Weldable uncoated 420mpa grade steel plate for marine atmospheric corrosion resistance and method of manufacturing the same

Abstract

The application provides a kind of easily welded and unpainted 420MPa marine atmosphere corrosion resistant steel plate and its manufacturing method, the steel plate composition is by weight percentage: C: 0.02%~0.04%, Si: 0.02%~0.10%, Mn: 0.50%~0.70%, P≤0.012%, S≤0.005%, Ni: 2.5%~3.5%, Cu: 0.35%~0.85%, Mo: 0.15%~0.35%, Nb: 0.005%~0.015%, Ti: 0.012%~0.030%, Al: 0.015%~0.040%, the balance is Fe and inevitable impurities.The steel component of the application satisfies 22≤15[Mo]+6[Ni]+5[Cu]≤26, and the Pcm value (%)≤0.167%.The production method of the steel plate includes smelting, slab continuous casting, slow cooling of casting blank, slab heating, controlled rolling, and controlled cooling.The above-mentioned offshore steel plate has yield strength≥420MPa, tensile strength≥540MPa, yield ratio≤0.72, elongation after fracture≥20%, and KV2 impact energy at-80℃≥200J.The finished steel plate thickness≤60mm, and can be used without painting under the condition that the chloride ion deposition amount is not higher than 0.61mdd.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials, and particularly relates to a bridge and building easy-to-weld and paint-free 420MPa-grade marine atmosphere corrosion-resistant steel plate. BACKGROUND

[0002] With the development of bridge construction in China, many coastal and cross-sea bridge engineering projects are under construction or planning. Since the bridge site is in a high-chloride corrosion environment, the use of conventional steel materials is limited, so the demand for marine atmosphere corrosion-resistant bridge steel is increasing. The content of Cl - in the marine atmosphere is high, and the environmental temperature and humidity will change repeatedly, which will destroy the passive film on the surface of the steel and cause serious corrosion of the steel.

[0003] The main load-bearing part of the bridge is mostly a steel structure. In the marine environment, corrosion will cause the thickness of the structure to decrease, and even stress corrosion cracking will occur, which will greatly affect the safe service of the steel structure. In recent years, there has been a growing demand for the implementation of minimal maintenance of bridge steel structures, so corrosion-resistant steel without painting has attracted much attention. It is very urgent to reduce the corrosion rate through a stable protective rust layer to achieve the demand for not using paint. At present, there are some studies on marine atmosphere corrosion-resistant structural steel at home and abroad. Through retrieval, some patents have been found, but the content, production method, performance, and product category described in the patents are obviously different from the technical solutions of the present application.

[0004] Chinese patent application No. CN202011325158.8 discloses "a building structural steel suitable for marine atmospheric environment and a production method thereof", which provides a building structural steel suitable for marine atmospheric environment, belonging to the technical field of steel plate production. The chemical composition and mass fraction of the building structural steel are as follows: carbon: 0.07%-0.17%, silicon: 0.6%-0.8%, manganese: 0.3%-1.0%, phosphorus: 0.08%-0.15%, sulfur: 0.005-0.035%, copper: 0.15%-0.2%, antimony: 0.1%-0.2%, cerium: 0.0025%-0.0045%; and optionally containing: tin: 0.01%-0.02%, vanadium: 0.05-0.1%, or any one or both of the above, and the rest is iron and unavoidable impurities. The building structural steel has excellent corrosion resistance, which can effectively improve the service life and safety of steel structure buildings. The limitations of this patent mainly include: the composition contains Sb and Sn elements, which increases the difficulty coefficient of smelting, and is not green and environmentally friendly. The I index is used to evaluate the weather resistance, which will cause large deviation due to the use range of the formula. The steel grade does not achieve paint-free use in terms of corrosion resistance.

