阅读说明：本技术 一种低剩磁、表面质量优异的船用5Ni钢板的制造方法 (Manufacturing method of marine 5Ni steel plate with low remanence and excellent surface quality ) 是由 刘朝霞 许晓红 白云 苗丕峰 刘俊 韩步强 周永浩 武金明 高俊 李国忠 狄梦龙 于 2020-03-30 设计创作，主要内容包括：本发明涉及一种低剩磁、表面质量优异的船用5Ni钢板的制造方法,工艺流程为转炉冶炼->LF精炼->RH高真空脱气->Ca处理->连铸->铸坯缓冷处理->铸坯表面清理->加热->轧制->两次淬火->回火。组分设计采用低碳、低硅,5％Ni,并加入V、Nb,改善淬透性提高低温韧性,提高奥氏体非再晶开始温度,让钢板可采用高温轧制、高温淬火,进而显著改善板型,使适用于薄规格产品的生产。本申请从元素设计、工艺控制、母材组织、中心偏析、淬回火温度及时间等角度进行优化,在实现5Ni钢板高强度的同时,保证钢板具有良好的延伸率、-130℃低温冲击韧性、优异表面质量以及低剩磁。(The invention relates to a manufacturing method of a marine 5Ni steel plate with low remanence and excellent surface quality, which comprises the following process flows of converter smelting- > L F refining- > RH high vacuum degassing- > Ca processing- > continuous casting- > casting blank slow cooling processing- > casting blank surface cleaning- > heating- > rolling- > twice quenching- > tempering, the component design adopts low carbon and low silicon, 5% of Ni, V, Nb is added, hardenability is improved, low temperature toughness is improved, austenite non-recrystallization starting temperature is improved, the steel plate can adopt high temperature rolling and high temperature quenching, and further the plate shape is obviously improved, so that the steel plate is suitable for production of thin specification products.)

1. A method for manufacturing a 5Ni steel plate for a ship with low remanence and excellent surface quality is characterized in that: the method comprises the following steps:

(1) smelting molten steel: c in percentage by mass: 0.07 to 0.10%, Si: 0.05-0.20%, Mn: 0.60-0.80%, Ni: 4.90-5.25%, P: less than or equal to 0.0070%, S: less than or equal to 0.0020 percent, Alt: 0.010-0.035%, V: 0.010-0.015%, Nb: 0.010 to 0.020%, Ca 0.0005 to 0.0030%, O: less than or equal to 0.0012 percent, N: less than or equal to 0.0040 percent, H: smelting molten steel with chemical components less than or equal to 0.00010 percent and the balance of Fe and inevitable impurity elements;

(2) continuous casting: pouring the molten steel into a continuous casting billet, controlling the casting superheat degree to be 5-25 ℃, and taking the casting billet with the center segregation not higher than C1.0 level as a qualified billet;

(3) slab slow cooling and grinding: carrying out slow cooling treatment after the casting blank is off line, wherein the slow cooling starting temperature is not lower than 600 ℃, and the slow cooling time is not lower than 72 hours; after slow cooling, polishing and cleaning the surface of a casting blank, respectively polishing the upper surface and the lower surface to a thickness of 1-2mm, and then coating paint on the surface to isolate air so as to prevent oxidation during reheating;

(4) reheating: heating to 1170-1220 ℃, controlling the heating time to be more than or equal to 0.32min/mm at 600-900 ℃, starting heat preservation timing when the core temperature of the casting blank reaches the surface temperature of the casting blank, and keeping the heat preservation time to be not less than 0.5 hour;

(5) hot rolling: after the billet is taken out of the furnace, the billet is subjected to high-pressure water descaling treatment, and two-stage controlled rolling of rough rolling and finish rolling is carried out: the initial rolling temperature of rough rolling is 1080-1150 ℃, and the reduction rate of three passes after rough rolling is more than or equal to 15%. When the temperature thickness is more than or equal to 1.8H, H is the thickness of the finished steel plate; the finish rolling adopts high-temperature rolling, the initial rolling temperature is 880-970 ℃, the finishing temperature is more than or equal to 800 ℃, the rolling target thickness is achieved, and the steel plate is air-cooled after the rolling is finished;

