EH40 super-thick marine steel resistant to corrosion of tropical marine climate and manufacturing method thereof

Abstract

The present application relates to the technical field of super-thick marine steel resistant to corrosion in tropical marine climate, in particular to an EH40 super-thick marine steel resistant to corrosion in tropical marine climate and a manufacturing method thereof. The steel is composed of the following chemical components by weight percentage: C: 0.065%-0.08%, Si: 0.15%-0.3%, Mn: 1.5%-1.7%, P≤0.02%, S≤0.01%, Als: 0.015%-0.03%, Ni: 0.4%-0.8%, Cu: 0.1%-0.2%, Co: 0.1%-0.15%, Sn: 0.05%-0.15%, V: 0.01%-0.03%, Ti: 0.01%-0.015%, N: 0.002%-0.004%, and the rest is Fe and inevitable impurities. The thickness of the finished steel plate can reach 100 mm, while maintaining the high strength, high toughness and excellent weldability of the EH40 grade, the corrosion resistance of the steel in the tropical marine climate environment is significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heavy tropical marine climate corrosion-resistant special thick marine steel, in particular to a EH40 special thick marine steel resistant to tropical marine climate corrosion and a manufacturing method thereof. BACKGROUND

[0002] Tropical marine climate environment, with its perennial high temperature, extreme high humidity, high salt deposition, strong ultraviolet radiation and frequent and severe dry-wet alternating cycle as the most prominent features, constitutes one of the most severe atmospheric corrosion environments for engineering metal materials. This environment can dramatically accelerate the electrochemical corrosion process of steel, induce severe uniform corrosion, pitting corrosion, crevice corrosion and even stress corrosion cracking (SCC), leading to rapid failure of key load-bearing parts of marine structures (such as large ships, deep sea platforms, cross-sea bridges, port machinery, etc.), not only greatly shortening the service life, but also bringing huge safety hazards and expensive maintenance costs. With the deepening of marine resource development and shipping activities to tropical waters, the demand for high-performance marine structural steel that can serve reliably in such harsh environments for a long time has become unprecedented urgent.

[0003] Among numerous marine structural materials, EH40 marine steel has become the preferred material for key parts of large ships (such as ultra-large container ships, liquefied gas ships) such as decks, sides, hatch surrounds and offshore platform legs, jacket, etc. due to its excellent strength (yield strength ≥ 400 MPa), good low-temperature toughness and acceptable welding performance. At the same time, in order to meet the design requirements of structural large-scale, deep-water and extreme load, these key load-bearing components often need to use special thick steel plates (usually referring to thickness ≥ 80 mm or even 100 mm or more).

[0004] However, when the standard EH40 steel is applied to special thick specifications in tropical marine environment, its inherent limitations are exposed: (1) The chemical composition system of standard EH40 steel is mainly designed to meet the requirements of strength, toughness and basic weldability, and lacks effective protective elements or optimized combinations for tropical marine high-salt and high-humidity corrosion environment. The rust layer formed is loose and porous, with poor protection, resulting in a much higher corrosion rate in tropical marine atmosphere and splash zone environment than in ordinary atmospheric environment, and early rusting, thinning, and even local perforation failure are prone to occur. (2) The center of the special thick plate is cooled slowly during rolling and cooling, which is easy to form coarse or poor structure (such as ferrite-pearlite). This non-uniformity of the structure not only affects the strength and toughness match, but also significantly reduces the overall corrosion resistance (coarse / poor phases are more prone to corrosion) and increases the risk of central hydrogen-induced delayed cracking (HIC). SUMMARY

[0005] In order to overcome the prior art, the present application provides a kind of EH40 special thick marine steel of resistance to tropical marine climate corrosion and a manufacturing method thereof, and the thickness of steel plate finished product can reach 100mm, while maintaining the high strength, high toughness and excellent weldability of EH40 grade, the corrosion resistance in tropical marine climate environment is significantly improved.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A kind of EH40 special thick marine steel of resistance to tropical marine climate corrosion, which is composed of the following weight percentage of chemical components:

[0008] C: 0.065%~0.08%, Si: 0.15%~0.3%, Mn: 1.5%~1.7%, P≤0.02%, S≤0.01%, Als: 0.015%~0.03%, Ni: 0.4%~0.8%, Cu: 0.1%~0.2%, Co: 0.1%~0.15%, Sn: 0.05%~0.15%, V: 0.01%~0.03%, Ti: 0.01%~0.015%, N: 0.002%~0.004%, the rest is Fe and inevitable impurities.

