Ship plate steel and preparation method thereof

Abstract

The invention relates to the technical field of manufacturing of structural steel for general-strength ship bodies, and discloses ship plate steel and a preparation method thereof. The ship plate steel comprises the following chemical components in percentage by weight: 0.050%-0.070% of C, 0.150%-0.250% of Si, 0.60%-0.80% of Mn, less than or equal to 0.02% of P, less than or equal to 0.008% of S, 0.020%-0.040% of Als, 0.005%-0.015% of Nb, 0.010%-0.020% of Ti, less than or equal to 0.006% of N and the balance of Fe and inevitable impurities. According to the ship plate steel, in the technical specification range required by the classification society, the component design of low C and Mn elements is adopted, the influence of segregation of the C and Mn elements on the ship plate steel is fundamentally relieved, the hot rolling technology is matched, formation of a banded structure is improved, the stretching layering phenomenon of the ship plate steel is effectively improved, and the production qualification rate of the ship plate steel is increased.

Description

Technical Field

[0001] The present invention relates to the technical field of manufacturing structural steel for general-strength hulls, and particularly relates to a ship plate steel and a preparation method thereof. Background Art

[0002] With the development of the shipbuilding industry, the demand for the diversity of structural steel plate specifications for hulls is increasing continuously. The consumption of ship plate steel with a thickness below 25 mm is gradually increasing. Using a medium-thick plate rolling mill to produce thin-specification ship plates has the disadvantages of low efficiency and high cost. Domestic steel mills generally use hot strip mills for production. With the process means of controlled rolling and controlled cooling of the hot strip mill, the produced ship plate steel has the advantages of high dimensional accuracy, good surface quality, low production cost and high efficiency, which can bring huge economic benefits to enterprises.

[0003] Ship plate steel can only be produced and sold after passing the certification of various classification societies. Currently, ship plate steel with a large domestic demand needs to pass the certification of the China Classification Society (CCS), among which the CCS-B has the largest consumption. During the final inspection of CCS-B ship plate steel, it is easy to have the rejudgment of tensile delamination defects, which affects the product qualification rate and order delivery rate, and increases the manufacturing cost of enterprises.

[0004] Searching for the research on tensile delamination by domestic and foreign researchers, the tensile delamination of hot-rolled ship steel plates is mainly caused by center segregation of the continuous casting billet, inclusions and banded structure formed after rolling.

[0005] Chinese Patent Application CN114934234A discloses a method for controlling the fracture of a tensile specimen of ship plate steel not to show delamination. The inventor found that there is an obvious correlation between the segregation sensitivity index Σ 敏感指数 = 10P + 2S + (Als + As + Sn + Cu + Sb + Ni + Cr) of ship plate steel and tensile fracture delamination. By controlling the generation of trace elements through the steelmaking process, the segregation phenomenon is improved, thereby controlling the tensile fracture delamination phenomenon.

[0006] Chinese Patent Application CN116043096A discloses a steelmaking method for reducing manganese segregation in ship plate steel billets. By controlling the precipitation temperature of manganese sulfide and adding calcium-magnesium alloy to purify the molten steel, the manganese segregation of the billet is reduced, and the banded structure is improved to reduce the tensile delamination phenomenon of ship plate steel.

[0007] The above two patent applications reduce the segregation of MnS and improve the tensile delamination phenomenon by controlling the steelmaking process and the amount of residual elements. Due to the different equipment conditions and technical levels of different steel mills, optimizing and controlling the continuous casting process cannot fundamentally solve this problem, and it may still produce continuous casting billets that do not meet the requirements, increasing the manufacturing cost.

[0008] Chinese Patent Application CN109930062A discloses a method for improving the tensile properties of extra-thick normalized ship plate steel. By adding an on-line ACC controlled cooling process after rolling the steel plate and optimizing the controlled cooling process parameters, the tensile properties of the normalized ship plate steel are effectively improved. The yield strength is ≥280 Mpa, the tensile strength is ≥450 Mpa, the average surplus of the yield strength and the tensile strength both reach more than 40 Mpa, and the finished product qualification rate reaches 100%. This patent application does not involve how to improve the defect of tensile delamination of the hull steel plate.

[0009] At present, for general strength ship plate steel in China, a relatively high C and Mn element composition design is generally adopted, with solid solution strengthening as the main strengthening method. Elements such as C and Mn are prone to cause central segregation of the continuous casting billet, and pearlite aggregation during the hot rolling process produces banded structure. During the tensile process, due to different plastic toughness of different structures, microcracks are generated, which in turn cause tensile delamination defects.

[0010] For grade B ship plate steel with H≤25 mm, the required impact energy is ≥27 J (0 °C). At present, the level of impact energy is in the range of 50 J to 100 J. The current relatively high C and Mn element composition design affects the impact performance due to reasons such as banded structure, resulting in large fluctuations in impact performance, and there are often cases where individual impact energy values are on the low side.

[0011] Therefore, although the relatively high C and Mn element composition design adopted for general strength ship plate steel at present has a relatively low alloy cost, it has an increased cost due to segregation-induced tensile delamination of hot-rolled finished products. At the same time, the C content of this composition design is generally within the peritectic transformation range (C: 0.09% to 0.17%), and corner cracks will occur in the continuous casting billet, and the method of flame cleaning the corners of the continuous casting billet is required to avoid edge cracks of the steel plate. This design not only requires flame cleaning of the continuous casting billet, but also has a relatively high proportion of tensile delamination, a long production cycle, and a high rejection rate. Summary of the Invention

[0012] The purpose of the present invention is to overcome the problems existing in the prior art that ship plate steel generally adopts a relatively high C and Mn element composition design, and elements such as C and Mn are prone to cause central segregation of the continuous casting billet, and pearlite aggregation during the hot rolling process produces banded structure, which leads to tensile delamination defects. A ship plate steel and its preparation method are provided. Within the technical specifications required by the classification society, this ship plate steel adopts a low C and Mn element composition design, fundamentally reducing the influence of C and Mn element segregation on the ship plate steel, and cooperating with the hot rolling process to improve the formation of banded structure, thereby effectively improving the phenomenon of tensile delamination of the ship plate steel, increasing the production qualification rate of the ship plate steel, reducing the manufacturing cost of the ship plate steel, and shortening its delivery cycle.

