Weldable steel plate for marine engineering containing magnesium oxide and method for manufacturing the same

Abstract

The application discloses a kind of magnesium oxide-containing easy-to-weld marine engineering steel plate and preparation method thereof, and its chemical composition mass percentage is C:0.05-0.09%, Si:0.15-0.30%, Mn:1.10-1.50%, P≤0.010%, S≤0.003%, Nb:0.008-0.020%, Ti:0.005-0.015%, Al:0.005-0.010%, Mg:0.0010-0.0025%, the rest components are Fe and inevitable impurities.The easy-to-weld marine engineering steel plate base metal yield strength is greater than or equal to 420MPa, tensile strength is greater than or equal to 550MPa, A is greater than or equal to 45%, -60℃ near surface, 1 / 4, heart transverse and longitudinal impact energy is greater than or equal to 200J, control CEV≤0.34, Pcm≤0.18;After welding of greater than or equal to 300kJ / cm line input, joint tensile strength is greater than or equal to 540MPa, thickness direction -60℃ impact energy is greater than or equal to 150J, fully meet the need of large line energy welding.The invention discloses the comprehensive mechanical properties of magnesium oxide-containing easy-to-weld marine engineering steel produced by metallurgical technology are excellent, alloy cost is low, production line adaptability is strong, and it has broad popularization prospect.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy and relates to a weldable steel plate for marine engineering containing magnesium oxide and its preparation method. Background Technology

[0002] Marine engineering steels are primarily high-strength low-alloy steels, requiring high strength and toughness, fatigue resistance, resistance to lamellar tearing, and good weldability. Currently, ships and marine engineering equipment are developing towards larger sizes and higher efficiency, often employing high heat input welding to improve production efficiency during construction. However, various marine engineering steel plates are prone to a significant decrease in low-temperature impact toughness in the heat-affected zone (HAZ) when using high heat input welding, severely restricting their application. Since low-temperature impact toughness is one of the important properties of marine engineering steels and a key factor that must be considered in the design of marine engineering equipment, improving the low-temperature impact toughness of the HAZ in high heat input welded steel plates is a crucial technical challenge that needs to be overcome for high-strength, thick-gauge marine engineering steels.

[0003] While there are numerous patents currently available for easily weldable steel plates used in marine engineering, these patents often employ multiple alloys, resulting in higher costs. Below is a brief introduction to several similar patents:

[0004] Chinese patent application CN 110004358 A discloses "A Low Pcm Value Thick and Easy-to-Weld Marine Engineering Steel Plate and Its Production Method." This patent is based on a low-carbon composition system and uses alloys such as Nb, V, Ti, Ni, and Mo to develop a low Pcm value marine engineering steel plate. The method disclosed in this patent reduces the Pcm value of the steel plate through various expensive alloys, resulting in a high overall cost. Furthermore, it is only limited to obtaining a lower Pcm value and does not fundamentally improve the performance of the weld heat-affected zone of the steel plate.

[0005] Chinese patent application CN 110791702 A discloses "A marine engineering steel plate with good weldability and a low yield strength ratio and its manufacturing method." This patent uses low-carbon micro-niobium technology and adds Cu, Ni, and Mo to improve the strength and low-temperature impact resistance of the steel plate, thereby achieving low yield strength ratio control. However, the overall Ceq ​​and Pcm content of the method described in this invention is still at a relatively high level, which limits the degree of improvement in weldability.

[0006] Chinese patent application CN 115323253 A discloses a "Production Method of S355NL Low-Temperature Impact Thick Plate by Low Compression Ratio Rolling". This patent employs oxide metallurgy in the steelmaking process and Ti-Mg nanoscale precipitate control technology to induce the online generation of fine second-phase particles-induced intragranular ferrite, enabling the production of 355MPa level medium-thick plates under low compression ratio rolling conditions. However, the method described in this patent uses a low-carbon, high-alloy design, resulting in lower overall strength and higher CEV, leading to poor weldability under high-voltage conditions.

