EH460 Grade Ship Plate Steel Suitable for High Heat Input Welding and Its Manufacturing Method

Abstract

The present invention discloses an EH460 grade ship plate steel suitable for large heat input welding and a preparation method thereof. Specifically, a design concept of "ultra-low carbon + oxide metallurgy + fine composition control" is adopted. By reducing the C content in the steel, the weldability of the steel is improved; the value of Mn + 0.4(Ni + Cu) is controlled to make up for the strength loss caused by the reduction of C, so as to ensure the strength of the steel. At the same time, Ti, Mg, Zr and REM elements are added to the steel for oxide metallurgy, and the Ti / N and (Mg + Zr + REM) / (Al + Ti) are controlled, so that ideal inclusion morphology, size and quantity are formed during the solidification process of the steel in large heat input welding, effectively pinning the prior austenite grain boundaries, hindering the growth of austenite grains, refining the grains, promoting the nucleation of intragranular acicular ferrite, and making the volume fraction of acicular ferrite in the structure of the welding heat affected zone ≥ 65%, significantly improving the low temperature toughness of the welded joint.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and particularly to an EH460 grade ultra-high strength TMCP ship plate steel suitable for large heat input welding and a manufacturing method thereof. Background Art

[0002] High-strength structural steel plates for ships and offshore engineering are key materials for the construction of high-tech ships and advanced offshore engineering. Their characteristics are both high strength and good low-temperature toughness. With the continuous development of the shipbuilding industry, the modernization level of ships has been continuously improved, and the requirements for the stability, safety and reliability of the hull structure have also been continuously increased, which has further increased the strength requirements for steel plates for ships and offshore engineering. Especially for the special structural components of new high-tech ships, ordinary high-strength grade shipbuilding steel plates are difficult to meet their service requirements. Therefore, ultra-high strength steel is the future development trend of shipbuilding structural steel plates. EH460 belongs to the ultra-high strength grade structural steel for ships and offshore engineering in GB / T712 and has currently been applied to the manufacture of special structural components of ships and offshore engineering.

[0003] At present, China's shipbuilding volume accounts for about one-third of the global total shipbuilding volume and has become the world's largest shipbuilding country. With the continuous improvement of China's shipbuilding capacity, the demand for structural steel plates for shipbuilding has been increasing year by year. At the same time, since the welding cost accounts for about 30% - 40% of the total manufacturing cost during the entire shipbuilding cycle, and the welding man-hour cost accounts for more than half of the welding cost, improving the welding efficiency and shortening the shipbuilding cycle are the key issues for reducing the shipbuilding cost. Generally, using large heat input welding in the shipbuilding field can play a significant role in "cost reduction and efficiency increase". However, with the increase of the welding heat input, the microstructure of the heat affected zone deteriorates continuously, and the low-temperature toughness decreases significantly. Therefore, how to ensure the low-temperature toughness of large heat input welding of structural steel for ships and offshore engineering has become a key problem that needs to be solved urgently in the iron and steel industry.

[0004] The "A steel plate with 550MPa grade that can be welded with large heat input and a manufacturing method thereof" provided by the publication number CN104046899A adopts the design idea of low C, Si, Al - high Mn - Nb, optimizes the TMCP process, and successfully develops a steel plate with a yield strength ≥ 465MPa, a tensile strength of 550MPa - 650MPa, an impact energy at - 60°C (single value) ≥ 100J, and an impact energy at - 40°C (single value) ≥ 100J in the heat affected zone of large heat input welding. However, this invention does not mention the heat input amount during large heat input welding. Therefore, the post-weld performance of this steel type cannot be accurately evaluated. And this patent selects Ca as a deoxidizer, and the smelting process is relatively complex.

[0005] The "EH460-grade ship plate steel for large heat input welding and its preparation method" provided by the publication number CN111440986A uses Ti-Mg composite deoxidation to form a large number of small-sized (0.5 - 4 μm) Ti2O3-MgO-Al2O3-MnS composite inclusions with a dispersed distribution. Under the condition that the heat input of welding is 100 - 300 kJ / cm, the average Charpy impact energy at -40 °C in the heat-affected zone of the steel plate is ≥120 J. However, this invention only mentions its applicability to welding with a heat input in the range of 100 - 300 kJ / cm, and does not mention the welding performance at a heat input above 300 kJ / cm.