[0013] To achieve the above purpose, on the one hand, the present invention provides a ship plate steel, which includes the following chemical components in weight percentage:

[0014] C: 0.050% - 0.070%, Si: 0.150% - 0.250%, Mn: 0.60% - 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% - 0.040%, Nb: 0.005% - 0.015%, Ti: 0.010% - 0.020%, N ≤0.006%, the balance being Fe and unavoidable impurities.

[0015] Preferably, the metallographic structure of the ship plate steel is polygonal ferrite and pearlite, wherein the ferrite proportion > 90%, the grain size ≥ grade 8, and the banded structure ≤ grade 1.0.

[0016] Preferably, the contents of Si, Mn, and C in the ship plate steel satisfy the relationship shown in formula (1):

[0017]

[0018] Preferably, the contents of Ti and N in the ship plate steel satisfy the relationship shown in formula (2):

[0019] w Ti / w N ≥3 Formula (2).

[0020] The second aspect of the present invention provides a preparation method of ship plate steel, which includes hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of the slab, hot continuous rolling, and laminar flow cooling carried out in sequence; wherein, after alloy fine-tuning, the components in the molten steel satisfy the following weight percentages:

[0021] C: 0.050% - 0.070%, Si: 0.150% - 0.250%, Mn: 0.60% - 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% - 0.040%, Nb: 0.005% - 0.015%, Ti: 0.010% - 0.020%, N ≤0.006%, the balance being Fe and unavoidable impurities.

[0022] Preferably, after hot metal pretreatment, the content of S element in the hot metal ≤ 0.005 wt%.

[0023] Preferably, during LF refining, the continuous casting temperature at the ladle furnace outlet is controlled to be 39°C - 47°C above the target liquidus temperature, wherein the target liquidus temperature is 1520°C - 1530°C.

[0024] Preferably, during continuous casting, the molten steel temperature in the tundish is controlled to be 15°C - 30°C above the target liquidus temperature, wherein the target liquidus temperature is 1520°C - 1530°C.

[0025] Preferably, during the hot charging process of the continuous casting billet, the hot charging rolling is adopted for the continuous casting billet, and the heating furnace is kept warm for 2 to 3 hours.

[0026] Preferably, during the hot continuous rolling process, rough rolling is carried out first, and then finish rolling is carried out for 5 to 10 passes;

[0027] Among them, the thickness range of the intermediate billet after rough rolling is 55 mm to 65 mm, and the deformation amount of the first 2 passes of finish rolling is more than 55%.

[0028] Preferably, during the laminar flow cooling process, the cooling rate is (30 - 50) °C / s.

[0029] The third aspect of the present invention provides a ship plate steel obtained by the preparation method described above.

[0030] Compared with the prior art, the present invention has at least the following technical effects:

[0031] (1) The ship plate steel described in the present invention contains lower contents of C and Mn elements. By optimizing the composition of the ship plate steel, the occurrence of segregation is fundamentally reduced, the phenomenon of tensile delamination of the ship plate steel is improved, the production qualification rate of the ship plate steel is increased, the manufacturing cost of the ship plate steel is reduced, and its delivery cycle is accelerated.

[0032] (2) The method described in the present invention, through the composition design with low C and Mn elements, fundamentally reduces the occurrence of segregation. Through hot continuous rolling and laminar flow cooling means, a uniform and fine structure is obtained, ensuring good strength, toughness, and impact properties of the ship plate steel while solving the tensile delamination problem and improving the product qualification rate.

[0033] (3) The method described in the present invention, by optimizing the composition of the ship plate steel, on the basis of not increasing the manufacturing cost, has a large production window for steelmaking and continuous casting processes, can effectively reduce the degree of billet segregation, and produces hot-rolled ship plate steel with an extremely low tensile delamination rate.

[0034] (4) The method described in the present invention, through a unique composition design, also has the following advantages: due to the reduction of C and Mn contents, it avoids the composition range of the peritectic region, the billet crack sensitivity is low, hot charging rolling can be carried out without cleaning the corners of the billet, the production process is accelerated, and the labor cost is saved.

[0035] (5) The thickness of the product obtained by the method described in the present invention is 6 mm to 25 mm, the yield strength R eH ≥235 MPa, the tensile strength R m is in the range of 400 MPa to 520 MPa, the elongation A5≥23%, the V-notch impact energy ≥100 J (-40 °C, the size of the impact specimen is 10 mm * 10 mm * 55 mm), the impact performance has a large improvement, the product performance has sufficient margin and small performance scatter, and it has good mechanical properties and low-temperature toughness. Brief Description of the Drawings

[0036] Figure 1 It is the macrostructure sample diagram of the continuous casting billet prepared in Example 1.

[0037] Figure 2 It is the metallographic structure diagram (center position in the thickness direction) of the ship plate steel prepared in Example 1.

[0038] Figure 3 It is the tensile fracture diagram of the ship plate steel prepared in Example 1.

[0039] Figure 4 It is the macrostructure sample diagram of the continuous casting billet prepared in Example 2.

[0040] Figure 5 It is the macrostructure sample diagram of the continuous casting billet prepared in Comparative Example 1.

