Low-cost 590mpa high-strength weathering steel under marine environment

Abstract

The application provides a kind of low-cost 590MPa high-strength weathering steel under marine environment, and particularly relates to the field of low-alloy steel.The chemical components include the following mass percentages:C:0.05-0.07%;Si:0.10-0.20%;Mn:0.5-1.6%;P:0.025-0.08%;S:≤0.002%;Nb:0.01-0.015%;Ti:0.01-0.02%;Mo:0.1-0.3%;Cr:0.25-0.8%;Ni:0.3-0.5%;Cu:0.1-0.32%;B:0.002-0.003%;Sc:0.001-0.003%;Als:0.001-0.01%, and the balance is Fe.The application reduces the content of Cr, Ni, Mo, Ti and Mn in the composition, increases trace B, Sc and Al, adjusts the component ratio of alloy elements, increases the strength of the material, adopts subsequent heat treatment method, and through tempering and corresponding forging and rolling process, finally makes the metallographic structure of the material be lath bainite structure+martensite structure+small amount of residual austenite structure, so that the metallographic structure of the metal material reaches the optimal state, the element distribution and effect are more reasonable, the corrosion resistance is improved and the structural strength is strengthened.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel, specifically relating to a low-cost, high-strength weathering steel with a strength of 590MPa for marine environments. Background Technology

[0002] Weathering steel, also known as low-alloy atmospheric corrosion resistant steel, is made by adding small amounts of alloying elements such as Cu, Cr, Ni, P, Mn, and Sn to low-carbon steel to improve its atmospheric corrosion resistance. Currently, due to the expanding applications of weathering steel, there is a need to develop more types of weathering steel to meet the demands of different application environments.

[0003] Marine environments, due to their proximity to the sea, are characterized by high humidity and salinity. They also feature a combination of high humidity, high heat, high salinity, and strong ultraviolet radiation, creating extremely harsh corrosive conditions. Ordinary carbon steel corrodes very quickly, severely shortening equipment lifespan. Especially near the coastline, sea breezes bring large amounts of salt and chloride ions, with Cl- deposition near the shore reaching up to 2 mdd. Simultaneously, the high temperature and heat environment causes rapid wet-dry cycles on material surfaces, placing even stricter demands on corrosion resistance. Therefore, the corrosion resistance of low-alloy structural steel becomes a crucial indicator in steel structure design for this region. Weathering steel resistant to marine corrosion has been a key research focus, and several high-performance weathering steels have been developed both domestically and internationally, such as the applications filed by the Iron and Steel Research Institute: "A High-Strength Weathering Steel Resistant to High Humidity and Heat in Marine Atmospheres" (CN106756602A), "High-Strength Weathering Steel Resistant to High Humidity and Heat in Marine Atmospheres and its Preparation Method" (CN106756476A), and "A Weathering Steel Used in Marine Environments" (CN107488818A). All three patents mimic the 3wt% Ni content of 3wt% Ni + 0.4wt% Cu weathering steel, improving the new steel's resistance to marine atmospheric corrosion. These new materials contain relatively high levels of precious metals, such as Ni, significantly increasing material costs. In low-alloy weathering steel, elements like Ni, Mo, and Ti effectively improve weather resistance, but their relatively high prices increase processing and manufacturing costs. Reducing the content of these relatively expensive elements inevitably affects the mechanical properties and corrosion resistance of weathering steel.

[0004] Balancing cost and performance requires a change in the design approach for weathering steel. Therefore, it is urgent to further optimize the alloy ratio of the material and temper it through subsequent heat treatment to improve the overall performance of the material and reduce its cost. Summary of the Invention

[0005] The purpose of this invention is to provide a novel, low-cost, high-strength weathering steel with a 590MPa strength suitable for marine environments, exhibiting excellent resistance to marine atmospheric corrosion, high strength, and good corrosion resistance. Through alloy ratio adjustment and optimization, and subsequent heat treatment and other processing steps, a high-performance weathering steel is obtained.

