A high-strength shipbuilding steel with a carbon equivalent of ≤0.32% and a production method in a CSP production line

Abstract

A high-strength shipbuilding steel with a carbon equivalent of ≤0.32%, the components and wt% of which are: C: 0.17-0.19%, Si: 0.10-0.20%, Mn: 0.50-0.65%, P: ≤0.012%, S: ≤0.008%, Als: 0.015-0.055%, Ti: 0.015-0.020%, N: ≤0.006%; production method: 1) molten iron is pre-treated by desulfurization; converter smelting; LF furnace refining; RH vacuum treatment; casting into billets; heating the billets; rolling; cooling by front-stage rapid cooling; coiling. The thickness specification of the invention is 2-8mm, the yield strength is not less than 458MPa, the tensile strength is not less than 552MPa, the elongation after fracture A≥27%, and the impact energy A at 0℃ kv ≥94J, carbon equivalent Ceq(%)≤0.275%, which fully meets the requirements of shipbuilding field.

Description

Technical Field

[0001] The invention relates to a steel for ship engineering and a production method thereof, in particular to a high-strength ship engineering steel with a carbon equivalent of ≤0.32% and a production method thereof. Background Art

[0002] The construction process of steel hull mainly includes material cutting, pretreatment and welding. Therefore, according to the processing characteristics of the shipbuilding industry, marine steel materials are required to have good processing performance and dimensional accuracy under certain strength conditions, and have good toughness and welding performance, especially the requirements for good welding performance of materials. When thin-gauge high-strength marine steel is produced on the hot rolling production line, the rolling load is large, the plate shape control ability is poor, the production efficiency is low, and the product yield rate is low. The CSP production line has advantages due to its short process, low energy consumption, and high production efficiency, especially in the production of thin-gauge products. The product dimensional accuracy is high and the plate shape is well controlled. Therefore, the use of CSP technology to produce thin-gauge high-strength marine steel products is an economical and efficient production method, and the products have the characteristics of high dimensional accuracy, good processing performance, and excellent welding performance.

[0003] Before the present invention was applied, there was a document with Chinese patent publication number CN104962813A, which disclosed "a method for producing hot-rolled steel plates for welding gas cylinders using thin slab continuous casting and rolling process", which includes the following steps in order: steelmaking → continuous casting → soaking → hot rolling → coiling. Compared with the prior art, the present invention has the following advantages: due to the use of CSP thin slab continuous casting and rolling process, there is no need for cooling inspection and secondary heating of the casting in the traditional process, and rolling is carried out after soaking in the soaking furnace, which shortens the process flow, improves production efficiency, and reduces production energy consumption and cost. Its energy consumption per ton of steel is 40% to 41% of that of the conventional process, and the continuous casting slab yield rate is increased from 94.6% of the conventional process to 97%, and the cost per ton of steel is reduced by 30 to 50 US dollars. The steel coil invented in this document belongs to the field of hot-rolled steel plates for welding gas cylinders, which is essentially different from ship steel products. Ship steel has specific composition requirements and process performance requirements, and must comply with the relevant standards of classification societies before it can be used as ship steel materials.

[0004] The document with Chinese patent application number CN200710020524.7 records "high-strength steel for hull structure and its plate and coil production method", adopting the process route of steelmaking → refining → CSP continuous casting → heating → rolling → cooling → coiling, and the mass percentage of steel is C = 0.03 ~ 0.075, Si = 0.01 ~ 0.35, Mn = 1.40 ~ 1.60, Als = 0.015 ~ 0.10, V = 0.020 ~ 0.080. However, the high Mn content and the addition of V significantly increase the alloy cost of the material and increase the difficulty of smelting. In addition, there is no clear specification for the processing performance of the material and no clear control means for the welding performance of the material. In the field of ship steel, which uses welding as the main processing method, the welding performance requirements are extremely strict. The use of this type of steel may cause welding problems, resulting in the failure of the material performance. After loading, it may affect the safety of the ship. Summary of the invention

[0005] The present invention aims to overcome the shortcomings of the prior art and provide a kind of 2-8mm thick material with a yield strength of not less than 458MPa, a tensile strength of not less than 552MPa, an elongation after fracture A≥27%, and an impact energy of A≥27% at 0℃. kv High-strength shipbuilding steel with a carbon equivalent of ≥94J and a carbon equivalent of Ceq (%) ≤0.275% and a production method thereof.