[0006] The "quenched and tempered offshore steel plate for large heat input welding of EH550MPa grade and its manufacturing method" provided by the publication number CN113322408A has a yield strength of the offshore steel ≥550 MPa, a tensile strength ≥670 MPa, the impact energy of the base metal at -40 °C (single value) ≥180 J, and the impact energy at -40 °C (single value) in the heat-affected zone at heat inputs of 50 kJ / cm, 100 kJ / cm, and 150 kJ / cm ≥80 J. However, this invention uses a heat treatment process of quenching + tempering, which has a long process flow and high production costs.

[0007] The "thick steel plate for offshore engineering of EH420 grade with large heat input welding and its preparation method" provided by the publication number CN109321847A contains elements such as Mn, Ti, Al, Mg, and Ca. It forms Al-Mg-Ti-Ca-Mn-O-S composite inclusions with a size of 0.5 - 5 μm, and the quantity of these inclusions accounts for more than 10%. These inclusions are conducive to fine and dispersed distribution and have a stronger ability to promote the nucleation of acicular ferrite, which can significantly improve the low-temperature toughness of large heat input welding. When the heat input of welding the steel plate is 200 - 400 kJ / cm, the impact toughness of the base metal and the heat-affected zone at -40 °C ≥100 J. However, the yield strength of the steel plate in this invention is lower than 460 MPa and does not reach the EH460 strength level.

[0008] The "thick steel plate with large heat input welding and its manufacturing method" provided by the publication number CN1804093A removes precious elements such as Ni, Cr, Mo, and Cu from the steel and optimizes the TMCP process to obtain a base metal and HAZ with excellent low-temperature toughness. However, due to the removal of Ni and Cr elements, the corrosion resistance of this steel is significantly reduced.

[0009] The above-mentioned prior arts are difficult to simultaneously meet high strength, high toughness, and high production economy under the condition of welding with a heat input above 300 kJ / cm. Summary of the Invention

[0010] In view of this, the present invention provides an EH460 grade ultra-high strength TMCP ship plate steel that can meet the requirements of large heat input welding, solving the problem that it is difficult to provide a ship plate steel that can simultaneously meet high strength, high toughness and high production economy under the condition of heat input welding above 300 kJ / cm in the prior art.

[0011] In addition, the present invention also provides a preparation method for preparing the ship plate steel.

[0012] In the first aspect, for the EH460 grade ship plate steel suitable for large heat input welding, by mass percentage, its chemical composition includes:

[0013] C: 0.03% - 0.05%, Si: 0.12% - 0.18%, Mn: 1.3% - 1.7%, P ≤ 0.02%, S ≤ 0.01%, Als: 0.01% - 0.05%, Ni: 0.5% - 0.8%, Cu: 0.3% - 0.6%, Cr: 0.25% - 0.5%, Nb: 0.03% - 0.07%, Ti: 0.006% - 0.03%, N: 0.004% - 0.012%, Mg: 0.005% - 0.02%, Zr: 0.008% - 0.025%, REM: 0.004% - 0.025%, and the balance is Fe and unavoidable impurities.

[0014] In the present disclosure and possible embodiments, the carbon equivalent C of the steel plate composition eq is 0.38% - 0.48%, the crack sensitivity index P cm is 0.16% - 0.22%, 1.75 ≤ Mn + 0.4(Ni + Cu) ≤ 2.15, 2.5 ≤ Ti / N ≤ 3.4, (Mg + Zr + REM) / (Al + Ti) ≥ 1.2.

[0015] In the present disclosure and possible embodiments, in the steel plate structure, the content of acicular ferrite is ≥ 65%, the inclusion type is (Al, Mg, Zr, REM)Ox - MnS composite inclusion, and the size is 0.67 - 3.43 μm; under the condition of heat input ≤ 400 kJ / cm, the average grain size of the original austenite grains in the coarse-grained heat-affected zone is below 10 μm.