[0007] The main advantages of this invention are: (1) The steel plate has high strength and toughness. The yield strength of the 60mm thick marine engineering steel plate produced by this invention is ≥420MPa, the tensile strength is ≥550MPa, A is ≥45%, and the transverse and longitudinal impact energy at -60℃ near the surface, 1 / 4, and the core is ≥200J, with excellent mechanical properties; (2) The Pcm designed by this invention is ≤0.18, with good weldability. At the same time, the Mg oxide in the steel can greatly improve the mechanical properties of the heat-affected zone of the weld. After welding under the condition of 300kJ / cm heat input, the tensile strength at the joint of the steel plate is still ≥550MPa, and the impact energy at -60℃ in the thickness direction is ≥150J. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, this invention provides a magnesium oxide-containing easy-to-weld marine engineering steel plate and its preparation method. This production method can produce a low-cost, high-strength and high-toughness steel plate suitable for high heat input welding in the field of shipbuilding and marine engineering. Its mechanical properties and welding performance at a heat input of 300 kJ / cm both meet application requirements. Furthermore, it establishes a specific magnesium oxide-containing easy-to-weld marine engineering steel composition and corresponding production process.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A type of easy-to-weld marine engineering steel plate containing magnesium oxide has the following chemical composition by mass percentage: C: 0.05-0.09%, Si: 0.15-0.30%, Mn: 1.10-1.50%, P≤0.010%, S≤0.003%, Nb: 0.008-0.020%, Ti: 0.005-0.015%, Al: 0.0050-0.010%, Mg: 0.0010-0.0025%, with the remainder being Fe and unavoidable impurities.

[0011] Composition design and smelting

[0012] The added chemical elements and their mechanisms of action in this invention are as follows:

[0013] (a) Carbon

[0014] Interstitial solid solution strengthening with carbon atoms is the most economical and effective strengthening method for steel materials. Carbon atoms enter the crystal lattice, causing lattice distortion and resulting in forward interactions that significantly improve the strength of the steel. However, as the carbon content increases, the toughness, elongation, and other properties of the steel decrease dramatically, while weldability deteriorates sharply. Therefore, the carbon content in this steel is controlled at 0.05-0.09%.

[0015] (b) Silicon

[0016] Si can dissolve in steel and cause lattice distortion, resulting in solid solution strengthening and increasing the strength of the steel plate. However, excessive Si content can worsen weldability; therefore, the Si content in this steel is controlled at 0.15-0.30%.

[0017] (c) Manganese

[0018] Mn can lower the γ-α phase transformation temperature of steel, refine the grains, and change the microstructure after the phase transformation. Mn can also refine the pearlite lamellae, improve the strength of pearlite in low-carbon steel, and at the same time, not significantly reduce ductility. Therefore, the Mn content in this steel is controlled at 1.10-1.50%.

[0019] (d) Niobium

[0020] Nitrogen (Nb) can refine grains and increase the grain coarsening temperature. Simultaneously, Nb in steel combines with C and N to form nano-sized Nb (C, N), resulting in a significant strengthening effect. Nb also improves the mechanical properties of the weld heat-affected zone. However, Nb alloys are relatively expensive; therefore, the addition amount in this steel is 0.008-0.020%.

[0021] (e) Titanium

[0022] Ti plays a significant role in inhibiting austenite grain growth, delaying austenite recrystallization, refining grain size, and precipitation strengthening, and is beneficial to the strength and toughness of steel plates. Ti can also significantly improve the tensile strength of weld metal, which is beneficial for improving plasticity and toughness. However, the Ti content must be controlled within a reasonable range; too little Ti has a weak strengthening effect, while too much Ti leads to a significant decrease in weld toughness. Therefore, the Ti content in this steel is controlled at 0.005-0.015%.

[0023] (f) Aluminum

[0024] Al is a strong deoxidizing element that can significantly reduce the oxygen content in steel, decrease the amount of oxide inclusions, and improve the overall performance of the steel. However, excessively high Al content in steel can hinder the production of Mg oxides, which is detrimental to oxide metallurgical control. Therefore, the Al content in this steel is controlled at 0.005-0.010%.