(6) quenching heat treatment process: carrying out water quenching at the first quenching temperature of 880 +/-10 ℃, keeping the temperature for 30-100 min after the furnace temperature reaches the temperature; the second quenching temperature is 760 +/-10 ℃, the holding time is 30-100 min after the furnace temperature reaches the temperature, and water quenching is carried out;

(7) and (3) tempering heat treatment process: the tempering temperature is 630 +/-10 ℃, and after the steel plate 1/2 reaches the temperature, the temperature is kept for 120-200 min, so that carbon in the quenched martensite is fully diffused, and the microstructure of the tempered sorbite is obtained.

2. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: the method is suitable for manufacturing 5Ni steel plates with the thickness of 6-50 mm.

3. The method of manufacturing 5Ni steel sheet for ships having low remanence and excellent surface quality as claimed in claim 1, wherein the step (1) of smelting molten steel includes converter smelting, RH refining, and L F refining.

4. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: and (2) carrying out argon protection and dynamic soft reduction control in the whole process of molten steel pouring.

5. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: and (3) stacking the casting blanks into a pit or covering the casting blanks during slow cooling.

6. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: and (4) heating the casting blank by adopting a walking beam furnace, wherein the average speed is 10-14 cm/min.

7. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: and after tempering heat treatment, the steel plates are hoisted by adopting a vacuum chuck and are independently stacked.

8. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: the surface quality of the steel plate finished product reaches SA 2.0, the average remanence of the steel plate corner is less than or equal to 15Guass, and ultrasonic flaw detection is carried out according to EN10160 standard: the plate body area meets the requirement of Class S3, and the edge area meets the requirement of Class E4.

9. The method for manufacturing a 5Ni steel sheet for a ship excellent in low remanence and surface quality according to claim 1, characterized in that: the yield strength of the steel plate is more than or equal to 520MPa, the tensile strength is 620-645 MPa, the yield ratio is less than or equal to 0.82, the elongation is more than or equal to 26 percent, and the impact toughness is more than or equal to 200J at-130 ℃; the structure of the steel plate is mainly a uniform tempered sorbite structure.

Technical Field

The invention relates to a manufacturing method of a steel plate for a ship, in particular to a manufacturing method of a 5Ni steel plate for a ship.

Background

The most important transportation mode of the liquefied Petroleum Gas (L iquefied Petroleum Gas, L PG for short) is marine transportation except pipeline transportation, and the transportation ships adopting L PG for ship storage and transportation, L PG for transportation and liquefied ethylene (L EG) obtained by Petroleum cracking and liquefaction are usually constructed by 5Ni steel with high strength, good low-temperature toughness and high elongation rate, wherein the 5Ni steel means that the content of Ni in the steel reaches about 5 percent.

The patent publication No. CN104195428A discloses a V-containing low-carbon high-strength 5Ni steel medium plate and a manufacturing method thereof, which is characterized in that a proper amount of V is added on the basis of low C and about 5 percent of Ni content, a 5Ni steel is obtained through two-stage controlled rolling and quenching and tempering heat treatment, and the steel plate flatness of a steel plate is introduced, the production thickness of the steel plate is between 25mm and 50mm, and the steel plate is not suitable for the production of thinner steel plates, and the steel plate flatness is difficult to ensure if the production thickness is reduced. Patent document No. CN102330031A discloses a method for manufacturing high-toughness-130 ℃ low-temperature steel, in which Nb, Mo, Cr, Zr, RE, etc. are added, the Nb and Mo belong to precious metals, and the addition of the elements Zr, RE, etc. significantly increases the cost of alloy raw materials, and significantly increases the difficulty of smelting molten steel, thereby making it difficult to realize batch production. The patent publication No. CN104388838A discloses a 5Ni steel plate for an ultralow temperature pressure vessel and a production method thereof, wherein the 5Ni steel plate is produced with the thickness of 8-50 mm, the component design is simplified, and the 5Ni steel plate with high strength and low-temperature toughness of-125 ℃ is obtained by adopting a heat treatment mode of controlled rolling, cooling, stacking cooling and twice quenching and tempering. Wherein the rolling temperature of the second stage of rolling is controlled to be less than or equal to 850 ℃, the finishing temperature is controlled to be less than or equal to 810 ℃, the semi-finished steel plate is cooled by water after rolling, the temperature of red return during water cooling is less than or equal to 650 ℃, the rolling temperature of the second stage of rolling is low, and the water cooling after rolling is not beneficial to the shape control of the thin plate. Patent document No. CN105331890A discloses a method for producing a high-toughness 5Ni steel medium plate by on-line quenching, which adopts a casting blank cogging method, performs two-stage rolling, on-line quenching to below 200 ℃, and tempering at 590-620 ℃ to obtain a 5Ni steel plate with a tempered martensite plus small amount of reverse transformation austenite mixed structure. The cogging of the casting blank has good effect on the structural property uniformity of the steel plate, but the production cost is obviously increased, and the production process is prolonged.