[0006] Measures to achieve the above objectives:

[0007] A high-strength shipbuilding steel with a carbon equivalent of ≤0.32%, the components and weight percentage contents of which are: C: 0.17-0.19%, Si: 0.10-0.20%, Mn: 0.50-0.65%, P: ≤0.012%, S: ≤0.008%, Als: 0.015-0.055%, Ti: 0.015-0.020%, N: ≤0.006%, and the rest are Fe and unavoidable inclusions.

[0008] Preferably, the weight percentage of C is 0.17-0.186%.

[0009] Preferably, the weight percentage of Mn is 0.50-0.61%.

[0010] Preferably, the weight percentage of Ti is 0.015-0.018%.

[0011] Preferably, the weight percentage content of N is controlled to be ≤0.004%.

[0012] A method for producing high-strength shipbuilding steel with a carbon equivalent of ≤0.32% in a CSP production line, comprising the following steps:

[0013] 1) After the molten iron is pretreated with desulfurization, the sulfur in the molten iron is controlled to be ≤ 0.008%;

[0014] 2) Converter smelting, and control the C content in the final molten steel to not less than 0.17%, and the tapping temperature is controlled at 1660-1680°C;

[0015] 3) Refining is carried out in LF furnace, and the leaving station temperature is controlled to be not less than 1650℃;

[0016] 4) Perform RH vacuum treatment, and control the N content to ≤ 0.004%, the Als content to 0.015-0.055%, and the treatment end temperature to 1560-1580°C;

[0017] 5) Casting into billets, during which: the billet drawing speed is controlled at 3.9-4.1 m / min; the edge cooling speed of the secondary cooling water section is controlled to be no more than 18°C / s; the cooling water volume is such that the temperature of the continuous casting thin slab before entering the soaking furnace is 850-1100°C;

[0018] 6) Heat the ingot, control the temperature of the soaking furnace at 1200-1240℃, and control the total soaking time to be no less than 22min; the temperature difference of the same plate is ≤20℃;

[0019] 7) Use the equal proportion crown rolling mode for rolling: control the starting rolling temperature at 1160-1180℃,

[0020] The pass reduction rate and rolling temperature are:

[0021] F1-F3: The reduction rate of each pass is 65-68%, and the rolling temperature is 1110-1170℃;

[0022] The reduction rate of F4 is 27-34%, and the rolling temperature is 1060-1120℃;

[0023] F5-F6: The reduction rate of each pass is 8-12%, and the rolling temperature is 940-1100℃;

[0024] The reduction rate of F7 is 4-8%, and the rolling temperature is 880-920℃;

[0025] And during the rolling process, ensure that the austenite is fully recrystallized;

[0026] 8) Use the front-stage rapid cooling method to cool to the coiling temperature at a cooling rate of 180-220°C / s, and open the side water spray cooling to make the temperature of the steel plate entering the middle section at 660-670°C;

[0027] 9) Coil the steel sheet and control the coiling temperature at 580-620°C.

[0028] The functions and mechanisms of various raw materials and main processes in the present invention are described as follows:

[0029] The carbon (C) content of the present invention is 0.170-0.190%. The strength of the steel increases linearly with the increase of the carbon content. The increase of the carbon content affects the toughness of the steel and significantly increases the toughness-brittleness transition temperature of the steel. At the same time, the carbon can react with the Ti element in the steel to form microalloy carbides, which plays a role in precipitation strengthening. Therefore, in order to reduce the alloy cost and make the steel plate have certain hardness, wear resistance, welding and other performance, the carbon content is achieved in the peritectic zone to produce welded high-strength ship steel in the CSP process. The carbon content is limited to 0.170%-0.190%, preferably 0.17-0.186%.