[0041] Figure 6 It is the metallographic structure diagram (center position in the thickness direction) of the ship plate steel prepared in Comparative Example 1.

[0042] Figure 7 It is the tensile fracture diagram of the ship plate steel prepared in Comparative Example 1.

[0043] Figure 8 It is the macrostructure sample diagram of the continuous casting billet prepared in Comparative Example 2. Detailed Description of the Invention

[0044] The following is a detailed description of the specific embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0045] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0046] In order to solve the problem of tensile delamination in the finished product inspection of existing ship plate steels with general strength, the applicant has conducted a large number of practical studies. By optimizing the design of the component ranges of existing ship plate steels with general strength, not only can the problem of tensile delamination be solved, but also the corner crack defects of the continuous casting billet are significantly reduced, and the basic performance of the ship plate steel can be further ensured. Thus, the present invention has been completed.

[0047] As described above, in the first aspect of the present invention, a ship plate steel is provided, and the ship plate steel includes the following chemical components by weight percentage:

[0048] C: 0.050% to 0.070%, Si: 0.150% to 0.250%, Mn: 0.60% to 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% to 0.040%, Nb: 0.005% to 0.015%, Ti: 0.010% to 0.020%, N ≤0.006%, and the balance is Fe and unavoidable impurities. In the present invention, the Als represents acid-soluble aluminum.

[0049] In the ship plate steel of the present invention, the contents of C and Mn are relatively low, which not only fundamentally reduces the occurrence of segregation, reduces the formation of banded structure, solves the problem of tensile delamination, but also has good mechanical properties and low-temperature toughness.

[0050] In some embodiments, the metallographic structure of the ship plate steel is polygonal ferrite and pearlite, wherein the ferrite ratio > 90%, the grain size ≥ 8 grades, the banded structure ≤ 1.0 grade. When inspected for hot-rolled finished products, there is no delamination in tension and it has good edge quality. In the present invention, the grain size of the ship plate steel ≥ 8 grades indicates that its grains are relatively fine, with high strength, toughness and plasticity, and can be applied to occasions with high strength and high toughness requirements; the banded structure ≤ 1.0 grade indicates that the banded structure is less and the tensile delamination rate is extremely low.

[0051] In some embodiments, the yield strength R of the ship plate steel provided by the present invention eH ≥235 MPa, the tensile strength R m ranges from 400 MPa to 520 MPa, the elongation A5 ≥ 23%, the V-notch impact energy ≥ 100 J (-40 °C, impact specimen size 10 mm * 10 mm * 55 mm), and it has good mechanical properties and low-temperature toughness.

[0052] The following details the contents of three components, Si, Mn and C, and the content relationship among them.

[0053] C is an element that is prone to segregation during the crystallization of molten steel. At the same time, it is also the most basic solid solution strengthening element in steel, playing a very important role in the strength, cold bending performance and welding performance of materials. However, within the C content range required by the classification society specifications, when C is high, the product strength can be significantly improved, but when C is high, it is in the peritectic reaction zone (C: 0.09% to 0.17%), which can significantly increase the crack sensitivity of the continuous casting billet, increase the cost of cleaning the corners of the continuous casting billet and extend the manufacturing cycle. Therefore, in the present invention, the C content is in the range of 0.050% to 0.070%, which can not only reduce the influence of C on segregation, but also avoid the peritectic reaction zone, and can also meet the product requirements.

[0054] Mn is a solid-solution strengthening element. At the same time, Mn is an element prone to segregation. During the solidification process of the continuous casting billet, due to the segregation of Mn, different Mn-depleted and Mn-rich zones of Ar3 are formed. Element C is prone to diffuse into the Mn-rich zone, and it is easy to form ferrite / pearlite bands to form banded structures after rolling. Since the bonding force between the ferrite and pearlite bands is weak, crack sources are easily generated during tensile deformation, and then delamination occurs. However, while Mn can increase the strength of the material, it can also expand the austenite phase region, lower the transformation temperature of supercooled austenite, be conducive to the refinement of the microstructure, improve the plasticity and toughness of the material, and lower the ductile-brittle transition temperature of the steel. Therefore, within the range of Mn content required by the classification society specifications, a Mn content of 0.60% - 0.80% can meet the product requirements.

[0055] Si is a deoxidizing element, which can improve the cleanliness of molten steel, has a solid-solution strengthening effect, and can significantly increase the strength and hardness of the steel. Si has the effect of reducing the activity of C, which can inhibit the diffusion of C into the Mn-rich zone, and can prevent the formation of banded structures to a certain extent. Through a large number of studies, the inventors found that for ordinary ship plate steel, When it is, the occurrence of banded structures can be effectively reduced, and then the defect of tensile delamination can be improved. Therefore, in the present invention, adding element Si not only increases the strength but also is conducive to the control of segregation. A Si content of 0.150% - 0.250% can meet the requirements.

[0056] Al is a deoxidizing element. Appropriate addition can form fine AlN particles, which is conducive to grain refinement, also inhibits the aging of low-carbon steel, improves the toughness of the steel at low temperatures, especially reduces the brittle transition temperature of the steel. A design of 0.020% - 0.040% is more appropriate.

[0057] Nb has a significant effect on grain refinement and phase transformation behavior. Solute Nb has a strong solute drag effect, which can greatly inhibit the growth of austenite during soaking. Nb combines with C and N to form fine carbonitrides, which can delay recrystallization. After Nb is strain-induced to precipitate during rolling, the nucleation points of ferrite are increased, the grains are refined, and the strength, plasticity and low-temperature toughness of the product are improved. However, the cost of Nb alloy is relatively high. Considering the cost, a Nb content of 0.005% - 0.015% can meet the product requirements.

