A low-temperature structural steel plate with excellent low-temperature impact toughness at -50℃ and a strength of 355MPa.

Abstract

This invention discloses a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃, belonging to the field of low-carbon microalloy steel production technology. The steel plate comprises: C: 0.14%–0.18%, Si: 0.25%–0.3%, Mn: 0.8%–0.9%, P≤0.025%, S≤0.015%, Nb: 0.01%–0.02%, Al: 0.015%–0.04%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities. This invention reduces alloy costs through simple composition design and achieves efficient and low-cost manufacturing of 355MPa low-temperature structural steel plates with excellent low-temperature impact toughness of -50℃ in thicknesses of 6–10 mm by controlling the steelmaking, continuous casting, heating, rolling controlled cooling, and normalizing processes. This ensures the service safety of engineering structural steel plates under -50℃ conditions.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed steel production technology, and more specifically, to a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃. Background Technology

[0002] Medium-thick plates of the 355MPa grade are the main type of medium-thick plates, with typical varieties including low-alloy Q345B / C / D / E. These steels are primarily based on carbon-manganese steel, with the addition of microalloying elements such as Nb, V, and Ti, which are carbide and nitride-forming elements. This is achieved through solid solution strengthening, precipitation strengthening, and grain refinement strengthening to improve the steel's strength and toughness. Statistics show that medium-thick plates of this strength grade account for 20-30% of annual production. Therefore, research into developing manufacturing technologies for this type of low-cost steel plate is of great significance. However, with increasingly stringent design specifications in engineering machinery applications such as bridges, pipelines, and railway vehicles, especially under extremely cold operating conditions, the requirements for low-temperature toughness and other indicators of steel plates are becoming increasingly stringent while ensuring their economic efficiency and safety. For example, the steel plate toughness must meet a transverse Charpy impact energy of ≥120J at -50℃. Currently, higher-grade engineering structural steels have lower carbon content and higher alloying element additions. This results in a continuous increase in the production cost of structural steel as its quality grade improves. For 355MPa grade engineering structural steel with special requirements for low-temperature toughness, the traditional design uses a "low-carbon" composition with a carbon content below 0.10%, a higher manganese content ranging from 1.05% to 1.55%, and the addition of alloying elements such as niobium, vanadium, and titanium to ensure performance, resulting in relatively high production costs. If a "medium- or high-carbon" composition is used instead of a "low-carbon" composition, and the addition of alloying elements is reduced, while optimizing the steelmaking and rolling processes, production costs will be significantly reduced. Currently, to ensure 355MPa grade low-temperature toughness, a higher alloy composition is typically used, a thicker intermediate billet is maintained for heating, and a two- or even three-stage controlled rolling process is employed, resulting in the final rolling temperature of the steel plate potentially being controlled below 800℃, or even lower; this significantly increases the difficulty of rolling. Firstly, the mill load, steel plate temperature uniformity, and plate shape control all face severe challenges, and the thickness of the steel plates is mainly above 10mm. Therefore, the key to developing thin-gauge, economical 355MPa steel plates lies in solving the problem of insufficient low-temperature toughness at -50℃ for 355MPa grade steel plates with a thickness of 6~10mm used in engineering structures.