For 5Ni steel plate, the surface quality and remanence are important supply requirements, and the two requirements are two most important points in the 5Ni supply technical requirement different from other steel types. Wherein the surface quality is an important factor restricting the production of thin gauge 5Ni steel sheets.

Disclosure of Invention

Aiming at a 5Ni steel plate, particularly a thin 5Ni steel plate, the microalloying component design of Ni and V + Nb with low C and 5 percent content is adopted, a casting blank is adopted in the production method for peeling, high-temperature hot rolling is carried out, water cooling is not carried out after rolling, then two times of quenching and tempering heat treatment are carried out, the first quenching temperature is increased, the segregation with high Ni content is improved, and a vacuum chuck is adopted for hoisting, so that the good strong toughness 5Ni steel is obtained, and the product has excellent surface quality and low residual magnetism.

The chemical components of the marine 5Ni steel are as follows by mass percent: 0.07 to 0.10%, Si: 0.05-0.20%, Mn: 0.60-0.80%, Ni: 4.90-5.25%, P: less than or equal to 0.0070%, S: less than or equal to 0.0020 percent, Alt: 0.010-0.035%, V: 0.010-0.015%, Nb: 0.010 to 0.020%, Ca 0.0005 to 0.0030%, O: less than or equal to 0.0012 percent, N: less than or equal to 0.0040 percent, H: less than or equal to 0.00010 percent, and the balance of Fe and inevitable impurity elements.

The arrangement of each element in the marine 5Ni steel is as follows:

the addition of C can increase the hardenability of steel, particularly the production of medium plate, can obviously improve the strength, but the excessive content of C is not beneficial to the ultralow temperature impact property of-110 to-130 ℃, the ultralow temperature strain aging property, the welding property and the corrosion resistance of the steel, so the carbon content is controlled to be 0.07 to 0.10 percent in the invention.

Si is mainly used for deoxidation, although the adding amount is determined according to different smelting modes, in order to obtain good steel plate performance, more than 0.05 percent is needed, if more than 0.30 percent, silicon aluminate spinel is easily formed in iron scales and is not easy to remove, and the surface quality of 5Ni steel is particularly important, so the invention adopts low Si content control, and the upper limit is regulated to be 0.20 percent.

Mn is an element that improves the hardenability of steel, and plays a role of solid solution strengthening to compensate for the loss of strength caused by the decrease of the C content in steel. When the Mn content in the steel is too low, the effect of securing strength cannot be sufficiently exerted, but when the Mn content is too high, the carbon equivalent thereof is increased to deteriorate the weldability. In addition, Mn is likely to segregate in the center of the steel sheet, and reduces the impact toughness at the center of the steel sheet. Therefore, the Mn content of the invention is controlled to be 0.60-0.80%.