[0058] The function of Ti is similar to that of Nb. It can combine with C and N, and has the effects of grain refinement and precipitation strengthening. During the continuous casting process, Ti has a stronger affinity for N than Nb does. Appropriate addition of Ti can fix the N element in the steel and reduce the influence of N on the corner cracks of the continuous casting billet. However, the precipitate Ti(C, N) of Ti is harmful to the toughness of the steel. Therefore, the addition of Ti needs to be strictly controlled. In the present invention, the function of Ti is only to fix N and does not play a strengthening role. It is required that w Ti / w N≥3, it is advisable to design the Ti content to be 0.010% - 0.020%. Since N is a harmful element in this steel grade, combined with the actual control level, it is required that N ≤ 0.006%.

[0059] During the cooling process of molten steel, P is a strong segregation element, which causes grain boundary segregation, weakens the intergranular structure and produces banded structure; S is also a relatively strong segregation element, with a weaker effect than P, but it combines with Mn to form plastic inclusions MnS, which is easy to be elongated along the rolling direction. During the finished product tensile test, the existence of MnS destroys the continuity of the structure, causes longitudinal cracks during the tensile process, and then forms delamination defects. Therefore, it is necessary to strictly control the contents of P and S elements. Combined with the actual control level, it is advisable that P ≤ 0.02% and S ≤ 0.008%.

[0060] Through the optimization of the ingredient ratio, the present invention ensures the basic properties of the ship plate steel, such as strength, plasticity, toughness and welding performance, while making the probability of tensile delamination extremely low. At the same time, the continuous casting billet produced by the ingredient ratio of the present invention has a large continuous casting process window and good corner quality of the billet, and can be hot charged and rolled without subsequent corner cleaning treatment, significantly improving the production efficiency and reducing the manufacturing cost.

[0061] The second aspect of the present invention provides a preparation method of ship plate steel, which includes the following steps carried out in sequence: hot metal pretreatment, converter smelting, alloy fine adjustment, LF refining, continuous casting, hot charging of the billet, hot continuous rolling and laminar flow cooling; among them, after alloy fine adjustment, the components in the molten steel meet the following weight percentages:

[0062] C: 0.050% - 0.070%, Si: 0.150% - 0.250%, Mn: 0.60% - 0.80%, P: ≤ 0.02%, S: ≤ 0.008%, Als: 0.020% - 0.040%, Nb: 0.005% - 0.015%, Ti: 0.010% - 0.020%, N ≤ 0.006%, and the balance is Fe and unavoidable impurities.

[0063] The preparation method described in the present invention fundamentally reduces the occurrence of segregation through the ingredient design of low C and Mn elements, and obtains a uniform and fine structure through hot continuous rolling and laminar flow cooling means. While ensuring that the ship plate steel has good strength, toughness and impact performance, it solves the problem of tensile delamination and improves the product qualification rate. Among them, the design reasons for the contents of each component in the molten steel are the same as those described above and will not be elaborated here.

[0064] In the present invention, it is required to carry out front slag skimming and back slag skimming in the hot metal pretreatment step, and after hot metal pretreatment, desulfurize to make the content of S element in the hot metal ≤ 0.005 wt%, so as to ensure that the content of S element in the obtained finished steel is relatively low.

[0065] During the LF refining process of the present invention, white slag operation is adopted, and the top slag is fully reduced, thereby effectively preventing secondary oxidation of molten steel and effectively adsorbing inclusions in the molten steel. In some embodiments, the continuous casting temperature at the end of the LF refining process is controlled to be 39°C to 47°C above the target liquidus temperature, where the target liquidus temperature is 1520°C to 1530°C.

[0066] In some embodiments, during the continuous casting process, the tundish molten steel temperature is controlled to be 15°C to 30°C above the target liquidus temperature, where the target liquidus temperature is 1520°C to 1530°C. In the present invention, controlling the tundish molten steel temperature within this range helps to form more uniform equiaxed crystals, reduce the proportion of columnar crystals, and control central segregation. Further, in the present invention, dynamic soft reduction is used during the continuous casting process, which can control central segregation and porosity of the slab.

[0067] During the hot charging process of the slab in the present invention, the slab is hot charged and rolled, and the heating furnace is kept warm for 2 to 3 hours. In the present invention, limiting the heat preservation time of the heating furnace to 2 to 3 hours can enable alloying elements to diffuse sufficiently and improve the segregation of alloying elements during the solidification process of the slab.

[0068] In one embodiment, the hot continuous rolling process first performs rough rolling and then 5 to 10 passes of finish rolling; among them, the thickness range of the intermediate slab after rough rolling is 55 mm to 65 mm, and the deformation amount of the first 2 passes of finish rolling is more than 50%. The addition of Nb element in the composition design inhibits the recrystallization process of deformed austenite. Adopting this hot continuous rolling method can increase the flattening of austenite grains, introduce a large number of deformation bands, dislocations and other defects in the austenite grains, effectively refine the structure after phase transformation, and thus refine the grains to improve strength and toughness.

[0069] In one embodiment, the laminar cooling process adopts a front-section concentrated cooling mode for rapid cooling, where the cooling rate is (30 - 50) °C / s. In the present invention, controlling the cooling rate in the laminar cooling process within this range can enable the strip steel to be rapidly cooled after leaving the rolling mill, inhibit the growth of ferrite grains, refine the microstructure, and improve strength and toughness.

[0070] In the present invention, the preparation method further includes coiling and transverse cutting. Among them, the coiling temperature can be 630°C to 650°C.

[0071] In the method of the present invention, steps that are not specifically described and mentioned are all conventional operations in the art and will not be elaborated.