[0025] (g) Magnesium

[0026] Mg is a strong deoxidizing element, exhibiting a stronger affinity for [O] in molten steel than deoxidizing elements such as Al and Ti. After entering molten steel, Mg can react directly with [O] to form MgO, or it can reduce the Al and Ti deoxidation products generated in the molten steel, replacing the deoxidizing elements within them. Furthermore, Mg has a significant modifying effect on oxide inclusions. As the Mg content in steel increases, it promotes the incremental refinement and precipitation of nano-sized TiN particles, enhancing their pinning effect and helping to suppress austenite grain growth during the high-temperature stage of welding thermal cycling. Based on this, the Mg content in this steel is controlled at 0.0010-0.0025%.

[0027] (h) Harmful elements

[0028] Sulfur readily combines with manganese to form MnS inclusions, which deform during rolling, severely affecting the low-temperature impact toughness of steel. Furthermore, sulfur increases the hot brittleness of weld metal, making it prone to hot cracking and porosity in the weld; its content should not exceed 0.003%.

[0029] Phosphorus: Phosphorus is an element that easily segregates in steel, making it prone to cold brittleness. Furthermore, phosphorus plays a similar role in welds as in steel, easily causing weld cracks. To ensure sufficient toughness in the weld metal, the phosphorus content should not exceed 0.013%.

[0030] A method for preparing a weldable marine engineering steel plate containing magnesium oxide, the method comprising the following steps:

[0031] 1) Smelting production

[0032] Converter smelting: Molten iron must be weighed and metered before entering the furnace, and the proportion of scrap steel added must be controlled to ≤20%; slag must be added 3 minutes before the end of the smelting process, and a single-stage carbon removal and single-slag deep dephosphorization process is adopted for smelting, with the final slag basicity controlled at R=3.5-4.5; when charging the steel, deoxidation is carried out using ferrosilicon and other materials, and metallic manganese, ferrosilicon, and ferroniobium alloys are added in batches. 500kg of top lime and 200kg of pre-melted slag are added along the steel flow. To ensure the deoxidation effect and full melting of the alloy, the charging process time is controlled to ≥3min.

[0033] LF Refining: Argon stirring is performed throughout the process, with lime added for slag formation. Ferrosilicon powder is used to adjust the free oxygen content. The top slag is yellow-white before sampling. The top slag must be yellow-white or white before leaving the station, and this yellow-white or white slag must be maintained for at least 8 minutes. The final slag basicity must be at least 2.3. During refining, oxygen content is controlled. When the free oxygen content is between 30-60 ppm, wire is fed and alloying is fine-tuned: ① Feed Ti wire at a speed of 1.5-2.5 m / s, with a length of 80-100 m; ② After feeding Ti wire, feed Mg wire at a speed of 1.0-3.0 m / s, with a length of 1200-1400 m; ③ Alloying is performed simultaneously with wire feeding, adjusting the composition to the control range. After feeding, argon stirring is performed for 3-5 minutes, oxygen content is controlled, and steel samples are taken. The refining process time is controlled to be at least 45 minutes, and the soft blowing time is at least 5 minutes.

[0034] Table 1 Chemical composition of Ti and Mg wires

[0035] Ti wire 0.075 72.2 2.68 0.13 1.35 0.18 0.22 0.026 0.09 the rest Mg wire 6.3 0.12 1.12 0.71 34.79 0.52 0.01 0.016 0 the rest

[0036] Note: Mg line: diameter Line weight ratio 330g / m; Ti wire: diameter Linear weight ratio 596g / m

[0037] RH refining: During the RH refining process, the vacuum degree is between 40-133Pa, the holding time is ≥15 minutes, and the pure degassing time is greater than 5 minutes; after the RH treatment is completed, calcium treatment is not allowed, the soft blowing time is ≥8 minutes, and the entire RH smelting cycle is controlled within 40-60 minutes.

