High-purity ship steel capable of reducing residual stress and smelting method of high-purity ship steel

Abstract

The invention discloses high-purity ship steel capable of reducing residual stress and a smelting method of the high-purity ship steel. In the smelting method, converter smelting is conducted, specifically, the final slag alkalinity is 3.8-4.3, the proportion of MgO is 8.0-10.0%, total iron is 15-17%, and the temperature of tapping molten steel is 1650-1670 DEG C; during tapping, alloying and slagging are carried out according to the sequence of lime, silicon-manganese alloy or medium-carbon manganese-iron alloy, aluminum ingots and lime, and the mass MAl of Al added through the aluminum ingots meets the formula that MAl = 0.0014 MO + 0.69; in the RH vacuum refining procedure, vacuum degassing, alloying, net circulation and vacuum breaking hydrogen determination are conducted in sequence, then tapping is conducted, the tapping temperature T is equal to TRH + TRHXZ, the value of TRH ranges from 1563 DEG C to 1583 DEG C, and TRHXZ is equal to k * (tRH-10).

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel material preparation, and relates to a shipbuilding steel and a smelting method thereof, in particular to a smelting method of high-purity shipbuilding steel capable of reducing residual stress. BACKGROUND

[0002] Shipbuilding steel plays an indispensable role in modern marine development and shipping fields, and is widely used. From large ocean-going cargo ships, luxury cruise ships to deep-sea drilling platforms and offshore wind power facilities, shipbuilding steel provides a solid structural support for various marine equipment with its excellent strength, toughness, corrosion resistance and weldability, ensuring its reliability and safety in complex marine environments. With the improvement of the automation level of downstream users, the requirements for shipbuilding steel are becoming higher and higher, especially the control requirements for steel plate residual stress.

[0003] Residual stress has a significant negative impact on the performance of steel plates, mainly manifested in poor dimensional stability, reduced machining accuracy, decreased pitting corrosion resistance and shortened fatigue life, etc. For example, ① after the steel plate is cut by fire, if the residual stress is large, deformation defects such as warping and side bending may occur, which cannot meet the use requirements; ② when the steel plate is bent and cold worked, due to the large residual stress or uneven distribution, waves or cracks may appear at the edge, resulting in the inability to weld; ③ after the steel plate is welded, the large residual stress may cause welding cold cracks; ④ after the steel plate is made into a structural member, the residual stress may cause stress concentration, thereby causing cracking, and in the service process, the stress concentration site is prone to pitting.

[0004] Therefore, controlling and reducing residual stress to obtain a low-residual-stress steel plate is a difficult problem to be solved in the field of shipbuilding steel. SUMMARY

[0005] The present application aims to provide a shipbuilding steel and a smelting method thereof.

[0006] To achieve the above-mentioned application purpose, an embodiment of the present application provides a smelting method of a shipbuilding steel. The chemical composition of the shipbuilding steel includes, in mass percent: C 0.05~0.11%, Si 0.09~0.32%, Mn 0.92~1.63%, Cr 0~0.54%, Ni 0~0.39%, Mo 0~0.31%, Cu 0~0.39%, Nb 0.009~0.056%, Ti 0.009~0.021%, Al 0.021~0.049%, P≤0.0180%, S≤0.0060%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and unavoidable impurities. The smelting method includes, in sequence, the KR desulfurization process for molten iron, the converter smelting process, the LF refining process, the RH vacuum refining process, and the billet casting process. In the converter smelting process: the basicity of the final slag is controlled at 3.8~4.3, the mass percentage of MgO is 8.0~10.0%, and the mass percentage of total iron is 15~17%. The tapping temperature of the molten steel is 1650~1670℃. During tapping, alloying and slag formation are carried out in the following order: lime, ferrosilicon or medium-carbon ferromanganese alloy, aluminum ingot, and lime again. The mass percentage of Al added from the aluminum ingot is controlled by the amount of Al added. Al Satisfy M Al =0.0014M O +0.69, where M O M is the mass percentage of oxygen (O) in the molten steel at tapping time, expressed in ppm. Al The unit is kg / t; In the RH vacuum refining process: vacuum degassing, alloying, net circulation, and hydrogen degassing are carried out in sequence, followed by steel tapping at a temperature T=T RH +T RHXZ T RH The value is taken from 1563~1583℃, T RHXZ =k×(t RH -10), k takes values ​​from 0.3 to 0.5, t RH This refers to the time from tapping the steel to pouring it, expressed in minutes.

[0007] Preferably, in the RH vacuum refining process: the T corresponding to the start-up of the casting furnace RH The value is taken from 1573~1583℃, and the corresponding T for the continuous casting furnace is... RH The value is taken from 1563 to 1573℃.

[0008] Preferably, in the LF refining process, the tapping temperature is 1598~1618℃.

[0009] Preferably, in the LF refining process: the tapping temperature corresponding to the start-up furnace is 1608~1618℃, and the tapping temperature corresponding to the continuous casting furnace is 1598~1608℃.

[0010] Preferably, in the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0 mbar, the degassing time is 20~25 min, titanium-iron alloy is added during alloying, the processing time during net circulation is 8~10 min, and the H mass percentage of hydrogen in the vacuum degassing process is ≤1.5 ppm.