[0030] The silicon (Si) content of the present invention is 0.10% to 0.20%. Si has a certain solid solution strengthening effect, which is beneficial to improving the strength of the steel but not conducive to the control of toughness. Under high temperature conditions, Si is first oxidized to SiO2, and then SiO2 reacts with FeO in the oxide layer to form fayalite phase Fe2SiO4 (2FeO·SiO2), which is easily enriched at the bonding surface of the iron oxide scale and the substrate, is not easy to remove and affects the rolling surface quality. Moreover, if the Si content is too high, micro cracks caused by the oxide pressing in are easily formed on the surface, which is not conducive to the control of the surface quality of the steel plate. Therefore, the Si content of the present invention is controlled to be 0.10% to 0.20%.

[0031] The manganese (Mn) content of the present invention is 0.50% to 0.65%. Manganese is the main solid solution strengthening element and has the effect of significantly refining grains. An appropriate amount of Mn content can reduce the γ→α phase transition temperature, help obtain fine phase transition products, and improve toughness. At the same time, since manganese and sulfur have a greater affinity, FeS can be avoided from precipitating at the grain boundary, reducing hot brittleness and improving hot processing performance. However, when the Mn content is high, (FeMn)2SiO4 is easily generated, which is easily enriched at the interface between the iron oxide scale and the substrate, causing the roughness of the interface between the iron oxide scale and the substrate, and forming defects such as the iron oxide scale. Therefore, the present invention controls the Mn content to 0.50% to 0.65%, preferably 0.50% to 0.61%.

[0032] The phosphorus (P) content of the present invention is ≤0.012%, and the sulfur (S) content is ≤0.008%. S mainly affects the plasticity of steel and is easy to combine with manganese to form MnS inclusions. P mainly affects the impact toughness and tough-brittle transition temperature of steel and is easy to cause segregation in steel. Sulfur usually exists in the weld in the form of MnS and FeS. When sulfur exists in the form of FeS, it is most harmful because it can be infinitely dissolved in liquid iron, while its solubility in the solid state is only 0.015% to 0.020%. Therefore, when the molten pool solidifies, FeS precipitates and forms low-melting eutectics with α-Fe and FeO. These low-melting eutectics gather at the grain boundaries in a liquid film, resulting in cracks at the grain boundaries, i.e., hot cracks. In addition, the impact toughness and corrosion resistance of the weld metal are reduced. P mainly exists in the form of iron phosphides Fe2P and Fe3P in the weld. Like S, it can form low-melting eutectics Fe3P+P with iron and gather at the grain boundaries to produce hot cracks. In addition, these phosphides also weaken the bonding force between grains, and they are hard and brittle, which increases the cold brittleness of the weld metal, reduces the impact toughness, and causes cold cracking. Therefore, the present invention should minimize the adverse effects of phosphorus and sulfur elements, and control the P and S contents of the steel to P: ≤0.012%, S: ≤0.008%.

[0033] The acid-soluble aluminum (ALs) content of the present invention is 0.015% to 0.055%. ALs is a main deoxidizing element, which can reduce oxide inclusions in steel, purify steel quality, refine grains, and at the same time, inhibit the aging of low-carbon steel, improve the toughness of steel at low temperatures, especially reduce the brittle transition temperature of steel, and improve the anti-oxidation performance of steel. Therefore, the ALs content of the present invention is in the range of 0.015% to 0.055%.

[0034] The titanium (Ti) content of the present invention is 0.015% to 0.020%. Ti has strong binding force with carbon, nitrogen and oxygen, and forms corresponding compounds with them, which has excellent hydrogen resistance, improves the strength of steel, improves the welding performance of steel, and increases toughness. However, too low Ti content cannot achieve the purpose of precipitation strengthening, and is limited to combining with N and S, and cannot generate effective TiC. Therefore, the Ti content of the present invention ranges from 0.015% to 0.020%, and preferably the content is 0.015% to 0.018%.

[0035] The nitrogen (N) content of the present invention is ≤0.006%. The energy of TiN generated by combining with N is very low, and it begins to precipitate in the solidification process of molten steel. The presence of N will seriously reduce the effective Ti content, affect the precipitation strengthening of TiC, and the formed TiN square particles will most likely become the source of cracks during material forming or impact, leading to the problem of material forming cracking. Therefore, the N content of the present invention is ≤0.006%, preferably ≤0.004%.