[0003] To date, there are very few reports, both domestically and internationally, on manufacturing methods for low-temperature structural steel plates with excellent low-temperature impact toughness of 355MPa at -50℃. Patent application No. 202010071364.4 discloses a thin-gauge, low-yield-strength-ratio, high-toughness single-rolled steel plate for bridges and its manufacturing method. Its chemical composition is: C: 0.10-0.12%; Si: 0.2-0.3%; Mn: 1.48-1.58%; P≤0.015%; S≤0.004%; Nb: 0.011-0.021%; Al: 0.02-0.04%; Ti: 0.009-0.015%; Ca: 0.001-0.004%; N≤0.004%. To achieve high strength and excellent low-temperature toughness, this design requires the addition of relatively high contents of Mn and Nb. The production process targets thin plates with a thickness of 6-8mm. However, the low-temperature toughness can only be guaranteed up to -20℃, failing to meet the performance requirements of -50℃. Patent application number 201611152263.X discloses a pressure vessel steel plate with a yield strength of 345MPa. Its main components include: C: 0.15-0.18%, Si: 0.15-0.35%, Mn: 1.40-1.50%, P≤0.008%, S≤0.002%, Al: 0.020-0.040%, Nb: 0.020-0.035%, O≤0.003%. This patent can only meet the performance requirements for use in an environment of -10℃ (the conventional 345MPa pressure vessel is usually used in an environment of -40-60℃). Patent application number 200810200100.3 discloses a high-strength, low-yield-strength-ratio steel plate, whose main components include: C: 0.15-0.20%, Si: 1.0-2.0%, Mn: 1.8-2.0%, Al≤0.036%, V: 0.05-0.10%, P≤0.01%, S≤0.005%, Cr: 0.8-1.0%. This patent uses low-alloying treatment and traditional TMCP rolling process to obtain a hot-rolled steel plate with high strength and low yield-strength-ratio; however, its strength is high but its low-temperature impact toughness is poor, which cannot meet the requirements of low-temperature use environments.

[0004] Although the steels disclosed in the above patent documents have achieved high strength and manufacturing thickness range, they cannot meet the requirements for low-temperature toughness at -50℃ and have high production costs. Therefore, they are not suitable for manufacturing thin-gauge 355MPa grade low-temperature structural steel plates with excellent low-temperature impact toughness at -50℃. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃. This invention enables the efficient production of low-temperature structural steel plates with a thickness of 6-10mm and excellent low-temperature impact toughness of 355MPa at -50℃ using continuous casting billets with a thickness of 135-170mm. This steel plate has low cost, high strength and toughness, and can meet the service safety requirements of engineering structural steel plates under -50℃ conditions.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃, comprising the following components by weight percentage:

[0008] C: 0.14%~0.18%, Si: 0.25%~0.3%, Mn: 0.8%~0.9%, P≤0.025%, S≤0.015%, Nb: 0.01%~0.02%, Al: 0.015%~0.04%, N: 0.003%~0.006%, the weight percentage ratio of C, Si, Al, Nb and Mn in the aforementioned low-temperature structural steel plate conforms to: 1.66≤1.3e （Nb+0.2Si） +0.5(1.5C+0.4Mn-Al)≤1.68, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%;

[0009] The method for preparing the 355MPa grade low-temperature structural steel plate with excellent low-temperature impact toughness at -50℃ includes: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling and normalizing;

[0010] S1. The smelting raw materials are sequentially subjected to KR hot metal pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel;

[0011] S2. In the continuous casting process, the superheat is 8-15℃, the billet pulling speed is 0.9-1.3m / min; the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 250-300A, the secondary cooling water volume is 1.5-2.0L / kg, and the proportion of columnar crystals in the continuous casting billet is controlled at ≤10%; after the billet is removed from the continuous casting stage, the billet is stacked and slowly cooled, the stacking temperature is ≥750℃, and the slow cooling time is ≥36h, to obtain the cooled billet.

[0012] S3. In the reheating of the billet, the cooled billet is passed sequentially through a preheating section, a heating section, and a soaking section before exiting the furnace to obtain a heated billet; wherein, the temperature range of the preheating section is 850-1000℃, the temperature range of the heating section is 1190-1210℃, the temperature range of the soaking section is 1100-1130℃, the total furnace time of the heating section and the soaking section is 3-3.5h, and the furnace time of the soaking section is ≥1h;

[0013] S4. In the rolling and cooling process, before the rolling begins, the heated billet is descaled by high-pressure water after exiting the furnace. The descaled billet is rolled into a steel plate. The reduction rate of each pass in the first to third passes is ≥30%, and the number of rolling passes is 5 to 7. Subsequently, laminar flow cooling is used for cooling. The initial cooling temperature is 730 to 750°C, the final cooling temperature is 620 to 650°C, and the cooling rate is 5 to 8°C / s.