[0011] Preferably, in the KR desulfurization process of molten iron: the temperature of the molten iron leaving the station is ≥1300℃, and S≤0.0025%.

[0012] Preferably, in the converter smelting process: before tapping, argon blowing is started at the bottom of the ladle, with an argon blowing pressure of 0.5~0.6MPa and a bright ring diameter of molten steel of 240~340mm; when 3 / 4 of the steel has been tapped, the argon blowing pressure is reduced to 0.42~0.52MPa and the bright ring diameter of molten steel of 120~220mm.

[0013] Preferably, in the LF refining process: after the ladle is in place, bottom blowing argon is first turned on and initial alloying is carried out, then main alloying and slag formation are carried out under strong stirring; then electric refining is carried out, during which alloys are added; finally, white slag is formed and steel is tapped.

[0014] Preferably, in the LF refining process: After connecting the bottom blowing argon, start the bottom blowing argon, and the blowing pressure is 0.4~0.6MPa; During the main alloying and slag formation processes, the bottom-blown argon flow rate is 700~860 NL / min, the argon blowing pressure is 0.50~0.60 MPa, and the strong stirring time is ≤6 min; When adding alloy, the flow rate of bottom-blown argon is 650~800 NL / min, the blowing pressure is 0.45~0.55 MPa, and the strong stirring time is ≤4 min; During the remaining time, the flow rate of bottom-blown argon is 300~400 NL / min, the blowing pressure is 0.40~0.50 MPa, and the diameter of the bright ring of molten steel is <150 mm.

[0015] Preferably, in the LF refining process: when making white slag, the CaO / SiO2 ratio in the slag is controlled to be 5.0±0.2 and the CaO / Al2O3 ratio to be 1.8±0.5, and the white slag is maintained for more than 15 minutes.

[0016] Preferably, in the billet casting process: a continuous casting machine is used to prepare the continuous casting billet, and full protective casting is used, with a superheat of 26~30℃.

[0017] Preferably, the chemical composition of the shipbuilding steel, by mass percentage, also satisfies any one, two, three, or all of the following conditions: CEV = C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15 is 0.227~0.604; Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15 is 0.107~0.276; The Ni / Cu ratio is 0.95~1.05; 5C+Si+Mn+Cr+Ni+Mo+Cu is 1.46~4.13; In the formula, the element symbols represent the mass percentage of the corresponding element.

[0018] To achieve the above-mentioned objective, one embodiment of the present invention provides a shipbuilding steel. The chemical composition of the shipbuilding steel, by mass percentage, includes: C 0.05~0.11%, Si 0.09~0.32%, Mn 0.92~1.63%, Cr 0~0.54%, Ni 0~0.39%, Mo 0~0.31%, Cu 0~0.39%, Nb 0.009~0.056%, Ti 0.009~0.021%, Al 0.021~0.049%, P≤0.0180%, S≤0.0060%, O≤0.0025%, N≤0.0048%, H≤0.0002%, with the remainder being iron and unavoidable impurities; The thickness of the shipbuilding steel is 200~320mm, and the number of inclusions with a size ≥15μm on its cross-section is ≤5 / cm. 2 The number of inclusions with a size ≥10μm and <15μm is ≤15 per cm. 2 The number of inclusions with a size ≥5μm and <10μm is ≤150 / cm². 2 .

[0019] Preferably, the shipbuilding steel has a center segregation of ≤0.5 grade, a manganese segregation ratio of ≤1.10 in the cross section, a center porosity of ≤0.5 grade, and inclusions of types A, B, C, D, and Ds are all rated ≤1 grade and their sum is ≤2.5 grade.

[0020] Compared with the prior art, the beneficial effects of one embodiment of this application are as follows: based on the designed chemical composition, by controlling the final slag in converter smelting and controlling the amount of Al added based on the O content, and combined with the precise control of the tapping temperature in RH vacuum refining, the purity of the molten steel can be optimized, that is, the purity of the molten steel can be greatly improved, thereby improving the internal quality of shipbuilding steel, reducing the content of inclusions, and thus greatly reducing the residual stress of the final shipbuilding steel. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them.

[0022] One embodiment of this application provides a method for smelting shipbuilding steel.

[0023] The chemical composition of the shipbuilding steel, by mass percentage, includes: C 0.05~0.11%, Si 0.09~0.32%, Mn 0.92~1.63%, Cr 0~0.54%, Ni 0~0.39%, Mo 0~0.31%, Cu 0~0.39%, Nb 0.009~0.056%, Ti 0.009~0.021%, Al 0.021~0.049%, P≤0.0180%, S≤0.0060%, O≤0.0025%, N≤0.0048%, H≤0.0002%, with the remainder being iron and unavoidable impurities.

[0024] The following is a detailed analysis and explanation of the main functions of each element and the selection of its dosage.

[0025] C: Carbon is the most economical strengthening element in steel, providing solid solution strengthening. It also forms carbides with niobium, titanium, chromium, molybdenum, etc., providing precipitation strengthening. Increased carbon content increases the material's hardness and strength, but reduces ductility and toughness. During quenching or rapid cooling, the formation of carbides and martensite causes significant volume expansion, thus increasing residual stress. Higher carbon content leads to greater hardenability of the steel and a greater tendency for residual stress to concentrate. In this invention, the carbon content is controlled at 0.05~0.11%.