[0038] Continuous casting: Casting speed and superheat are crucial to the core quality of easily weldable marine engineering steel plates. In actual production, the casting speed for 250mm cross-section is controlled at 1.0-1.3m / min, and the casting speed for 300mm cross-section is controlled at 0.7-0.9m / min. Peritectic steel protective slag is used in the crystallizer. Argon blowing protection is used throughout the casting process. The slow cooling time after the billet is removed from the line is ≥72 hours to completely eliminate the influence of hydrogen in the steel.

[0039] 2) Rolling

[0040] Slab heating regime

[0041] The slab is heated using a cold-charged billet heating method. Due to the low-alloy, low-Pcm design of this steel grade, low-temperature heating is required to prevent coarse grains. The temperature of the soaking zone in the heating furnace is controlled at 1130-1200℃, the tapping temperature at 1130-1170℃, and the heating rate at ≥ (9-11) min / cm. When heating thick cold billets (h≥300mm), the minimum heating rate is 9-10 min / cm. This heating rate and temperature can homogenize the original austenite structure in the billet and fully dissolve alloying elements such as Ti and Nb in the steel, while not damaging the oxide metallurgical effect. The preferred slab heating regime for this steel grade is: soaking zone temperature 1160℃, tapping temperature 1160℃, and heating rate 10 min / cm.

[0042] Rolling process

[0043] The steel plate is produced using controlled rolling and controlled cooling, followed by MULPIC water cooling after finishing rolling. The specific process is shown in Table 2. The roughing stage is completed in 5-7 passes depending on the target steel plate thickness, with the reduction rate gradually increasing with each pass. The first pass reduction rate is ≥7%, and the reduction rates for the last two passes should both be ≥20%. During the roughing stage, the temperature of this steel grade remains in the recrystallization zone. The gradually increasing reduction rate effectively breaks down the austenite grains, promoting complete recrystallization and refining the austenite grains. When producing 60mm steel plates using a 300mm thick slab, the preferred roughing pass for this steel grade is 5 passes, with a first pass reduction rate of 7.5% and the last two passes reductions of 20% and 21%, respectively.

[0044] In the finishing rolling stage, which is in the non-recrystallization zone, the rolling method still adopts a method of gradually increasing the reduction per pass. The specific operation is as follows: The number of finishing rolling passes is controlled to be 7-9 based on the finished product thickness. The reduction rate in the first pass is ≥8%, and the reduction rate increases with each subsequent pass. The rolling temperature is controlled at at least 3 passes at 760-790℃, with a cumulative deformation ≥25%. Increasing the deformation rate per pass in the finishing rolling stage ensures the penetration effect of the core rolling, refines the grain size of the core structure, and improves uniformity. Simultaneously, the NbC, TiC, and other second-phase nanoparticles precipitated during finishing rolling form Cotillard atmospheres that pin austenite grain boundaries, hindering the recrystallization process and inhibiting abnormal austenite grain growth. When producing 60mm steel plates using 300mm thick slabs, this steel grade is preferably finished rolled in 9 passes, with a combined reduction rate of 27% over 3 passes at 760-780℃.

[0045] After rolling, the steel sheet is rapidly fed into the MULPIC water cooler for controlled cooling. The temperature of the steel sheet entering the cooler is 760-780℃, and the cooling rate is controlled between 12-26℃ / s depending on the thickness of the steel sheet. This process completely transforms the austenite into a mixed microstructure of ferrite, pearlite, and bainite, resulting in a steel sheet with high strength and toughness. The preferred cooling process for this steel grade adopts the line control shown in Table 2.

[0046] Table 2 Rolling and Water Cooling Processes

[0047]

[0048] The 60mm thick marine engineering steel plate produced using this invention has a yield strength ≥420MPa, tensile strength ≥550MPa, A ≥45%, and transverse and longitudinal impact energy ≥200J at near surface, 1 / 4, and core under -60℃ conditions, exhibiting excellent mechanical properties.