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

[0007] A low-cost, 590MPa high-strength weathering steel for marine environments comprises the following chemical composition by mass percentage: C: 0.05–0.07%; Si: 0.10–0.20%; Mn: 0.5–1.6%; P: 0.025–0.08%; S: ≤0.002%; Nb: 0.01–0.015%; Ti: 0.01–0.02%; Mo: 0.1–0.3%; Cr: 0.25–0.8%; Ni: 0.3–0.5%; Cu: 0.1–0.32%; B: 0.002–0.003%; Sc: 0.001–0.003%; Als: 0.001–0.01%, with the balance being Fe.

[0008] Furthermore, the following forging and rolling process is adopted, which includes the following steps:

[0009] (1) Weathering steel billet is obtained by smelting according to the chemical composition, and then forged into square billet;

[0010] (2) Heat the forged billet to the austenitizing temperature, hold it at the temperature to make the billet completely austenitized; forge and roll it into a forging billet or plate.

[0011] (3) Use a rapid cooling system to cool the forging billet or plate obtained in step (2) to room temperature, then heat and hold the forging billet or plate to room temperature;

[0012] (4) Heat the forging billet or plate obtained in step (3) and keep it warm, then cool it to room temperature in the furnace to obtain low-cost 590MPa high-strength weathering steel for marine environments.

[0013] Furthermore, in step (2), the heating temperature is 1180℃~1220℃, and the holding time is 0.5h~1h.

[0014] Furthermore, in step (3), the heating temperature is 700-750℃ and the holding time is 60-90min.

[0015] Furthermore, in step (3), the rapid cooling system has a cooling rate of 40-60℃ / s for the plate and 50-70℃ / s for the forging billet. The rapid cooling system uses a water cooling system.

[0016] Furthermore, in step (4), the temperature of heating the forged billet or plate obtained in step (3) and holding it is 680℃±20℃, and the holding time is 1h.

[0017] Furthermore, the low-cost 590MPa high-strength weathering steel obtained in step (4) in the marine environment has a granular bainitic structure + martensite structure + a small amount of retained austenite structure.

[0018] The beneficial effects of this invention are:

[0019] 1. This invention provides a new composition ratio of low-cost 590MPa high-strength weathering steel for marine environments. Compared with ordinary Q500 steel, its corrosion rate is reduced by 5% to 15%, achieving excellent atmospheric corrosion resistance.

[0020] 2. The weathering steel of the present invention can be used in hot and humid environments with bare surfaces or light coatings, resulting in low maintenance costs, long product lifespan, and reduced overall lifespan costs.

[0021] 3. This invention reduces the content of Cr, Ni, Mo, Ti, and Mn in the composition, increases trace amounts of B, Sc, and Al, adjusts the composition ratio of alloying elements, increases the strength of the material, and adopts subsequent heat treatment methods. Through modulated heat treatment and corresponding forging and rolling processes, the final metallographic structure of the material is lath bainite + martensite + a small amount of retained austenite, so that the metallographic structure of the metal material reaches the optimal state, the element distribution and function are more reasonable, and the corrosion resistance and structural strength are improved. Attached Figure Description

[0022] Figure 1 The corrosion rate of Examples 1-4 and Comparative Example 1 (Q500NH) varies with time.

[0023] Figure 2 The material is the one obtained in Example 1.

[0024] Figure 3 Metallographic photograph of Q500NH material.

[0025] Figure 4 The material is obtained by processing component 3 in Comparative Example 2. Detailed Implementation

[0026] The specific embodiments are provided to further illustrate the present invention, but are not intended to limit the scope of the invention. Unless otherwise specified, the materials involved in the specific embodiments are commercially available products.

[0027] Carbon (C) is an effective strengthening element in steel, and increasing the C content is beneficial to improving strength. However, excessively high carbon content can lead to the precipitation of carbide particles, reducing plasticity and toughness. Therefore, the C mass percentage content in this invention is designed to be 0.05–0.07 wt%.