Ni is an element that improves the hardenability of the steel sheet and can significantly improve its low-temperature toughness, and has a good influence on impact toughness and ductile-brittle transition temperature. However, when the Ni content is too high, scale with high viscosity is easily generated on the surface of the plate blank and is difficult to remove, and the surface quality of the steel plate is affected. In addition, Ni is also a precious metal, and too high a content increases the cost. Therefore, on the premise of meeting the standards of classification society and international universal standards, the content of the compound is controlled to be 4.90-5.25%, and the optimal cost performance is favorably achieved.

The solute dragging effect of Nb and the pinning effect of Nb (C, N) on austenite grain boundaries both inhibit the recrystallization of deformed austenite and expand the austenite non-recrystallization interval, so that the finish rolling initial rolling temperature can be increased, the plate shape of a steel plate is ensured, high-temperature finish rolling is easy to realize, the yield ratio of the steel plate is reduced, and the temperature interval formed by secondary iron scale is avoided. However, too much Nb can also cause the formation of fine grains of the steel plate and improve the yield ratio, so the content of Nb is controlled to be 0.010-0.020% in a narrow region.

V is a strong carbon and nitride forming element, second phase particles such as VC, V (CN) and the like are formed in steel, crystal grains can be refined, the strength and the low-temperature toughness of steel are improved, but the content of V is too high, the weldability of the steel is reduced, and the content of V is controlled to be 0.010-0.015%.

Ca treatment is a necessary treatment link of the steel grade, 0.0005-0.0030 percent of Ca can reduce the performance harm caused by sulfide and can be sharp Al₂O₃The inclusion denaturation is spherical low-melting-point inclusion, thereby reducing the sharp angle of hard inclusion in the rolling process of the steel plateThe generation of micro-cracks is avoided, and the impact toughness of the steel plate is improved.

P improves corrosion resistance, but decreases low-temperature toughness and affects weldability of steel sheet, which is not suitable for structural steel, and the present invention provides that it is controlled to 0.0070% or less.

S forms MnS inclusions, causes center segregation, and adversely affects corrosion resistance, and is controlled to 0.0020% or less.

Al mainly plays roles in fixing nitrogen and deoxidizing. AlN formed by bonding Al with N is effective in refining grains, but too high a content impairs the toughness of the steel. Therefore, the content (Alt) of the high-performance liquid crystal is controlled to be 0.010-0.035%.

O, N: harmful gas elements, high content and more inclusions, and reduce the plasticity, toughness and welding bending property of the steel plate. The invention strictly controls the O content not to be higher than 0.0012%; the content of N is not higher than 0.0040%.

H: and (4) harmful gas elements. High H content, easy generation of white spots, reduction of plasticity and toughness of the steel plate and serious harm to the service performance of the steel plate. In order to improve the comprehensive performance of the steel plate, the H content is strictly controlled within 0.00010%.

The preparation method of the 5Ni steel plate of the invention has the following specific processes,

the smelting continuous casting process comprises the steps of sequentially carrying out converter smelting, RH refining and L F refining on smelting raw materials, carrying out low-superheat-degree pouring for controlling the internal looseness and segregation of a steel plate, controlling the pouring superheat degree at 5-25 ℃, carrying out argon protection pouring in the whole process, and controlling dynamic soft reduction so as to ensure that the central segregation of a casting blank is not higher than C1.0 level.

The slab slow cooling and cleaning and grinding process comprises the following steps: after the casting blank is off-line, slow cooling treatment is carried out, and the casting blank is stacked into a pit for slow cooling or covered for slow cooling. The slow cooling starting temperature is not lower than 600 ℃, and the slow cooling time is not lower than 72 hours. After the slow cooling is finished, the surface of a casting blank is cleaned by polishing, the thickness of the upper surface and the lower surface of the casting blank is respectively 1-2mm, and after polishing, coating (high-temperature-resistant emulsion paint) is coated, wherein the purpose of the coating is to seal the surface of the casting blank and prevent the surface of the casting blank from being oxidized when the casting blank is exposed in the air and reheated.