[0026] Silicon (Si): As a deoxidizer and solid solution strengthening element, silicon can improve the strength and hardness of steel, but it reduces plasticity and toughness. The addition of silicon increases the elastic limit and yield strength of steel, but has little direct impact on residual stress. High silicon content increases grain boundary segregation of elements such as phosphorus and sulfur, reducing low-temperature toughness and weldability. Furthermore, excessive silicon easily forms Fe2SiO4 on the surface of continuously cast billets, which is detrimental to the control of steel plate surface quality. In one embodiment, the silicon content is controlled at 0.09~0.32%.

[0027] Mn: Manganese plays a solid solution strengthening role in steel, improving its hardenability and strength. However, increasing manganese content increases residual stress, especially during rapid cooling. Excessive manganese can lead to center segregation in the cast billet, negatively impacting toughness and increasing the banding level, resulting in a more uneven microstructure. Furthermore, manganese readily combines with sulfur to form manganese sulfide inclusions. Center segregation, banding, and inclusions all increase residual stress in the steel. In one embodiment, the manganese content is controlled at 0.92~1.63%.

[0028] Cr: Chromium plays a solid solution strengthening role in steel, significantly improving its hardenability and hardness. Chromium can form stable carbides and increase the stability of martensite, resulting in significant residual stress after quenching or rapid cooling. When the chromium content is too high, the distribution of residual stress after cooling becomes more complex.

[0029] One embodiment does not contain chromium, for example, chromium alloys are not actively added in the form of alloying during the production process so that the chromium content in the steel is 0, or chromium is introduced only as an impurity from the raw materials.

[0030] Preferably, in another embodiment, chromium is added, and the chromium content can be controlled at 0.06~0.54%.

[0031] Ni: Nickel acts as a solid solution strengthening agent in steel, increasing its strength without significantly increasing its hardness. Nickel expands the austenite phase region, thereby reducing residual stress; furthermore, nickel can improve the toughness of steel and reduce stress concentration.

[0032] One embodiment does not contain nickel, for example, by not actively adding nickel alloys in the form of alloying during the production process so that the nickel content in the steel is 0, or by nickel being introduced as an impurity from the raw materials.

[0033] In another embodiment, nickel is added, with the nickel content controlled at 0.06~0.39%.

[0034] Molybdenum (Mo) significantly improves the hardenability of steel, enhances its strength and toughness, and also refines grain size and improves corrosion resistance. Molybdenum can improve the tempering stability of steel and reduce the increase of residual stress. In this application, molybdenum may be selectively added or not.

[0035] One embodiment does not contain molybdenum, for example, by not actively adding molybdenum alloys in the form of alloying during the production process so that the molybdenum content in the steel is 0, or by molybdenum being introduced as an impurity from the raw materials.

[0036] Preferably, in another embodiment, molybdenum is added, and the molybdenum content is controlled at 0.02~0.31%.

[0037] Cu: Copper can promote the precipitation of niobium and compensate for the strength loss caused by the decrease in carbon content. Copper mainly exists in steel in a solid solution state. When tempered, it can precipitate as ε-Cu, which has little effect on residual stress.

[0038] One embodiment does not contain copper, for example, copper alloys are not actively added in the form of alloying during the production process so that the copper content in the steel is 0, or copper is introduced only as an impurity in the raw materials.

[0039] Preferably, in another embodiment, copper is added, and the copper content is controlled at 0.06~0.39%.

[0040] Niobium (Nb) is an important grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and precipitation within austenite, pinning austenite grain boundaries and refining recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the microstructure after phase transformation and further improving the strength and toughness of the steel. Niobium can reduce residual stress through grain refinement and precipitation strengthening. In this application, the niobium content is controlled at 0.009~0.056%.

[0041] Ti: Titanium is a nitrogen-fixing element in steel. It can form dispersed carbonitrides, which inhibit austenite grain coarsening and refine grains during billet heating and hot rolling, thereby reducing residual stress. However, when the titanium content is high, large carbonitride precipitates are easily formed in the core of the billet, affecting the low-temperature toughness of the steel plate. Furthermore, large TiN particles can easily lead to stress concentration. In this application, the titanium content is controlled at 0.009~0.021%.

[0042] Al: Aluminum is a deoxidizing element in steel. Excessive aluminum can increase the number of Al2O3 inclusions in steel, affecting its low-temperature toughness. Furthermore, large inclusions can lead to discontinuous microstructure and stress concentration. In this application, the aluminum content is controlled at 0.021~0.049%.

[0043] P, S, N, O, and H are all impurity elements in steel that can cause deterioration in the properties of steel plates, including but not limited to increased residual stress. In this application, P is controlled to be ≤0.0180%, S ≤0.0060%, O ≤0.0025%, N ≤0.0048%, and H ≤0.0002%.

[0044] Preferably, the P content is controlled at 0.0150% or less, 0.0120% or less, and can also be controlled at 0.0050% or more.

[0045] Preferably, the sulfur content is controlled at 0.0050% or less, 0.0040% or less, or even 0.0005% or more.