[0016] In the second aspect, the preparation method of the ship plate steel described in the first aspect includes a smelting process and a continuous casting process, wherein:

[0017] In the smelting process, molten steel is smelted according to the chemical composition and the weight percentages. The pre-desulfurized hot metal is smelted in a converter to reduce the contents of P, S and impurity elements, where the P content is ≤ 0.02% and the S content is ≤ 0.01%. Secondary refining is carried out in an LF furnace to adjust the composition of the molten steel, and vacuum degassing is carried out in an RH furnace to control the O content ≤ 0.005% until the composition of the molten steel is qualified. During the process of smelting the molten steel, Ni-Mg master alloy wire is used for adding Mg element, and the Ni content in the alloy wire is 65% - 80%.

[0018] In the continuous casting process, the superheat of the molten steel is controlled at 30°C - 40°C, and the qualified molten steel is cast into a billet, and the casting speed is 1.2 m / min - 1.4 m / min.

[0019] In the present disclosure and possible embodiments, the preparation method further includes:

[0020] The billet heating process and the rolling process;

[0021] In the billet heating process, after the billet is taken off the production line and stacked for slow cooling for 36 - 48 h, the billet is heated to 1180 ± 40°C at a rate of 5 ± 1°C / min and kept warm for 40 - 80 min.

[0022] The rolling process adopts the TMCP process, and the rolling process includes rough rolling, finish rolling and controlled cooling;

[0023] The starting rolling temperature in the rough rolling process is 1160 - 1200°C, and the final plate thickness after rough rolling is (1.8 - 2.5)T;

[0024] The starting rolling temperature in the finish rolling process is 800 - 880°C, the single-pass reduction rate is 8 - 15%, and the finishing rolling temperature is 750 - 850°C;

[0025] In the controlled cooling process, the steel plate is cooled by spraying water. The starting cooling temperature is 720 - 800°C, the cooling rate is 8 - 15°C / s, and the recrystallization temperature is 380 - 520°C.

[0026] The beneficial effects of the present invention:

[0027] The present invention adopts the design concept of "ultra-low carbon + oxide metallurgy + fine composition control" to design an EH460 grade ultra-high strength ship plate steel suitable for welding with high heat input. Specifically, by reducing the C content in the steel to improve its weldability, and controlling the value of Mn + 0.4(Ni + Cu) to make up for the strength loss caused by C reduction, thereby ensuring the strength of the steel. At the same time, Ti, Mg, Zr and REM elements are added to the steel for oxide metallurgy, and the ratios of Ti / N and (Mg + Zr + REM) / (Al + Ti) are controlled, so that ideal inclusion morphology, size and quantity are formed during the solidification process of the steel during welding with high heat input, effectively pinning the prior austenite grain boundaries, hindering the growth of austenite grains, refining the grains, promoting the nucleation of intragranular acicular ferrite, and making the volume fraction of acicular ferrite in the heat affected zone of the weld ≥ 65%, significantly improving the low-temperature toughness of the welded joint. The thickness of the ship plate steel for welding with high heat input described in the present invention is ≤ 60 mm, the yield strength of the base metal is ≥ 460 MPa, the tensile strength is ≥ 570 MPa, the Charpy impact energy KV2 at -40 °C is ≥ 120 J. Under the condition that the welding heat input is not more than 400 kJ / cm, the heat affected zone of the steel plate has good performance with KV2 ≥ 80 J / -40 °C, effectively solving the problem that it is difficult for the prior art to provide a ship plate steel that simultaneously meets high strength, high toughness and high production economy under the welding heat input condition above 300 kJ / cm. Detailed implementation manners

[0028] The following describes the present disclosure based on embodiments, but it should be noted that the present disclosure is not limited to these embodiments. In the following detailed description of the present disclosure, some specific details are described in detail. However, those skilled in the art can also fully understand the present disclosure for the parts that are not described in detail.