A heating process: and (3) feeding the casting blank into a stepping heating furnace, heating to 1170-1220 ℃ at an average speed of 10-14 cm/min, and preserving heat when the core temperature reaches the surface temperature for not less than 0.5 hour, so that alloy elements in the steel are fully dissolved to ensure the uniformity of the components and the performance of the final product.

The rolling process comprises the following steps: after the billet is taken out of the furnace, the billet is subjected to high-pressure water descaling treatment and then is subjected to two-stage controlled rolling of rough rolling and finish rolling: the initial rolling temperature of rough rolling is 1080-1150 ℃, the reduction rate of three passes after rough rolling is more than or equal to 15%, and the thickness after temperature is more than or equal to 1.8H, wherein H is the thickness of a finished product. And (3) rolling at high temperature in the finish rolling process, wherein the initial rolling temperature is 880-970 ℃, the final rolling temperature is not lower than 800 ℃, and air cooling and water cooling cannot be performed after rolling.

Quenching heat treatment process: the first quenching (first quenching) temperature is 880 +/-10 ℃, the holding time is 30-100 min after the furnace temperature reaches the temperature, and water quenching is carried out. And the temperature of the second quenching (second quenching) is 760 +/-10 ℃, the holding time is 30-100 min after the furnace temperature reaches the temperature, and water quenching is carried out. The temperature control precision is +/-10 ℃ to ensure the uniformity of the steel plate.

And (3) tempering heat treatment process: after the steel plate is quenched, tempering is carried out at 630 +/-10 ℃, the temperature of the core part of the steel plate is kept for 120-200 min after the core part of the steel plate reaches the temperature, enough time is given to fully diffuse carbon in the quenched martensite, a tempered sorbite is obtained, the strength and toughness matching of the steel plate is ensured, and the engineering applicability of the steel plate is improved.

The tempered steel plate is a finished steel plate product, the product is hoisted in vacuum, and the performance is sampled and detected after tempering.

Compared with the prior art, the invention has the advantages that:

(1) the low-carbon and 5% Ni is adopted, and a proper amount of V is added, so that the hardenability of the steel plate is ensured, the tensile strength of the steel plate is improved, the low-temperature impact toughness of the steel plate at minus 130 ℃ to minus 150 ℃ is ensured, a proper amount of Nb is added, the austenite non-recrystallization starting temperature is improved, the steel plate is rolled at high temperature, particularly, the high-temperature finish rolling is carried out, and the shape of the steel plate, particularly the thin steel plate, is ensured.

(2) The invention adopts the modes of low Si content, high temperature rolling, grinding of the upper and lower surfaces of the casting blank, coating of the surface, high pressure water descaling and the like, and is beneficial to improving the surface quality of the steel plate.

(3) According to the invention, good plate shape of the 5Ni steel plate is ensured by adding Nb, controlling sectional heating, rolling at high temperature and air cooling (without water cooling) after rolling, and the steel plate is prevented from being scratched due to straightening, thereby being beneficial to protecting the surface quality of the steel plate.

(4) Although the austenite ferrite phase transformation point of the 5Ni steel is much lower than that of other common steels, in order to ensure that the steel is produced together with other steels, the first quenching temperature is increased to 880 +/-10 ℃ in the heat treatment process, a large-scale production heat treatment furnace is not needed to be greatly heated and cooled, and the first quenching temperature is increased, so that the segregation degree of the high-Ni-content steel is improved.

The 5Ni steel plate manufacturing method can produce the steel plate with the thickness of 6-50 mm, the yield strength of more than or equal to 520MPa, the tensile strength of 620-645 MPa, the yield ratio of less than or equal to 0.82, the elongation of more than or equal to 26 percent, and the impact toughness of more than or equal to 200J at the temperature of minus 130 ℃; the surface quality is excellent: reaching the level of SA 2.0. The average remanence of the corners of the steel plate is less than or equal to 15 Guass. The ultrasonic flaw detection is carried out according to EN10160 standard, the plate body area meets the requirement of Class S3, and the edge area meets the requirement of Class E4. The structure of the steel plate is uniform tempered sorbite structure, and the method is suitable for batch production.