[0028] In steel metallurgy, Si plays a similar role to P, both shrinking the γ-phase region and forming a γ-phase ring. Its solid solution strengthening effect on ferrite is second only to P. Si also increases the resistivity of steel and enhances its corrosion resistance under natural conditions. It synergistically improves the corrosion resistance of steel with other alloying elements (such as Cu, Cr, P, Ni, etc.). However, excessively high Si content makes descaling during rolling difficult and also leads to decreased weldability. Therefore, the Si mass percentage content designed in this invention is 0.10–0.20 wt%.

[0029] Mn has a strong solid solution strengthening effect, can significantly reduce the phase transformation temperature of steel, and refine the microstructure of steel, making it an important strengthening and toughening element. Mn can improve the corrosion resistance of steel in marine atmospheres, but excessive Mn can easily cause billet cracks during casting and also reduce the weldability of steel. Therefore, the Mn mass percentage in this invention is designed to be 0.5%–1.6%.

[0030] An appropriate phosphorus (P) content can significantly improve the atmospheric corrosion resistance of steel. When P and Cu are added to steel together, a better synergistic weather resistance effect is observed. However, excessively high P content will significantly reduce the plasticity and low-temperature toughness of steel. Therefore, the P mass percentage in this invention is designed to be 0.025–0.08%.

[0031] Cr can improve the hardness and wear resistance of high-carbon steel without making it brittle; at higher contents, it gives the steel good high-temperature oxidation resistance and resistance to corrosion in oxidizing media, and also increases the steel's thermal strength. Therefore, the Cr mass percentage in this invention is designed to be 0.25%–0.8%.

[0032] The primary role of Cu in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steels, especially when used in combination with P. High Cu content is detrimental to hot deformation and can lead to copper embrittlement during hot working. Therefore, this invention designs the Cu mass percentage to be 0.1–0.32%.

[0033] In this invention, the proportions of the main basic elements of weathering steel are controlled within a reasonable range. Cr:Mn:Cu:P≈10:20:5:1, wherein the P content is not higher than 0.08%.

[0034] S is a poor weather-resistant element, and its proportion in the alloy should be minimized. Therefore, the mass percentage of S in this invention is designed to be ≤0.002%.

[0035] Nitrogen (Nb) is both a strong ferrite-forming element and a strong carbonitride compound. Under prolonged heating, it readily forms metallic compounds, which enhance mechanical properties and refine grain size. Therefore, the Nb mass percentage in this invention is designed to be 0.01–0.015%.

[0036] Ti is a strong carbide-forming element and also a strong ferrite-forming element, which can significantly improve the plasticity and toughness of low-alloy steel. Therefore, the mass percentage of Ti in this invention is designed to be 0.01–0.02%.

[0037] Mo in steel can improve hardenability and hot strength, prevent temper brittleness, effectively inhibit cementite aggregation at 450–600℃, and promote the precipitation of special carbides. However, Mo is expensive and its content is relatively high. Therefore, the Mo mass percentage in this invention is designed to be 0.1–0.3%.

[0038] Ni strengthens ferrite and refines pearlite in steel, increasing its strength while having less impact on toughness, plasticity, and other processing properties compared to other alloying elements. Higher Ni content can significantly improve the steel's resistance to marine atmospheric corrosion. However, higher Ni content also increases material cost. Therefore, this invention uses a Ni mass percentage of 0.3% to 0.5%.

[0039] The main function of boron (B) in steel is to increase its hardenability, thereby saving on other rarer and more expensive metals, such as nickel, chromium, and molybdenum. Therefore, the mass percentage of boron in this invention is designed to be 0.002–0.003%.

[0040] Sc belongs to the rare earth transition element group and can effectively enhance the strength and hardness of alloys, while also improving their thermal stability and wear resistance. This element is abundant but its distribution is relatively dispersed. Therefore, the mass percentage of Sc in this invention is designed to be 0.002–0.003%.