[0009] The role of selecting the above alloy element type and its content:

[0010] 1, C element can effectively improve the strength and hardness of steel plate, and C content is too low, which will lead to the reduction of C solid solution content and carbide content, the reduction of dislocation strengthening and fine grain strengthening effect, and the insufficient strength of steel plate. C content is too high, which will produce a large number of hardened structure, and the impact toughness is reduced. At the same time, too high C element in steel will lead to the decline of corrosion resistance of steel plate, so the content of C element in steel should be accurately controlled. Therefore, the content of C element is accurately controlled in 0.065%~0.08% in the present application.

[0011] 2, Si can improve the strength and hardness of steel plate, refine the grain size, so as to improve the elastic limit, yield strength and fatigue resistance of steel plate. At the same time, Si as a deoxidizer can reduce O content, and the deoxidation effect is not obvious when Si content is less than 0.15%. Si element can form a surface film with O, which can resist atmospheric corrosion to some extent, and Si element can be appropriately increased. Therefore, the content of Si element is accurately controlled in 0.15%~0.3% in the present application.

[0012] 3, Mn element is similar to Fe atomic radius, which can be solid-solved in Fe matrix in large amount, so as to improve the strength, wear resistance and hot workability of steel plate. At the same time, Mn element can increase the heat transfer capacity of thick steel plate. When the mass percentage of Mn element is greater than 1.7%, the segregation of Mn element will make the low temperature toughness of thick plate core poor. Therefore, the content of Mn element is accurately controlled in 1.5%~1.7% in the present application.

[0013] 4. P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to ≤0.02% and S to ≤0.01%.

[0014] 5. Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is poor, and microalloying elements such as Ti cannot refine the grains due to oxidation. For ultra-high strength steel plates with large thickness, the Al content in the steel needs to be appropriately increased. Conversely, if the Al content is too high, large inclusions will form. Therefore, this invention precisely controls the Al content at 0.015%~0.03%.

[0015] 6. The role of Ni is to improve the toughness of steel plates. Ni can strengthen ferrite and refine and increase pearlite, thereby improving the strength of steel. Adding a large amount can achieve a lower ductile-brittle transition temperature, improving the low-temperature toughness of the steel plate. Adding a certain amount of Ni to steel can refine ferrite grains and prevent intergranular corrosion. The carbon content of Ni-containing steel can be appropriately reduced, thus improving toughness and plasticity. Simultaneously, the addition of Ni can reduce the hot cracking tendency of Cu in steel. Therefore, this invention precisely controls the Ni content to 0.4%~0.8%.

[0016] 7. Cu (Cu) can improve the strength and hardness of steel, especially low-carbon steel. By promoting the formation of grain boundary hindrance and dispersed precipitates, it enhances the cold work hardening ability and tempering stability of steel, thereby increasing its tensile strength, yield strength, and hardness while maintaining good plasticity and toughness. Cu compounds in steel exhibit high resistance to oxidation and sulfide corrosion, particularly in corrosive media containing seawater. Cu can also reduce the tendency of steel to intergranular corrosion and stress corrosion cracking. Cu can work synergistically with Ni (Ni) to lower the ductile-brittle transition temperature of steel plates and improve low-temperature toughness. However, adding excessive Cu alone can lead to a decrease in low-temperature toughness and cause hot brittleness. Therefore, this invention precisely controls the Cu content to 0.1%~0.2%.

[0017] 8. Co can work synergistically with Mn to significantly enhance solid solution strengthening, thereby improving the strength and hardness of the steel plate. However, excessive addition will reduce the low-temperature toughness of the steel plate. Co can also improve the oxidation resistance of the steel plate. Therefore, this invention precisely controls the Co content to 0.1%~0.15%.

