A-grade and B-grade marine steel and production method thereof

Abstract

The invention provides A-grade and B-grade marine steel and a production method thereof, and the A-grade and B-grade marine steel comprises the following chemical elements in percentage by mass: 0.10 wt% < = C < = 0.18 wt%, 0.10 wt% < = Si < = 0.30 wt%, 0.80 wt% < = Mn < = 1.00 wt%, less than or equal to 0.025 wt% of P, less than or equal to 0.008 wt% of S, 0.018 wt% < = Als < = 0.050 wt%, less than or equal to 0.0065 wt% of N, and the balance of iron and other inevitable impurities. By accurately controlling the content of each element and setting rolling process parameters, the quality requirements of stable mechanical properties, non-deformation of materials in laser cutting and welding processes and the like for special processes such as a fire calibration process in the material use process are met. The balance of strength, toughness, corrosion resistance and weldability can be realized, so that the overall performance of the marine steel is improved, and the service life of the marine steel is prolonged.

Description

Technical Field

[0001] The invention belongs to the field of hot-rolled steel plates, and in particular relates to A- and B-grade ship steel and a production method thereof. Background Art

[0002] The material grade of A and B grade marine steel plates is relatively low, and they are mainly used for ordinary structural components for shipbuilding, etc., and the requirements for materials are not high. However, with the popularization of automation such as laser cutting and automatic welding in downstream processing and supporting plants and the improvement of the requirements for building appearance beautification, it is necessary to use normalizing heat treatment process for A and B grade marine steel products in the production process as a process guarantee measure for the high performance stability of A and B grade marine steel to meet the process requirements for the use of materials in fire calibration. As we all know, conventional normalizing heat treatment requires high temperature guarantee, and the addition of heat treatment process prolongs the production line and production cycle, and the production cost of the product is relatively high, resulting in high prices for rolled Kaiping marine steel plates.

[0003] Based on this, it is necessary to provide a kind of A, B grade shipbuilding steel and its production method to alleviate or solve the above problems. Summary of the invention

[0004] The invention aims to solve the technical problem that the A and B grade shipbuilding steels in the above-mentioned common technologies need to adopt a normalizing heat treatment process, which leads to an extension of the production line and the production cycle, resulting in a rise in costs. The present invention provides A and B grade shipbuilding steels, whose chemical elements, measured by mass fraction, include: 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the rest are iron and other inevitable impurities.

[0005] Furthermore, the microstructure of the A and B grade marine steels includes ferrite and uniformly dispersed pearlite, and the grain size of the overall microstructure of the A and B grade marine steels reaches above grade 10.5.

[0006] The present invention provides a method for producing grade A and B ship steel, comprising the steps of:

[0007] The steel is tapped from a converter, and is successively subjected to LF refining and continuous casting to obtain a cast billet; the composition of the cast billet comprises, by mass fraction, 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the remainder is iron and other inevitable impurities;

[0008] The ingot is heated, rolled and then coiled to obtain the A and B grade shipbuilding steels, and the coiling temperature is not less than 780°C.

[0009] Furthermore, in the step of heating the ingot, rolling it and then coiling it to obtain the A and B grade shipbuilding steel, the coiled steel coil is subjected to a flattening treatment to obtain the A and B grade shipbuilding steel, and the flattening treatment reduces the pressure reduction at the rough straightening entrance to -5mm to -10mm.

[0010] Furthermore, the flattening process also includes: when the thickness requirement of the A and B grade shipbuilding steel is not more than 12 mm, the straightness is not more than 3 mm / m; when the thickness requirement of the A and B grade shipbuilding steel is more than 12 mm, the straightness is not more than 4 mm / m, or not more than 6 mm / 2m.

[0011] Furthermore, when the overall temperature of the coiled steel coil is lower than 60° C., the steel coil is subjected to a flattening process.

[0012] Furthermore, the rolling process includes initial rolling and finishing rolling, wherein the finishing rolling temperature is not lower than 900°C, the reduction rate during the finishing rolling process is not less than 50% to 70%, and the finishing rolling speed is not lower than 6-8m / s.

[0013] Furthermore, no water cooling is used during the finishing rolling, layer cooling and coiling processes, a heat preservation cover is used in the processes from the initial rolling to the finishing rolling, and the interval time between the initial rolling and the finishing rolling is 30 to 55 seconds.