[0014] S5. In the normalizing process, the cooled steel plate is subjected to offline normalizing at a temperature of 880–920°C and a holding time of 1.2–1.5 mm / min.

[0015] Optionally, the steel plate has a transverse tensile yield strength of 370–405 MPa, a tensile strength of 510–550 MPa, an elongation of ≥25%, and a transverse Charpy impact energy of ≥135 J at -50℃.

[0016] Optionally, the thickness of the steel plate is 6~10mm.

[0017] Optionally, in the metallographic structure of the steel plate, the volume fraction of polygonal ferrite is 80-90%, the volume fraction of bainite is 5-15%, the volume fraction of M / A phase is ≤5%, and the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the metallographic structure is ≥60%.

[0018] Optionally, step S1 includes: pretreating the smelting raw materials with KR molten iron to control the S content ≤0.015%, and then feeding them into the converter after slag removal; using a double-slag method to remove P during converter smelting to control the P content ≤0.025%, controlling the C content to be 0.14%~0.18% at the end of converter smelting, and blowing argon gas for 20~30 minutes when tapping steel from the converter; subsequently performing LF refining and RH vacuum degassing in sequence, with RH vacuum degassing maintained for ≥20 minutes.

[0019] Optionally, in step S2, the thickness of the continuously cast billet is 135~170mm.

[0020] Optionally, in step S4, the high-pressure water descaling time is 1 to 2 minutes, and the pressure of the high-pressure water descaling is 20 to 25 MPa.

[0021] Implementing the embodiments of the present invention will have the following beneficial effects:

[0022] 1. After deep desulfurization pretreatment of KR hot metal, slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and controlling the continuous casting speed can improve macroscopic segregation in the continuously cast billet, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, help reduce billet segregation, reduce internal structural defects, refine austenite grains, and reduce the width of ferrite bands in the banded structure after rolling. Optimizing the electromagnetic stirring current intensity and the secondary cooling water ratio in the secondary cooling zone can reduce the average carbon segregation index, suppress segregation, and reduce the proportion of columnar crystals, keeping it below 10%. Limiting the secondary cooling intensity suppresses the tendency of central segregation and crack deterioration in the billet. Billet stacking helps reduce central segregation and effectively reduces the internal H content of the billet.

[0023] 2. Reheating of the billet: The temperature and time of the billet in the preheating section, heating section and soaking section are limited to ensure that the carbides and nitrides of Nb are quickly and fully dissolved in the matrix and fully diffused. At the same time, it promotes the diffusion of alloying elements Mn and C, and reduces their influence on microstructure and properties due to compositional segregation. Meanwhile, the furnace time in the heating section and soaking section is controlled to inhibit the excessive growth of the original austenite grains and increase the contribution of fine grain strengthening to the strength and toughness of the steel plate.

[0024] 3. The composition of this invention is reasonable, and the amount of alloy added is low. By replacing dislocation and precipitation strengthening with fine grain strengthening, and with the addition of a small amount of microalloying element Nb, the alloy cost and resistance to high-temperature deformation during the rolling stage are greatly reduced. This is beneficial to increasing the reduction per pass and ensuring the comprehensive performance of the rolled steel plate. At the same time, the hot rolling process improves rolling efficiency. In addition, the use of a lower initial cooling temperature ensures that the volume fraction of polygonal ferrite in the steel plate after final cooling is 80-90%. The fine polygonal ferrite structure presents large-angle grain boundaries, which can effectively reduce the average effective grain size of the steel. At the same time, the slow cooling rate avoids the formation of M / A structure in the microstructure, ensuring that the M / A phase ratio is within 5%, and inhibits carbon diffusion and segregation to avoid the formation of banded structure. The steel plate is subjected to post-rolling cooling treatment to ensure strength margin and leave room for subsequent normalizing treatment.