[0046] Preferably, the nitrogen content is controlled at 0.0022% or more.

[0047] Preferably, the O content is controlled at 0.0011% or more.

[0048] Preferably, the H content is controlled at 0.00018% or less, and can also be controlled at 0.00005% or more.

[0049] Furthermore, the chemical composition of the shipbuilding steel, by mass percentage, also satisfies any one, two, three, or all of the following: ①, ②, ③, and ④: ①CEV=C+Mn / 6+(Cr+Mo) / 5+(Cu+Ni) / 15 is 0.227~0.604; ②Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15 is 0.107~0.276; ③ The Ni / Cu ratio is 0.95~1.05; ④5C+Si+Mn+Cr+Ni+Mo+Cu is 1.46~4.13.

[0050] In each formula, the element symbol represents the mass percentage of the corresponding element. For example, if the C element content in the shipbuilding steel is 0.05%, then "C" in the formula represents a mass percentage of 0.05.

[0051] The smelting method includes, in sequence: KR desulfurization of molten iron, converter smelting, LF refining, RH vacuum refining, and billet casting, wherein: In the converter smelting process: the basicity of the final slag is controlled at 3.8~4.3, the mass percentage of MgO is 8.0~10.0%, and the mass percentage of total iron is 15~17%. The tapping temperature of the molten steel is 1650~1670℃. During tapping, alloying and slag formation are carried out in the following order: lime, ferrosilicon or medium-carbon ferromanganese alloy, aluminum ingot, and lime again. The mass percentage of Al added from the aluminum ingot is controlled by the amount of Al added. Al Satisfy M Al =0.0014M O +0.69, where M O M is the mass percentage of oxygen (O) in the molten steel at tapping time, expressed in ppm. Al The unit is kg / t; In the RH vacuum refining process: vacuum degassing, alloying, net circulation, and hydrogen degassing are carried out in sequence, followed by steel tapping at a temperature T=T RH +T RHXZ T RH The value is taken from 1563~1583℃, T RHXZ =k×(t RH -10), k takes values ​​from 0.3 to 0.5, t RH This refers to the time from tapping the steel to pouring it, expressed in minutes.

[0052] Thus, based on the designed chemical composition, by controlling the final slag in converter smelting and controlling the amount of Al added based on the O content, combined with precise control of the tapping temperature in RH vacuum refining, the purity of the molten steel can be optimized, that is, the purity of the molten steel can be greatly improved, thereby improving the internal quality of shipbuilding steel, reducing the content of inclusions, and thus greatly reducing the residual stress of the final shipbuilding steel.

[0053] In the converter smelting process, the composition of the molten steel can be analyzed during tapping to obtain the mass percentage (M) of oxygen in the molten steel. O Based on this value, the mass of Al added to the aluminum ingot, M, is then determined. Al For example, if the detected O mass percentage is 80 ppm, then formula M... Al =0.0014M O M in +0.69 O Substitute 80.

[0054] Here, the "mass M of Al added to the aluminum ingot" is mentioned. Al "M" refers to the mass of Al element added through aluminum ingots. Al The unit kg / t refers to the Al mass per ton of molten steel.

[0055] Furthermore, the aforementioned "t" RH "Time from tapping to pouring" refers to the time from the tapping moment to the start of pouring. The tapping moment refers to the moment when tapping ends (i.e., the ladle car leaves the converter position) in the RH vacuum refining process; the pouring start moment refers to the moment when the molten steel reaches the predetermined level in the tundish of the continuous casting machine and the nozzle is opened (so that the molten steel can begin to be poured into the crystallizer).

[0056] The t determined by the tapping time and the start time of pouring RH This can be determined in advance based on the PLC control program during production. Thus, based on the t determined by production control... RH This allows for confirmation of T before steel is tapped. RHXZ This allows for precise control of the tapping temperature T.

[0057] Furthermore, in the KR desulfurization process for molten iron: the temperature of the molten iron leaving the station is ≥1300℃, and the sulfur content is ≤0.0025%. This ensures the purity of the molten iron and reduces the difficulty of subsequent steelmaking.

[0058] To reduce the production difficulty of the KR desulfurization process, the temperature and composition of the molten iron arriving at the station can also be controlled. For example, the temperature of the molten iron arriving at the station should be ≥1395℃, and the P content in the molten iron should be ≤0.15%, S ≤0.045%, 0.3% ≤ Si ≤0.6%, and Mn ≤0.40%.

[0059] Furthermore, in the KR desulfurization process of molten iron, the slag removal level can be controlled at Grade A or above, thereby minimizing the amount of desulfurized slag entering the converter and causing a rebound in sulfur content.

[0060] Next, in the converter smelting process: before tapping, argon blowing is initiated at the bottom of the ladle at a pressure of 0.5~0.6 MPa, resulting in a bright ring diameter of 240~340 mm for the molten steel; when 3 / 4 of the steel has been tapped, the argon blowing pressure is reduced to 0.42~0.52 MPa, and the bright ring diameter of the molten steel is 120~220 mm. This promotes the flotation of slag in the molten steel, improves the purity of the steel, and ensures a low phosphorus content at the converter's final stage.

[0061] In addition, slag retention technology can also be used in the converter smelting process, for example, the amount of slag retained is 50 to 65% of the total slag amount of the previous furnace.

