Low yield ratio uncoated 500mpa grade marine atmospheric corrosion resistant steel and method of manufacturing the same

Abstract

The application provides a kind of low yield ratio 500MPa grade marine atmospheric corrosion resistant steel and its manufacturing method, steel plate composition is according to percentage by weight: C:0.04%~0.06%, Si:0.10%~0.30%, Mn:0.60%~0.80%, P≤0.012%, S≤0.005%, Ni:2.5%~3.0%, Cu:0.35%~0.75%, Mo:0.35%~0.55%, Nb:0.03%~0.04%, Ti:0.006%~0.03%, Al:0.015%~0.040%, the balance is iron and inevitable impurities.The said component satisfies 24≤15[Mo]+ 6[Ni]+ 5[Cu]≤26, Pcm Value (%) ≤0.198%. The production method of the steel plate includes smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling, heat treatment, the yield strength of the above-mentioned steel plate is greater than or equal to 500MPa, the tensile strength is greater than or equal to 630MPa, the elongation after fracture is greater than or equal to 18%, the impact energy at-80 ℃ KV 2 The steel plate can be used without coating under the condition that the thickness of the steel plate is less than or equal to 64mm, 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, paint-free, 500MPa grade marine atmospheric corrosion resistant structural steel for bridges and buildings. 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 CN202210444394.4 discloses "A weather-resistant and fire-resistant structural steel for marine environments in the South China Sea". The chemical composition (wt%) of this structural steel is: C: ≤0.12%; Si: 0.1-0.3%; Mn: 0.5-1.7%; P: ≤0.06%; S: ≤0.003%; Nb: 0.01-0.04%; Ti: ≤0.03%; Mo: ≤0.4%; Cr: 0.1-0.5%; Ni: 0.2-0.5%; Cu: 0.2-0.5%; B: 0-0.003%; Als < 0.02%, with the balance being Fe. The low-alloy steel of this invention can achieve a strength level of 460 MPa and exhibits superior fire resistance and corrosion resistance compared to ordinary Q460 steel, with a corrosion rate reduced by 5%-15%. With excellent performance, it can be widely used in construction, bridge building, and transportation, and has good application value. The limitations of this patent are mainly reflected in the following: the addition of Cr element causes the corrosion resistance of the steel plate to reverse under long-term corrosion in marine environments, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0006] Chinese patent application CN202210570652.3 discloses "A production method for Q550 grade high corrosion-resistant and high-strength near-shore structural steel". The process steps are: hot metal pretreatment → converter steelmaking → ladle refining (LF) → vacuum treatment (RH) → continuous casting → heating → rolling → controlled rolling cooling → tempering → finishing → performance testing → ultrasonic flaw detection. The chemical composition of the steel is as follows (mass percentage): C=0.03~0.07, Si=0.15~0.40, Mn=1.20~1.30, 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, Cr=0.15~0.30, with the remainder being Fe and unavoidable impurity elements. Suitable for production of materials with a thickness ≤60mm, featuring common technologies for resistance to marine atmospheric corrosion, yield strength ≥550MPa, tensile strength ≥660MPa, and low-temperature impact toughness at -60℃. KV 2 ≥120J, low-temperature impact toughness at -60℃ at 1 / 2 thickness of steel plate. KV 2 Structural steel plates with a performance requirement of ≥120J and a lamellar tear resistance Z≥35%. The limitations of this patent are mainly reflected in the fact that the addition of Cr element causes the corrosion resistance of the steel plate to reverse under long-term corrosion in a marine environment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0007] Chinese patent application CN202210570654.2 discloses a "production method of Q500 grade high corrosion-resistant and high-strength near-shore structural steel". The process steps are: hot metal pretreatment → converter steelmaking → ladle refining (LF) → vacuum treatment (RH) → continuous casting → heating → rolling → controlled rolling cooling → tempering → finishing → performance testing → ultrasonic flaw detection. The steel's mass percentage composition is: C=0.03~0.07, Si=0.15~0.40, Mn=1.20~1.30, 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, Cr=0.15~0.30, with the remainder being Fe and unavoidable impurity elements. The steel plates produced have a thickness ≤60mm and possess common technologies for resistance to marine atmospheric corrosion environments. They exhibit a yield strength ≥500MPa, tensile strength ≥600MPa, low-temperature impact toughness (-60℃) KV2 ≥120J, low-temperature impact toughness (at 1 / 2 thickness) KV2 ≥120J at -60℃, and resistance to lamellar tearing (Z ≥35%). The limitations of this patent are mainly reflected in the following: the addition of Cr causes a reversal of corrosion resistance in the steel plate under long-term corrosion in a marine environment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0008] Chinese patent application CN202110711870.X discloses "a marine atmospheric corrosion resistant steel and its manufacturing method". This steel, in addition to Fe and unavoidable impurity elements, contains the following chemical elements in the following mass percentages: C: 0.02–0.2%, Si: 0.2–0.8%, P: 0.001–0.03%, Mn: 0.4–1.5%, Cu: 0.05–0.4%, Cr: 0.8–9.0%, Mo: 0.05–0.3%, Al: 0.03–0.05%; wherein the marine atmospheric corrosion resistant steel does not contain Ni. Furthermore, this invention also discloses a manufacturing method for the above-mentioned marine atmospheric corrosion resistant steel, comprising the steps of: (1) smelting and casting; (2) heating; (3) rolling, including rough rolling and finish rolling; and (4) air cooling to room temperature. The marine atmospheric corrosion-resistant steel described in this invention not only has low production costs but also excellent resistance to marine atmospheric corrosion. It can be effectively applied in industries and fields such as coastal construction, factory structures, cross-sea bridges, and offshore platforms, offering significant economic and social benefits. The limitations of this patent are mainly reflected in the following: the addition of chromium causes a reversal of corrosion resistance in the steel plate under long-term corrosion in a marine environment, and the steel does not achieve paint-free use due to its corrosion resistance.

