Low yield ratio uncoated 345mpa grade marine atmospheric corrosion resistant steel plate and method of manufacturing the same

Abstract

The application provides a low yield ratio 345MPa-grade marine atmosphere corrosion resistant steel plate without coating and a manufacturing method thereof. The steel plate comprises the following components in percentage by weight: C: 0.02%-0.04%, Si: 0.10%-0.30%, Mn: 0.30%-0.50%, P: ≤0.012%, S: ≤0.005%, Ni: 2.8%-3.5%, Cu: 0.35%-0.85%, Mo: 0.05%-0.15%, Ti: 0.006%-0.03%, Al: 0.015%-0.040%, and the balance of iron and inevitable impurities. 22≤15[Mo]+6[Ni]+5[Cu]≤26, and the Pcm value (%) is ≤0.16%. The production method of the steel plate comprises smelting, slab continuous casting, slab heating, controlled rolling, and controlled cooling. The yield strength of the steel plate is ≥345MPa, the tensile strength is ≥490MPa, the yield ratio is ≤0.77, the elongation after fracture is ≥20%, the KV2 impact energy at-80℃ is ≥200J, and the thickness of the steel plate is ≤64mm, and the steel plate can be used without coating under the condition that the chloride ion deposition amount is not higher than 0.61mdd.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and in particular relates to a low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant structural steel for bridges and buildings, and its manufacturing method. Background Technology

[0002] With the development of bridge construction in China, numerous coastal and cross-sea bridge projects are under construction or planned. Due to the high chloride corrosion environment at the bridge sites, conventional steel materials are limited in their use, thus increasing the demand for bridge steel resistant to marine atmospheric corrosion. - High concentrations of certain substances, coupled with fluctuating ambient temperature and humidity, can damage the passivation film on the steel surface, leading to severe corrosion.

[0003] The main load-bearing components of bridges are mostly steel structures. In marine environments, corrosion can cause thinning of the structure and even stress corrosion cracking, all of which significantly impact the safe service life of the steel structure. In recent years, there has been a growing demand for minimal maintenance of bridge steel structural components, making paint-free corrosion-resistant steel a focus of attention. The need to reduce corrosion rates through a stable protective rust layer, thereby eliminating the need for paint, is urgent. Currently, there is some research on structural steel resistant to marine atmospheric corrosion both domestically and internationally, and some patents have been found. However, the content described in these patents differs significantly from the technical solution of this invention in terms of composition, production methods, performance, and product categories.

[0004] Chinese Patent Application No. CN202011325158.8 discloses "A Structural Steel for Marine Atmospheric Environments and Its Production Method," which provides a structural steel suitable for marine atmospheric environments, belonging to the field of steel plate production technology. The chemical composition and mass fraction of the 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 selectively includes any one or two of tin (0.01%–0.02%) and vanadium (0.05%–0.1%), with the remainder being iron and unavoidable impurities. This structural steel exhibits excellent corrosion resistance, effectively improving the lifespan and safety of steel structure buildings. The limitations of this patent are mainly reflected in the following aspects: the composition contains Sb and Sn elements, making smelting difficult and not environmentally friendly. The I-index is used to evaluate weather resistance, but due to the limited range of application of the formula, it can lead to significant deviations. Furthermore, the steel grade does not achieve paint-free application in terms of corrosion resistance.

[0005] Chinese patent application CN202110952035.5 discloses "a weathering structural steel with a stabilized rust layer and its preparation method," whose chemical composition and mass percentages are: C≤0.15%, Si≤0.20%, Mn 0.80~1.20%, P0.040~0.070%, S≤0.003%, Ni 0.35~0.45%, Cu 0.22~0.32%, Ce 0.015~0.030%, Sb The steel contains 0.02% to 0.10% Fe and unavoidable impurities. The preparation process involves first producing steel plates through smelting, rolling, and cooling, followed by stabilization of the rust layer. This invention features a simple manufacturing process, low production cost, environmental friendliness, and high operability. It can quickly and economically form weather-resistant structural steel with a stabilized rust layer, with the rust layer forming within the steel structure manufacturing cycle, meeting the manufacturing requirements of structural components such as buildings, towers, and offshore platforms. The limitations of this patent are mainly reflected in the presence of Sb and Ce elements, resulting in a high smelting difficulty and a lack of environmental friendliness. Furthermore, the steel does not achieve paint-free use in terms of corrosion resistance.

