 0.1~0.15%; Cr: 19.5~21.5%; Mn: 4.0~5.0%; Cu: 0.1~0.3%; Ni: 1.0~1.4%; Si: 0.3~0.6%, the balance is Fe and unavoidable trace elements; The yield strength ratio YS of the duplex stainless steel plate along the rolling direction and the transverse direction RD : YS TD The ratio is 1.05~1.20; The method for manufacturing the duplex stainless steel plate comprises the following steps: S1. Melting: Put the raw materials into the smelting equipment and heat them to 1500~1700℃ under a protective gas atmosphere to obtain a steel billet; S2, hot rolling: heating the steel billet to 1100-1300° C. to obtain a duplex stainless steel hot-rolled steel plate; S3, solid solution: S4, cold rolling: the duplex stainless steel hot-rolled steel plate after solid solution treatment in S3 is cold rolled for 4 to 5 times to obtain a cold-rolled steel plate, the reduction amount of a single cold rolling is 20 to 30%, and the total reduction amount is 70 to 80%; S5, two-phase zone quenching: Use thermodynamic calculation software to calculate the temperature T corresponding to the α+γ two-phase ratio of 50% D The cold rolled steel sheet after S4 treatment is subjected to two-phase solid solution treatment at a solid solution temperature of T D ±30℃, solution treatment for 20~60min, then water-cooling to below 300℃ to obtain duplex stainless steel plate; S6. Post-treatment: Rinse the duplex stainless steel plate obtained in S5 with clean water, then put the rinsed duplex stainless steel plate into 50% nitric acid at 60-80°C, keep it for 10-30 minutes, and then wash the duplex stainless steel plate with clean water.

2. The duplex stainless steel plate with strong corrosion resistance and low anisotropy according to claim 1, characterized in that: The S3 comprises the following steps: S301, pre-solidification: the duplex stainless steel hot-rolled steel plate is pre-solidified at 1100~1200℃ for 30~60min; S302, Secondary solid solution: According to the calculation results of Thermo-Calc software, the ferrite transformation temperature T is obtained. α ; The hot-rolled duplex stainless steel plate after pre-solidification in S301 is heated to (T α +T0) solution treatment for 10 minutes, then out of the furnace and water-cooled to below 300°C, wherein the T0 value is 10~30°C.

3. The duplex stainless steel plate with strong corrosion resistance and low anisotropy as claimed in claim 1, characterized in that: In the above-mentioned S4, the number of cold rolling passes is 5.

4. The duplex stainless steel plate with strong corrosion resistance and low anisotropy as claimed in claim 1, characterized in that: The cold rolling of S4 uses a cold rolling mill, and the cold rolling mill is equipped with an ultrasonic vibration device, and the ultrasonic frequency of the ultrasonic vibration device is 20-50kHz.

5. The duplex stainless steel plate with strong corrosion resistance and low anisotropy as claimed in claim 1, characterized in that: In S1, the protective gas is nitrogen or argon.

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