[0009] Chinese patent application number CN202110726233.X discloses "a high-performance steel plate resistant to marine atmospheric corrosion and its production method". The weight percentage of the chemical composition of the steel plate is as follows: C: 0.11%~0.14%, Si: 0.51%~0.80%, Mn: 0.50%~1.00%, P: 0.020%~0.040%, S: ≤0.005%, V: 0.055%~0.065%, Ti: 0.020%~0.040%, Cr: 0.40%~0.60%, Cu: 0.15%~0.35%, Sb: 0.03%~0.05%, Zr: 0.02%~0.04%, Als: 0.015%~0.045%. The furnace temperature for billet loading is 700–800℃, and the soaking zone temperature is 1200–1220℃. The roughing rolling initial temperature is 1180–1200℃, the reduction rate per pass is 15%–25%, and the roughing rolling speed is 1.0–1.2 m / s. The intermediate billet thickness is 1.5–2 times the finished product thickness for preheating, the secondary rolling temperature is 910–920℃, the final rolling temperature is 860–890℃, and after rolling, it undergoes laminar flow cooling with an initial cooling temperature of 820–840℃ and a reheating temperature of 620–640℃. The steel plate produced by this invention exhibits a seawater corrosion resistance rate of less than 0.10 mm / a, a yield strength of 500–550 MPa, and an impact energy of greater than or equal to 200 J at -60℃. The limitations of this patent are mainly reflected in the presence of sulfur (Sb) in the composition, which increases the difficulty of smelting and is not environmentally friendly. The addition of Cr causes the steel plate to undergo a reversal of corrosion resistance during long-term corrosion in a marine environment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0010] Although China currently possesses marine atmospheric corrosion-resistant steels, research on them is still insufficient, mainly in the following aspects:

[0011] (1) Using the I value to evaluate the weather resistance of low-alloy steels with high Ni and Cr content is not entirely correct. 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) 2The 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 after exceeding these ranges may lead to erroneous conclusions.

[0012] (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.

[0013] (3) The problem of "corrosion resistance reversal" of Cr element under marine atmospheric corrosion needs to be addressed in marine 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 showed "corrosion resistance reversal" after long-term exposure in seawater near Naos Island in the Panama Canal Zone. That is, 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 a lot of research on the seawater corrosion of Cr steel. Through long-term exposure tests, it was found 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 steel materials for weathering steel used in cross-sea bridges.

[0014] (4) Regarding corrosion resistance, none of the relevant patented products that have been searched so far have the characteristic of being paint-free.

[0015] (5) The addition of corrosion-resistant elements Sb, Sn, and rare earth elements increases the difficulty and cost of steelmaking due to low yield, 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, there are also some problems. 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 temperature. The existing method of adding block antimony alloy has a low yield, 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.

[0016] 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.

[0017] In summary, current technologies for research on structural steel resistant to marine atmospheric corrosion are still insufficient. Summary of the Invention

[0018] To address the shortcomings of existing technologies, the present invention aims to provide a low yield strength ratio, paint-free 500MPa grade marine atmospheric corrosion-resistant steel 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 500MPa grade weather-resistant 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.