[0005] Chinese patent application number CN202211078494.6 discloses "a low-temperature service environment high crack arrest toughness ship and ocean engineering structure steel and its production method". The structure steel contains C: 0.03%~0.07%, Si: 0.05%~0.17%, Mn: 1.90%~3.00%, Nb: 0.01%~0.05%, Ti: 0.005%~0.025%, Ni: 1.00%~2.00%, Cu: 1.00%~2.00%, Cr: 0.10%~0.19%, Ce: 0.0030%~0.0070%, Zr: 0.0030%~0.0070%, Alt≤0.018%, P≤0.010%, S≤0.005%, N≤40ppm, O≤20ppm, H≤2ppm, the rest is Fe and inevitable impurities. The invention greatly improves the crack arrest toughness of the steel plate after welding in a low-temperature service environment. The limitations of this patent mainly manifest in: the composition contains Ce element, the smelting difficulty coefficient is large, and it is not green and environmentally friendly. The addition of Cr element causes the steel plate to reverse in corrosion resistance when it is corroded in the marine environment for a long time, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0006] Chinese patent application number CN202210570630.7 discloses "a production method of Q420-grade high-corrosion-resistance high-strength offshore structure steel", the process steps are hot metal pretreatment → converter steelmaking → LF secondary refining → RH vacuum treatment → continuous casting → heating → rolling → controlled rolling and cooling → tempering → finishing → performance testing → ultrasonic flaw detection. Its characteristics are: the mass percentage composition of the steel is C=0.03~0.07, Si=0.15~0.40, Mn=0.95~1.05, P≤0.020, S≤0.003, Nb=0.04~0.06, Ti=0.01~0.02, Als=0.02~0.05, Cu=0.30~0.70, Ni=0.7~1.5, the rest is Fe and inevitable impurity elements. It is suitable for producing steel with a thickness specification ≤60mm, resistant to marine atmospheric corrosion environment, with a yield strength ≥420MPa, a tensile strength ≥540MPa, a low-temperature-60℃ impact toughness KV2≥120J, a low-temperature-60℃ impact toughness KV2≥120J at 1 / 2 thickness of the steel plate, and a lamination tear resistance Z≥35%. The limitations of this patent mainly manifest in: the production of 420MPa weathering steel requires tempering heat treatment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0007] Chinese patent application number CN201810908224.0 discloses "a 420MPa grade low yield ratio marine atmospheric corrosion resistant bridge steel and its production method", the steel contains the following mass percentage content of chemical components: C: 0.04~0.20%, Si: 0.15~0.45%, Mn: 0.85~1.80%, Alt: 0.015~0.050%, Ni: 2.50~4.75%, Cu: 0.20~0.60%, Nb: 0.020~0.080%, Ti: 0.015~0.036%, Ca: 0.0020~0.0050%, P: ≤0.020%, S: ≤0.002%, N: ≤0.004%, the balance is Fe and other inevitable impurities; the steel of the present application has excellent corrosion resistance in marine atmosphere, high strength and low yield ratio, and can be used to manufacture various bridges in marine atmosphere environment, and the safety is greatly improved. The limitations of this patent mainly lie in: the production of 420MPa weathering steel needs tempering heat treatment, and the steel grade does not realize the use of non-coating in corrosion resistance.

[0008] Chinese patent application number CN202211198488.4 discloses "a marine atmospheric corrosion resistant structural steel plate and preparation method", its chemical composition and mass percentage are as follows: C: 0.04%~0.08%, Si: 0.15%~0.35%, Mn: 0.40%~0.90%, P: ≤0.015%, S ≤0.002%, Nb: 0.020%~0.040%, Ti: 0.010%~0.020%, V: 0.010%~0.030%, Cu: 0.30%~0.60%, Ni: 1.00%~1.20%, Mo: 0.15%~0.40%, Alt: 0.02%~0.04%, the balance is Fe and inevitable inclusions. The steps are as follows: smelting, continuous casting, soaking, rolling, relaxation, cooling and offline tempering. The present application obtains 420MPa grade low yield ratio weathering bridge steel with low yield ratio, high toughness and high ductility through scientific component design and matching controlled rolling and controlled cooling + tempering manufacturing method; the yield strength is above 480MPa, the tensile strength is above 585MPa, the yield ratio is ≤0.85, the Akv at-60℃ is above 250J, the elongation is ≥22%, the corrosion resistance alloy index V is ≥1.20, and the comprehensive performance is good. The limitations of this patent mainly lie in: the production process is complex and needs tempering heat treatment, and the steel grade does not realize the use of non-coating in corrosion resistance.

[0009] At present, although there are marine atmospheric corrosion resistant steels in China, there are still some deficiencies in the research, mainly as follows:

[0010] (1) Using the I value to evaluate the weather resistance of low-alloy steels with high Ni and Cr content is not entirely accurate. The formula for calculating the I value is: I = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.10 (% Ni)(% P) - 33.39 (% Cu) 2 The industry generally recognizes that weather-resistant steel has an I-value ≥ 6.0, with a higher I-value indicating stronger weather resistance. However, the I-value formula is based on a large amount of data published by Larrabee and Coburn, obtained through regression and correction. Therefore, when using this formula, the chemical composition of the steel must meet the range specified in the Larrabee-Coburn test: Cu 0.012–0.510%; Ni 0.05–1.10%; Cr 0.10–1.30%; Si 0.10–0.64%; P 0.01–0.12%. Currently, most publicly available marine atmospheric corrosion-resistant structural steels have Ni and Cr elements exceeding the formula's range. Using the formula outside this range may lead to erroneous conclusions.