[0025] 4. The rolled steel plate is normalized to ensure that the rolled steel plate is re-austenitic, the grains are refined, the microstructure is uniform, and the toughness is improved.

[0026] 5. This invention reduces alloy costs through simple composition design and achieves efficient and low-cost manufacturing of a 6-10mm thick, 355MPa-grade low-temperature structural steel plate for engineering structures by controlling the steelmaking, continuous casting, heating, rolling controlled cooling, and normalizing processes. The steel plate's polygonal ferrite volume fraction is controlled at 80-90%, bainite volume fraction at 5-15%, and M / A phase volume fraction below 5%. The proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the microstructure is ≥60%. This steel plate is low-cost, high-strength, and tough, meeting the service safety requirements for engineering structures at -50℃. Specific properties include: transverse tensile yield strength between 370-405MPa, tensile strength between 510-550MPa, elongation ≥25%, and transverse Charpy impact energy ≥135J at -50℃. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0028] This invention discloses a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃, comprising the following components by weight percentage: C: 0.14%~0.18%, Si: 0.25%~0.3%, Mn: 0.8%~0.9%, P≤0.025%, S≤0.015%, Nb: 0.01%~0.02%, Al: 0.015%~0.04%, N: 0.003%~0.006%. The weight percentage ratio of C, Si, Al, Nb, and Mn in the low-temperature structural steel plate conforms to: 1.66≤1.3e （Nb+0.2Si） +0.5(1.5C+0.4Mn-Al)≤1.68, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0029] Specifically, the roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0030] Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.14% to 0.18%.

[0031] Mn: It enhances the strength of steel through solid solution strengthening and also lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to the production of fine low-temperature phase transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, which is detrimental to improving the low-temperature toughness of the steel plate and cannot guarantee the uniformity of the cross-sectional microstructure. Therefore, the Mn content range of this invention is designed to be 0.8% to 0.9%.

[0032] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. However, excessive Si will reduce the toughness of the heat-affected zone in the base metal. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial for reducing the Bauschinger effect in the matrix material. Therefore, the Si content is set at 0.25% to 0.3% in this invention.

[0033] Nitrogen (Nb) exhibits excellent grain refinement and precipitation strengthening effects; it also delays austenite recrystallization. However, excessive Nb increases production costs and complicates continuous casting process control. This invention selects an Nb content range of 0.01% to 0.02%.

[0034] N: In steel, the element N has no other significant role besides forming fine NbN particles to refine the austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.003% to 0.006%.

[0035] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.04%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0036] P and S are unavoidable impurity elements in steel, and their levels should be as low as possible. However, due to considerations of smelting costs and processes, they cannot be kept infinitely low. Therefore, this invention controls P ≤ 0.025% and S ≤ 0.015%.

[0037] Furthermore, this invention also adjusts the relationship: 1.66 ≤ 1.3e （Nb+0.2Si） +0.5(1.5C+0.4Mn-Al)≤1.68, by precisely controlling the ratio of C, Si, Mn, Nb and Al elements, we can balance low cost and high strength and toughness, and meet the service safety requirements of steel plates for engineering structures under -50℃ conditions.

[0038] In one specific embodiment, the steel plate has a transverse tensile yield strength of 370–405 MPa, a tensile strength of 510–550 MPa, an elongation of ≥25%, and a transverse Charpy impact energy of ≥135 J at -50℃.

[0039] In one specific embodiment, the thickness of the steel plate is 6~10mm.

[0040] In one specific embodiment, the metallographic structure of the steel plate has a volume fraction of 80-90% for polygonal ferrite, a volume fraction of 5-15% for bainite, and a volume fraction of ≤5% for the M / A phase, wherein the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the metallographic structure is ≥60%.