[0014] Furthermore, the pulling speed during the continuous casting process is 1.2-1.4 m / min, the solidification end is pressed down by 4.5 mm in three stages, and the specific water content is controlled at 0.65-0.80 L / kg.

[0015] Furthermore, in the step of coiling the billet after heating and rolling, the thickness of the billet is 230 mm, the temperature of the billet after heating is 1170-1210° C., and the heating time of the billet is 150-300 min.

[0016] Compared with the prior art, the present invention has at least the following advantages:

[0017] The present invention provides a grade A and B ship steel, which comprises, by mass fraction, 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the remainder is iron and other inevitable impurities. By precisely controlling the content of each element, a balance of strength, toughness, corrosion resistance and weldability can be achieved, thereby improving the overall performance and service life of the ship steel.

[0018] The present invention controls the carbon content between 0.10% and 0.18%. As a main alloying element in steel, the limitation of carbon content has an important influence on the strength, hardness and weldability of steel.

[0019] The present invention controls the silicon (Si) content between 0.10% and 0.30% to improve the strength and hardness of the steel, and also helps to improve the oxidation resistance and corrosion resistance of the steel.

[0020] The present invention controls the manganese (Mn) content between 0.80% and 1.00%, which can improve the strength and toughness of the steel, and also help improve the wear resistance and corrosion resistance of the steel. By accurately controlling the content of elements such as carbon, silicon, and manganese, the toughness of the steel can be improved while ensuring the strength of the steel. This balance is crucial for ship steel because the hull needs to withstand complex stresses and environmental conditions. Elements such as silicon and manganese help improve the corrosion resistance of steel to adapt to the use environment of ship steel in the marine environment.

[0021] The present invention controls the content of acid-soluble aluminum (Als) between 0.018% and 0.050%. As a deoxidizer, acid-soluble aluminum can refine the grains of steel, improve the strength and toughness of steel, and further improve the mechanical properties and fatigue resistance of ship steel.

[0022] The present invention controls the contents of nitrogen, phosphorus and sulfur to ≤0.0065wt%, ≤0.025wt% and ≤0.008wt% respectively. By controlling the contents of elements such as carbon, phosphorus and sulfur, the weldability of the steel can be improved, ensuring the welding quality of the hull during construction and maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0024] Figure 1 The metallographic structure diagram of grade A and B marine steel prepared in Example 1 of the present invention;

[0025] Figure 2 The metallographic structure diagram of grade A and B marine steel prepared in comparative example 1 of the present invention. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Furthermore, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that they can be implemented by ordinary technicians in the field. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0028] When numerical ranges are given in the embodiments, it should be understood that unless otherwise specified in the present invention, both endpoints of each numerical range and any value between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present invention are familiar to those skilled in the art and the description of the present invention, and any method, device and material of the prior art similar or equivalent to the method, device and material in the embodiments of the present invention can also be used to implement the present invention.

[0029] The invention provides A and B grade ship steel, which comprises, by mass fraction, 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the rest is iron and other inevitable impurities.

[0030] In the marine steel product standard GB / T 712-2022 "Structural Steel for Ships and Offshore Engineering", Grades A and B belong to ordinary marine steels with consistent yield and tensile strength mechanical properties. The main difference is that Grade A does not require impact toughness testing, while Grade B does.

[0031] The present invention controls the carbon content between 0.10% and 0.18%. As a main alloying element in steel, the limitation of carbon content has an important influence on the strength, hardness and weldability of steel.

[0032] The present invention controls the silicon (Si) content between 0.10% and 0.30% to improve the strength and hardness of the steel, and also helps to improve the oxidation resistance and corrosion resistance of the steel.

[0033] The present invention controls the manganese (Mn) content between 0.80% and 1.00%, which can improve the strength and toughness of the steel, and also help improve the wear resistance and corrosion resistance of the steel. By accurately controlling the content of elements such as carbon, silicon, and manganese, the toughness of the steel can be improved while ensuring the strength of the steel. This balance is crucial for ship steel because the hull needs to withstand complex stresses and environmental conditions. Elements such as silicon and manganese help improve the corrosion resistance of steel to adapt to the use environment of ship steel in the marine environment.