[0029] In addition, those of ordinary skill in the art should understand that what is provided is only to illustrate the purpose, features and advantages of the present disclosure.

[0030] At the same time, unless the context clearly requires otherwise, words such as "including" and "comprising" in the entire specification and claims should be interpreted as having an inclusive meaning rather than an exclusive or exhaustive meaning; that is, it is the meaning of "including but not limited to".

[0031] The present invention adopts the chemical composition design of "ultra-low carbon + oxide metallurgy + fine composition control", and through a low-cost TMCP rolling process, an EH460 grade ultra-high strength ship plate steel that can meet the welding with high heat input below 400 kJ / cm is prepared.

[0032] The present disclosure elaborates on the ship plate steel of the present invention and its preparation method through 6 embodiments to further illustrate the technical solution of the present invention and the resulting inventive effects. Specifically, the chemical composition of the ship plate steel is designed according to the following scheme in the following embodiments. By weight percentage, its composition includes:

[0033] C: 0.03% - 0.05%, Si: 0.12% - 0.18%, Mn: 1.3% - 1.7%, P ≤ 0.02%, S ≤ 0.01%, Als: 0.01% - 0.05%, Ni: 0.5% - 0.8%, Cu: 0.3% - 0.6%, Cr: 0.25% - 0.5%, Nb: 0.03% - 0.07%, Ti: 0.006% - 0.03%, N: 0.004% - 0.012%, Mg: 0.005% - 0.02%, Zr: 0.008% - 0.025%, REM: 0.004% - 0.025%, and the balance is Fe and unavoidable impurities; wherein: the carbon equivalent Ceq of the steel plate composition eq is 0.38% - 0.48%, and the crack sensitivity index Pcm cm is 0.16% - 0.22%, 1.75 ≤ Mn + 0.4(Ni + Cu) ≤ 2.15, 2.5 ≤ Ti / N ≤ 3.4, (Mg + Zr + REM) / (Al + Ti) ≥ 1.2; the content of acicular ferrite is ≥ 65%, the inclusion type is (Al, Mg, Zr, REM)Ox - MnS composite inclusion, and the size is 0.67 - 3.43 μm; under the condition that the welding line energy ≤ 400 kJ / cm, the average grain size of the original austenite grains in the coarse - grained heat - affected zone is not greater than 10 μm.

[0034] The above - mentioned Als is acid - soluble aluminum, and REM is the English abbreviation of rare earth metal.

[0035] The calculation method of the above - mentioned Ceq is shown in formula (1), and the calculation method of the crack sensitivity index Pcm is shown in formula (2):

[0036] Ceq = C + Mn / 6+(Cr + Mo + V) / 5+(Ni + Cu) / 15 (1);

[0037] Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2).

[0038] The inventive concept of the optimized design of the chemical composition of the steel grade in the present invention is as follows:

[0039] (1) As a basic strengthening element in steel, C mainly exists in the form of cementite in steel and is the main element to ensure the strength and hardness of steel. However, if the C content is too high, it will increase the hardenability sensitivity of steel, cause welding cracks, and reduce the weldability of steel. Therefore, on the premise of ensuring the strength and hardness of steel, the content of C element in steel should be reduced as much as possible. Therefore, the present invention adopts the design idea of ultra-low C and controls the C content within 0.03% - 0.05%.

[0040] (2) Si has a strong solid solution strengthening effect on the ferrite phase, can significantly improve the strength of steel. At the same time, as a deoxidizer, Si can reduce the O content. However, when the content is relatively high, it will cause grain coarsening, welding spatter, and cold brittleness, reducing the weld quality. Therefore, the present invention controls the Si content within 0.12% - 0.18%.

[0041] (3) Mn is a commonly used deoxidizing and desulfurizing element and also the main strengthening element in low-alloy steel. It can be dissolved in austenite and ferrite in large amounts, playing a role in expanding the austenite phase region and improving the stability of austenite. When the Mn content is too low, the strength cannot be guaranteed; when the Mn element content is too high, Mn segregation will occur, resulting in uneven structure of the steel plate and significantly reducing the welding performance of local positions. Therefore, the present invention controls the Mn content within 1.3% - 1.7%.