[0036] The reason why the present invention controls the molten iron desulfurization pretreatment requirement S: ≤0.008%; the converter molten steel smelting requirement C ≥0.17%, the steel tapping temperature is 1660-1680°C; deoxidation and slag making in the LF furnace, the leaving station temperature is ≥1650°C; the molten steel enters the RH furnace for decarburization and deoxidation; calcium treatment improves the slag property, N is controlled to be ≤0.004%, Als is controlled to be 0.015-0.055%, and the steel tapping temperature is 1560-1580°C is that too high a steel tapping temperature will greatly increase the oxygen content in the steel, reduce the alloy yield, bring about an increase in deoxidizer, and thus increase the inclusions in the steel, which is unfavorable to the physical quality. Therefore, in order to obtain the alloy yield and the quality of the molten steel, a reasonable steel tapping temperature must be set and the deoxidizer must be used reasonably.

[0037] The invention controls the billet drawing speed to 3.9-4.1 m / min, adopts the edge weak cooling system in the second cooling water section, controls the cooling water volume to ensure that the temperature of the continuous casting thin slab entering the equalizing furnace is 850-1100°C, so as to reduce the generation of crack defects on the edge of the billet, and can ensure that the billet has a higher temperature when entering the equalizing furnace, thereby improving the thermal efficiency of the furnace and ensuring the temperature uniformity.

[0038] The reason why the soaking furnace time is controlled to be ≥22 minutes and the temperature out of the soaking furnace is controlled to be 1200-1240℃ is that the factors affecting the thickness accuracy and plate shape are mainly temperature unevenness and the stability of the roller system. Temperature unevenness will cause changes in rolling force and plasticity coefficient, resulting in thickness deviation and thus causing the steel plate to be stretched to the sides or up and down, so the temperature difference of the same plate is required to be ≤20℃

[0039] The invention controls the start rolling temperature to 1160-1180°C, adopts the equal proportion crown rolling mode, and the reduction rate and rolling temperature of each pass are respectively F1-F3: 65-68%, 1130-1170°C; F4: 27-34%, 1060-1120°C; F5-F6: 8-12%, 940-1100°C; F7: 4-8%, 880-920°C; during the rolling process, the opening of the finishing mill water is strictly controlled to be ≤2, reducing the influence of the mill water on the strip temperature, improving the control of the final rolling temperature, and also improving the uniformity of the temperature of the edge of the coil. Avoid uneven deformation of the slab due to uneven temperature during the rolling process, thereby generating large uneven deformation resistance, resulting in large uneven deformation stress and thermal stress, and also ensure that the austenite fully recrystallizes, eliminates the mixed crystal defects in the organization, and ensures the final rolled plate shape quality and the use and welding performance of the material.

[0040] The invention controls cooling by adopting front-end fast cooling, controls the cooling speed at 180-220°C / s, sprays water on the side to ensure cooling uniformity, the intermediate cooling temperature is 660-670°C, and the coiling temperature is controlled at 580-620°C, so that the material is cooled uniformly during the cooling process, and the cooling deformation stress, temperature field stress and phase change stress are uniformized, ensuring that the material has good organizational properties.

[0041] Compared with the prior art, the thickness specification of the present invention is 2-8mm, the yield strength is not less than 458MPa, the tensile strength is not less than 552MPa, the elongation after fracture A≥27%, and the impact energy A at 0℃ kv ≥94J, carbon equivalent Ceq(%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.275%, which fully meets the requirements of the shipbuilding field. DETAILED DESCRIPTION

[0042] The present invention is described in detail below:

[0043] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0044] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0045] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0046] Each embodiment of the present invention is produced according to the following steps

[0047] 1) After the molten iron is pretreated with desulfurization, the sulfur in the molten iron is controlled to be ≤ 0.008%;

[0048] 2) Converter smelting, and control the C content in the final molten steel to not less than 0.17%, and the tapping temperature is controlled at 1660-1680°C;

[0049] 3) Refining is carried out in LF furnace, and the leaving station temperature is controlled to be not less than 1650℃;

[0050] 4) Perform RH vacuum treatment, and control the N content to ≤ 0.004%, the Als content to 0.015-0.055%, and the treatment end temperature to 1560-1580°C;

[0051] 5) Casting into billets, during which: the billet drawing speed is controlled at 3.9-4.1 m / min; the edge cooling speed of the secondary cooling water section is controlled to be no more than 18°C / s; the cooling water volume is such that the temperature of the continuous casting thin slab before entering the soaking furnace is 850-1100°C;