[0062] Furthermore, in the converter smelting process, the basicity of the slag is controlled at 2.0~2.2, and the total iron content in the slag is 14~17%.

[0063] Furthermore, in the LF refining process: after the ladle is in place, bottom blowing argon is first turned on and initial alloying is carried out, then main alloying and slag formation are carried out under strong stirring; then electric refining is carried out, during which alloys are added; finally, white slag is formed and steel is tapped.

[0064] Specifically, in the LF refining process: after the ladle is in place, bottom blowing argon is started after connection, with a blowing pressure of 0.4~0.6MPa; during the main alloying and slag formation, large bottom blowing technology is adopted, specifically, the bottom blowing argon flow rate is 700~860NL / min, the blowing pressure is 0.50~0.60MPa, and the strong stirring time is ≤6min; during the addition of alloys, medium bottom blowing technology is adopted, specifically, the bottom blowing argon flow rate is 650~800NL / min, the blowing pressure is 0.45~0.55MPa, and the strong stirring time is ≤4min; for the remaining time, small bottom blowing technology is adopted, specifically, the bottom blowing argon flow rate is 300~400NL / min, the blowing pressure is 0.40~0.50MPa, and the bright ring diameter of the molten steel is <150mm.

[0065] In this way, by optimizing the flow rates of large, medium, and small bottom blowing, the stirring of the molten pool can be strengthened, promoting rapid melting of slag and uniformity of steel composition and temperature. At the same time, optimizing the setting of small-flow soft blowing operation can promote the flotation and removal of inclusions, further reducing the inclusion content in the molten steel, improving the morphology of inclusions, and reducing the formation of large-sized inclusions.

[0066] The flow rate of bottom-blown argon is the largest during the main alloying and slag formation processes, followed by the flow rate during the addition of alloys, while the flow rate of bottom-blown argon is relatively small at other times.

[0067] Furthermore, deoxidation can be carried out using slag surface deoxidizers during the main alloying and slag formation processes, but this is not the only option.

[0068] During electrorefining, lime, fluorite, and other slag materials can be used to adjust the slag; in addition, calcium carbide can be added to diffuse deoxidize the molten steel.

[0069] In addition, during the LF refining process, when making white slag, the CaO / SiO2 ratio in the slag is controlled to be 5.0±0.2 and the CaO / Al2O3 ratio to be 1.8±0.5, and the white slag is maintained for more than 15 minutes.

[0070] Preferably, in the LF refining process, the tapping temperature is 1598~1618℃.

[0071] More specifically, in the LF refining process: the tapping temperature corresponding to the open casting furnace is 1608~1618℃, and the tapping temperature corresponding to the continuous casting furnace is 1598~1608℃.

[0072] Next, preferably, in the RH vacuum refining process: the T corresponding to the start-up of the casting furnace RH The value is taken from 1573~1583℃, and the corresponding T for the continuous casting furnace is... RH The temperature is set between 1563 and 1573℃. This helps to obtain purer molten steel, reduce inclusions, and thus facilitate the control of residual stress in shipbuilding steel.

[0073] Furthermore, in the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0 mbar, the degassing time is 20~25 min, titanium-iron alloy is added during alloying, the processing time during net circulation is 8~10 min, and the H mass percentage of hydrogen in the degassing process is ≤1.5 ppm. In this way, through deep degassing, the gas content and inclusion content in the molten steel can be further reduced, thereby improving the purity of the molten steel.

[0074] In addition, oxygen blowing can be completely prohibited during the RH vacuum refining process.

[0075] After the RH vacuum refining process, the ladle is left to stand on the rotary table for 13-18 minutes before casting, which can further promote the floating of inclusions.

[0076] Next, in the billet casting process: a continuous casting machine is used to prepare the continuous casting billet, and full protective casting is adopted with a superheat of 26~30℃.

[0077] In this way, by using fully protected casting and low-superheat casting, the internal quality of the continuously cast billet is improved, the central segregation, central porosity and inclusion content are reduced, and the quality of the continuously cast billet is ensured to meet the requirements of subsequent low residual stress steel plate production.

[0078] Specifically, the specific operations of full protective casting may include: using a long sprue nozzle, tundish covering agent, immersion nozzle, and argon sealing.

[0079] High-purity argon gas is introduced into the long nozzle of the ladle, with the flow rate controlled at 180~220L / min. Its lower end is inserted 220~280mm below the surface of the molten steel in the tundish, effectively isolating air and stabilizing the steel flow.

[0080] Meanwhile, an independent argon gas channel is set in the sliding gate mechanism area, with an argon gas flow rate of 4~8L / min at the upper gate and 4~12L / min at the mechanism body; argon gas is also introduced at the stopper rod and the submersible gate, with the flow rate maintained at 4~8L / min, to prevent air intake and the generation of impurities.

[0081] All argon-sealed interfaces maintain a back pressure of no less than 0.05 Bar to ensure reliable sealing, thereby strictly controlling the nitrogen increase during continuous casting to within ≤0.0002% (i.e., 2ppm).