[0042] (4) The P and S elements are not beneficial to the mechanical properties and welding performance of the steel plate. It should be controlled that P ≤ 0.02% and S ≤ 0.01%.

[0043] (5) Al is the main deoxidizing element in steel and can also play a role in refining grains and corrosion resistance. Al will also form AlN with nitrogen element in steel, thus fixing the nitrogen in the steel and significantly improving the impact toughness of the steel. If the Al content is too low, the deoxidation effect is not good, the micro-alloying elements in the steel are over-oxidized, seriously reducing the welding performance; on the contrary, if the Al content is too high, it is easy to form large-size and multi-component composite inclusions. Therefore, the present invention controls the Als content to be 0.01% - 0.05%.

[0044] (6) Ni can be infinitely soluble with the Fe matrix. Solid solution in the Fe matrix can increase the strength, and at the same time, it can also reduce the ductile-brittle transition temperature and improve the toughness of the steel. The addition of Ni can appropriately reduce the C in the steel, thereby improving the weldability of the steel, and can also significantly improve the corrosion resistance of the steel. Therefore, the present invention controls the Ni content to be 0.5% - 0.8%.

[0045] (7) The addition of Cu can improve the strength, toughness, and fatigue performance of steel by forming precipitation phases, can also improve the hardenability of steel, and increase the atmospheric corrosion resistance of steel. However, too much Cu will cause hot brittle cracking during the hot working process of steel. Therefore, the present invention controls the Cu content to be 0.3% - 0.6%.

[0046] (8) The addition of Cr can significantly improve the strength, hardness, wear resistance, oxidation and corrosion resistance of steel, and increase the ductile-brittle transition temperature. However, when the chromium content is relatively high, the impact toughness of the steel drops sharply. Therefore, in the present invention, the Cr content is controlled to be 0.25% - 0.5%.

[0047] (9) Nb is an important microalloying element in steel. By forming stable carbonitrides with C and N elements in the steel, it plays a role in precipitation strengthening and grain refinement, significantly improving the strength of the steel. It can also effectively reduce the low-temperature ductile-brittle transition temperature of the steel, improving the toughness and welding performance of the steel. Therefore, in the present invention, the Nb content is controlled to be 0.03% - 0.07%.

[0048] (10) Ti has a very strong affinity with C, O, and N and is a good deoxidizer and degasser. At the same time, it forms stable, fine and dispersed carbonitride oxides, playing a role in fixing oxygen and nitrogen and grain refinement, improving the strength of the steel. At the same time, fine TiN particles can effectively prevent the growth of prior austenite grains in the heat-affected zone of welding and promote the nucleation of intragranular acicular ferrite, significantly improving the performance of the heat-affected zone (HAZ) of welding. Therefore, by precisely controlling the Ti and N contents and the Ti / N ratio, a reasonable content of TiN precipitation phase is obtained, thereby improving the mechanical properties and welding performance of the steel. In the present invention, the Ti content is controlled to be 0.006% - 0.03%, the N content is controlled to be 0.004% - 0.012%, and 2.5 ≤ Ti / N ≤ 3.4.

[0049] (11) The Mg element can refine the inclusions precipitated in the steel and effectively modify oxides and sulfides, and the reaction products are not easily aggregated into large clusters. Mg can also react with Ti2O3 and Al2O3 at high temperatures to form MgO, and the reaction temperature is exactly the thermodynamic dissolution temperature of TiN. Since the mismatch degree between MgO and TiN is small, MgO will precipitate on the surface of TiN particles, thereby inhibiting the dissolution of TiN, enhancing the stability of pinned particles, hindering the growth of prior austenite grains in the heat-affected zone, refining the microstructure, promoting the nucleation of acicular ferrite, and significantly improving the welding performance. In addition, there is a relationship of competitive deoxidation between Mg and Ti. If the Mg content is too low, there are too few MgO microparticles, which is not conducive to the uniform distribution of inclusions. Therefore, in the present invention, the Mg content is controlled to be 0.005% - 0.02%.