[0052] 6) Heat the ingot, control the temperature of the soaking furnace at 1200-1240℃, and control the total soaking time to be no less than 22min; the temperature difference of the same plate is ≤20℃;

[0053] 7) Use the equal proportion crown rolling mode for rolling: control the starting rolling temperature at 1160-1180℃,

[0054] The pass reduction rate and rolling temperature are:

[0055] F1-F3: The reduction rate of each pass is 65-68%, and the rolling temperature is 1110-1170℃;

[0056] The reduction rate of F4 is 27-34%, and the rolling temperature is 1060-1120℃;

[0057] F5-F6: The reduction rate of each pass is 8-12%, and the rolling temperature is 940-1100℃;

[0058] The reduction rate of F7 is 4-8%, and the rolling temperature is 880-920℃;

[0059] And during the rolling process, ensure that the austenite is fully recrystallized;

[0060] 8) Use the front-stage rapid cooling method to cool to the coiling temperature at a cooling rate of 180-220°C / s, and open the side water spray cooling to make the temperature of the steel plate entering the middle section at 660-670°C;

[0061] 9) Coil the steel sheet and control the coiling temperature at 580-620°C.

[0062] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0063]

[0064]

[0065] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0066]

[0067] Table 2

[0068]

[0069]

[0070] Table 3 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0071]

[0072] As can be seen from Table 3, the thickness specification of the present invention is 2-8 mm, the yield strength is not less than 458 MPa, the tensile strength is not less than 552 MPa, the elongation after fracture A ≥ 27%, and the impact energy A at 0 ° C kv ≥94J, carbon equivalent Ceq(%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.275%.

[0073] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A method for producing high-strength shipbuilding steel with a carbon equivalent of ≤0.32%, comprising the following steps: 1) After the molten iron is pretreated by desulfurization, the S content in the molten iron is controlled to be ≤0.008%; 2) Converter smelting, and control the C content in the final molten steel to not less than 0.17%, and the tapping temperature is controlled at 1660~1680℃; 3) Refining is carried out in LF furnace, and the temperature leaving the station is controlled to be not less than 1650℃; 4) Perform RH vacuum treatment, and control the N content to ≤0.004%, Als to 0.015~0.055%, and the treatment end temperature to 1560~1580℃; 5) Casting into billets, during which: the billet drawing speed is controlled at 3.9~4.1m / min; the edge cooling speed of the secondary cooling water section is controlled to be no more than 18℃ / s; the cooling water volume is such that the temperature of the continuous casting thin slab before entering the soaking furnace is 850~1100℃; 6) Heat the ingot, control the temperature of the soaking furnace at 1200~1240℃, and control the total soaking time to be no less than 22min; the temperature difference of the same plate is ≤20℃; 7) Use equal proportion crown rolling mode for rolling: control the start rolling temperature at 1160~1180℃, and the reduction rate and rolling temperature of each pass are as follows: F1~F3: The reduction rate of each pass is 65~68%, and the rolling temperature is 1110~1170℃; The reduction ratio of F4 is 27~34%, and the rolling temperature is 1060~1120℃; F5~F6: The reduction rate of each pass is 8~12%, and the rolling temperature is 940~1100℃; The reduction rate of F7 is 4~8%, and the rolling temperature is 880~920℃; And during the rolling process, ensure that the austenite is fully recrystallized; 8) Use the front-stage rapid cooling method to cool to the coiling temperature at a cooling rate of 201~220℃ / s, and open the side water spray cooling to make the temperature of the steel plate entering the middle section at 660~670℃; 9) Coil the steel sheet and control the coiling temperature at 580-587°C; The high-strength shipbuilding steel with a carbon equivalent of ≤0.32% has the following components and weight percentage contents: C: 0.17-0.19%, Si: 0.135-0.20%, Mn: 0.541-0.589%, P: ≤0.012%, S: ≤0.008%, Als: 0.032-0.034% or Als: 0.036-0.037% or Als: 0.052%, Ti: 0.015~0.018%, N: ≤0.006%, the rest are Fe and inevitable inclusions.

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