[0006] Chinese patent application CN201310115964.6 discloses "a structural steel plate with excellent resistance to marine atmospheric corrosion and its production method". The chemical composition, by weight percentage, is: C 0.001-0.08%, Si 0.10-0.50%, Mn 0.50-1.60%, P≤0.080%, S≤0.025%, Ni 0.01-3.50%, Mo 0.01-0.50%, Cu 0.01-1.20%, Nb 0.001-0.060%, Ca 0.0005-0.0040%, Al 0.010-0.080%, N≤0.0080%, with the balance being Fe and unavoidable impurities. The steel plate also contains one or more elements selected from at least one group of the following groups (1) to (2): (1) one or more elements of Cr 0.01 to 5.50%, RE 0.001 to 0.080%, Sb 0.005 to 0.10%; (2) one or more elements of Ti 0.001 to 0.090%, V 0.001 to 0.080%. A controlled rolling and cooling process is adopted, with the pre-rolling continuous casting billet heating temperature between 1100℃ and 1250℃, the roughing finishing temperature between 990℃ and 1080℃, and the finishing rolling starting temperature between 850℃ and 970℃; laminar flow cooling is adopted after rolling, with a final cooling temperature of 250℃ to 750℃ and a cooling rate of 5℃ to 40℃ / s. This invention ensures that the steel grade has excellent resistance to the marine atmospheric environment by using low-carbon and alloyed components. The limitations of this patent are mainly reflected in the large differences in composition, and the protected components contain elements such as Cr, RE, and V. The addition of Cr reverses the corrosion of the steel plate in a long-term marine environment, while the addition of RE increases the difficulty of smelting. The steel grade does not achieve paint-free use due to its corrosion resistance.

[0007] Chinese patent application CN201511009728.1 discloses "A corrosion-resistant bridge bearing steel suitable for marine environments," characterized in that the steel composition comprises, by mass percentage: C: 0.08–0.15%, Si: 0.16–1.0%, Mn: 0.5–1.6%, Cr: 0.60–11.0%, Ni: 0.30–2.0%, Al: 0.01–0.5%, P≤0.015%, S≤0.015%, Cu≤0.80%, V≤0.06%, and Mo≤0.35%, with the balance being Fe and unavoidable impurity elements. The steel undergoes the following heat treatment process: heating in a furnace from room temperature to 860–950°C, holding at that temperature for 3.0–5.0 hours, and then cooling to room temperature. By adjusting the composition ratio and heat treatment process, the corrosion resistance and mechanical properties are improved, meeting the engineering requirements for different corrosion resistance and strength grades. The limitations of this patent are mainly reflected in the following aspects: Firstly, it describes a cast steel production process, with significantly different composition and processes compared to this patent. Secondly, the patent uses a high carbon (C) composition and adds chromium (Cr). The high C content reduces the steel's impact toughness and weldability, and the addition of Cr causes the steel plate's corrosion resistance to reverse under long-term marine corrosion. Thirdly, subsequent heat treatment is required, increasing the production process. Finally, the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0008] Chinese patent application CN202010017496.9 discloses "marine weathering bridge steel plate," whose smelting composition, by mass percentage, is controlled within the following ranges: C: 0.05–0.11%, Si: 0.15–0.5%, Mn: 1.10–1.50%, P: ≤0.009%, S: ≤0.005%, Nb: 0.020–0.035%, Cu: 0.30–0.80%, Mo: 0.15–0.20%, Ni: 1.10–1.3%. The composition is 0%, N: ≤0.008%, Als: 0.015~0.050%, V: ≤0.10%, Ti: ≤0.04%. This invention's product exhibits excellent fatigue resistance, corrosion resistance, weldability, and low notch sensitivity. It can replace imported 1Ni steel plates and is a key steel grade used in spherical bearings for coastal bridges and other marine bridge equipment and key components in my country, significantly reducing production and construction costs. Therefore, this invention's product can be widely applied in the field of marine alloy steel plates. The limitations of this patent are mainly reflected in: differences in composition compared to this patent; to ensure strength, high Mn content is used, and Nb and V elements are added. Furthermore, the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0009] Although China currently possesses marine atmospheric corrosion-resistant steels, research on them is still insufficient, mainly in the following aspects:

[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 low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel plate for bridges, buildings, and other steel structures, and 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 steel 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 345MPa grade weathering steel with excellent strength and toughness can be obtained. The advantages of this invention are that the material has a low yield strength ratio, excellent resistance to marine atmospheric corrosion, and low-temperature impact resistance, and 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 low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant structural steel is as follows: C: 0.02%–0.04%, Si: 0.10%–0.30%, Mn: 0.30%–0.50%, P≤0.012%, S≤0.005%, Ni: 2.8%–3.5%, Cu: 0.35%–0.85%, Mo: 0.05%–0.15%, Ti: 0.006%–0.03%, Al: 0.015%–0.040%, and 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 iron and unavoidable impurities.

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

[0021] Furthermore, the component satisfies the condition that the Pcm value is ≤0.16%, and Pcm(%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0022] Furthermore, the microstructure of the steel plate is ferrite + bainite, wherein the volume percentage content of ferrite is 85% to 90%.

[0023] Furthermore, the steel plate has a yield strength ≥345MPa, tensile strength ≥490MPa, yield-to-tensile ratio ≤0.77, elongation after fracture ≥20%, impact energy at -80℃ KV2 ≥200J, and a thickness ≤64mm.

[0024] Furthermore, the corrosion rate of the steel plate in a marine atmospheric environment over a one-year period is 0.021 mm / a to 0.039 mm / a.

[0025] The rationale for the design of the components in this invention is as follows:

[0026] 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.

[0027] Si (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 deteriorate the toughness of steel. 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 controlled at 0.10% to 0.30%.

[0028] 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 0.30% to 0.50%.

[0029] 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%.

[0030] 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. Furthermore, the addition of Ni can prevent the hot working cracking problem of Cu and improve the low-temperature toughness of the steel. This invention suggests controlling the Ni content at 2.8%–3.5%.

[0031] 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 recommends controlling the Cu content between 0.35% and 0.85%.

[0032] 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 0.05% and 0.15% is most suitable.

[0033] Ti can solidify nitrogen, forming a TiN-dominant precipitate that inhibits austenite grain growth at high temperatures. Due to its low solid solubility, Ti readily precipitates as interphase during the austenite-ferrite transformation, thus increasing strength. However, excessive Ti reduces the toughness of the steel. This invention suggests that controlling the Ti content between 0.006% and 0.030% is most 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 345MPa grade marine atmospheric corrosion resistant steel described in this invention shall have the following composition ratio of weather-resistant elements Mo, Ni and Cu: H = 15[Mo] + 6[Ni] + 5[Cu], and shall ensure that 22 ≤ H ≤ 26.

[0036] To ensure good weldability, the Pcm value (%) should be ≤0.16% and C ≤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 a low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel, including smelting, slab continuous casting, slab heating, 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 the solid solution of 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) Controlled 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%–34%. The final rolling temperature in the recrystallization zone is ≥980℃, 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 ≥8%, the cumulative reduction rate is ≥60%, the final rolling temperature is 770℃~800℃, and the finished product thickness is ≤64mm.

[0041] 4) Controlled Cooling: Water cooling is used after rolling, with an initial cooling temperature of 680℃~740℃. During the relaxation and warm-up stage before initial cooling, some ferrite structure will form in the steel plate, mainly to ensure that the steel plate has high plasticity and toughness. 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 350℃~450℃, the steel plate is air-cooled to room temperature. After the above accelerated cooling process, the steel plate can form about 10%~15% 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 would cause poor plate shape. The final microstructure of the steel plate of this invention is ferrite + bainite, wherein the volume percentage content of ferrite structure is 85%~90%.