[0072] After the present invention has been applied for some time, the applicant conducts production inspection statistics, and the incidence rate of tensile delamination defects is reduced from about 11% before optimization to less than 0.5%. The slab steel manufactured by the method of the present invention has good corner quality of the slab and can be used for hot charging production.

[0073] The third aspect of the present invention provides a ship plate steel prepared by the preparation method described above.

[0074] In a preferred embodiment, the ship plate steel comprises the following chemical components by weight percentage: C: 0.050% - 0.070%, Si: 0.150% - 0.250%, Mn: 0.60% - 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% - 0.040%, Nb: 0.005% - 0.015%, Ti: 0.010% - 0.020%, N ≤0.006%, and the balance is Fe and inevitable impurities.

[0075] In a preferred embodiment, the metallographic structure of the ship plate steel is polygonal ferrite and pearlite, wherein the ferrite proportion > 90%, the grain size ≥ grade 8, and the banded structure ≤ grade 1.0.

[0076] In a preferred embodiment, the contents of Si, Mn and C in the ship plate steel satisfy the relational expression shown in formula (1):

[0077]

[0078] In a preferred embodiment, the contents of Ti and N in the ship plate steel satisfy the relational expression shown in formula (2):

[0079] w Ti / w N ≥3 Formula (2).

[0080] In the present invention, within the technical specification requirements of the classification society, the ship plate steel adopts a composition design with low C and Mn elements, fundamentally reducing the influence of the segregation of C and Mn elements on the ship plate steel. Combined with the hot rolling process, it improves the formation of the banded structure, thereby effectively improving the phenomenon of tensile delamination of the ship plate steel, increasing the production qualification rate of the ship plate steel, reducing the manufacturing cost of the ship plate steel, and shortening its delivery cycle.

[0081] The present invention will be described in detail below through examples.

[0082] Example 1

[0083] This example is used to illustrate the preparation process of E-class ship plate steel.

[0084] The D-class ship plate steel provided in this example has the following chemical composition:

[0085] C: 0.0558%, Si: 0.2196%, Mn: 0.7019%, P: 0.0141%, S: 0.0024%, Als: 0.03%, Nb: 0.0158%, Ti: 0.0148%, N: 0.0022%, the balance being Fe and unavoidable impurities.

[0086] The preparation method of E-grade ship plate steel includes hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of the slab, hot strip rolling, and laminar flow cooling carried out in sequence.

[0087] Among them, after the hot metal is pretreated, the content of S element in the hot metal is 0.003%.

[0088] After alloy fine-tuning, the composition of the molten steel meets the following weight percentages:

[0089] C: 0.0558%, Si: 0.2196%, Mn: 0.7019%, P: 0.0141%, S: 0.0024%, Als: 0.03%, Nb: 0.0158%, Ti: 0.0148%, N: 0.0022%, the balance being Fe and unavoidable impurities.

[0090] During the LF refining process, white slag operation is adopted, the top slag is fully reduced, the liquidus temperature of the molten steel is 1526 °C, and the tapping temperature for continuous casting is 1571 °C.

[0091] During the continuous casting process, it is cast into a continuous casting slab with a thickness of 230 mm by the conventional continuous casting method. The liquidus temperature of the molten steel is 1526 °C, the tundish molten steel temperature is 1549 °C, dynamic soft reduction is applied, the slab thickness is 230 mm, the center segregation grade of the continuous casting slab is B0.5, and the corner quality is good without cracks.

[0092] Samples are taken from the continuous casting slab, macro specimens are made according to the method of GB / T226 - 2015, and the macrostructure of the slab is rated according to YB / T4003 - 2016. The macro pickling specimens of the continuous casting slab are shown Figure 1 as shown, and the macro rating results of the slab are shown in Table 1.

[0093] Table 1 Segregation and corner crack conditions of the slab

[0094] Slab information Center segregation grade Center porosity grade Center crack grade Corner crack Example 1 B0.5 0.5 0 None

[0095] During the hot strip rolling process, the slab is heated to 1218 °C and then undergoes 3 + 5 passes of rough rolling. The thickness of the rough rolling intermediate slab is 60 mm, the entry temperature of the finishing mill is 1030 °C, 7 passes of finishing rolling are carried out on a 2250 mm hot strip mill. The deformation of the first 2 passes of finishing rolling is 58%, the thickness of the finished product after finishing rolling is 12.1 mm, the finishing rolling temperature is controlled at 878 °C, and after finishing rolling, it undergoes laminar flow cooling with a cooling rate of 30 °C / s and a coiling temperature of 634 °C.

[0096] For the ship plate products obtained in this example, tensile specimens were taken and prepared according to GB / T 2975, and tensile tests were carried out according to GB / T 228.1; metallographic specimens were prepared according to GB / T 13298, the microstructure was evaluated according to GB / T 13299, the grain size was evaluated according to GB / T 6394, and the banded structure was evaluated according to GB / T 34474.1; the microstructure was polygonal ferrite and pearlite, the proportion of ferrite was more than 95%, the grain size was 9.0 grades, there was no banded structure, and the metallographic structure is shown in Figure 2 . Its tensile mechanical properties: yield strength 361 MPa, tensile strength 458 MPa, elongation 33%. The fracture of the tensile specimen was observed by the naked eye, and there was no delamination phenomenon, as shown in Figure 3 . For the V-notch impact test (impact specimen size 10 mm * 10 mm * 55 mm), the impact temperature was -40 °C. Three groups of specimens were taken for the impact test, and the impact energy values were 271 J / 269 J / 273 J.

[0097] Example 2

[0098] This example is used to illustrate the preparation process of B-grade ship plate steel.