[0011] (2) Using accelerated corrosion tests or electrochemical tests to evaluate the weather resistance of steel materials is insufficient to reflect the actual situation. These two methods can produce a "comparative result" in a very short time, which is feasible in the steel composition screening process; however, they cannot truly reflect the corrosion behavior and characteristics of materials in actual environments. Due to the high concentration of Cl in the marine atmosphere... - Due to its high content and high humidity, the corrosive environment is more severe than that of ordinary atmospheric environment. Therefore, to determine whether a material has long-term corrosion resistance, it is necessary to conduct exposure tests in a real environment to evaluate and predict the corrosion status of the material.

[0012] (3) The problem of "corrosion resistance reversal" of Cr in marine environments needs to be addressed in marine atmospheric corrosion-resistant structural steels. Among seawater corrosion-resistant steels, Cr-containing low-alloy steels are an important series, and the corrosion behavior of Cr steels in seawater is complex. In 1970, Southwcll et al. reported that 3% and 5% Cr steels exhibited "corrosion resistance reversal" after long-term exposure in seawater near Naos Island in the Panama Canal Zone. This means that the corrosion rate of Cr steel in seawater is lower than that of carbon steel in the short term, but higher in the long term. Chinese researchers have conducted extensive studies on the seawater corrosion of Cr steel. Long-term exposure tests have revealed that Cr steel exhibits corrosion resistance reversal in the seawater of Qingdao, Sanya, and Zhanjiang. Therefore, caution is needed when considering whether to use Cr-containing low-alloy steels for weathering steel materials used in cross-sea bridges.

[0013] (4) Regarding corrosion resistance, none of the relevant patented products that have been searched so far have the characteristic of being paint-free.

[0014] (5) The addition of corrosion-resistant elements Sb, Sn, and rare earth elements increases the difficulty and cost of steelmaking due to low yields, and is not conducive to green and environmentally friendly manufacturing. Although the addition of Sb, Sn, and rare earth elements in a certain range can effectively improve the corrosion performance of materials, some problems also exist. In steelmaking, Sb element is only added to the molten steel in the ladle in block form during the tapping process of the converter, or added to the molten steel in block form from the vacuum chamber of the RH refining furnace. After the block antimony alloy is added to the molten steel, the antimony alloy is heated and melted, and then dissolved into the molten steel. However, since it takes a long time for the block antimony alloy to completely melt, the solid solution form of antimony in the molten steel is displacement solid solution. If the melting process of the antimony alloy occurs on the surface of the molten steel, the melted antimony alloy will volatilize into the air due to the low boiling point of antimony and its easy oxidation at high temperatures. The existing methods of adding block antimony alloy have low yields, with the Sb yield being about 20-80%. The Sb element released into the air is toxic and will seriously endanger the health of steelmaking workers. For most steel grades, the effect of rare earth element microalloying is unstable, and flocculation and inconsistent yield during steel casting are significant challenges. Rare earth steels, in particular, are prone to nozzle blockage (clogging) and severe low-magnification inclusion defects in billets, leading to a reduced yield. Therefore, incorporating Sb, Sn, and rare earth elements increases the difficulty of the steelmaking process and may even increase smelting costs.

[0015] Sb reduces the strength and increases the brittleness of steel; Sn steel exhibits segregation and grain boundary aggregation during the solidification process of continuously cast billets, which harms the quality and performance of the steel and significantly reduces its mechanical properties. Therefore, to compensate for the performance reduction caused by the addition of Sb and Sn, it is necessary to add other alloying elements. This ultimately leads to an increase in cost.

[0016] In summary, current technologies for research on structural steel resistant to marine atmospheric corrosion are still insufficient. Summary of the Invention

[0017] To address the shortcomings of existing technologies, the present invention aims to prepare a weldable, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate for bridges, buildings, and other steel structures, as well as its manufacturing method. This invention does not add Sb, Sn, rare earth elements, or Cr, reducing smelting difficulty and avoiding the reversal effect of Cr on the steel's corrosion resistance in marine environments. By adding certain amounts of Ni, Cu, Mo, and other elements, and employing controlled rolling and controlled cooling processes, a 420MPa grade weathering steel with excellent strength and toughness can be obtained. The advantages include a low yield strength ratio, excellent resistance to marine atmospheric corrosion, and low-temperature impact resistance, even under appropriate Cl conditions. - It can be used without coating at low deposition levels.

[0018] The objective of this invention is achieved as follows:

[0019] The chemical composition (by weight percentage) of a weldable, paint-free 420MPa grade marine atmospheric corrosion resistant steel plate is as follows: C: 0.02%–0.04%, Si: 0.02%–0.10%, Mn: 0.50%–0.70%, P≤0.012%, S≤0.005%, Ni: 2.5%–3.5%, Cu: 0.35%–0.85%, Mo: 0.15%–0.35%, Nb: 0.005%–0.015%, Ti: 0.012%–0.030%, Al: 0.015%–0.040%, ensuring that 22≤15[Mo]+6[Ni]+5[Cu]≤26 (the values ​​of each element in the formula do not include %, for example, 0.05≤[Mo]≤0.15); the balance is Fe and unavoidable impurities.