Drawings

FIG. 1 is a macroscopic structure of the test steel of example 4 of the present invention, in which the center segregation caused by continuous casting is significantly improved;

FIG. 2 is a metallographic structure of 1/4 mm in thickness of the test steel of example 1 of the present invention, which is a tempered sorbite structure;

FIG. 3 shows a metallurgical structure at 1/4 mm thickness of the test steel of example 4 of the present invention, which is a tempered sorbite structure.

Detailed Description

The present invention will be described in further detail with reference to examples.

The production process flow of the marine 5Ni steel plate comprises the following steps of converter smelting- > L F refining- > RH high vacuum degassing- > Ca treatment- > continuous casting- > casting blank slow cooling treatment- > casting blank surface cleaning- > heating- > rolling- > twice quenching- > tempering, and the production process flow comprises the following specific steps:

(1) smelting, namely selecting high-quality raw materials, smelting by adopting a 150-ton converter, carrying out RH high vacuum degassing treatment, then sending the raw materials into an L F furnace for refining, breaking the vacuum for Ca treatment, and then carrying out RH vacuum degassing, wherein the alloy components of the examples 1-4 are controlled as shown in Table 1.

(2) Continuous casting: and casting the smelted molten steel into a continuous casting billet with the thickness of 150mm, controlling the casting temperature to be 5-25 ℃ above the liquidus, carrying out argon protection casting in the whole process, and implementing dynamic soft reduction in the casting process. The continuous casting process parameters are shown in a table 2, and the casting blank with the center segregation not higher than C1.0 grade is qualified.

(3) Casting blank slow cooling treatment: and (3) placing the continuous casting slab into a pit for stacking, slowly cooling and hydrogen diffusion, wherein the slowly cooling starting temperature is more than or equal to 600 ℃, and the time is 72 hours. The pit entry temperature and the slow cooling time are shown in Table 2. After slow cooling, the surface of the casting blank is polished and cleaned by a machine, the thickness of the upper surface and the lower surface is respectively cleaned to be 1.5mm, and high-temperature coating (high-temperature resistant emulsion paint) is coated to isolate air and prevent oxidation before reheating.

(4) Reheating: and (4) putting the continuous casting slab obtained in the step (3) into a stepping heating furnace, heating to 1180-1250 ℃ at an average speed of 10-14 cm/min, and preserving heat for more than 1 hour when the core temperature reaches the surface temperature. The alloy elements in the steel are fully dissolved in solid solution to ensure the uniformity of the components and the performance of the final product. And controlling the heating time to be more than or equal to 0.32min/mm at the temperature of 600-900 ℃.

(6) Rolling: after the steel billet is taken out of the furnace and is subjected to high-pressure water descaling treatment, two stages of rough rolling and finish rolling are carried out: the initial rolling temperature of rough rolling is 1080-1150 ℃, the reduction rate of three passes after rough rolling is more than or equal to 15%, and the thickness after temperature is more than or equal to 1.8H, wherein H is the thickness of a finished product. And (3) high-temperature rolling is adopted for finish rolling, the start rolling temperature of the finish rolling is 880-970 ℃, and the finish rolling finishing temperature is more than or equal to 800 ℃. And after the rolling is finished, the steel plate is not subjected to accelerated water cooling by an ACC unit and is subjected to air cooling. The process parameters of the rolling stage are shown in table 3.

(7) Quenching: the first quenching temperature of the steel plate is 880 +/-10 ℃, and the holding time is 30-100 min after the furnace temperature reaches the temperature; the secondary quenching temperature is 760 +/-10 ℃, and the holding time is 30-100 min after the furnace temperature reaches the temperature; the quenching medium is water.

(8) Tempering: the tempering temperature of the steel plate is 630 +/-10 ℃, and the heat preservation time is 120-200 min.

(9) Hoisting the steel plate after secondary quenching and tempering by adopting a vacuum chuck, and separately stacking.

(10) And (4) carrying out transverse stretching and transverse impact tests on the tempered steel plate, and measuring the residual magnetism and the surface quality of the steel plate.