[0099] The B-grade ship plate steel provided in this example has the following chemical composition:

[0100] C: 0.0626%, Si: 0.2018%, Mn: 0.6460%, P: 0.0104%, S: 0.0040%, Als: 0.045%, Nb: 0.0134%, Ti: 0.0126%, N: 0.0019%, and the balance is Fe and inevitable impurities.

[0101] The preparation method of the B-grade ship plate steel includes the following steps carried out in sequence: hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of the slab, hot strip rolling, and laminar flow cooling.

[0102] Among them, after the hot metal is pretreated, the content of S element in the hot metal is 0.0035%.

[0103] After alloy fine-tuning, the composition of the molten steel meets the following weight percentages:

[0104] C: 0.0626%, Si: 0.2018%, Mn: 0.6460%, P: 0.0104%, S: 0.0040%, Als: 0.045%, Nb: 0.0134%, Ti: 0.0126%, N: 0.0019%, and the balance is Fe and inevitable impurities.

[0105] During the LF refining process, white slag operation is adopted, the top slag is fully reduced, the liquidus temperature of the molten steel is 1526 °C, and the temperature of the molten steel for continuous casting at the station is 1573 °C.

[0106] In the continuous casting process, a continuous casting slab with a thickness of 230 mm is cast by the conventional continuous casting method. The liquidus temperature of the molten steel is 1526 °C, the tundish molten steel temperature is 1550 °C, dynamic soft reduction is applied, the slab thickness is 230 mm, the center segregation grade of the continuous casting slab is C1.0, and the corner quality is good without cracks.

[0107] The sampling and inspection methods of the macrostructure specimens of the continuous casting slab are the same as those in Example 1. The macrostructure pickling specimens of the continuous casting slab are shown in Figure 4 As shown, the macrostructure grading results of the slab are shown in Table 2.

[0108] Table 2 Segregation and corner crack conditions of the slab

[0109] Slab information Center segregation grade Center porosity grade Center crack grade Corner crack Example 2 C1.0 0.5 0 None

[0110] During hot continuous rolling, the slab is heated to 1215 °C and then undergoes 3 + 3 passes of rough rolling. The thickness of the rough rolled intermediate slab is 60 mm, the entry temperature of finish rolling is 1030 °C, 7 passes of finish rolling are carried out on a 2250 mm hot continuous rolling mill. The deformation of the first 2 passes of finish rolling is 59%, the thickness of the finish rolled product is 10.09 mm, the finishing temperature is controlled at 877 °C, and after finish rolling, laminar cooling is carried out with a cooling rate of 30 °C / s and the coiling temperature is 645 °C.

[0111] The sampling and inspection methods of the finished ship plate obtained in this example are the same as those in Example 1. The microstructure of the product is polygonal ferrite and pearlite, the ferrite ratio is more than 95%, the grain size is 9.0 grade, and there is no banded structure. The mechanical properties of the obtained product are: yield strength 359 MPa, tensile strength 451 MPa, elongation 34%. By observing the fracture of the tensile specimen with the naked eye, there is no tensile delamination phenomenon. V-notch impact test (impact specimen size 10 mm * 10 mm * 55 mm), impact temperature: 0 °C, 3 groups of specimens are taken for impact test, and the impact energy values are 200 J / 274 J / 209 J.

[0112] Example 3

[0113] This example is used to illustrate the preparation process of D-class ship plate steel.

[0114] The D-class ship plate steel provided in this example has the following chemical composition:

[0115] C: 0.0644%, Si: 0.2013%, Mn: 0.6572%, P: 0.0142%, S: 0.0045%, Als: 0.045%, Nb: 0.0139%, Ti: 0.0126%, N: 0.0017%, and the balance is Fe and unavoidable impurities.

[0116] The preparation method of D-class ship plate steel includes hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of the slab, hot strip rolling and laminar cooling carried out in sequence.

[0117] Among them, after the hot metal is pretreated, the content of S element in the hot metal is 0.0035%.

[0118] After alloy fine-tuning, the components in the molten steel meet the following weight percentages:

[0119] C: 0.0644%, Si: 0.2013%, Mn: 0.6572%, P: 0.0142%, S: 0.0045%, Als: 0.045%, Nb: 0.0139%, Ti: 0.0126%, N: 0.0017%, and the balance is Fe and inevitable impurities.

[0120] During the LF refining process, white slag operation is adopted, the top slag is fully reduced, the liquidus temperature of the molten steel is 1526°C, and the temperature of the molten steel for continuous casting at the station is 1567°C.

[0121] During the continuous casting process, it is cast into a continuous casting slab with a thickness of 230 mm by the conventional continuous casting method. The liquidus temperature of the molten steel is 1526°C, the temperature of the molten steel in the tundish is 1547°C, dynamic soft reduction is put into use, the thickness of the slab is 230 mm, the central segregation grade of the continuous casting slab is C1.0, and the corner quality is good without cracks.

[0122] The sampling and inspection methods of the macrostructure specimens of the continuous casting slab are the same as those in Example 1, and the macrostructure grading results of the slab are shown in Table 3.

[0123] Table 3 Segregation and corner crack conditions of the slab

[0124]

[0125]

[0126] During the hot strip rolling process, the slab is heated to 1240°C and then undergoes 3 + 5 passes of rough rolling. The thickness of the intermediate billet after rough rolling is 58 mm, the entry temperature of the finish rolling is 1029°C, 7 passes of finish rolling are carried out on a 2250 mm hot strip mill. The deformation amount of the first 2 passes of finish rolling is 60%, the thickness of the finish rolled product is 10.07 mm, the finish rolling temperature is controlled at 878°C. After finish rolling, it undergoes laminar cooling, the cooling rate is 32°C / s, and the coiling temperature is 635°C.