[0082] In addition, the fluctuation range of the molten steel level in the crystallizer is controlled within ±2.0 mm to ensure uniform growth of the billet shell and reduce surface defects. The temperature of the molten steel in the tundish is precisely controlled within the range of 1536~1550℃, and through strict raw material control and baking procedures, the hydrogen content in the steel is kept below 2.0 ppm.

[0083] The tundish insulation can adopt a double-layer covering structure: the bottom layer is a low-carbon alkaline covering agent, which is used to adsorb floating inclusions and inhibit the oxidation of molten steel; the top layer is covered with carbonized rice husks, which provides excellent thermal insulation performance, reduces temperature drop and maintains thermal stability.

[0084] Next, during the casting process, the fluctuation range of the molten steel surface in the crystallizer is controlled to not exceed ±5mm; an electromagnetic stirring device is installed in the secondary cooling zone, with its working frequency set at 5.0~5.5Hz and current intensity controlled at 550~600A, which effectively promotes the formation of equiaxed crystals and breaks up dendrite networks, suppressing compositional segregation.

[0085] In the final solidification region of continuous casting, dynamic light reduction technology is implemented. The reduction position and amount are dynamically adjusted according to the real-time solidification state of the billet. The total reduction is controlled between 5% and 7% of the billet thickness to compensate for solidification shrinkage, compact the central looseness, and significantly reduce central segregation.

[0086] The taper of the fan-shaped section of the continuous casting machine is designed to be 1.1%~1.3%. The cooling system of the crystallizer adopts high-precision water control parameters: the cooling water flow rate is maintained at 450~550L / min, the inlet water temperature is controlled at 26~38℃, and the outlet water temperature difference is maintained at 5~8℃, ensuring stable heat conduction and uniform billet shell growth, providing a reliable guarantee for the production of high-quality continuous casting billets.

[0087] In one embodiment of this application, the shipbuilding steel is prepared by the smelting method and has a thickness of 200-320 mm, for example, a continuously cast billet of 220 mm or 320 mm.

[0088] The number of inclusions with a size ≥15μm on the cross-section of the shipbuilding steel is ≤5 per cm. 2 The number of inclusions with a size ≥10μm and <15μm is ≤15 per cm. 2 The number of inclusions with a size ≥5μm and <10μm is ≤150 / cm². 2 In other words, the inclusions are few in number and small in size, which avoids stress concentration and thus helps to reduce residual stress, meeting the requirements for subsequent low residual stress processing.

[0089] Furthermore, the shipbuilding steel exhibits a center segregation of ≤0.5 grade, a manganese segregation ratio of ≤1.10 in the cross-section, and a center porosity of ≤0.5 grade. In other words, the shipbuilding steel possesses excellent center quality, further preventing stress concentration and thus facilitating the reduction of residual stress.

[0090] Furthermore, the inclusions of type A, B, C, D, and Ds in the shipbuilding steel are all rated ≤1 and the sum is ≤2.5, that is, the sum of the grades of type A, B, C, D, and Ds inclusions is ≤2.5.

[0091] In this application, the types and ratings of non-metallic inclusions in the shipbuilding steel are tested according to GB / T 10561-2023 "Determination of Non-metallic Inclusion Content in Steel - Microscopic Examination Method with Standard Rating Chart"; the size distribution and quantity per unit area of ​​inclusions are statistically analyzed based on the same metallographic sample, using an automatic image analysis system to quantify them according to the size classification principles in the appendix of this standard. The center segregation grade and center porosity grade are evaluated according to GB / T 1979-2001 "Rating Chart of Low-Magnification Structure Defects in Structural Steel"; the manganese segregation ratio is obtained by sampling at the center line and 1 / 4 radius position of the billet cross-section, preparing chemical analysis samples according to GB / T 20066-2006 "Sampling and Sample Preparation Methods for Determination of Chemical Composition of Steel and Iron", and calculating the manganese content after determining it according to GB / T223.63-2022 "Determination of Manganese Content in Iron and Steel and Alloys - Sodium (Potassium) Periodate Spectrophotometric Method".

[0092] Furthermore, the shipbuilding steel prepared by the smelting method is a continuously cast billet (i.e., shipbuilding steel billet). The continuously cast billet can be further processed into steel plate products (i.e., shipbuilding steel plates). Based on the continuously cast billet, the steel plate products can have low residual stress and small residual stress fluctuations.

[0093] In one embodiment, the continuously cast billet can be further processed and prepared into a steel plate product with low residual stress using existing technologies, which will not be described in detail here.

[0094] The residual stress of the steel plate product is ≤80MPa.

[0095] More preferably, the residual stress of the steel plate does not exceed 70 MPa, or does not exceed 60 MPa, or does not exceed 50 MPa, or does not exceed 40 MPa, or does not exceed 30 MPa.

[0096] The steel plate not only has low residual stress, but also exhibits small fluctuations and uniform distribution of residual stress throughout the plate.

[0097] For example, the residual stress difference between any two points in the head, middle, tail, upper, and lower parts is ≤30MPa.

[0098] In this application, the steel can be sampled and its residual stress tested in accordance with GB / T 31310-2014 "Determination of Residual Stress in Metallic Materials - Drilling Strain Method".