[0019] The objective of this invention is achieved as follows:

[0020] The chemical composition (by weight percentage) of a low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant structural steel is as follows: C: 0.04%–0.06%, Si: 0.10%–0.30%, Mn: 0.60%–0.80%, P≤0.012%, S≤0.005%, Ni: 2.5%–3.0%, Cu: 0.35%–0.75%, Mo: 0.35%–0.55%, Nb: 0.03%–0.04%, Ti: 0.006%–0.03%, Al: 0.015%–0.040%, ensuring that 24≤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.

[0021] Furthermore, the component satisfies 24≤ H ≤26, of which H =15[Mo]+6[Ni]+5[Cu].

[0022] Furthermore, the components satisfy... PCM Value (%) ≤ 0.198%, of which PCM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0023] Furthermore, the microstructure of the steel plate is ferrite + bainite, wherein the volume percentage of ferrite is 45%~54%.

[0024] Furthermore, the steel plate has a yield strength ≥ 500 MPa, tensile strength ≥ 630 MPa, elongation after fracture ≥ 18%, and a temperature resistance of -80℃. KV 2 Impact energy ≥200J, steel plate thickness ≤64mm.

[0025] Furthermore, the corrosion rate of the steel plate in a marine atmospheric environment over a one-year period is 0.019 mm / a to 0.037 mm / a.

[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 hardenability during welding, preventing cracking and improving weldability. Therefore, this invention considers a C content of 0.04%–0.06% to be suitable.

[0027] Si is one of the deoxidizing elements in steel. Si also has a strong solid solution strengthening effect, which can purify ferrite and reduce the content of pearlite, thus helping to 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 best controlled between 0.10% and 0.30%.

[0028] Mn: It can improve the strength of steel through solid solution strengthening, reduce 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 0.60% to 0.80%.

[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 at ≤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 significantly. 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 that controlling the Ni content between 2.5% and 3.0% is most suitable.

[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 suggests that controlling the Cu content between 0.35% and 0.75% is most suitable.

[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 0.35% and 0.55% is most suitable.

[0033] Nb: The functions of 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. This invention believes that in order to achieve a strength index of 500MPa, it is more appropriate to control the Nb content at 0.03% to 0.04%.

[0034] 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.

[0035] 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%.

[0036] The 500MPa 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 ensure that 24≤ H ≤26.

[0037] To ensure good weldability, PCM Value (%) ≤ 0.198%, and C ≤ 0.06%, where PCM(%) =C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0038] The second technical solution of the present invention is to provide a method for manufacturing a low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel, including smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling, and heat treatment.

[0039] 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.

[0040] 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.

[0041] 3) Rolling: In the recrystallization zone, a longitudinal-transverse rolling process is employed. The initial rolling temperature is 1100–1150℃, with a cumulative reduction rate of 29%–52% in longitudinal rolling and 17%–30% in transverse rolling. 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 longitudinal and transverse rolling is to improve the banded structure and texture orientation generated within the steel plate during rolling, weakening the influence of the rolled structure on the longitudinal and transverse 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 cause austenite grains to recrystallize and inhibit grain growth. Utilizing the cumulative effect of multi-pass high-reduction deformation, austenite recrystallization is promoted, achieving the grain refinement target. The initial rolling temperature in the non-recrystallization zone is controlled at 810℃~890℃, the reduction rate per rolling pass is ≥10%, the cumulative reduction rate is ≥60%, the final rolling temperature is 800℃~850℃, and the finished product thickness is ≤64mm.

[0042] 4) Controlled Cooling Process Characteristics: After rolling, water cooling is used for cooling. The initial cooling temperature is 720℃~810℃. 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 ≥7℃ / s. Compared with direct air cooling after rolling, accelerated cooling can reduce the tendency of austenite growth, resulting in finer austenite grains, thereby improving the strength and toughness of the steel plate. After the final cooling temperature reaches 300℃~350℃, the steel plate is air-cooled to room temperature. Through the above accelerated cooling process, a certain amount of bainite structure can be formed in the steel plate, 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.

[0043] 5) Heat treatment process characteristics: Due to the high strength of 500MPa grade steel, in order to eliminate the internal stress caused by controlled cooling and promote the homogenization of microstructure, tempering heat treatment is performed after rolling. The tempering heating temperature is 430℃~530℃, and the total furnace time is 2.8min / mm~3.2min / mm. The final state microstructure of the steel plate of this invention is ferrite + bainite, wherein the volume percentage content of ferrite is 45%~54%, and the volume percentage content of bainite is 46%~55%.