[0018] 9. The main functions of Sn in steel include improving its strength and corrosion resistance, as well as its processing and weldability. Sn can form a solid solution with iron atoms in steel, thereby increasing its hardness and strength, and inhibiting intergranular corrosion, thus enhancing its corrosion resistance. Therefore, this invention precisely controls the Sn content to be between 0.05% and 0.15%.

[0019] 10、V is an important additive element of the present application. V can form fine compounds with C and N, increase the strength and hardness of the steel plate, and the V C and N compounds are distributed on the austenite grain boundaries, which can hinder the growth of austenite grains during heating, effectively inhibit the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, refine the grains, and improve the fatigue resistance of the steel. At the same time, V can promote the formation of a passivation film, slow down the oxidation reaction on the surface of the steel, and enhance the corrosion resistance. Therefore, the content of V is accurately controlled to be 0.01% to 0.03%.

[0020] 11、The addition of Ti element is to form TiN by Ti and N to prevent grain growth and improve the toughness of the steel plate. A certain content of Ti element can improve the intergranular corrosion resistance of the steel. Therefore, the content of Ti is accurately controlled to be 0.01% to 0.015%.

[0021] 12、N element can cooperate with V and Ti elements to form fine and dispersed CN compounds, increase the nucleation sites of intracrystalline ferrite, and play a role in refining the grains. Increasing the content of N can increase the content of TiN in the steel. However, when the solid solution N content is too large, the toughness of the steel plate decreases. Therefore, the content of N is accurately controlled to be 0.002% to 0.004%.

[0022] The above-mentioned EH40 super-thick marine steel plate resistant to hot and marine climate corrosion has a maximum thickness of 100 mm, a yield strength of ≥420 MPa, a tensile strength of 600 to 680 MPa, a transverse elongation of ≥26%, a Charpy impact energy of the core of the steel plate at 20°C of ≥200 J, and a Charpy impact energy of the core of the steel plate at -40°C of ≥200 J. The seawater corrosion resistance of the steel plate is 40% or less of the performance of a conventional 40 MPa grade marine steel plate, and the marine atmospheric corrosion resistance is 50% or less of the performance of a conventional 40 MPa grade marine steel plate.

[0023] The microstructure at 1 / 2 of the thickness of the steel plate is 30% to 40% polygonal ferrite and 60% to 70% acicular ferrite, and the grain size of the microstructure is 10 to 12.

[0024] The manufacturing method of the above-mentioned EH40 super-thick marine steel plate resistant to hot and marine climate corrosion adopts a high-cleanliness alloying smelting + casting machine heavy pressing + electroslag remelting + two-stage low-temperature controlled rolling + low-temperature tempering process, and specifically includes the following steps:

[0025] 1、Steel refining:

[0026] The molten steel is refined by a converter, an LF furnace, an RH or VD furnace to further reduce the contents of P, S and non-metallic inclusions.

[0027] 2、Continuous casting:

[0028] Full protection casting, the superheat of molten steel is 3-10℃, the total press-down amount of the casting machine is ≥8mm; the proportion of equiaxed crystal in the core of the continuous casting billet is ≥90%, and the center segregation rating is ≤C1.0 level. The purpose of the heavy press-down of the casting machine is to improve the quality and performance of the billet by applying a large pressure during continuous casting. The heavy press-down technology fully utilizes the thickness temperature difference of the billet by applying a large deformation press-down at the solidification end of the continuous casting machine, realizes efficient transmission of the press-down amount to the core thereof, and thus significantly improves the segregation, porosity and shrinkage hole of the billet and improves the density and uniformity of the chemical composition of the billet.

[0029] 3. Electroslag remelting:

[0030] Si-Al-Ca ternary slag system is adopted, the slag melting time is 75-100min, argon is opened 30-40min in advance, and argon is blown for protection throughout the process; the inner cavity of the crystallizer is a cube with a thickness of 450-800mm; the cooling water flow rate of the crystallizer is 35-50m 3 / h; the feeding time is ≥3h, and the slow cooling time is 36-72h.