[0127] The sampling and inspection methods for the finished ship plate obtained in this example are the same as those in Example 1. The microstructure of the product is polygonal ferrite and pearlite, the proportion of ferrite is more than 95%, the grain size is 9.5 grades, and there is no banded structure. The mechanical properties of the obtained product are as follows: yield strength 378 MPa, tensile strength 460 MPa, elongation 36%. By observing the fracture of the tensile specimen with the naked eye, there is no tensile delamination phenomenon. For the V-notch impact test (impact specimen size 10 mm * 10 mm * 55 mm), the impact temperature is -20 °C. Three groups of specimens are taken for the impact test, and the impact energy values are 246 J / 236 J / 216 J.

[0128] Comparative Example 1

[0129] This comparative example is used to illustrate the preparation process of B-grade ship plate steel.

[0130] The B-grade ship plate steel provided in this comparative example has the following chemical composition:

[0131] C: 0.1204%, Si: 0.1893%, Mn: 0.9024%, P: 0.0154%, S: 0.0061%, Als: 0.0337%, and the balance is Fe and unavoidable impurities.

[0132] The preparation method of B-grade ship plate steel includes hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of the billet, hot strip rolling, and laminar flow cooling, which are carried out in sequence.

[0133] After the hot metal is pretreated, the content of S element in the hot metal is 0.0040%.

[0134] After alloy fine-tuning, the composition of the molten steel meets the following weight percentages:

[0135] C: 0.1204%, Si: 0.1893%, Mn: 0.9024%, P: 0.0154%, S: 0.0061%, Als: 0.0337%, and the balance is Fe and unavoidable impurities.

[0136] For LF refining, white slag operation is adopted, the top slag is fully reduced, the liquidus temperature of the molten steel is 1521 °C, and the temperature of the molten steel for continuous casting at the station is 1567 °C.

[0137] It is cast into a continuous casting slab with a thickness of 230 mm by the conventional continuous casting method. The liquidus temperature of the molten steel is 1521 °C, the temperature of the molten steel in the tundish is 1547 °C, and dynamic soft reduction is applied. The thickness of the billet is 230 mm. The central segregation grade of the continuous casting billet is B1.5, the central porosity and crack grades are relatively high, there are transverse cracks at the corners, and the corners of the billet are cleaned after cooling.

[0138] The sampling and inspection methods for the macrostructure specimen of the continuous casting billet are the same as those in Example 1. The macrostructure pickling sample at the central position in the thickness direction of the continuous casting billet is shown inFigure 5 As shown, the quality of the continuous casting billet is shown in Table 4.

[0139] Table 4 Segregation and corner crack conditions of continuous casting billets

[0140] Slab information Center segregation grade Center porosity grade Center crack grade Corner crack Comparative example 1 B1.5 1.0 0.5 With transverse crack

[0141] The continuous casting billet is heated to 1229 °C and then undergoes 3 + 3 passes of rough rolling. The thickness of the rough rolling intermediate billet is 42 mm, the entry temperature of finish rolling is 1029 °C, and 7 passes of finish rolling are carried out on a 2250 mm hot tandem mill. The deformation amount of the first 2 passes of finish rolling is 46%, the thickness of the finish rolled product is 10.1 mm, the finishing temperature is controlled at 881 °C, and after finish rolling, it undergoes laminar cooling with a cooling rate of 31 °C / s and a coiling temperature of 630 °C.

[0142] For the obtained ship plate, tensile specimens are taken according to GB / T 2975 and tensile tests are carried out according to GB / T 228.1; metallographic specimens are prepared according to GB / T 13298, the microstructure is evaluated according to GB / T 13299, the grain size is evaluated according to GB / T 6394, and the banded structure is evaluated according to GB / T 34474.1; the microstructure of the ship plate product is ferrite + pearlite, the proportion of ferrite is 85% - 95%, the grain size is 8.5 grades, the banded structure is 1.5 grades, and the metallographic structure is shown in Figure 6 . The yield strength of the tensile mechanical properties is 306 MPa, the tensile strength is 446 MPa, and the elongation is 35.5%. Through visual observation, there is a delamination phenomenon on the tensile fracture surface, and the fracture surface of the tensile specimen is shown in Figure 7 As shown. V-notch impact test (impact specimen size 10 mm * 10 mm * 55 mm), impact temperature: 0 °C, 3 groups of specimens are taken for impact test, and the impact energy values are 94 J / 87 J / 97 J.

[0143] Comparative Example 2

[0144] This comparative example is used to illustrate the preparation process of B-grade ship plate steel.

[0145] The B-grade ship plate steel provided in this comparative example has the following chemical composition:

[0146] C: 0.1458%, Si: 0.1949%, Mn: 0.8708%, P: 0.0073%, S: 0.0041%, Als: 0.0388%, and the balance is Fe and unavoidable impurities (billet number: D231093411530, coil number: H231922190).

[0147] The preparation method of B-grade ship plate steel includes, in sequence, hot metal pretreatment, converter smelting, alloy fine-tuning, LF refining, continuous casting, hot charging of continuous casting billets, hot tandem rolling, and laminar cooling.

[0148] After hot metal pretreatment, the content of S element in hot metal is 0.0038%.

[0149] After the alloy is fine-tuned, the components in the molten steel meet the following weight percentages:

[0150] C: 0.1458%, Si: 0.1949%, Mn: 0.8708%, P: 0.0073%, S: 0.0041%, Als: 0.0388%, and the balance is Fe and inevitable impurities

[0151] LF refining adopts white slag operation, the top slag is fully reduced, the liquidus temperature of the molten steel is 1520 °C, and the temperature of the molten steel for continuous casting at the station is 1566 °C.