[0041] This invention also discloses a method for preparing a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃, as described above, comprising: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling, and normalizing; specifically including:

[0042] S1. The smelting raw materials are sequentially subjected to KR molten iron pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel.

[0043] In one specific embodiment, step S1 includes: pretreating the smelting raw materials with KR molten iron to control the S content ≤0.015%, and then feeding them into the converter after slag removal; using the double slag method to remove P during converter smelting to control the P content ≤0.025%, controlling the C content to be 0.14%~0.18% at the end of converter smelting, and blowing argon gas for 20~30 minutes when tapping steel from the converter; subsequently performing LF refining and RH vacuum degassing in sequence, with RH vacuum degassing maintained for ≥20 minutes.

[0044] S2. In continuous casting, the superheat is 8-15℃, and the casting speed is 0.9-1.3m / min. The electromagnetic stirring current intensity in the secondary cooling zone during continuous casting is controlled at 250-300A, and the secondary cooling water volume is 1.5-2.0L / kg to reduce the average carbon segregation index and suppress segregation. By limiting the intensity of secondary cooling, the tendency of segregation and crack deterioration in the center of the billet is suppressed, and the proportion of columnar crystals is reduced, controlling the proportion of columnar crystals in the continuous casting billet to ≤10%. After continuous casting, the billet is stacked and slowly cooled at a stacking temperature ≥750℃ and a slow cooling time ≥36h to promote the diffusion of Mn, H, and C elements and reduce their influence on the microstructure and properties due to component segregation, resulting in a cooled billet.

[0045] In one specific embodiment, in step S2, the thickness of the continuously cast billet is 135~170mm.

[0046] In one specific embodiment, in step S2, a continuously cast billet with a thickness of 135~170mm is used for production on a medium-thick plate reciprocating rolling mill, and water is used as the cooling medium.

[0047] S3. In the reheating of the billet, the cooled billet is sent into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace to obtain a heated billet. The temperature range of the preheating section is 850-1000℃, which promotes the rapid and complete dissolution of Nb carbides and nitrides into the matrix and their full diffusion. The temperature range of the heating section is 1190-1210℃, and the temperature range of the soaking section is 1100-1130℃. The total furnace time in the heating and soaking sections is 3-3.5 hours, and the furnace time in the soaking section is ≥1 hour. This invention appropriately increases the heating temperature of the heating section and ensures the furnace time in the heating and soaking sections to further promote the diffusion of alloying elements Mn, C, and H, and reduce their impact on microstructure and properties due to component segregation. At the same time, the temperature and time of the soaking section are controlled to inhibit the excessive growth of the original austenite grains and ensure the contribution of fine grain strengthening to improving the strength and toughness of the steel plate.

[0048] S4. During rolling and cooling, before the initial rolling, the heated billet is descaled using high-pressure water. The descaled billet is then rolled into steel plates. The reduction rate in each of the first to third passes is ≥30%, with 5 to 7 rolling passes. Increasing the reduction rate per pass ensures core penetration and promotes dynamic recrystallization of austenite grains. Subsequently, laminar flow cooling is used, with an initial cooling temperature of 730–750℃ and a final cooling temperature of 620–650℃, at a cooling rate of 5–8%. The relatively low starting cooling temperature (℃ / s) ensures that the volume fraction of polygonal ferrite in the steel plate after final cooling is 80-90%. The fine polygonal ferrite structure with large-angle grain boundaries can effectively reduce the average effective grain size of the steel. At the same time, the slow cooling rate avoids the formation of M / A phase in the structure, ensuring that the M / A phase ratio is within 5%. It also inhibits the diffusion and segregation of carbon elements and avoids the formation of banded structure. The steel plate is cooled after rolling to ensure a strength margin and leave room for subsequent normalizing treatment.

[0049] In one specific embodiment, in step S4, the high-pressure water descaling time is 1 to 2 minutes, and the high-pressure water descaling pressure is 20 to 25 MPa.