[0034] The present invention controls the content of acid-soluble aluminum (Als) between 0.018% and 0.050%. As a deoxidizer, acid-soluble aluminum can refine the grains of steel, improve the strength and toughness of steel, and further improve the mechanical properties and fatigue resistance of ship steel.

[0035] The present invention controls the contents of nitrogen, phosphorus and sulfur to ≤0.0065wt%, ≤0.025wt% and ≤0.008wt% respectively. By controlling the contents of nitrogen, phosphorus, sulfur and other elements, the weldability of the steel can be improved, ensuring the welding quality of the hull during construction and maintenance.

[0036] For example, the carbon content in grade A and B shipbuilding steels may be 0.13wt% to 0.15wt%; the carbon content in grade A and B shipbuilding steels may be 0.10wt% to 0.15wt%; the carbon content in grade A and B shipbuilding steels may be 0.13wt% to 0.18wt%.

[0037] For example, the silicon content in grade A and B shipbuilding steels may be 0.2wt% to 0.3wt%; the silicon content in grade A and B shipbuilding steels may be 0.20wt% to 0.25wt%.

[0038] For example, the manganese content in grade A and B shipbuilding steels may be 0.95wt% to 1.00wt%; the manganese content in grade A and B shipbuilding steels may be 0.90wt% to 1.00wt%; the manganese content in grade A and B shipbuilding steels may be 0.95wt% to 0.96wt%.

[0039] For example, the phosphorus content in grade A and B shipbuilding steels may be 0.012wt% to 0.016wt%; the phosphorus content in grade A and B shipbuilding steels may be 0.012wt% to 0.025wt%; the phosphorus content in grade A and B shipbuilding steels may be 0.012wt% to 0.02wt%.

[0040] For example, the sulfur content in grade A and B shipbuilding steels may be 0.002wt% to 0.004wt%; the sulfur content in grade A and B shipbuilding steels may be 0.002wt% to 0.005wt%.

[0041] For example, the acid-soluble aluminum content in grade A and B shipbuilding steels may be 0.02wt% to 0.04wt%; the acid-soluble aluminum content in grade A and B shipbuilding steels may be 0.020wt% to 0.035wt%; the acid-soluble aluminum content in grade A and B shipbuilding steels may be 0.020wt% to 0.050wt%.

[0042] For example, the nitrogen content in grade A and B shipbuilding steels may be 0.005wt% to 0.006wt%; the nitrogen content in grade A and B shipbuilding steels may be 0.005wt% to 0.0055wt%; the nitrogen content in grade A and B shipbuilding steels may be 0 to 0.0052wt%.

[0043] The microstructure of the A and B grade marine steels includes ferrite and uniformly dispersed pearlite, and the grain size of the overall microstructure of the A and B grade marine steels reaches above level 10.5, ensuring the strength and excellent low-temperature toughness of the AB grade marine steels.

[0044] The present invention provides a method for producing grade A and B ship steel, comprising the steps of:

[0045] S1. The steel is tapped from a converter and subjected to LF refining and continuous casting treatment in sequence to obtain a cast billet; the composition of the cast billet comprises, by mass fraction, 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the remainder is iron and other inevitable impurities.

[0046] The rating of center segregation and center porosity of the ingot shall not exceed C1.5.

[0047] Exemplarily, the converter includes a top and bottom blown converter.

[0048] In some embodiments, the casting speed during the continuous casting process can be 1.2-1.4 m / min, and the solidification end is pressed down by 4.5 mm in three stages, and the specific water content is controlled at 0.65-0.80 L / kg.

[0049] For example, the casting speed during the continuous casting process can be 1.4 m / min, with a three-stage reduction of 4.5 mm at the end of solidification, and the specific water content is controlled at 0.8 L / kg.

[0050] S2. The ingot is heated, rolled and then coiled to obtain the A and B grade shipbuilding steel.

[0051] In the step of coiling the billet after heating and rolling, the billet has a thickness of 230 mm, a temperature of 1170-1210° C. after heating, and a heating time of 150-300 min.

[0052] Exemplarily, the thickness of the ingot may be 230 mm, the temperature of the ingot after heating may be 1195° C., and the heating time of the ingot may be 182 min.