[0020] Furthermore, the components satisfy 22≤H≤26, where H=15[Mo]+6[Ni]+5[Cu].

[0021] Furthermore, the component satisfies that the Pcm value (%) ≤ 0.167%, wherein Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0022] Furthermore, the steel plate has a yield strength ≥420MPa, tensile strength ≥540MPa, yield-to-tensile ratio ≤0.72, elongation after fracture ≥20%, impact energy at -80℃ KV2 ≥200J, and finished steel plate thickness ≤60mm.

[0023] Furthermore, the microstructure of the steel plate is ferrite + bainite, with the ferrite microstructure having a volume percentage content of 59% to 68%.

[0024] The rationale for the design of the components in this invention is as follows:

[0025] Carbon (C) can strengthen the steel through interstitial solid solution, effectively improving tensile strength. However, the C content should not be too high, as increased C content is detrimental to toughness, especially low-temperature toughness. The steel of this invention incorporates a certain amount of alloying elements, enhancing strength through solid solution strengthening and precipitation strengthening; therefore, excessive C addition is unnecessary. Furthermore, a lower C content reduces the hardenability of the steel during welding, preventing cracking and improving weldability. Therefore, this invention considers a C content of 0.02%–0.04% to be suitable.

[0026] Si is one of the deoxidizing elements in steel. Si also has a strong solid solution strengthening effect, which can purify ferrite, reduce the content of pearlite, and help reduce the Bauschinger effect in the matrix material. However, excessive Si will worsen the toughness of steel and also reduce its weldability. Given that an appropriate amount of Si can have a beneficial effect on the corrosion resistance of steel, the Si content in this invention is best controlled between 0.02% and 0.10%.

[0027] Mn: It can improve the strength of steel through solid solution strengthening, lower the austenite phase transformation temperature, inhibit the growth of phase transformation grains before accelerated cooling of steel plates, play a role in refining grains, and improve the strength of steel plates; however, excessive Mn content can induce segregation and deteriorate the uniformity of steel plate structure. This invention believes that it is more appropriate to control the Mn content at 0.50% to 0.70%.

[0028] P and S: These are harmful impurity elements in this invention, and the lower the content, the better. Excessive P can lead to microstructure segregation and have a significant adverse effect on low-temperature toughness. In this invention, P is controlled to be ≤0.012%. Increased S content will promote the formation and growth of inclusions and deteriorate low-temperature performance. Therefore, S ≤0.005%.

[0029] Ni (Ni) is a relatively stable element. Adding Ni can shift the self-corrosion potential of steel in a positive direction, increasing its stability. This invention has shown through experiments that Ni is an effective alloying element for resisting marine atmospheric corrosion, with significant effects observed at a content of approximately 1.0–3.5%, providing resistance to various atmospheric corrosions. Under high-salt environmental conditions and after long-term natural exposure, when the Ni content reaches approximately 1.5%, the average corrosion depth decreases substantially. This is because the Ni enriched in the stabilizing rust layer effectively inhibits Cl-. - The intrusion of ions promotes the formation of a protective rust layer and reduces the corrosion rate of steel. In addition, the addition of Ni can avoid the hot working cracking problem of Cu and can also improve the low-temperature toughness of steel. This invention suggests that controlling the Ni content at 2.5% to 3.5% is more suitable.

[0030] Cu (Cu): As the most important alloying element in weathering steel, its purpose is to improve the steel's corrosion resistance. Adding Cu to steel results in superior corrosion resistance compared to ordinary carbon steel in rural, industrial, and marine atmospheres. It is worth noting that Cu significantly counteracts the harmful effects of sulfur (S) in steel; however, the Cu content should not be too high. During high-temperature heating (1100–1200℃), copper-containing steel is susceptible to selective oxidation of the base iron, leading to the accumulation of a layer of liquid copper beneath the iron oxide layer. This "copper-rich liquid phase" penetrates along the austenite grain boundaries, easily causing surface cracks during rolling. Given the significant effect of Cu in reducing the corrosion rate of steel, this invention suggests that controlling the Cu content between 0.35% and 0.85% is most suitable.

[0031] Mo (Mo) is an alloying element that effectively improves resistance to atmospheric corrosion. When steel contains an appropriate amount of Mo, the corrosion rate can be reduced by more than half in atmospheric corrosion environments (including industrial, marine, and rural atmospheres, especially industrial atmospheres). Adding Mo can also effectively improve the pitting corrosion resistance of steel. In marine environments, Mo decomposes from the steel to form molybdate ions, which react with Cl-, which is also an anion. - This generates a repulsive effect, inhibiting Cl from an electrochemical perspective. - Mo can effectively eliminate temper brittleness, giving it good impact toughness, while also improving the hardenability of the steel. Considering both strength and durability, this invention suggests that controlling the Mo content between 0.15% and 0.35% is most suitable.