[0050] (12) Zr is a strong deoxidizer and decarburizer, and also a carbide-forming element. It can refine austenite grains, and can also combine with S to form ZrS to prevent the steel from suffering from hot short cracking during hot working. During the smelting process, Zr can combine with oxygen to form fine ZrO2 particles, and the fine ZrO2 particles can be evenly suspended in the liquid steel without floating on the surface. During the high-input welding process, ZrO2 can effectively promote the precipitation of MnS, form ZrO2-MnS composite oxides, hinder the growth of prior austenite grains, and at the same time promote the nucleation of acicular ferrite, improving the welding performance of the steel. Therefore, the content of Zr in the present invention is controlled to be 0.008% to 0.025%.

[0051] (13) REM can purify the molten steel in the steel, and can also change the morphology and distribution of inclusions, playing a role in refining grains. The addition of REM also has a certain effect on resisting hydrogen-induced stress corrosion (HISC). REM has a certain solid solubility in the steel, and its segregation at the grain boundaries can inhibit the segregation of P, S and low-melting-point impurities at the grain boundaries, purify and strengthen the grain boundaries, hinder the formation and propagation of intergranular cracks, and play a microalloying role. In addition, the addition of REM can improve the welding performance of the steel. REM can refine the size of Nb and Ti carbonitride particles, enhance their pinning effect, inhibit the growth of prior austenite grains, and at the same time increase the proportion of large-angle grain boundaries, increase the energy barrier for crack propagation, and significantly improve the toughness of the heat-affected zone (HAZ) of the weld. Therefore, the content of REM in the present invention is controlled to be 0.004% to 0.025%.

[0052] (14) In order to obtain good welding performance in the present invention, the carbon equivalent Ceq is controlled to be 0.35% to 0.45%, and the crack sensitivity index Pcm is controlled to be 0.16% to 0.22%.

[0053] (15) By precisely controlling the elements of Mn, Cu, and Ni in the present invention, the steel described in the present invention can obtain good welding performance, especially the toughness of the heat-affected zone of the weld, on the premise of obtaining sufficient strength. Therefore, 1.75 ≤ Mn + 0.4(Ni + Cu) ≤ 2.15 is controlled in the present invention.

[0054] (16) In order to obtain an appropriate amount of inclusions and promote the nucleation of acicular ferrite, Mg, Zr, and REM elements are added to the steel, and (Mg + Zr + REM) / (Al + Ti) ≥ 1.2 is controlled to obtain (Al, Mg, Zr, REM)Ox-MnS composite inclusions with a size of 0.67 - 3.43 μm. These inclusions promote the nucleation of acicular ferrite in the heat-affected zone of welding, making the content of acicular ferrite reach more than 65%. At the same time, they can also pin the original austenite grain boundaries and refine the grains. When the input welding line energy is 100 - 400 kJ / cm, the average grain diameter of the original austenite grains in the coarse-grained heat-affected zone (GCHAZ) is below 10 μm, and the crack propagation ability is significantly reduced, thereby obtaining a welded joint with excellent toughness.

[0055] Regarding the inclusions, the present invention uses field emission SEM-EDS to determine the types and sizes of the inclusions. Among them, the morphology and distribution of the inclusions are observed by SEM at a magnification of 2000 times, and 10 fields of view are continuously selected, with a total of 80 inclusions for measurement and statistics to determine the inclusion size; the X-ray energy spectrometer (EDS) is used to determine the composition of 100 inclusions in 10 consecutive fields of view to determine the types of inclusions.

[0056] The present invention uses an optical microscope (OM) to observe the ferrite structure in the heat-affected zone of welding. Five fields of view at a magnification of 1000 times are randomly selected, and the content of the acicular ferrite structure in each field of view is statistically analyzed to determine the average content of the acicular ferrite structure in the heat-affected zone.