[0041] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention shall not be limited to the scope of the embodiments described herein.

[0042] Example 1

[0043] (1) Steel billets are obtained by smelting according to the chemical composition of component 1 in Table 1, and then forged into square billets;

[0044] (2) Heat the forged billet to the austenitizing temperature of 1200℃ and hold for 0.8h to make it fully austenitized; then forge and roll it into a plate.

[0045] (3) Use a water-cooled rapid cooling system to cool the plate to room temperature at 60℃ / s, then heat the plate to 750℃ and hold for 90 minutes, and quench it to room temperature.

[0046] (4) Reheat to 680℃ and hold for 1 hour, then cool to room temperature with the furnace.

[0047] The final sheet microstructure consists of lath bainite, martensite, and a small amount of retained austenite.

[0048] Example 2

[0049] (1) Steel billets are obtained by smelting according to the chemical composition of component 1 in Table 1, and then forged into square billets;

[0050] (2) Heat the forged billet to the austenitizing temperature of 1180℃ and hold for 0.5h to make it fully austenitized; then forge and roll it into a plate.

[0051] (3) Use a water-cooled rapid cooling system to cool the plate to room temperature at 40℃ / s, then heat the plate to 710℃ and hold for 60 minutes, and quench it to room temperature.

[0052] (4) Reheat to 700℃ and hold for 1 hour, then cool to room temperature with the furnace.

[0053] The final sheet microstructure consists of lath bainite, a small amount of granular bainite, martensite, and a small amount of retained austenite.

[0054] Example 3

[0055] (1) Steel billets are obtained by smelting according to the chemical composition of component 1 in Table 1, and then forged into square billets;

[0056] (2) Heat the forged square billet to the austenitizing temperature of 1200℃ and hold for 1 hour to make it fully austenitized; then forge and roll it into a steel billet.

[0057] (3) The steel billet was cooled to room temperature at 70℃ / s using a water-cooled rapid cooling system. Then the steel billet was heated to 750℃ and held for 90 minutes, and then quenched to room temperature.

[0058] (4) Reheat to 660℃ and hold for 1 hour, then cool to room temperature with the furnace.

[0059] The final steel billet microstructure consists of lath bainite, martensite, and a small amount of retained austenite.

[0060] Example 4

[0061] (1) Steel billets are obtained by smelting according to the chemical composition listed in component 2 of Table 1, and then forged into square billets;

[0062] (2) Heat the forged billet to the austenitizing temperature of 1220℃ and hold for 1 hour to make it fully austenitized; then forge and roll it into a thin plate.

[0063] (3) Use a water-cooled rapid cooling system to cool the weathering steel to room temperature at 60℃ / s, then heat the weathering steel to 720℃ and hold for 70 minutes, and quench it to room temperature;

[0064] (4) Reheat to 680℃ and hold for 1 hour, then cool to room temperature with the furnace.

[0065] The final material microstructure consists of lath bainite, martensite, and a small amount of retained austenite.

[0066] Comparative Example 1

[0067] Ordinary weathering steel Q500NH (produced by Nanjing Iron & Steel Co., Ltd.) was purchased externally. The specific material composition ratio is shown in Table 1 (Q500). According to the product description, the processing and performance of the purchased material conform to the national standard "GB / T4171 Weathering Structural Steel". Before leaving the factory, it undergoes a two-stage rolling process: the roughing temperature is controlled at 1200℃, and the finishing temperature is controlled at 1100℃; the finishing rolling temperature is controlled at 950℃, and then rapidly cooled to 600℃ at a rate of 20℃ / s, followed by air cooling to room temperature. The resulting material microstructure is bainitic + ferrite.

[0068] Comparative Example 2

[0069] Steel billets were obtained by smelting according to the chemical composition listed in component 3 of Table 1, and then forged into square billets. The billets were then subjected to heat treatment according to the method for Q500NH in the comparative example, i.e., the rough rolling temperature was controlled at 1200℃, the finishing temperature at 1100℃, the initial rolling temperature at 950℃, and rapid cooling to 600℃ at 20℃ / s, followed by air cooling to room temperature. The resulting material microstructure was granular bainite + ferrite.