[0053] In some embodiments, the heated ingot may be subjected to dephosphorization. During the heating process, iron oxide scale will be generated on the surface of the ingot. If this layer of scale is not removed, it will be pressed into the surface of the product during the subsequent rolling process, forming surface defects such as indentations and pits. Through descaling, the iron oxide scale on the surface of the ingot can be effectively removed to avoid its influence on the surface quality of the product. At the same time, the surface of the product is made smoother and neater, which is conducive to improving the overall appearance quality of the product.

[0054] In some embodiments, in the step of heating the ingot and then rolling it to obtain the A and B grade shipbuilding steel, the coiled steel coil is subjected to a flattening treatment to obtain the A and B grade shipbuilding steel, and the flattening treatment reduces the pressure at the entrance of the rough straightening to -5mm to -10mm.

[0055] The flattening treatment also includes: when the thickness of the A and B grade ship steel is required to be no more than 12 mm, the straightness is no more than 3 mm / m;

[0056] When the thickness of grade A and B marine steel is required to be greater than 12 mm, the straightness shall not be greater than 4 mm / m, or not greater than 6 mm / 2m.

[0057] In other embodiments, after the overall temperature of the coiled steel coil is lower than 60° C., the steel coil is subjected to a flattening process.

[0058] For example, the coiled steel coils can be stacked for 24 hours before being transported out of the hot rolling warehouse, and the stacking of the steel coils can be standardized to ensure uniform cooling of the entire steel coil and avoid repeated heating of the steel coils.

[0059] The rolling process may include preliminary rolling and finishing rolling, wherein the finishing rolling temperature is not less than 900° C., the reduction ratio during the finishing rolling is not less than 50%, and the finishing rolling speed is not less than 6 m / s.

[0060] In some embodiments, the temperature of the finish rolling may be no higher than 950°C.

[0061] For example, the temperature of the finish rolling may be 900-950°C.

[0062] The coiling temperature is not less than 780℃.

[0063] In some embodiments, the coiling temperature may be no higher than 850°C.

[0064] An exemplary coiling temperature may be 780-850°C.

[0065] In some embodiments, water cooling is not performed during the finishing rolling, layer cooling and coiling processes, and a heat preservation cover is used for the initial rolling and the finishing rolling.

[0066] For example, after the coiled steel coil is naturally cooled to 60° C., the cooled steel coil may be flattened and cut into plates.

[0067] As the main material for the hull structure of seagoing and inland vessels, Grade A and B marine steels are required to have certain strength, toughness, low temperature resistance and corrosion resistance, as well as good welding performance. These performance requirements determine that marine steels in common technologies must undergo strict heat treatment processes.

[0068] The functions of normalizing heat treatment process are as follows: first, the strength and toughness of steel parts can be accurately controlled to achieve the best balance state, meeting the dual requirements of strength and toughness of marine steel; second, the structure of steel parts after normalizing treatment is more uniform, which is beneficial to improve the corrosion resistance of steel and extend the service life of the hull structure; finally, the hardness of steel parts after normalizing treatment is moderate, which is beneficial to heat conduction and weld formation during welding, ensuring welding quality.

[0069] In common technology, normalizing is a heat treatment process in which steel parts are heated to an appropriate temperature (30-50°C above Ac3 for hypoeutectoid steel and above Accm for hypereutectoid steel), kept warm for a certain period of time, and then cooled in air.

[0070] In comparison, the A and B grade ship steel materials produced by the technical solution of the present invention can resist high temperature use processes such as high temperature fire calibration and welding process during use, and the mechanical properties of the materials remain highly stable during this process. Specifically, the present invention adopts the normalizing temperature rolling process of high temperature coiling, which achieves the same quality effects as the subsequent coiling of the flat plate and then conventional heat treatment production, reduces the subsequent normalizing heat treatment production process, saves production costs and low-carbon process production requirements, and has a huge cost reduction and efficiency improvement effect and application space.

[0071] In order to facilitate those skilled in the art to further understand the present invention, examples are given below:

[0072] Example 1

[0073] This embodiment provides a short-process production method for high-performance stability A and B grade shipbuilding steel, and the specific implementation process includes:

[0074] S1. Composition of molten steel in the middle package: C: 0.143wt%, Si: 0.203wt%, Mn: 0.956wt%, P: 0.0152wt%, S: 0.0032wt%, Als: 0.0256wt%, N: 0.0055wt%, and the rest are iron and unavoidable elements.