[0049] The steel plate can be welded under high wire input conditions of ≥300kJ / cm, and preheating is not required before welding. The welding process adopts double-wire submerged arc welding with a single-sided V-groove. The interpass temperature is controlled at ≤200℃, and the number of welding passes is determined according to the thickness of the steel plate. After welding, the tensile strength at the joint is ≥540MPa, and the impact energy at -60℃ in the weld fusion metal area near the surface, at 1 / 4 of the thickness, and in the core is ≥120J.

[0050] For high heat input welding, Mg, a strong deoxidizing element, has a stronger affinity for [O] in molten steel than deoxidizing elements such as Al and Ti. After entering the molten steel, Mg can react directly with [O] to form MgO, or it can reduce the Al and Ti deoxidation products generated in the molten steel, replacing the deoxidizing elements and forming Mg-containing inclusions. Since there is no excess oxygen in the steel to combine with Ti to form Ti oxides, large-sized TiN particles that affect the strength and toughness of the weld heat-affected zone (HAZ) cannot be formed. Furthermore, Mg has a strong modifying effect on oxide inclusions. Added Mg can modify Al2O3 inclusions in the steel into magnesium-aluminum spinel inclusions, and can also promote the incremental refinement and precipitation of nano-sized TiN particles in the steel, enhancing the pinning effect. The HAZ microstructure retains complete pre-austenite grain boundaries (PAGB), and the intragranular microstructure is high-strength, low-toughness lath bainite (LB), with fine MA islands within the LB lath structure. MA island morphology significantly affects HAZ toughness: small, dot-shaped MA islands are harmless to toughness; elongated MA islands worsen HAZ toughness. Since PAG size directly affects MA size, and thus HAZ toughness, the oxide formed by Mg added to steel greatly promotes the formation of nano-sized TiN particles, thereby pinning boundaries, inhibiting the growth of proto-austenite grain boundaries (PAGB), and further promoting the formation of dot-shaped MA islands, thus improving HAZ toughness.

[0051] The present invention describes a method for refining the grain size of easily weldable marine engineering steel plates containing magnesium oxide by adding microalloying elements such as Nb and Ti, resulting in significant grain refinement and precipitation strengthening effects. Combined with a rolling mode that increases the reduction rate per pass during roughing and controlled cumulative deformation per pass within the 760-790℃ range during finishing, the dislocation density in the steel plate can be significantly increased, enhancing the strength and toughness of the steel. The added Mg element can inhibit the precipitation of large-sized TiN and has a strong modifying effect on inclusions. Furthermore, Mg can promote the refinement of the original austenite grain boundaries (PAGB), thus improving the overall strength and toughness of the weld. By implementing the above key technologies, the 60mm thick magnesium oxide-containing easy-to-weld marine engineering steel plate base material exhibits a yield strength ≥420MPa, tensile strength ≥550MPa, A ≥45%, and transverse and longitudinal impact energy greater than 200J at near-surface, 1 / 4, and core temperatures (-60℃), while controlling CEV ≤0.38 and Pcm ≤0.18. After welding with a wire input ≥300kJ / cm, the joint tensile strength ≥540MPa and the impact energy at -60℃ in the thickness direction ≥150J, fully meeting the requirements for high heat input welding. The easy-to-weld marine engineering steel produced by the magnesium oxide-containing metallurgical technology disclosed in this invention has excellent comprehensive mechanical properties, low alloy cost, strong production line adaptability, and broad prospects for promotion. Attached Figure Description

[0052] Fig. 1The metallographic structure (near surface) of the easy-to-weld marine engineering steel plate containing magnesium oxide in Example 1 of the present invention;

[0053] Fig. 2 The metallographic structure (1 / 4 thickness) of the easy-to-weld marine engineering steel plate containing magnesium oxide in Example 1 of the present invention;

[0054] Fig. 3 The metallographic structure (1 / 2 thickness) of the easy-to-weld marine engineering steel plate containing magnesium oxide in Example 1 of the present invention;