[0044] The beneficial effects of this invention are as follows:

[0045] 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. In terms of composition design, low C and low Mn improve the material's toughness, while Nb and Ti elements are used to inhibit austenite grain growth and promote nucleation during austenite transformation to refine grains, reduce segregation, and improve microstructure uniformity. Combined with appropriate production processes, this solves the problem of achieving high strength while maintaining good weather resistance and low-temperature toughness. A reasonable Ni, Cu, and Mo ratio enhances the steel's weather resistance. Simultaneously, appropriate smelting, heating, rolling, and cooling processes yield 500MPa grade high weather-resistant bridge steel for cross-sea bridges, possessing excellent weather resistance, low yield strength ratio, and low-temperature toughness.

[0046] This invention employs a reasonable cooling and tempering heat treatment process after rolling to obtain an appropriate amount of (Nb,Ti)(C,N) second-phase precipitates, compensating for the insufficient strength caused by low C and Mn content. The final result is a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥18%, yield-to-tensile ratio ≤0.85, and a strength at -80℃. KV 2 Impact energy ≥200J.

[0047] By employing a rational ratio of corrosion-resistant elements such as Cu, Ni, and Mo, the material exhibits excellent resistance to marine atmospheric corrosion, particularly in Cl... - In a marine atmospheric environment with a sedimentation amount of 0.61 mdd, the corrosion rate of the steel of this invention under a one-year cycle in the marine atmospheric environment, as determined by the exposure test results, is 0.019 mm / a to 0.037 mm / a.

[0048] The atmospheric corrosion development of steel follows a power function law, and the following formula is used to predict the corrosion rate:

[0049] Y = A · X B

[0050] 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 predict that the corrosion depth of the steel of this invention is <0.10mm after 50 years. Therefore, according to the provisions of 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.

[0051] 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 4.34%, 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. At the same time, the production process of the marine atmospheric corrosion-resistant structural steel described in this invention is simple. Attached Figure Description

[0052] Figure 1 This is the corrosion morphology after one year of outdoor exposure in Example 1 of the present invention.

[0053] Figure 2 This is the metallographic structure of Example 1 of the present invention. Detailed Implementation

[0054] The present invention will be further illustrated below through examples.

[0055] 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 promotes nucleation during austenite transformation, thus refining the grains. Second, they improve the weather resistance of the material. Extensive testing has shown that the composite addition of multiple elements is more effective in achieving corrosion resistance than the addition of a single element. Therefore, the proportions of weather-resistant elements Mo, Ni, and Cu in the composition must maintain the following relationship: H =15[Mo]+6[Ni]+5[Cu], and ensure that 24≤ H High weather resistance can be achieved with a temperature of ≤26.

[0056] According to the component ratio of the technical solution, the embodiments of the present invention involve smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling, and heat treatment. The invention is characterized by...

[0057] Slab heating

[0058] The billet is placed into the heating furnace at a furnace temperature of 650-750℃. During the subsequent heating process, the heating rate of the billet is controlled at 4-6℃ / min. The temperature of the heating section of the continuous casting billet is 1200-1240℃, the temperature of the soaking section is 1150-1190℃, and the holding time of the soaking section is 3.7-5.3h.

[0059] Rolling

[0060] The recrystallization zone rolling stage adopts a longitudinal-transverse rolling process, with an initial rolling temperature of 1100-1150℃, a cumulative reduction rate of 29%-52% in longitudinal rolling, a cumulative reduction rate of 17%-30% 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 810℃-890℃, with a rolling pass reduction rate of ≥10%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 800℃-850℃.

[0061] Controlled cooling

[0062] After rolling, the steel plate is cooled by water cooling. The initial cooling temperature is 720℃~810℃, and the accelerated cooling rate is ≥7℃ / s. After the final cooling temperature reaches 300℃~350℃, the steel plate is air cooled to room temperature.

[0063] Heat treatment

[0064] After rolling, the product undergoes tempering heat treatment at a temperature of 430℃ to 530℃, with a total furnace time of 2.8 min / mm to 3.2 min / mm.

[0065] Furthermore, the target superheat of the tundish during the continuous casting process is ≤35℃; the casting process is protected throughout, the casting speed of the continuous casting billet is controlled at 0.6~0.8m / min, the billet thickness is 200~300mm, electromagnetic stirring or light pressing is used during continuous casting, and the continuous casting billet is stacked and slowly cooled or slowly cooled in a slow cooling pit after it leaves the line.

[0066] Furthermore, the smelting process includes RH refining furnace refining, RH vacuum circulation time ≥15min, and through long-term vacuum treatment, the molten steel [N] ≤0.0040%, [O] ≤0.0010%, [H] ≤0.00015%.

[0067] 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.