[0031] The electroslag remelting process has remarkable effects in reducing alloy segregation of steel plate, improving corrosion resistance and improving core mechanical properties of thick steel plate. By means of protective slag, argon protection and deoxidizer, the chemical composition can be precisely controlled, the invasion of harmful gas can be inhibited, and the oxidation of alloy can be reduced. The accurate and uniform control of alloy composition can effectively reduce the internal alloy potential difference of the steel plate, enhance its corrosion resistance, and be more beneficial to optimizing the core mechanical properties of the thick steel plate.

[0032] 4. Two-stage controlled rolling:

[0033] The billet is loaded into the heating furnace at 600-700℃ and is kept for 60-75min, the heating temperature is 1150-1250℃, and the holding time after reaching the temperature is 60-90min.

[0034] The purpose of ensuring the holding time after reaching the temperature is to make the core of the large-thickness billet reach the target temperature. The purpose of controlling the soaking time is to prevent abnormal growth of the billet structure and affect the core strength and low-temperature toughness of the rolled steel plate.

[0035] The one-stage rolling temperature is 1140-1220℃, and the total deformation amount is ≥55%. The intermediate billet cooling rate is 5-8℃ / s, the two-stage opening rolling temperature is 770-830℃, the last three passes of the press-down amount is ≥10mm, and the final rolling temperature is 700-750℃.

[0036] The purpose of the two-stage controlled rolling is to increase the pass reduction rate to the maximum under the condition of low hardness of the steel plate at a relatively high temperature, further break the as-cast grains, increase the deformation storage energy and grain nucleation site of the blank, and improve the low-temperature toughness of the core of the super-thick plate. During the low-temperature controlled rolling of the steel plate, the transformation of austenite to bainite is inhibited or delayed due to the low temperature, thereby avoiding the increase of the strength of the steel plate and the decrease of the low-temperature toughness under the high-temperature condition.

[0037] 5. Tempering:

[0038] The tempering temperature is 420-500 DEG C, and the tempering time is 3-4.5 min / mm. The purpose of the tempering heat treatment is to ensure that the steel plate obtains uniform and fine precipitated phase organization by reasonable temperature and time setting, improve the low-temperature toughness of the steel plate, and at the same time, the tempering homogenization and stress relief of the steel microstructure can also greatly improve the marine environment corrosion resistance of the steel plate.

[0039] Compared with the prior art, the beneficial effects of the present application are:

[0040] 1. The present application adopts C, Mn, Ni, Cu, Co, Sn elements and V, Ti and other micro-alloy phase matching components. The Mn element is added to improve the strength, wear resistance and hot workability of the steel plate, and increase the heat transfer capacity of the super-thick steel plate. The Ni and Cu elements are added to strengthen the ferrite and refine and increase the pearlite, improve the strength of the steel, a large amount of addition to obtain a lower ductile-brittle transition temperature, improve the low-temperature toughness of the steel plate, refine the ferrite grains and avoid intergranular corrosion. The Co element is added to have a significant solid solution strengthening effect, improve the strength and hardness of the steel plate. The Sn element is added to improve the strength and corrosion resistance of the steel, improve the processing performance and welding performance. V is an important added element in the present application, V and C, N form a fine compound, improve the strength and hardness of the steel plate, effectively inhibit the recrystallization of deformed austenite, refine the grains and improve the fatigue resistance of the steel; at the same time, V promotes the formation of passivation film, slows down the oxidation reaction on the surface of the steel and enhances the corrosion resistance.

[0041] 2. The purpose of the heavy pressing of the casting machine is to improve the quality and performance of the casting blank by applying a large pressure during continuous casting. The heavy pressing technology applies a large deformation reduction at the solidification end of the continuous casting machine, fully utilizes the thickness temperature difference of the casting blank, realizes the efficient transmission of the reduction to the core, thereby significantly improves the segregation, porosity and shrinkage hole and other problems of the casting blank, and improves the density and uniformity of the chemical composition of the casting blank.