[0055] Fig. 4 This is the macroscopic metallographic structure of the welded area of ​​the easy-to-weld marine engineering steel plate containing magnesium oxide in Embodiment 1 of the present invention. Detailed Implementation

[0056] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0057] Example 1:

[0058] A 60mm thick, easily weldable marine engineering steel plate containing magnesium oxide comprises the following chemical composition: C: 0.07%, Si: 0.25%, Mn: 1.20%, P: 0.007%, S: 0.003%, Nb: 0.015%, Ti: 0.010%, Al: 0.006%, Mg: 0.0015%, with the remainder being Fe and unavoidable impurities. The steel plate base material has a yield strength of 433 MPa, a tensile strength of 572 MPa, an alumina of 48%, and average transverse and longitudinal impact energy values ​​at near-surface, 1 / 4 of the length, and core at -60℃ of 313 J, 287 J, and 267 J, respectively; CEV: 0.27; and Pcm: 0.14.

[0059] The manufacturing method includes the following steps: molten iron is smelted in a converter, refined by LF, and degassed by RH before continuous casting. A 250mm or 300mm thick continuous casting billet is used. After continuous casting, the billet needs slow cooling. During the smelting process, when the free oxygen content in the refining process reaches 45ppm, wire is fed and alloy fine-tuning is performed: ① Ti wire is fed at a speed of 2m / s and a length of 85m; ② After the Ti wire is fed, Mg wire is fed at a speed of 2.0m / s and a length of 1273m; ③ Alloying is performed simultaneously with wire feeding, adjusting the composition to the control range. After feeding, argon is blown and stirred for 5 minutes, with a soft blowing time ≥9 minutes. During the RH refining process, the vacuum level is ≤70Pa, held for 15 minutes, and the pure degassed time is 10 minutes. The continuous casting process uses a 300mm billet casting speed.

[0060] The steel billet is heated, rolled, and then water-cooled to 510℃ using a MULPIC system to obtain the aforementioned marine engineering steel plate. During heating, the billet soaking temperature is 1160℃, the tapping temperature is 1150℃, and the heating rate is 10 / min / cm. The rough rolling stage is completed in 5 passes, with the reduction rate gradually increasing with each pass. The first pass reduction rate is 7.5%, and the reduction rates for the last two passes are 20% and 21%, respectively. The finish rolling stage requires 7 passes, again employing a progressively increasing reduction rate per pass. The first pass reduction rate is 8.2%, with the reduction rate increasing with each pass. The final three rolling temperatures are 787℃, 767℃, and 760℃, with a cumulative deformation of 26%.

[0061] Figs. 1-3 The figure reflects the changes in microstructure along the thickness direction of the steel plate. As shown in the figure, the steel plate produced in this embodiment of the invention has a microstructure of ferrite / pearlite with some bainite in the core. The microstructure is uniform in the thickness direction and no grain coarsening is observed, indicating that the smelting composition and rolling process are properly controlled. The steel plate adopts a single-sided V-groove and is welded by double-wire submerged arc welding. The tensile strength at the weld joint is 570 MPa. The average impact energy at -60℃ in the near-surface, 1 / 4 thickness, core, and lower surface areas of the weld fusion metal region are 209 J, 192 J, 178 J, and 202 J, respectively, indicating excellent comprehensive mechanical properties at the weld. The welding process and mechanical properties are shown in Tables 3 and 4.

[0062] Table 3 Example 1: Double-wire submerged arc welding of steel plates

[0063]

[0064] Table 4 Mechanical properties of steel plates after welding in Example 1

[0065]

[0066] Example 2

[0067] A 80mm thick, easily weldable marine engineering steel plate containing magnesium oxide comprises the following chemical composition: C: 0.09%, Si: 0.26%, Mn: 1.49%, P: 0.006%, S: 0.003%, Nb: 0.020%, Ti: 0.013%, Al: 0.008%, Mg: 0.0023%, with the remainder being Fe and unavoidable impurities. The steel plate base material has a yield strength of 417 MPa, a tensile strength of 568 MPa, an alumina of 45%, and average transverse and longitudinal impact energy values ​​at near-surface, 1 / 4 of the length, and core at -60℃ of 271 J, 235 J, and 217 J, respectively; CEV: 0.34; and Pcm: 0.17.