[0075] S2. Casting speed 1.35m / min, dynamic soft pressure reduction 4.5mm for the ingot, specific water content 0.78L / kg. Center segregation of the ingot C1.5.

[0076] S3. Rolling process: Finishing rolling temperature is 920°C, coiling temperature is 790°C, ingot thickness is 230mm, continuous casting ingot outlet temperature is 1195°C, and total heating time is 182 minutes.

[0077] S4. According to the hot rolling process, the rolled steel coil is shipped out for 24 hours to be cooled. When the steel coil is naturally cooled to below 60℃, it is processed for cutting and flattening. The production specification of the order thickness is 8mm, and the pressure reduction at the entrance of the rough straightening is reduced to -4mm. The straightness of the steel plate does not exceed 3mm / 2m.

[0078] Example 2

[0079] This embodiment provides a short-process production method for high-performance stability A and B grade shipbuilding steel, and the specific implementation process includes:

[0080] S1. Composition of molten steel in the middle package: C: 0.135wt%, Si: 0.223wt%, Mn: 0.96wt%, P: 0.0122wt%, S: 0.0022wt%, Als: 0.0326wt%, N: 0.0052wt%. The rest are iron and unavoidable elements.

[0081] S2. Casting speed is 1.40m / min, dynamic soft pressure is 4.5mm for casting, specific water volume is 0.76L / Kg. Center segregation of casting shall not exceed C1.5.

[0082] S2. Rolling process: Finishing rolling temperature is 922°C, coiling temperature is 810°C, ingot thickness is 230mm, continuous casting ingot outlet temperature is 1195°C, and total heating time is 166 minutes.

[0083] S3 is rolled according to the hot rolling process. The rolled steel coil is shipped out of the warehouse for 24 hours to be cooled. After the steel coil is naturally cooled to below 60℃, it is processed for flattening and cutting. The production specification of the order thickness is 10mm, and the pressure reduction at the entrance of the rough straightening is reduced to -8mm. The straightness of the steel plate does not exceed 5mm / 2m.

[0084] Comparative Example 1

[0085] This comparative example provides a conventional heat-treated A and B grade shipbuilding steel production method, and the specific implementation process includes:

[0086] S1. Composition of molten steel in the middle package: C: 0.152wt%, Si: 0.153wt%, Mn: 0.95wt%, P: 0.0112wt%, S: 0.0042wt%, Als: 0.0226wt%, N: 0.0052wt%. The rest are iron and unavoidable elements.

[0087] S2. Rolling process: Finishing rolling temperature is 850℃~870℃, coiling temperature is 610~630℃, laminar water ratio is 1:1.1 and laminar water height is set. The thickness of the casting is 230mm, the temperature of the continuous casting is 1205℃, and the total heating time is 176 minutes.

[0088] S3. Rolling is controlled according to the hot rolling process. The rolled steel coil is placed in the hot coil warehouse for more than 48 hours. The steel coil is naturally cooled to below 60℃ for flattening and cutting. The pressure at the entrance of rough straightening is reduced to -5mm, and the straightness of the steel plate is 5mm / 2m.

[0089] S4. The coiled flat steel plate adopts normalizing heat treatment process, normalizing temperature: 850℃, normalizing time: 2min / mm.

[0090] Comparative Example 2

[0091] This comparative example provides a conventional heat-treated A and B grade shipbuilding steel production method, and the specific implementation process includes:

[0092] S1. Composition of molten steel in the middle package: C: 0.142wt%, Si: 0.193wt%, Mn: 0.90wt%, P: 0.0162wt%, S: 0.0032wt%, Als: 0.0286wt%, N: 0.0056wt%. The rest are iron and unavoidable elements.

[0093] S2. Rolling process: Finishing rolling temperature is 850℃~870℃, coiling temperature is 610~630℃, laminar water ratio is 1:1.1 and laminar water height is set. The thickness of the casting is 230mm, the temperature of the continuous casting is 1195℃, and the total heating time is 216 minutes.

[0094] S3. Rolling is controlled according to the hot rolling process. The rolled steel coil is placed in the hot coil warehouse for more than 48 hours. The steel coil is naturally cooled to below 60℃ for flattening and cutting. The pressure at the entrance of rough straightening is reduced to -5mm, and the straightness of the steel plate is 5mm / 2m.