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

[0043] This invention improves material toughness through a low-C and low-Mn design, refines grains by utilizing Ti to inhibit austenite grain growth and promote nucleation during austenite transformation, reduces segregation, and improves microstructure uniformity. Combined with appropriate production processes, it solves the challenge of achieving high strength while maintaining good weather resistance and low-temperature toughness. The smelting and continuous casting process of this invention achieves low P and low S control, improving billet quality and thus enhancing the performance of the final product, resulting in excellent low-temperature toughness and a low yield strength ratio.

[0044] 1. This invention employs a reasonable cooling process after rolling, combined with solution strengthening, to compensate for the insufficient strength caused by low C and Mn content. The final product is a low yield-to-strength ratio, paint-free, marine atmospheric corrosion-resistant structural steel with a yield strength ≥345MPa, tensile strength ≥490MPa, yield-to-strength ratio ≤0.77, elongation after fracture ≥20%, and -80℃ KV2 impact energy ≥200J.

[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 0.021mm / a~0.039mm / 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] The A value represents the corrosion rate of the present invention in a marine atmospheric environment over a one-year period, which is mainly related to the environment; the B value represents 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, and the result predicts that the corrosion depth of the steel of the present invention is <0.13mm after 50 years.

[0049] Therefore, according to the Japanese "Construction Guidelines for Unpainted Weathering Steel Bridges," which stipulates that the corrosion rate of the steel plate thickness over 50 years must be <0.4 mm, the steel grade of this invention can be used without painting in this environment. The product of this invention has green and environmentally friendly characteristics and can be used without painting when the chloride ion deposition rate is no higher than 0.61 mdd.

[0050] 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, and Mo does not exceed 4.5%, 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. The production process of the marine atmospheric corrosion resistant structural steel described in this invention is simple. Attached Figure Description

[0051] Figure 1 This is the corrosion morphology after one year of outdoor exposure in Example 4 of the present invention.

[0052] Figure 2 This is the metallographic structure of Example 4 of the present invention. Detailed Implementation

[0053] The present invention will be further illustrated below through examples.

[0054] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, slab continuous casting, slab heating, controlled rolling, and controlled cooling.

[0055] Slab heating

[0056] The billet is placed into the heating furnace at a furnace temperature of 650-750℃, and the heating rate is controlled at 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.

[0057] Controlled rolling

[0058] 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%-34% in transverse rolling, and a final rolling temperature of ≥980℃ in the recrystallization zone. The intermediate billet thickness is 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 ≥8%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 770℃-800℃.

[0059] Controlled cooling

[0060] After rolling, water cooling is used for cooling. The initial cooling temperature is 680℃~740℃, the cooling rate is ≥6℃ / s, the final cooling temperature is 350℃~450℃, and then air cooling is used.

[0061] Furthermore, the target superheat of the tundish during the continuous casting process is controlled at ≤35℃; the entire process is protected during casting; the billet thickness during the continuous casting process is 200-300mm; electromagnetic stirring or light reduction technology 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.

[0062] The chemical composition of the embodiments of the present invention is 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.

[0063] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0064] Example C Si Mn P S Ni Cu Mo Ti Al H Pcm% 1 0.040 0.15 0.34 0.007 0.003 2.8 0.82 0.07 0.006 0.032 22.0 0.154 2 0.025 0.23 0.38 0.008 0.003 3.2 0.38 0.15 0.018 0.030 23.4 0.134 3 0.031 0.10 0.50 0.007 0.002 2.9 0.64 0.10 0.007 0.040 22.1 0.146 4 0.030 0.30 0.34 0.007 0.003 3.5 0.85 0.05 0.030 0.015 26.0 0.160 5 0.020 0.22 0.30 0.008 0.003 3.5 0.35 0.06 0.009 0.033 23.7 0.123

[0065] Table 2 Heating process of embodiments of the present invention

[0066]

[0067] Table 3 Rolling and Cooling Processes of Embodiments of the Invention

[0068]

[0069] Table 4 Performance of the embodiments of the present invention

[0070]

[0071] The atmospheric corrosion development of steel follows a power function law, and the following formula is used to predict the corrosion rate:

[0072] Y = A·X B

[0073] In this embodiment of the invention, value A represents the corrosion rate under a one-year cycle in a marine atmospheric environment, which is mainly related to the environment; value B represents 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 is predicted after 50 years, as shown in Table 5.