[0032] Nb: The functions of Nb in this invention include (1) precipitation strengthening, precipitation during rolling and before accelerated cooling, pinning grain boundaries, promoting nucleation, effectively refining grains, thereby improving strength and toughness; (2) reducing the austenite phase transformation temperature, which can refine grains; however, excessive Nb content will increase costs, and this invention believes that controlling the Nb content at 0.005% to 0.015% is more appropriate.

[0033] Ti can exert a solidification effect on nitrogen, forming a TiN-dominant precipitate that inhibits austenite grain growth under high-temperature conditions. Due to its low solid solubility, Ti readily precipitates as interphase during the austenite-ferrite transformation, thus improving strength. Ti can form fine TiC and TiN compounds in weld metal, which promote grain refinement and improve the plasticity and toughness of the weld metal; however, excessive Ti will reduce the toughness of the steel. This invention suggests that controlling the Ti content between 0.012% and 0.030% is more suitable.

[0034] Al (Al) is a strong deoxidizing element and can combine with nitrogen (N) to form AlN, which can refine grains, improve low-temperature impact toughness, and lower the brittle transition temperature of steel. When the Al content exceeds 0.040%, 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 such as Ti will form oxides. This invention suggests that the Al content should be controlled between 0.015% and 0.040%.

[0035] The composition ratio of weather-resistant elements Mo, Ni, and Cu in the 420MPa grade marine atmospheric corrosion-resistant steel described in this invention must ensure the following relationship: H = 15[Mo] + 6[Ni] + 5[Cu], and 22 ≤ H ≤ 26.

[0036] To ensure that the steel plate has a strength of 420MPa and good weldability, its Pcm value (%) is ≤0.167% and C is ≤0.04%, where Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0037] The second technical solution of the present invention is to provide a method for manufacturing an easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion resistant steel plate, including smelting, continuous casting of slabs, slow cooling of slabs, heating of slabs, controlled rolling, and controlled cooling.

[0038] 1) Smelting: During refining, the RH degassing time of this invention is controlled, with an RH vacuum circulation time ≥15min. Through prolonged vacuum treatment, the [N] content of the molten steel can be controlled to ≤0.0040%, [O] ≤0.0010%, and [H] ≤0.00015%. The continuous casting features of this patent are: target superheat in the tundish controlled to ≤35℃; full-process protective casting; and control of the continuous casting billet pulling speed to 0.6~0.8m / min. The billet thickness is 200~300mm. Electromagnetic stirring or light reduction is used during continuous casting to reduce center segregation. To avoid billet cracking, the continuously cast billets are stacked and slowly cooled after leaving the production line, or slowly cooled in a slow cooling pit, with the aim of effectively removing hydrogen content and casting internal stress from the billet.

[0039] 2) Slab Heating: The slab is placed into the heating furnace at a temperature of 650–750℃ to ensure consistent internal and external temperatures during the low-temperature stage, preparing for uniform microstructure in the high-temperature section. During subsequent heating, the heating rate is controlled at 4–6℃ / min to prevent uneven heating due to excessively rapid heating. The continuous casting slab heating section temperature is 1200–1240℃, and the soaking section temperature is 1150–1190℃, with a soaking time of 3.7–5.3 hours. This allows for the complete dissolution of C and N compounds, especially Nb and Mo, preventing abnormal growth of the as-cast microstructure. Simultaneously, it ensures sufficient austenite grain growth, providing adequate deformation dynamics for austenite deformation. The billet size design should ensure that the width of the slab after transfer does not exceed the length of the rolling mill rolls.

[0040] 3) Rolling: In the recrystallization zone rolling stage, a longitudinal-transverse rolling process is adopted. The initial rolling temperature is 1100–1130℃, the cumulative reduction rate in longitudinal rolling is 27%–50%, and the cumulative reduction rate in transverse rolling is 20%–32%. The final rolling temperature in the recrystallization zone is ≥950℃, and the intermediate slab thickness is 2.5–3 times the finished product thickness. The purpose of using longitudinal and transverse rolling is to improve the banded structure and texture orientation generated inside the steel plate during rolling, and to weaken the influence of the rolling structure on the transverse and longitudinal mechanical properties. High-temperature transverse rolling can improve the as-cast structure of the slab. The rolling temperature and deformation process in the recrystallization zone rolling stage cause austenite grains to recrystallize and inhibit grain growth. Utilizing the superimposed effect of multi-pass high reduction rate deformation, the recrystallization of austenite is promoted, achieving the grain refinement target. The initial rolling temperature in the non-recrystallization zone is controlled at 800℃~890℃, the reduction rate per rolling pass is ≥10%, the cumulative reduction rate is ≥60%, the final rolling temperature is 800℃~830℃, and the finished product thickness is ≤60mm.