[0068] The manufacturing method includes the following steps: molten iron is smelted in a converter, refined by LF, and degassed by RH before continuous casting. A 300mm thick continuous casting billet is selected, and the billet needs to be slowly cooled after continuous casting. During the smelting process, when the free oxygen content in the refining process is 50ppm, wire is fed and alloy fine-tuning is performed: ① Ti wire is fed at a speed of 2m / s and a length of 106m; ② After the Ti wire is fed, Mg wire is fed at a speed of 2.0m / s and a length of 1854m; ③ Alloying is performed simultaneously with wire feeding, and the composition is adjusted to the control range. After wire feeding, argon is blown and stirred for 5 minutes, and the soft blowing time is 12 minutes. During the RH refining process, the vacuum degree is ≤60Pa, the holding time is 15 minutes, and the pure degassed time is 10 minutes. The continuous casting process uses a 300mm billet casting speed of 0.95m / min.

[0069] The steel billet is heated, rolled, and then water-cooled to 450℃ using a MULPIC system to obtain the aforementioned marine engineering steel plate. During heating, the billet soaking temperature is 1160℃, the tapping temperature is 1150℃, and the heating rate is 9-10 / min / cm. The rough rolling stage is completed in 5 passes, with the reduction rate gradually increasing with each pass. The first pass reduction rate is 5.2%, and the reduction rates for the last two passes are 17.2% and 24.20%, respectively. The finishing rolling stage requires 8+1 passes, again employing a progressively increasing reduction rate per pass. The first pass reduction rate is 3.5%, with the reduction rate increasing with each pass. The final three rolling temperatures with the maximum reduction are 763℃, 755℃, and 748℃, respectively, with a cumulative deformation of 18%.

[0070] The steel plate is welded with a single-sided V-groove and exhibits good strength and toughness at the weld joint after double-wire submerged arc welding. The welding process is similar to that in the previous embodiment, and the mechanical properties of the steel plate after welding in Example 2 are shown in Table 5.

[0071] Table 5 Mechanical properties of steel plates after welding in Example 2

[0072]

[0073] The upper and lower limits of the process parameters (such as rolling deformation, holding time, etc.) and the range values ​​of the present invention can all achieve this method, and examples are not listed here.

[0074] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A weldable steel plate for marine engineering containing magnesium oxide, characterized in that, The chemical composition (by mass percentage) of the easily weldable marine engineering steel plate is as follows: C: 0.05-0.09%, Si: 0.15-0.30%, Mn: 1.10-1.50%, P≤0.010%, S≤0.003%, Nb: 0.008-0.020%, Ti: 0.005-0.015%, Al: 0.005-0.010%, Mg: 0.0010-0.0025%, with the remainder being Fe and unavoidable impurities. 1) Smelting production Converter smelting: The molten iron is metered and weighed before entering the furnace, and the proportion of scrap steel added is controlled to be ≤20%; during the smelting process, the slag is added 3 minutes before the end point, and the smelting process of single-stage carbon removal and single-slag deep dephosphorization is adopted. The basicity of the final slag is controlled at R=3.5-4.5; when the steel is discharged, ferrosilicon deoxidation is adopted, and metallic manganese, ferrosilicon, and ferroniobium alloy are added in batches. Lime and pre-melted slag are added along the steel flow. In order to ensure the deoxidation effect and the full melting of the alloy, the steel discharge process time is controlled to be ≥3 minutes. LF Refining: Argon stirring is performed throughout the process, with lime added for slag formation. Ferrosilicon powder is used to adjust the free oxygen content. The top slag is yellowish-white before sampling. The top slag must be yellowish-white or white before leaving the station, and this condition must be maintained for at least 8 minutes. The final slag basicity must be at least 2.