[0099] The foregoing has described the technical principles and basic details of various embodiments of the present invention. Several embodiments are provided below to demonstrate the beneficial effects of this application. Of course, these embodiments are only a part of the numerous variations contained in this invention, and not all of them.

[0100] First, the shipbuilding steel billet is prepared using the smelting method described in this application, specifically including: 1) Desulfurization of molten iron (KR) The temperature of the molten iron leaving the station must be ≥1300℃, and the sulfur content (S) must be ≤0.0025%. 2) Converter smelting The final slag should be controlled with a basicity of 3.8–4.3, an MgO mass percentage of 8.0–10.0%, and a total iron mass percentage of 15–17%. The tapping temperature of the molten steel should be 1650–1670℃. During tapping, alloying and slag formation should be carried out in the following order: lime, ferrosilicon or medium-carbon ferromanganese alloy, aluminum ingot, and lime again. The mass of Al added from the aluminum ingot should be controlled by the amount of Al added. Al Satisfy M Al =0.0014M O +0.69; 3) LF Refining After the ladle is in place, first connect the bottom blowing argon and then start the bottom blowing argon, with a blowing pressure of 0.4~0.6MPa; Initial alloying is performed, followed by main alloying and slag formation under strong stirring. The bottom-blown argon flow rate is 700~860 NL / min, the argon blowing pressure is 0.50~0.60 MPa, and the strong stirring time is ≤6 min. Then, electro-refining is carried out, during which alloys are added. The flow rate of bottom-blown argon is 650~800 NL / min, the argon blowing pressure is 0.45~0.55 MPa, and the strong stirring time is ≤4min. Finally, white slag is produced by controlling the CaO / SiO2 ratio to be 5.0±0.2 and the CaO / Al2O3 ratio to be 1.8±0.5 in the slag, and the white slag is maintained for more than 15 minutes; then the steel is tapped at a temperature of 1598~1618℃. 4) RH vacuum refining The steel is then subjected to vacuum degassing, alloying, net circulation, and hydrogen degassing in sequence, followed by tapping at a temperature T=T RH +T RHXZ T RH The value is taken from 1563~1583℃, T RHXZ =k×(t RH -10), k takes values ​​from 0.3 to 0.5; 5) Cast billet Continuous casting billets were prepared using a continuous casting machine, employing fully protected casting with a superheat of 26-30°C. See Table 1, which shows the chemical composition of shipbuilding steel billets for several embodiments. In the table, "-" indicates that the corresponding element was not added, and the content of the corresponding element is 0 or present as an impurity element maintained below 0.02%.

[0101] [Table 1]

[0102] Each shipbuilding steel billet was sampled and tested. The measurement results are shown in Table 2 below. In the table, “≥15”, “[10,15)” and “[5,10)” represent “the number of inclusions with a cross-sectional size ≥15μm”, “the number of inclusions with a cross-sectional size ≥10μm and <15μm”, and “the number of inclusions with a cross-sectional size ≥5μm and <10μm”, respectively.

[0103] [Table 2]

[0104] It is evident that the shipbuilding steel billets from furnace numbers 1# to 7# of this application are all prepared using the smelting method of this application. They all have the advantage of fewer and smaller inclusions, and further reduce central segregation and central porosity. This avoids discontinuous microstructure caused by central segregation, central porosity, and large particle inclusions, thereby avoiding stress concentration and increasing residual stress. As a result, the shipbuilding steel billets are easier to control for residual stress when subsequently used to prepare steel plate products, thus obtaining steel plate products with low residual stress.

[0105] [Table 3]

[0106] For example, as shown in Table 3, based on shipbuilding steel billets from furnace numbers 1# to 7#, when rolling steel plate products using existing known technologies (such as the conventional technology of continuous casting billet heating-two-stage hot rolling-cooling), controlling residual stress is easy, and steel plate products with low residual stress and small fluctuations can be easily obtained. Tests were conducted in accordance with GB / T 2975-2018 "Sampling Location and Specimen Preparation for Mechanical Property Testing of Steel and Steel Products" and GB / T According to GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature", sampling and mechanical property testing of the steel materials revealed that the residual stress of finished steel plates with yield strengths of 235MPa, 315MPa, 355MPa, 390MPa, 420MPa, 460MPa, and 500MPa was as low as 30MPa, 40MPa, 40MPa, 45MPa, 60MPa, 70MPa, and 80MPa, respectively. The fluctuation of residual stress across the entire plate was as low as 10MPa, 15MPa, 15MPa, 15MPa, 20MPa, 25MPa, and 30MPa, respectively. Furthermore, the residual stress of no steel plate product exceeded 80MPa.