[0041] 4) Controlled Cooling Process Characteristics: After rolling, water cooling is used for cooling. The initial cooling temperature is 780℃~820℃, and the accelerated cooling rate is ≥6℃ / s. Compared with direct air cooling after rolling, accelerated cooling can reduce the tendency of austenite growth, resulting in finer austenite grains and improving the strength and toughness of the steel plate. After the final cooling temperature reaches 500℃~550℃, the steel plate is air-cooled to room temperature. The higher final cooling temperature not only ensures that the steel plate has a certain proportion of ferrite soft phase, but also achieves the purpose of self-tempering, eliminating stress and eliminating the need for subsequent tempering. After the above accelerated cooling process, the steel plate can form approximately 32%~41% bainite structure, thereby improving the matrix strength. At the same time, the final cooling temperature of this invention is also very suitable, avoiding excessive straightening force during hot straightening of the steel plate, which could lead to poor plate shape. The final microstructure of the steel plate of this invention is ferrite + bainite, wherein the ferrite structure has a volume percentage content of 59%~68%.

[0042] The beneficial effects of this invention are as follows:

[0043] The composition of this invention improves the toughness of the material through a low C and low Mn design, and uses Ti element to inhibit austenite grain growth and promote nucleation during austenite transformation to refine the grains, reduce segregation, and improve the uniformity of the microstructure; coupled with the corresponding production process, it solves the problem of achieving high strength while maintaining good weather resistance and low-temperature toughness.

[0044] 1. The smelting and continuous casting process of this invention achieves low P and low S control, improving the quality of the cast billet and thus enhancing the performance of the final product. Furthermore, a reasonable cooling process is adopted after rolling to compensate for the insufficient strength caused by low C and Mn content. Ultimately, a weldable, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate with a yield strength ≥420MPa, tensile strength ≥540MPa, yield-to-tensile ratio ≤0.72, elongation after fracture ≥19%, and -80℃ KV2 impact energy ≥200J is obtained. The microstructure of the steel plate is ferrite + bainite, with a ferrite volume percentage of 59%–68%.

[0045] 2. Based on the technical characteristics of the steel plate of this invention, the alloy element ratio selected through long-term field exposure tests is combined with the TMCP process, resulting in steel plates with excellent resistance to marine atmospheric corrosion. Through a reasonable ratio of corrosion-resistant elements Cu, Ni, and Mo, the material exhibits excellent resistance to marine atmospheric corrosion, especially in Cl... - In a marine atmospheric environment with a sedimentation amount ≤0.61mdd, the corrosion rate of the steel of this invention in a marine atmospheric environment over a one-year period was 0.019mm / a~0.029mm / a, as determined by exposure tests.

[0046] The atmospheric corrosion development of steel follows a power function law, and the following formula is used to predict the corrosion rate:

[0047] Y = A·X B

[0048] In this invention, value A represents the corrosion rate in a marine atmospheric environment over a one-year period, and is mainly related to the environment; value B characterizes the corrosion development trend. After obtaining the values ​​A and B of the material corrosion life prediction curve, the corrosion reduction thickness of the material after 50 years is predicted. The results show that the corrosion depth of the steel of this invention is <0.12mm after 50 years. Therefore, according to the Japanese "Construction Guidelines for Unpainted Weathering Steel Bridges," the corrosion amount of the steel plate thickness is <0.4mm over 50 years, indicating that the steel grade of this invention can be used without painting in this environment.

[0049] 3. The material of this invention does not contain Sn, Sb, or rare earth elements, and the total upper limit of precious alloying elements Ni, Cu, Mo, and Nb does not exceed 4.715%, which reduces alloy costs and facilitates smelting and production. In particular, it avoids the toxic pollution caused by the addition of Sb, which is conducive to green and environmentally friendly manufacturing. Furthermore, the production process of the marine atmospheric corrosion resistant structural steel described in this invention is simple. Attached Figure Description

[0050] Figure 1 This is the corrosion morphology after one year of outdoor exposure in Example 1 of the present invention.

[0051] Figure 2This is the metallographic structure of Example 1 of the present invention. Detailed Implementation

[0052] The present invention will be further illustrated below through examples.

[0053] The effects of alloying elements in this invention are divided into two categories. First, they enhance the strength of the material. Elements such as C, Mn, Nb, and Ti improve the strength of steel through solid solution and precipitation. Nb precipitation during rolling and before accelerated cooling inhibits grain growth, thereby improving strength and toughness. Ti inhibits austenite grain growth during billet heating and refines grains through its nucleation-promoting effect during austenite transformation. Second, they improve the material's weather resistance.