3. During refining, oxygen content is controlled. When the free oxygen content is between 30-60 ppm, wire is fed and alloying is fine-tuned: Ti wire is fed at a speed of 1.5-2.5 m / s, with a length of 80-100 m. After the Ti wire is fed, Mg wire is fed at a speed of 1.0-3.0 m / s, with a length of 1200-1400 m. Alloying is performed simultaneously with wire feeding, adjusting the composition to the control range. After feeding, argon stirring is performed for 3-5 minutes, oxygen content is controlled, and steel samples are taken. The refining process time is controlled to be at least 45 minutes, and the soft blowing time is at least 5 minutes. RH refining: During the RH refining process, the vacuum degree is between 40-133Pa, the holding time is ≥15 minutes, and the pure degassing time is greater than 5 minutes; after the RH treatment, calcium treatment is not allowed, the soft blowing time is ≥8 minutes, and the entire RH smelting cycle is controlled within 40-60 minutes. Continuous casting: The casting speed is controlled at 7.0-1.3 m / min; all crystallizers use peritectic steel protective slag; argon blowing protection is used throughout the casting process; the slow cooling time after the billet is removed from the line is ≥72 hours; 2) Rolling Slab heating regime The slab is heated using cold-charged billets, with the temperature of the soaking zone of the heating furnace controlled at 1130-1200℃, the tapping temperature at 1130-1170℃, and the heating rate at 9-11 min / cm. The steel plate is produced by controlled rolling and controlled cooling. In the finishing rolling stage, the steel plate is in the non-recrystallization zone, and the rolling method still adopts the method of gradually increasing the reduction in each pass. After rolling, the steel plate is quickly fed into the MULPIC water cooler for controlled cooling. The temperature of the steel plate when it enters the water cooler is 760-780℃. The cooling rate is controlled at 12-26℃ / s according to the thickness of the steel plate, so that the austenite is completely transformed into a mixed structure of ferrite, pearlite and bainite.

2. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, The weldable marine engineering steel plate has a base material yield strength ≥420MPa, tensile strength ≥550MPa, A ≥45%, and transverse and longitudinal impact energy ≥200J at near surface, 1 / 4, and core at -60℃, with CEV ≤0.34 and Pcm ≤0.

18. After welding with a wire input of ≥300kJ / cm, the tensile strength at the joint is ≥540MPa, and the impact energy at -60℃ in the thickness direction is ≥150J.

3. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, The thickness of the weldable marine engineering steel plate is 40-60mm.

4. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, During continuous casting, the casting speed for 250mm section is controlled at 1.0-1.3 m / min, and the casting speed for 300mm section is controlled at 0.7-0.9 m / min.

5. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, The soaking zone temperature is 1160℃, the tapping temperature is 1160℃, and the heating rate is 10 min / cm.

6. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, The roughing stage is completed in 5-7 passes depending on the target steel plate thickness. The reduction rate of each pass is gradually increased, with the reduction rate of the first pass being ≥7% and the reduction of the last two passes being ≥20%.

7. The easy-to-weld steel plate for marine engineering according to claim 6, characterized in that, When producing 60mm steel plates using a 300mm thick slab, the roughing rolling passes are 5, with the first pass having a reduction rate of 7.5%, and the last two passes having reduction rates of 20% and 21%, respectively.

8. The easy-to-weld steel plate for marine engineering according to claim 1, characterized in that, The specific operation of the method of gradually increasing the reduction is as follows: control the finishing rolling passes to 7-9 passes according to the thickness of the finished product, the reduction rate of the first pass is ≥8%, the reduction rate increases with each pass, and control the rolling temperature of at least 3 passes to 760-790℃, and the cumulative deformation is ≥25%.

9. The easy-to-weld steel plate for marine engineering according to claim 8, characterized in that, When producing 60mm steel plates using a 300mm thick slab blank, a 9-pass finishing rolling process is used, with a combined reduction rate of 27% across 3 passes at 760-780℃.

Images