[0074] Table 5. Corrosion life prediction of uncoated 345MPa grade marine atmospheric corrosion resistant structural steel with low yield strength ratio.

[0075] Example Chloride ion deposition amount mdd A value (mm / a) B value 50-year projected average reduction in plate thickness (mm) 1 0.61 0.030 0.364 0.125 2 0.61 0.032 0.288 0.099 3 0.61 0.039 0.274 0.114 4 0.61 0.028 0.302 0.091 5 0.61 0.021 0.352 0.114

[0076] 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 to obtain a steel plate with a yield strength ≥345MPa, tensile strength ≥490MPa, yield-to-tensile ratio ≤0.77, elongation after fracture ≥20%, and -80℃ KV2 impact energy ≥200J, with a steel plate thickness ≤64mm. 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. This results in the steel plate having excellent resistance to marine atmospheric corrosion, excellent low-temperature toughness, low yield-to-tensile ratio, and low Pcm value. Therefore, the product has a high cost-performance ratio and market competitiveness, and possesses high technical trade value. In summary, by using a reasonable Ni, Cu, and Mo ratio to improve the weather resistance of the steel, and combining it with appropriate smelting, heating, rolling, and cooling production processes, a 345MPa grade high weather-resistant bridge steel for cross-sea bridges is obtained, possessing excellent weather resistance, low yield-to-tensile ratio, and low-temperature toughness.

[0077] 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.

Claims

1. A low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel plate, characterized in that, By weight percentage, it comprises the following components: C: 0.02%–0.031%, Si: 0.10%–0.23%, Mn: 0.30%–0.38%, P≤0.012%, S≤0.005%, Ni: 2.8%–3.5%, Cu: 0.64%–0.85%, Mo: 0.05%–0.10%, Ti: 0.018%–0.03%, Al: 0.015%–0.040%, with the balance being iron and unavoidable impurities; The component satisfies 22≤ H ≤26, H =15[Mo]+6[Ni]+5[Cu]; The manufacturing method of the low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel plate includes smelting, slab continuous casting, slab heating, controlled rolling, and controlled cooling. Slab heating The billet is placed into the heating furnace at a furnace temperature of 650-750℃, and the heating rate is controlled at 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. Controlled 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%-34% in transverse rolling, and a final rolling temperature of ≥980℃ in the recrystallization zone. The intermediate billet thickness is 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 ≥8%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 770℃-800℃. Controlled cooling After rolling, water cooling is used. The initial cooling temperature is 680℃~740℃, the cooling rate is ≥6℃ / s, and the final cooling temperature is 350℃~450℃, followed by air cooling. The microstructure of the steel plate is ferrite + bainite, wherein the volume percentage of ferrite is 85%~90%. The steel plate has a yield strength ≥345MPa, tensile strength ≥490MPa, yield ratio ≤0.77, elongation after fracture ≥20%, and a temperature of -80℃. KV 2 Impact energy ≥200J, steel plate thickness ≤64mm; The corrosion rate of the steel plate in a marine atmospheric environment over a one-year period is 0.021~0.039 mm / a.

2. The low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel plate according to claim 1, characterized in that, The components satisfy Pcm Value ≤ 0.16%, Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 +Cr / 20 + Mo / 15 + V / 10 + 5B.

3. The low yield strength ratio, uncoated 345MPa grade marine atmospheric corrosion resistant steel plate according to claim 1, characterized in that, During continuous casting, the target superheat of the tundish is controlled at ≤35℃; the entire process is protected during casting; the thickness of the billet during continuous casting is 200-300mm; electromagnetic stirring or light reduction technology is used during continuous casting; after the billet is removed from the line, it is stacked for slow cooling or slow cooling in a slow cooling pit.

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