[0054] First, the composition ratio of weather-resistant elements Mo, Ni, and Cu in the steel of this invention must ensure the following relationship: H = 15[Mo] + 6[Ni] + 5[Cu], and 22 ≤ H ≤ 26 to achieve high weather resistance. Second, in terms of composition design, the toughness of the material is improved by using low C and low Mn, and the grains are refined by using Nb and Ti elements to inhibit austenite grain growth and promote nucleation during austenite transformation; a reasonable Ni, Cu, and Mo ratio is used to improve the weather resistance of the steel. At the same time, with appropriate smelting, heating, rolling, and cooling production processes, a 420MPa grade high weather-resistant bridge steel for cross-sea bridges is obtained, which combines excellent weather resistance, low yield strength ratio, and low-temperature toughness.

[0055] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, slab continuous casting, slab slow cooling, slab heating, controlled rolling, and controlled cooling.

[0056] Slab heating

[0057] The billet is placed into the heating furnace at a furnace temperature of 650-750℃. The heating rate of the billet during the subsequent heating process is 4-6℃ / min. The temperature of the continuous casting billet heating section is 1200-1240℃, the temperature of the soaking section is 1150-1190℃, and the soaking section holding time is 3.7-5.3h.

[0058] Rolling

[0059] The recrystallization zone rolling stage adopts a longitudinal-transverse rolling process, with an initial rolling temperature of 1100-1130℃, a cumulative reduction rate of 27%-50% in longitudinal rolling, a cumulative reduction rate of 20%-32% in transverse rolling, a final rolling temperature of ≥950℃ in the recrystallization zone, and an intermediate billet thickness of 2.5-3 times the finished product thickness. The initial rolling temperature in the non-recrystallization zone is controlled at 800℃-890℃, with a rolling pass reduction rate of ≥10%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 800℃-830℃.

[0060] Controlled cooling

[0061] After rolling, the steel plate is cooled by water cooling. The initial cooling temperature is 780℃~820℃, the accelerated cooling rate is ≥6℃ / s, and the final cooling temperature is 500℃~550℃. The steel plate is then air-cooled to room temperature.

[0062] Furthermore, the smelting process includes RH refining furnace refining, with an RH vacuum circulation time of ≥15 min. Through long-term vacuum treatment, the molten steel's [N] ≤0.0040%, [O] ≤0.0010%, and [H] ≤0.00015% can be controlled.

[0063] Furthermore, the target superheat of the tundish during the continuous casting process is ≤35℃; the casting process is protected throughout, and the billet casting speed is controlled at 0.6~0.8m / min.

[0064] Furthermore, the thickness of the billet in the continuous casting process is 200-300mm. Electromagnetic stirring or light pressure is used during continuous casting. After the continuous casting billet is removed from the line, it is stacked for slow cooling or slow cooling in a slow cooling pit.

[0065] The chemical composition and alloy index of the embodiments of the present invention are shown in Table 1; the heating process of the corresponding embodiments is shown in Table 2; the rolling and cooling processes of the corresponding embodiments are shown in Table 3; the performance of the corresponding embodiments is shown in Table 4; and the corrosion life prediction results of the corresponding embodiments are shown in Table 5.

[0066] Table 1 Chemical composition (wt%) and alloy index of the embodiments of the present invention

[0067] Examples C Si Mn P S Ni Cu Mo Ti Nb Al H Pcm% 1 0.02 0.03 0.50 0.007 0.003 2.9 0.85 0.15 0.018 0.005 0.025 23.9 0.147 2 0.02 0.02 0.70 0.005 0.002 2.5 0.36 0.35 0.012 0.010 0.027 22.1 0.139 3 0.04 0.04 0.62 0.010 0.002 3.0 0.49 0.30 0.016 0.015 0.015 25.0 0.167 4 0.03 0.08 0.60 0.005 0.002 3.0 0.57 0.30 0.030 0.005 0.040 25.4 0.161 5 0.04 0.10 0.53 0.006 0.002 3.5 0.35 0.21 0.027 0.010 0.030 25.9 0.160 6 0.03 0.10 0.59 0.006 0.002 3.1 0.46 0.19 0.021 0.010 0.020 23.8 0.150

[0068] Table 2 Heating process of embodiments of the present invention

[0069]

[0070] Table 3 Rolling and Cooling Processes of Embodiments of the Invention

[0071]

[0072] Table 4 Performance of the embodiments of the present invention

[0073]

[0074] The atmospheric corrosion development of steel follows a power function law, and the following formula is used to predict the corrosion rate:

[0075] Y = A·X B

[0076] Where X represents the exposure period (years), Y represents the average reduction in plate thickness (mm), and A and B are constant coefficients that vary depending on the environment and steel composition. The A value corresponds to the corrosion rate in the first year and is mainly related to the environment; the B value characterizes the corrosion development trend. After obtaining the A and B values ​​of the material corrosion life prediction curve, the corrosion reduction thickness of the material after 50 years is predicted, as shown in Table 5.