[0042] 3、The electric-shock remelting process is adopted in the application, and remarkable effects are achieved in reducing alloy segregation of the steel plate, improving corrosion resistance and improving mechanical properties of the core of the super-thick steel plate. By means of protective slag, argon protection and deoxidizer, the chemical composition can be precisely controlled, the invasion of harmful gas can be inhibited and the oxidation of the alloy can be reduced. The accurate and uniform control of the alloy composition can effectively reduce the internal alloy potential difference of the steel plate, enhance the corrosion resistance and be more beneficial to optimizing the mechanical properties of the core of the super-thick steel plate.

[0043] 4、The two-stage controlled rolling is adopted in the application, and the purpose of the two-stage controlled rolling is to maximize the pass reduction rate under the condition that the hardness of the steel plate at a relatively high temperature is low, to further break the as-cast grains, to increase the deformation storage energy and grain nucleation sites of the blank and to improve the low-temperature toughness of the core of the super-thick plate. During the low-temperature controlled rolling of the steel plate, the transformation of austenite to bainite is inhibited or delayed due to the low temperature, and the low-temperature toughness is avoided to be reduced under the condition of high-temperature transformation of austenite to bainite.

[0044] 5、The tempering heat treatment is adopted in the application, and the purpose of the tempering heat treatment is to ensure that uniform and fine precipitated phase organizations are obtained in the steel plate by reasonable temperature and time setting, to improve the low-temperature toughness of the steel plate, and at the same time, to improve the marine environment corrosion resistance of the steel plate by tempering homogenization and elimination of internal stress of the steel microstructure.

[0045] In summary, the super-high-strength EH40 offshore steel with a maximum thickness of 100 mm is developed by the application through the composition design of C, Mn, Ni, Cu, Co, Sn elements and V, Ti and other micro-alloying phases, combined with continuous casting, electric-shock remelting, two-stage rolling and tempering processes. While the high strength, high toughness and excellent weldability of the EH40 grade are maintained, the corrosion resistance of the steel plate in the tropical marine climate environment is significantly improved.

[0046] The yield strength of the steel plate is greater than or equal to 400 MPa, the tensile strength is 600-680 MPa, the transverse elongation is greater than or equal to 26%, the Charpy impact energy of the core of the steel plate at 20 DEG C is greater than or equal to 200 J, the Charpy impact energy of the core of the steel plate at -40 DEG C is greater than or equal to 200 J. The seawater corrosion resistance of the steel plate is 40% or less of the performance of the conventional 40 MPa grade offshore steel, and the marine atmospheric corrosion resistance is 50% or less of the performance of the conventional 40 MPa grade offshore steel. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 It is the metallographic structure diagram of Example 1 of the application. DETAILED DESCRIPTION

[0048] This invention discloses an EH40 extra-thick marine steel resistant to tropical marine climate corrosion and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0049] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1; the continuous casting and electroslag remelting process in the embodiments of the present invention is shown in Table 2; the rolling and tempering process in the embodiments of the present invention is shown in Table 3; the mechanical properties of the steel plate in the embodiments of the present invention are shown in Table 4; and the corrosion resistance of the embodiments of the present invention and the comparative examples in the marine environment is shown in Table 5.

[0050] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention

[0051]

[0052] Table 2 Continuous casting and electroslag remelting processes in embodiments of the present invention

[0053]

[0054] Table 3 Rolling and tempering processes in embodiments of the present invention

[0055]

[0056] Table 4 Mechanical properties of steel plates in embodiments of the present invention

[0057]

[0058] Table 5. Resistance to marine corrosion in the embodiments and comparative examples of the present invention

[0059]

[0060] Note: The full immersion test reference standard JBT 7901, and the salt spray test reference standard GBT 10125. The comparison steel composition is 0.06C-0.3Si-1.50Mn-0.3Ni-0.2Cr-0.02Nb-0.02V-0.01Ti.

[0061] like Figure 1 As shown, the metallographic structure of Example 1 has 30%~40% polygonal ferrite and 60%~70% acicular ferrite at 1 / 2 thickness of the steel plate, with a grain size of 10~12 and good mechanical properties.