[0152] It is cast into a continuous casting slab with a thickness of 230 mm by the conventional continuous casting method. The liquidus temperature of the molten steel is 1520 °C, the temperature of the molten steel in the tundish is 1548 °C, dynamic soft reduction is applied, and the thickness of the slab is 230 mm. The central segregation grade of the continuous casting slab is B1.5, the central porosity grade is 1.0, there are transverse cracks at the corners, and the corners of the slab are cleaned after cooling.

[0153] The method for sampling and inspecting the macrostructure specimen of the continuous casting slab is the same as that in Example 1. The macrostructure pickling sample at the central position in the thickness direction of the continuous casting slab is shown in Figure 8 as shown, and the quality of the slab is shown in Table 5.

[0154] Table 5 Segregation and corner crack conditions of the slab

[0155] Slab information Center segregation grade Center porosity grade Center crack grade Corner crack Comparative example 2 B1.0 1.0 0 With transverse crack

[0156] The slab is heated to 1238 °C and then undergoes 3 + 3 passes of rough rolling. The thickness of the intermediate billet after rough rolling is 42 mm. The entry temperature for finish rolling is 1029 °C. It is finish-rolled in 7 passes on a 2250 mm hot strip mill. The deformation in the first 2 passes of finish rolling is 46%. The thickness of the finish-rolled product is 8.06 mm. The final rolling temperature is controlled at 879 °C. After finish rolling, it undergoes laminar cooling with a cooling rate of 33 °C / s and a coiling temperature of 639 °C.

[0157] In this comparative example, tensile specimens are taken and prepared according to GB / T 2975, and tensile tests are carried out according to GB / T 228.1. The mechanical properties of the obtained product are as follows: yield strength 364 MPa, tensile strength 481 MPa, elongation 33%. Through visual observation, there is a delamination phenomenon on the tensile fracture surface. For the V-notch impact test (impact specimen size 7.5 mm * 10 mm * 55 mm), the impact temperature is 0 °C. Three groups of specimens are taken for the impact test, and the impact energy values are 65 J / 67 J / 56 J.

[0158] According to the process in the embodiment, except for no delamination in stretching, the impact performance is greatly improved compared with that in the comparative example. Compared with Embodiments 1 and 2, although the component design of Comparative Examples 1 and 2 has a lower alloy cost, the component design is in the peritectic region. The continuous casting billet needs to be chamfered and rolled, and the central segregation grade of the continuous casting billet is high, the central porosity grade of the continuous casting billet is relatively high, and there are intermediate cracks, etc. There is delamination during the finished product tensile inspection. In Embodiments 1 and 2, increasing the thickness of the intermediate billet to increase the deformation amount and the rapid cooling process are adopted to obtain a fine grain structure. According to the process in Embodiment 1, D-class ship plate steel with better quality can be obtained.

[0159] It should be understood that the parts not elaborated in detail in this specification belong to the prior art.

[0160] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A ship plate steel, characterized in that: The ship plate steel includes the following chemical compositions in weight percentage: C: 0.050% ~ 0.070%, Si: 0.150% ~ 0.250%, Mn: 0.60% ~ 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% ~ 0.040%, Nb: 0.005% ~ 0.015%, Ti: 0.010% ~ 0.020%, N ≤ 0.006%, the balance is Fe and unavoidable impurities.

2. The ship plate steel according to claim 1, characterized in that: The metallographic structure of the ship plate steel is polygonal ferrite and pearlite, wherein the ferrite ratio is greater than 90%, the grain size is greater than or equal to level 8, and the banded structure is less than or equal to level 1.

0.

3. The ship plate steel according to claim 1 or 2, characterized in that: The contents of Si, Mn and C in the ship plate steel satisfy the relationship shown in formula (1): And / or, the contents of Ti and N in the ship plate steel satisfy the relationship shown in formula (2): w Ti / w N ≥3Formula (2).

4. A method for preparing ship plate steel, characterized in that: The preparation method comprises the following steps: molten iron pretreatment, converter smelting, alloy fine adjustment, LF refining, continuous casting, hot charging of ingots, hot continuous rolling and laminar cooling; wherein, after alloy fine adjustment, the components in the molten steel meet the following weight percentages: C: 0.050% ~ 0.070%, Si: 0.150% ~ 0.250%, Mn: 0.60% ~ 0.80%, P: ≤0.02%, S: ≤0.008%, Als: 0.020% ~ 0.040%, Nb: 0.005% ~ 0.015%, Ti: 0.010% ~ 0.020%, N ≤ 0.006%, the balance is Fe and unavoidable impurities.

5. The preparation method according to claim 4, characterized in that: After the molten iron is pretreated, the content of S element in the molten iron is ≤0.005 weight %.

6. The preparation method according to claim 4 or 5, characterized in that: During the LF refining process, the exit continuous pouring temperature is controlled to be 39°C to 47°C above the target liquidus temperature, wherein the target liquidus temperature is 1520°C to 1530°C; And / or, during the continuous casting process, the temperature of the molten steel in the tundish is controlled to be 15°C to 30°C above the target liquidus temperature, wherein the target liquidus temperature is 1520°C to 1530°C.

7. The preparation method according to claim 4, characterized in that: During the hot charging process of the ingot, the ingot is hot-charged and rolled, and the heating furnace is kept warm for 2 to 3 hours.

8. The preparation method according to claim 4 or 5, characterized in that: In the hot rolling process, rough rolling is first performed, followed by 5 to 10 passes of finishing rolling; The thickness of the intermediate billet after rough rolling is in the range of 55 mm to 65 mm, and the deformation of the first two passes of finishing rolling is more than 55%.

9. The preparation method according to claim 4, characterized in that: During the laminar cooling process, the cooling rate is (30-50)°C / s.

10. Ship plate steel produced by the production method according to any one of claims 4 to 9.

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