[0077] Table 5. Corrosion Life Prediction of Easy-to-Weld, Uncoated 420MPa Grade Marine Atmospheric Corrosion Resistant Steel Plates

[0078] Examples Chloride deposition amount mdd A value (mm / a) B value 50 year forecast mm 1 0.61 0.021 0.297 0.067 2 0.61 0.029 0.361 0.119 3 0.61 0.022 0.378 0.097 4 0.61 0.020 0.343 0.077 5 0.61 0.019 0.356 0.076 6 0.61 0.020 0.288 0.062

[0079] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

[0080] Based on the technical characteristics of the steel plate of this invention, the alloy element ratio selected through long-term field exposure tests is combined with the TMCP process, resulting in a steel plate with excellent resistance to marine atmospheric corrosion, excellent low-temperature toughness, low yield strength ratio, and low Pcm value. The steel plate of this invention has a yield strength ≥420MPa, tensile strength ≥540MPa, yield strength ratio ≤0.72, elongation after fracture ≥19%, and an impact energy of -80℃ KV2 ≥200J. It is an easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion resistant steel plate. The microstructure of the steel plate is ferrite + bainite, with a ferrite volume percentage of 59%–68%. Simultaneously, the product has green and environmentally friendly characteristics, and can be used without coating when the chloride ion deposition is not higher than 0.61mdd. Therefore, the product has a high cost-performance ratio and market competitiveness, and possesses high technical trade value.

Claims

1. A weldable, paint-free 420MPa grade marine atmospheric corrosion resistant steel plate, characterized in that, The composition, by weight percentage, comprises the following components: C: 0.02%–0.03%, Si: 0.02%–0.10%, Mn: 0.50%–0.70%, P ≤ 0.012%, S ≤ 0.005%, Ni: 2.5%–3.5%, Cu: 0.46%–0.85%, Mo: 0.19%–0.35%, Nb: 0.005%–0.010%, Ti: 0.016%–0.030%, Al: 0.015%–0.030%, with the balance being Fe and unavoidable impurities; the components satisfy 22 ≤ H ≤ 26, where H = 15[Mo] + 6[Ni] + 5[Cu]. The method for manufacturing a weldable, paint-free 420MPa grade marine atmospheric corrosion resistant steel plate includes smelting, continuous casting of slabs, slow cooling of slabs, heating of slabs, controlled rolling, and controlled cooling. Slab heating The billet is placed into the heating furnace at a furnace temperature of 650-750℃. The heating rate of the billet during the subsequent heating process is 4-6℃ / min. The temperature of the continuous casting billet heating section is 1200-1240℃, the temperature of the soaking section is 1150-1190℃, and the soaking section holding time is 3.7-5.3h. Rolling The recrystallization zone rolling stage adopts a longitudinal-transverse rolling process, with an initial rolling temperature of 1100-1130℃, a cumulative reduction rate of 27%-50% in longitudinal rolling, a cumulative reduction rate of 20%-32% in transverse rolling, a final rolling temperature of ≥950℃ in the recrystallization zone, and an intermediate billet thickness of 2.5-3 times the finished product thickness. The initial rolling temperature in the non-recrystallization zone is controlled at 800℃-890℃, with a rolling pass reduction rate of ≥10%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 800℃-830℃. Controlled cooling After rolling, the steel plate is cooled by water cooling. The initial cooling temperature is 780℃~820℃, the accelerated cooling rate is ≥6℃ / s, and the final cooling temperature is 500℃~550℃. The steel plate is then air-cooled to room temperature.

2. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The component satisfies the condition that the Pcm value (%) is ≤0.167%, where Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

3. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The steel plate has a yield strength ≥420MPa, tensile strength ≥540MPa, yield ratio ≤0.72, elongation after fracture ≥20%, impact energy at -80℃ KV2 ≥200J, and finished steel plate thickness ≤60mm.

4. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The microstructure of the steel plate is ferrite + bainite, with the ferrite content ranging from 59% to 68% by volume.

5. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The smelting process includes refining in an RH refining furnace, with an RH vacuum circulation time of ≥15 min. Through long-term vacuum treatment, the molten steel's [N] ≤0.0040%, [O] ≤0.0010%, and [H] ≤0.00015% can be controlled.

6. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The target superheat of the tundish during the continuous casting process is ≤35℃; the casting process is protected throughout, and the casting speed of the continuous casting billet is controlled at 0.6~0.8m / min.

7. The easy-to-weld, paint-free 420MPa grade marine atmospheric corrosion-resistant steel plate according to claim 1, characterized in that, The continuous casting process involves a billet thickness of 200-300 mm. Electromagnetic stirring or light pressure is used during continuous casting. After the billet is removed from the production line, it is stacked for slow cooling or slow cooling in a slow cooling pit.

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