[0050] S5. In normalizing, the cooled steel plate undergoes offline normalizing at a temperature of 880–920℃ and a holding time of 1.2–1.5 mm / min. Normalizing ensures that the rolled steel plate is re-austenitic, refines the grain size, maintains microstructure uniformity, and improves toughness.

[0051] By employing the above-mentioned composition and KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling, and normalizing treatment, a low-temperature structural steel plate with a thickness of 6-10 mm and excellent low-temperature impact toughness of 355 MPa was efficiently and cost-effectively manufactured for engineering structures. The steel plate has a multiphase microstructure dominated by polygonal ferrite, with the M / A phase ratio less than 5%. This steel plate possesses low cost, high strength and toughness, and can meet the service safety requirements for engineering structural steel plates at -50℃.

[0052] The following are specific embodiments.

[0053] Examples 1-6

[0054] The preparation method of the 355MPa grade low-temperature structural steel plate with excellent low-temperature impact toughness at -50℃ in Examples 1-6 includes: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling and normalizing; specifically including:

[0055] S1. After the raw materials for smelting are pretreated by KR hot metal for deep desulfurization, the slag is removed and the raw materials are then fed into the converter. During the converter smelting, the double slag method is used to remove phosphorus. The carbon content is controlled at the end of the converter smelting. Argon gas is blown when the steel is tapped from the converter. Then, LF refining and RH vacuum degassing are carried out in sequence to obtain molten steel.

[0056] S2. Molten steel is continuously cast. After the billet is produced, it is stacked and cooled slowly to obtain a cooled billet.

[0057] S3. In the reheating of the billet, the cooled billet is sent into a walking beam furnace for heating. After passing through the preheating section, heating section and soaking section in sequence, the billet is taken out of the furnace to obtain the heated billet.

[0058] S4. During rolling and cooling, before the start of rolling, the heated billet is descaled by high-pressure water after being taken out of the furnace. The descaled billet is rolled into steel plate and then cooled by laminar flow cooling.

[0059] S5. During normalizing, the steel plate is normalized offline after cooling.

[0060] Table 1 shows the chemical composition of the steels in Examples 1-6; Table 2 shows the smelting and stacking process parameters of the steels in Examples 1-6; Table 3 shows the process parameters for reheating the cast billets and high-pressure water descaling of the continuously cast billets before rolling in Examples 1-6; Table 4 shows the rolling and cooling parameters of the steels in Examples 1-6; and Table 5 shows the performance and heat treatment parameters of the steel plates in Examples 1-6.

[0061] Table 1. Chemical composition (wt, %) of Examples 1-6 of the present invention

[0062]

[0063] Note: Impurity elements in steel: P≤0.025%; S≤0.015%; total amount of other impurity elements less than 0.05%.

[0064] Table 2. Smelting and stacking process parameters of steels in Examples 1-6

[0065]

[0066] Table 3 Heating regime and high-pressure water descaling process parameters for steel billets in Examples 1-6

[0067]

[0068] Table 4 Rolling and cooling parameters for steels in Examples 1-6

[0069]

[0070] Table 5. Heat treatment parameters and properties of steel plates in Examples 1-6 of this invention

[0071]

[0072] Therefore, by employing the composition design and steelmaking, continuous casting, heating, and controlled rolling and cooling schemes of this invention, a low-temperature structural steel plate with a thickness of 6-10 mm and excellent low-temperature impact toughness of 355 MPa can be manufactured efficiently and at low cost. In the metallographic structure of the steel plate, the volume fraction of polygonal ferrite is 80-90%, the volume fraction of bainite is 5-15%, and the volume fraction of M / A phase is ≤5%. Furthermore, the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the metallographic structure is ≥60%. This steel plate possesses low cost, high strength and toughness, and can meet the service safety requirements for structural steel plates under conditions of -50℃.