[0057] The present invention uses field emission SEM-EBSD to determine the average grain diameter of the original austenite grains in the coarse-grained heat-affected zone. Among them, the EBSD (electron backscatter diffraction method) and the equivalent circle method are used to measure the average grain diameter of the original austenite grains. The closed area surrounded by grain boundaries with a crystallographic orientation difference greater than 15° is identified as the original austenite grain. Five fields of view at a magnification of 2000 times are continuously selected, with a total of 50 grains for measurement and statistics to determine the average grain diameter of the original austenite grains.

[0058] In addition, in each embodiment of the present disclosure, on the basis of the above chemical composition design, the low-cost TMCP rolling process is continued to prepare the ship plate steel of the present invention, which specifically includes smelting - continuous casting - billet heating - rolling. The steps of each process are as follows:

[0059] 1. Smelting and continuous casting

[0060] The molten steel is smelted according to the above chemical composition design and weight percentage, and then made into slabs through the continuous casting process. During smelting, the pre-desulfurized hot metal is smelted in a converter to reduce the contents of P, S and impurity elements, where the P content is ≤0.02%, the S content is ≤0.01%, and then secondary refining is carried out in an LF furnace to further adjust the composition of the molten steel. Then vacuum degassing is carried out in an RH furnace to make the O content ≤0.005%. Subsequently, the molten steel with qualified composition is cast into continuous casting billets. Among them, the addition of Mg element adopts a Ni-Mg master alloy wire, and the Ni content in the Ni-Mg master alloy wire is 65% - 80%. The Ni-Mg master alloy can reduce the activity of Mg, reduce the burning loss of Mg during smelting, and improve the recovery rate of Mg.

[0061] During continuous casting, the superheat of the molten steel is controlled at 30 - 40°C, and the casting speed is 1.2 - 1.4 m / min.

[0062] 2. Heating of the casting billet

[0063] First, the casting billets stacked offline for 36 - 48 h are loaded into the heating furnace, and then the billets are heated to 1180 ± 40°C at a rate of 5 ± 1°C / min and held for 40 - 80 min to make the inside and outside of the billets reach the set temperature, preparing for the subsequent rolling work.

[0064] 3. Rolling

[0065] The rolling adopts the TMCP process, and the rolling process is "rough rolling - finish rolling - controlled cooling". Among them, the rough rolling process starts at a temperature of 1160 - 1200°C, and the final plate thickness after rough rolling is (1.8 - 2.5)T (T is the finished plate thickness). The finish rolling process starts at a temperature of 800 - 880°C, the reduction rate per pass is 8 - 15%, and the final rolling temperature is 750 - 850°C. Subsequently, the steel plate is cooled by spraying water, the starting cooling temperature is 720 - 800°C, the cooling rate is 8 - 15°C / s, and the recrystallization temperature is 380 - 520°C.

[0066] Examples

[0067] The chemical compositions and contents of the shipbuilding steel plates designed in the embodiments (1 - 6) of the present disclosure are shown in Table 1:

[0068] Table 1 Chemical compositions of the shipbuilding steel plates in each embodiment wt%

[0069]

[0070] The rolling process parameters of the shipbuilding steel plates in the embodiments (1 - 6) of the present disclosure are shown in Table 2:

[0071] Table 2 Rolling process parameters of the shipbuilding steel plates in each embodiment

[0072]

[0073] The mechanical properties of the ship plate steel in the embodiments (1 - 6) of the present disclosure are shown in Table 3:

[0074] Mechanical properties of the ship plate steel in each embodiment in Table 3

[0075]

[0076] The large heat input welding properties of the ship plate steel in the embodiments (1 - 6) of the present disclosure are shown in Table 4:

[0077] Large heat input welding properties of the ship plate steel in each embodiment in Table 4

[0078]

[0079] As shown in Table 4, when the thickness of the finished steel plate ≤ 60 mm and the welding heat input is 100 - 400 kJ / cm, in the microstructure of the heat affected zone obtained, the volume fraction of acicular ferrite ≥ 65%, the average grain size of prior austenite ≤ 9.6 μm, and the welded joint of the steel plate of the present invention has excellent low - temperature toughness. The yield strength of the base metal ≥ 460 MPa, the tensile strength ≥ 570 MPa, the Charpy impact energy KV2 at - 40°C ≥ 120 J. Under the condition that the welding heat input does not exceed 400 kJ / cm, the heat affected zone of the steel plate welding has good performance with KV2 ≥ 80 J / -40°C.