[0062] As shown in Table 5, the application is a kind of EH40 super-thick marine steel resistant to tropical marine climate corrosion, the thickness of the finished steel plate can reach 100mm, the yield strength is greater than or equal to 400MPa, the tensile strength is 600~680MPa, the transverse elongation is greater than or equal to 26%, the Charpy impact energy of the core of the steel plate at 20℃ is greater than or equal to 200J, and the Charpy impact energy of the core of the steel plate at-40℃ is greater than or equal to 200J. The seawater corrosion resistance rate of the steel plate is less than 40% of the performance of conventional 40MPa grade marine steel, and the marine atmospheric corrosion resistance rate is less than 50% of the performance of conventional 40MPa grade marine steel. While maintaining the high strength, high toughness and excellent weldability of EH40 grade, the corrosion resistance of the application in the tropical marine climate environment is significantly improved.

[0063] The above is only a preferred specific embodiment of the application, but the protection scope of the application is not limited to this, any person skilled in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the application within the technical range disclosed by the application, which should be covered in the protection scope of the application.

Claims

1. A method of manufacturing an EH40 extra heavy marine steel resistant to corrosion in tropical marine climates, characterized in that, The offshore steel is composed of the following chemical components by weight percentage: C: 0.065%~0.08%, Si: 0.15%~0.3%, Mn: 1.5%~1.7%, P≤0.02%, S≤0.01%, Als: 0.015%~0.03%, Ni: 0.4%~0.8%, Cu: 0.1%~0.2%, Co: 0.1%~0.15%, Sn: 0.05%~0.15%, V: 0.01%~0.03%, Ti: 0.01%~0.015%, N: 0.002%~0.004%, and the rest is Fe and inevitable impurities; The offshore steel manufacturing method specifically comprises the following steps: 1) steel refining; 2) continuous casting: full protection casting, steel overheating degree 3~10℃, total press-down amount of casting machine ≥8mm; 3) electroslag remelting: Si-Al-Ca ternary slag system is adopted, slag melting time 75~100min, argon is opened 30~40min in advance, and argon is blown in the whole process; The inner cavity of the crystallizer is cuboid, and the thickness is 450-800 mm; the cooling water flow of the crystallizer is 35-50 m 3 / h; 4) two-stage controlled rolling: one-stage rolling temperature 1140~1220℃; intermediate blank cooling rate 5~8℃ / s, two-stage open rolling temperature 770~830℃, and final rolling temperature 700~750℃; 5) tempering: tempering temperature 420~500℃, and tempering time 3~4.5min / mm.

2. The offshore steel manufacturing method according to claim 1, wherein the maximum thickness of the steel plate product is 100mm, the yield strength is ≥420MPa, the tensile strength is 600~680MPa, the transverse elongation rate is ≥26%, the Charpy impact energy of the steel plate core at 20℃ is ≥200J, and the Charpy impact energy of the steel plate core at -40℃ is ≥200J.

3. The offshore steel manufacturing method according to claim 1, wherein the microstructure at the thickness of 1 / 2 of the steel plate is 30%~40% polygonal ferrite and 60%~70% acicular ferrite, and the microstructure grain size is 10~12 levels.

4. The offshore steel manufacturing method according to claim 1, wherein in 1), the molten steel is refined by a converter, an LF furnace, an RH or a VD furnace.

5. The offshore steel manufacturing method according to claim 1, wherein in 2), the equiaxed crystal ratio of the continuous casting blank core is ≥90%, and the center segregation rating is ≤C1.0 level.

6. The offshore steel manufacturing method according to claim 1, wherein in 3), the feeding time is ≥3h, and the slow cooling time is 36~72h.

7. The offshore steel manufacturing method according to claim 1, wherein in 4), the blank is loaded into a heating furnace at 600~700℃, and the holding time is 60~75min, the heating temperature is 1150~1250℃, and the holding time after reaching the temperature is 60~90min. ​ ​ ​ ​ ​ ​ 8. The manufacturing method of the EH40 super-thick marine steel resistant to corrosion of hot and marine climate according to claim 1, characterized in that, 4) in the first stage, the total deformation is greater than or equal to 55%.

9. The manufacturing method of the EH40 super-thick marine steel resistant to corrosion of hot and marine climate according to claim 1, characterized in that, 4) in the second stage, the last three passes are greater than or equal to 10 mm.

Images