[0073] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a low-temperature structural steel plate with excellent low-temperature impact toughness of 355 MPa at -50℃, characterized in that, The steel plate is a steel plate for engineering structures, comprising the following components by weight percentage: C: 0.14%~0.18%, Si: 0.26%~0.3%, Mn: 0.8%~0.9%, P≤0.025%, S≤0.015%, Nb: 0.01%~0.017%, Al: 0.015%~0.04%, N: 0.003%~0.006%, the weight percentage ratio of C, Si, Al, Nb and Mn in the aforementioned low-temperature structural steel plate conforms to: 1.66≤1.3e （Nb+0.2Si） +0.5(1.5C+0.4Mn-Al)≤1.68, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%; The method for preparing the 355MPa grade low-temperature structural steel plate with excellent low-temperature impact toughness at -50℃ includes: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling and normalizing; S1. The smelting raw materials are sequentially subjected to KR hot metal pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel; S2. In the continuous casting process, the superheat is 8-15℃, the billet pulling speed is 0.9-1.3m / min; the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 250-300A, the secondary cooling water volume is 1.5-2.0L / kg, and the proportion of columnar crystals in the continuous casting billet is controlled at ≤10%; after the billet is removed from the continuous casting stage, the billet is stacked and slowly cooled, the stacking temperature is ≥750℃, and the slow cooling time is ≥36h, to obtain the cooled billet. S3. In the reheating of the billet, the cooled billet is passed sequentially through a preheating section, a heating section, and a soaking section before exiting the furnace to obtain a heated billet; wherein, the temperature range of the preheating section is 850-1000℃, the temperature range of the heating section is 1190-1210℃, the temperature range of the soaking section is 1100-1130℃, the total furnace time of the heating section and the soaking section is 3-3.5h, and the furnace time of the soaking section is ≥1.1h; S4. In the rolling and cooling process, before the rolling begins, the heated billet is descaled by high-pressure water after exiting the furnace. The descaled billet is rolled into a steel plate. The reduction rate of each pass in the first to third passes is ≥30%, and the number of rolling passes is 5 to 7. Subsequently, laminar flow cooling is used for cooling. The initial cooling temperature is 730 to 750°C, the final cooling temperature is 620 to 650°C, and the cooling rate is 5 to 8°C / s. S5. In the normalizing process, the cooled steel plate is subjected to offline normalizing at a temperature of 880–920°C and a holding time of 1.2–1.5 mm / min. The steel plate has a transverse tensile yield strength of 370-405 MPa, a tensile strength of 510-550 MPa, an elongation of ≥25%, and a transverse Charpy impact energy of ≥180 J at -50℃. The thickness of the steel plate is 6~9mm; In the metallographic structure of the steel plate, the volume fraction of polygonal ferrite is 80-90%, the volume fraction of bainite is 5-15%, the volume fraction of M / A phase is ≤5%, and the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the metallographic structure is ≥60%.

2. The method for preparing a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃ according to claim 1, characterized in that, Step S1 includes: pretreating the smelting raw materials with KR molten iron to control the S content ≤0.015%, and then feeding them into the converter after slag removal; using a double slag method to remove P during converter smelting to control the P content ≤0.025%, controlling the C content to be 0.14%~0.18% at the end of converter smelting, and blowing argon gas for 20~30 minutes when tapping steel from the converter; then performing LF refining and RH vacuum degassing in sequence, with RH vacuum degassing maintained for ≥20 minutes.

3. The method for preparing a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃ according to claim 1, characterized in that, In step S2, the thickness of the continuously cast billet is 135~170mm.

4. The method for preparing a low-temperature structural steel plate with excellent low-temperature impact toughness of 355MPa at -50℃ according to claim 1, characterized in that, In step S4, the high-pressure water descaling time is 1 to 2 minutes, and the pressure of the high-pressure water descaling is 20 to 25 MPa.

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