[0095] S4. The coiled flat steel plate adopts normalizing heat treatment process, normalizing temperature: 870℃, normalizing time: 2min / mm.

[0096] Analysis Example: Implementation Example and Comparative Example Test Data:

[0097] Mechanical properties comparison:

[0098]

[0099] Example Performance Stability Test

[0100] The short-process production materials of the rolling process of the online high-temperature coiling process were tested after the 850℃ normalizing process. The mechanical properties of the materials did not show a downward trend, and were basically consistent with the properties of the raw materials. They met the performance requirements of the conventional offline heat treatment production materials and met the product quality requirements. The mechanical strength properties of the materials in Comparative Examples 1 and 2 were reduced by about 50MPa after the fire correction process, which deteriorated the material quality.

[0101]

[0102] Metallographic structure comparison

[0103] Mainly ferrite + pearlite, with banded structure in the entire thickness section. The structure of the short-process production material produced by online normalizing in Example 1 ( Figure 1 ) and the offline conventional normalizing heat treatment process production material group prepared in comparative example 1, the short process production material of online normalizing ( Figure 2 ) are basically the same.

[0104] The above technical solutions of the present invention are only preferred embodiments of the present invention, and the patent scope of the present invention is not limited thereto. All equivalent structural changes made by using the contents of the present invention specification and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A grade A or B marine steel, characterized in that: Calculated by mass fraction, the chemical elements include: 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the rest are iron and other inevitable impurities.

2. The A and B grade ship steel according to claim 1, characterized in that: The microstructure of the A-grade and B-grade shipbuilding steels includes ferrite and uniformly dispersed pearlite, and the grain size of the overall microstructure of the A-grade and B-grade shipbuilding steels reaches above grade 10.

5.

3. A method for producing grade A and B shipbuilding steel, characterized in that: Includes steps: The steel is tapped from the converter and then goes through LF refining and continuous casting to obtain ingots. The composition of the ingot includes, by mass fraction, 0.10wt%≤C≤0.18wt%, 0.10wt%≤Si≤0.30wt%, 0.80wt%≤Mn≤1.00wt%, P≤0.025wt%, S≤0.008wt%, 0.018wt%≤Als≤0.050wt%, N≤0.0065wt%, and the rest is iron and other inevitable impurities; The ingot is heated, rolled and then coiled to obtain the A and B grade shipbuilding steels, and the coiling temperature is not less than 780°C.

4. The method for producing grade A and B shipbuilding steel according to claim 3, characterized in that: In the step of heating the ingot, rolling it and then coiling it to obtain the A and B grade shipbuilding steel, the coiled steel coil is subjected to a flattening treatment to obtain the A and B grade shipbuilding steel, and the flattening treatment reduces the pressure reduction at the entrance of the rough straightening to -5mm to -10mm.

5. The production method according to claim 4, characterized in that: The flattening process also includes: when the thickness requirement of the A and B grade shipbuilding steel is not more than 12 mm, the straightness is not more than 3 mm / m; when the thickness requirement of the A and B grade shipbuilding steel is more than 12 mm, the straightness is not more than 4 mm / m, or not more than 6 mm / 2m.

6. The production method according to claim 4, characterized in that: When the overall temperature of the coiled steel coil is lower than 60° C., the steel coil is subjected to a flattening process.

7. The method for producing grade A and B shipbuilding steel according to claim 3, characterized in that: The rolling process includes initial rolling and finishing rolling, wherein the finishing rolling temperature is not less than 900°C, the reduction rate during the finishing rolling process is not less than 50% to 70%, and the finishing rolling speed is not less than 6-8m / s.

8. The production method according to claim 7, characterized in that: No water cooling is performed during the finishing rolling, layer cooling and coiling. A heat preservation cover is used in the processes from the initial rolling to the finishing rolling. The interval between the initial rolling and the finishing rolling is 30 to 55 seconds.

9. The method for producing grade A and B shipbuilding steel according to claim 3, characterized in that: The pulling speed during the continuous casting process is 1.2-1.4 m / min, and the solidification end is pressed down by 4.5 mm in three stages, and the specific water content is controlled at 0.65-0.80 L / kg.

10. The production method according to claim 3, characterized in that: In the step of coiling the billet after heating and rolling, the billet has a thickness of 230 mm, a temperature of 1170-1210° C. after heating, and a heating time of 150-300 min.

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