Claims

1. A method for smelting shipbuilding steel, characterized in that, The chemical composition of the shipbuilding steel, by mass percentage, includes: C 0.05~0.11%, Si 0.09~0.32%, Mn 0.92~1.63%, Cr 0~0.54%, Ni 0~0.39%, Mo 0~0.31%, Cu 0~0.39%, Nb 0.009~0.056%, Ti 0.009~0.021%, Al 0.021~0.049%, P≤0.0180%, S≤0.0060%, O≤0.0025%, N≤0.0048%, H≤0.0002%, with the remainder being iron and unavoidable impurities; The smelting method includes, in sequence, the KR desulfurization process for molten iron, the converter smelting process, the LF refining process, the RH vacuum refining process, and the billet casting process. In the converter smelting process: the basicity of the final slag is controlled at 3.8~4.3, the mass percentage of MgO is 8.0~10.0%, and the mass percentage of total iron is 15~17%. The tapping temperature of the molten steel is 1650~1670℃. During tapping, alloying and slag formation are carried out in the following order: lime, ferrosilicon or medium-carbon ferromanganese alloy, aluminum ingot, and lime again. The mass percentage of Al added from the aluminum ingot is controlled by the amount of Al added. Al Satisfying M Al =0.0014M O +0.69, where M O M is the mass percentage of oxygen (O) in the molten steel at tapping time, expressed in ppm. Al The unit is kg / t; In the RH vacuum refining process: vacuum degassing, alloying, net circulation, and hydrogen degassing are carried out in sequence, followed by steel tapping at a temperature T=T RH +T RHXZ T RH The value is taken from 1563~1583℃, T RHXZ =k×(t RH -10), k takes values ​​from 0.3 to 0.5, t RH This refers to the time from tapping the steel to pouring it, expressed in minutes.

2. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the RH vacuum refining process: T corresponding to the start-up of the casting furnace RH The value is taken from 1573~1583℃, and the corresponding T for the continuous casting furnace is... RH The value is taken from 1563 to 1573℃.

3. The method for smelting shipbuilding steel according to claim 2, characterized in that, In the LF refining process: the tapping temperature is 1598~1618℃.

4. The method for smelting shipbuilding steel according to claim 2, characterized in that, In the LF refining process: the tapping temperature corresponding to the open casting furnace is 1608~1618℃, and the tapping temperature corresponding to the continuous casting furnace is 1598~1608℃.

5. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0mbar, the degassing time is 20~25min, titanium-iron alloy is added during alloying, the processing time during net circulation is 8~10min, and the H mass percentage of hydrogen in the vacuum degassing process is ≤1.5ppm.

6. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the KR desulfurization process of molten iron: the temperature of the molten iron leaving the station is ≥1300℃ and S≤0.0025%.

7. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the converter smelting process: before tapping, argon blowing is started at the bottom of the ladle, with a blowing pressure of 0.5~0.6MPa and a bright ring diameter of molten steel of 240~340mm; when 3 / 4 of the steel has been tapped, the blowing pressure is reduced to 0.42~0.52MPa and the bright ring diameter of molten steel of 120~220mm.

8. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the LF refining process: after the ladle is in place, bottom blowing argon is first turned on and initial alloying is carried out, then main alloying and slag formation are carried out under strong stirring; then electric refining is carried out, during which alloys are added; finally, white slag is formed and steel is tapped.

9. The method for smelting shipbuilding steel according to claim 8, characterized in that, In the LF refining process: After connecting the bottom blowing argon, start the bottom blowing argon, and the blowing pressure is 0.4~0.6MPa; During the main alloying and slag formation processes, the bottom-blown argon flow rate is 700~860 NL / min, the argon blowing pressure is 0.50~0.60 MPa, and the strong stirring time is ≤6 min; When adding alloy, the flow rate of bottom-blown argon is 650~800 NL / min, the blowing pressure is 0.45~0.55 MPa, and the strong stirring time is ≤4 min; During the remaining time, the flow rate of bottom-blown argon is 300~400 NL / min, the blowing pressure is 0.40~0.50 MPa, and the diameter of the bright ring of molten steel is <150 mm.

10. The method for smelting shipbuilding steel according to claim 8, characterized in that, In the LF refining process: when making white slag, control the CaO / SiO2 ratio in the slag to be 5.0±0.2 and the CaO / Al2O3 ratio to be 1.8±0.5, and maintain the white slag for more than 15 minutes.

11. The method for smelting shipbuilding steel according to claim 1, characterized in that, In the billet casting process: a continuous casting machine is used to prepare the continuous casting billet, and full protective casting is adopted with a superheat of 26~30℃.

12. The method for smelting shipbuilding steel according to claim 1, characterized in that, The chemical composition of the shipbuilding steel, by mass percentage, also meets any one, two, three, or all of the following conditions: CEV = C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15 is 0.227~0.604; Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15 is 0.107~0.276; The Ni / Cu ratio is 0.95~1.05; 5C+Si+Mn+Cr+Ni+Mo+Cu is 1.46~4.13; In the formula, the element symbols represent the mass percentage of the corresponding element.

13. A type of shipbuilding steel, characterized in that, The shipbuilding steel is prepared by the smelting method according to any one of claims 1 to 12; The thickness of the shipbuilding steel is 200~320mm, and the number of inclusions with a size ≥15μm on its cross-section is ≤5 / cm. 2 The number of inclusions with a size ≥10μm and <15μm is ≤15 per cm. 2 The number of inclusions with a size ≥5μm and <10μm is ≤150 / cm². 2 .

14. The shipbuilding steel according to claim 13, characterized in that, The shipbuilding steel has a center segregation of ≤0.5 grade, a manganese segregation ratio of ≤1.10 in the cross section, a center porosity of ≤0.5 grade, and inclusions of types A, B, C, D, and Ds are all rated ≤1 grade and their sum is ≤2.5 grade.

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