[0080] The above - described embodiments are only for expressing the implementation manners of the present disclosure, and the description is relatively specific and detailed, but it should not be construed as a limitation to the patent scope of the present disclosure. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present disclosure, several deformations, equivalent substitutions, improvements, etc. can be made, and these all belong to the protection scope of the present disclosure. Therefore, the protection scope of the patent of the present disclosure should be subject to the appended claims.

Claims

1. An EH460 grade ship plate steel suitable for high-input energy welding, characterized in that: Its chemical composition by weight percentage includes: C: 0.030%~0.038%, Si: 0.12%~0.18%, Mn: 1.3%~1.7%, P≤0.02%, S≤0.01%, Als: 0.01%~0.05%, Ni: 0.51%~0.80%, Cu: 0.51%~0.60%, Cr: 0.25%~0.5%, Nb: 0.054%~0.070%, Ti: 0.027%~0.030%, N: 0.0055%~0.0120%, Mg: 0.008%~0.020%, Zr: 0.008%~0.025%, REM: 0.004%~0.025%, the rest is Fe and unavoidable impurities; Steel plate composition carbon equivalent C eq is 0.38%~0.48%, crack sensitivity index P cm 0.16%~0.22%, 1.75%≤Mn+0.4(Ni+Cu)≤2.15%, 2.5≤Ti / N≤3.4, (Mg+Zr+REM) / (Al+Ti)≥1.2; The acicular ferrite content is ≥65%, the inclusion type is (Al, Mg, Zr, REM)Ox-MnS composite inclusions, and the size is 0.67~3.43 μm; under the condition of welding line energy of 50 kJ / cm~400 kJ / cm, the average grain size of the original austenite grains in the coarse-grained heat affected zone is not more than 10 μm.

2. The method for preparing ship plate steel according to claim 1, comprising a smelting process and a continuous casting process, characterized in that: In the smelting process, molten steel is smelted according to the chemical composition and the weight percentage, the pre-desulfurized molten iron is smelted in a converter, the P content is ≤0.02%, the S content is ≤0.01%, the LF furnace is used for secondary refining to adjust the composition of the molten steel, and the RH furnace is used for vacuum degassing to control the O content to ≤0.005%, until the composition of the molten steel is qualified; in the process of smelting the molten steel, the Mg element is added by using a Ni-Mg intermediate alloy wire, and the Ni content in the alloy wire is 65%~80%; In the continuous casting process, the superheat of the molten steel is controlled to be 30°C to 40°C, and the molten steel with qualified composition is cast into a casting billet at a billet drawing rate of 1.2 m / min to 1.4 m / min.

3. The method for preparing ship plate steel according to claim 2, characterized in that: Also includes: Ingot heating process and rolling process; In the casting heating process, after the casting is stacked and slowly cooled for 36 to 48 hours, the casting is heated to 1180 ± 40 ° C at a rate of 5 ± 1 ° C / min and kept at this temperature for 40 to 80 minutes; The rolling process adopts TMCP technology, and the rolling process includes rough rolling, finish rolling and controlled cooling; The starting rolling temperature of the rough rolling process is 1160-1200°C, and the final plate thickness of the rough rolling is (1.8-2.5)T; The start rolling temperature of the finishing rolling process is 800-880°C, the single-pass reduction rate is 8-15%, and the final rolling temperature is 750-850°C; The controlled cooling process adopts a water spray cooling method for the steel plate, the start cooling temperature is 720~800℃, the cooling rate is 8~15℃ / s, and the red-return temperature is 380~520℃.

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