[0068] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0069]

[0070] Table 1 (continued)

[0071]

[0072] Table 2 Heating process of embodiments of the present invention

[0073]

[0074] Table 3 Rolling and cooling processes of embodiments of the present invention

[0075]

[0076] Table 4 Performance of embodiments of the present invention

[0077]

[0078] The atmospheric corrosion development of steel follows a power function law, and the following formula is used to predict the corrosion rate:

[0079] Y = A · X B

[0080] 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 ​​from the material corrosion life prediction curve, the corrosion reduction thickness of the material is predicted after 50 years, as shown in Table 5.

[0081] Table 5. Corrosion life prediction of low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant structural steel

[0082]

[0083] 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 Values. The steel of this invention has a yield strength ≥ 500 MPa, tensile strength ≥ 630 MPa, elongation after fracture ≥ 18%, yield strength ratio ≤ 0.85, and withstands temperatures down to -80℃. KV 2 Impact energy ≥200J. The product is environmentally friendly and can be used without coating when the chloride ion deposition is no higher than 0.61mdd. Therefore, the product has a high cost-performance ratio and market competitiveness, and possesses significant technical trade value.

[0084] 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, paint-free, 500MPa grade marine atmospheric corrosion-resistant steel, characterized in that, The composition, by weight percentage, comprises the following components: C: 0.04%–0.055%, Si: 0.10%–0.27%, Mn: 0.60%–0.80%, P ≤ 0.012%, S ≤ 0.005%, Ni: 2.5%–3.0%, Cu: 0.35%–0.75%, Mo: 0.35%–0.55%, Nb: 0.03%–0.04%, Ti: 0.006%–0.03%, Al: 0.015%–0.040%, with the balance being iron and unavoidable impurities; the components satisfy 24 ≤ H ≤26, of which H =15[Mo]+6[Ni]+5[Cu]; The manufacturing method of the low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel includes smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling, and heat treatment. Slab heating The billet is placed into the heating furnace at a furnace temperature of 650-750℃. During the subsequent heating process, the heating rate of the billet is controlled at 4-6℃ / min. The temperature of the heating section of the continuous casting billet is 1200-1240℃, the temperature of the soaking section is 1150-1190℃, and the holding time of the soaking section is 3.7-5.3h. Rolling The recrystallization zone rolling stage adopts a longitudinal-transverse rolling process, with an initial rolling temperature of 1100-1150℃, a cumulative reduction rate of 29%-52% in longitudinal rolling, a cumulative reduction rate of 17%-30% 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 810℃-890℃, with a rolling pass reduction rate of ≥10%, a cumulative reduction rate of ≥60%, and a final rolling temperature of 800℃-850℃. Controlled cooling After rolling, the steel plate is cooled by water cooling. The initial cooling temperature is 720℃~810℃, and the accelerated cooling rate is ≥7℃ / s. After the final cooling temperature reaches 300℃~350℃, the steel plate is air cooled to room temperature. Heat treatment After rolling, the product undergoes tempering heat treatment at a temperature of 430℃ to 530℃, with a total furnace time of 2.8 min / mm to 3.2 min / mm.

2. The low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel according to claim 1, characterized in that, The components satisfy Pcm Value (%) ≤ 0.198%, of which Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

3. The low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel according to claim 1, characterized in that, The steel plate has a microstructure of ferrite and bainite, wherein the ferrite microstructure has a volume percentage content of 45% to 54%.

4. The low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel according to claim 1, characterized in that, The steel plate has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥18%, and a temperature of -80℃. KV 2 Impact energy ≥200J, steel plate thickness ≤64mm.

5. The low yield strength ratio, paint-free, 500MPa grade marine atmospheric corrosion-resistant steel according to claim 1, characterized in that, The corrosion rate of the steel plate in a marine atmospheric environment over a one-year period is 0.019 mm / a to 0.037 mm / a.

6. The low yield strength ratio, paint-free, 500MPa grade marine atmospheric corrosion-resistant steel according to claim 1, characterized in that, The target superheat of the tundish during the continuous casting process is ≤35℃; the entire process is protected during casting, the casting speed of the continuous casting billet is controlled at 0.6~0.8m / min, the billet thickness is 200~300mm, electromagnetic stirring or light pressing is used during continuous casting, and the continuous casting billet is stacked and slowly cooled or slowly cooled in a slow cooling pit after it leaves the line.

7. The low yield strength ratio, uncoated 500MPa grade marine atmospheric corrosion resistant steel according to claim 1, characterized in that, The smelting process includes RH refining furnace refining, RH vacuum circulation time ≥15min, and through long-term vacuum treatment, the molten steel [N]≤0.0040%, [O]≤0.0010%, [H]≤0.00015% can be controlled.

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