[0070] According to ISO 16539-2013, indoor accelerated dry-wet alternation simulation tests were conducted on Examples 1-4 and Comparative Example 1 to compare their corrosion resistance performance relative to Comparative Example 1. The specific experimental procedures are as follows:

[0071] (I) Weigh the initial mass of the sample. The analytical balance used is a Sartorius BS224S with an accuracy of d = 0.1 mg.

[0072] (II) Place the test sample in the PR-2KP constant temperature and humidity test chamber for 30 minutes for pre-humidification. The temperature inside the test chamber is 35℃ and the relative humidity is 90%.

[0073] (III) Remove the test sample from the test chamber and apply 40 μL / cm to its surface. 2 Add the corrosion simulation solution dropwise and spread it evenly. The simulated marine environment used in this experiment is a 3wt% NaCl aqueous solution, which takes about 10 minutes.

[0074] (IV) The test sample is returned to the test chamber for 344 minutes, ensuring that the total time of the first stage is 6.4 hours. This process simulates a high temperature and high humidity environment.

[0075] (V) The chamber temperature is controlled at 40℃ and the relative humidity at 40% for 1.6 hours via program settings, simulating a dry environment. The total time is 8 hours, representing one test cycle. The wet-dry environment time ratio is 4:1, simulating the atmospheric corrosion environment of the South China Sea with a relative humidity of 80%.

[0076] (VI) Every 3 test cycles, i.e., after 24 hours, the test sample is removed and washed with distilled water to prevent salt particles from accumulating on the surface of the test sample. After the surface of the test sample is dry, steps (III), (IV), and (V) are repeated until the set wet-dry alternation cycle is reached. The test cycle is 30 days, totaling 720 hours. The corrosion weight loss measurement times are: 120, 240, 360, 480, 600, and 720 hours, for a total of 6 measurements.

[0077] The average corrosion rate was determined by calculating the corrosion weight loss per unit area of ​​three test samples. The reduction in corrosion rate of the example samples relative to the comparative samples was then calculated.

[0078] Table 1 Chemical composition of high-strength weathering steel (wt.%)

[0079]

[0080] like Figure 1 As shown in the average corrosion rate curves of Examples 1-4 and Comparative Example 1, the corrosion rates of the materials in Examples 1-4 and Comparative Example 1 both show a trend of first increasing and then decreasing. After 720 hours of corrosion testing, the corrosion rate of the materials in Examples 1-4 is lower than that of the material in Comparative Example 1.

[0081] Strength measurements showed that the tensile strength of the materials in Examples 1-4 was ≥590 MPa. The tensile strength of Comparative Example 1 was 500 MPa.

[0082] Figure 2 The material is the same as that obtained in Example 1. Metallographic examination showed that it consisted of lath bainite, martensite, and a small amount of retained austenite.

[0083] Figure 3 The image shows a metallographic photograph of Q500NH material. The results indicate a bainitic + ferrite structure.

[0084] Figure 4 This is the material obtained from component 3 in Comparative Example 2 after processing. Phase analysis shows it to be a granular bainitic structure + ferrite structure.

[0085] Matters not covered in this invention are common knowledge.

[0086] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A low-cost, 590MPa high-strength weathering steel for marine environments, characterized in that, The chemical composition includes the following percentages by mass: C: 0.05~0.07%; Si: 0.10~0.20%; Mn: 0.5~1.6%; P：0.025~0.08%； S: ≤0.002%; Nb: 0.01~0.015%; Ti: 0.01~0.02%; Mo: 0.1~0.3%; Cr：0.25~0.8%； Ni: 0.3~0.5%; Cu: 0.1~0.32%; B: 0.002~0.003%; Sc: 0.001~0.003%; Als: 0.001~0.01%, balance Fe; The forging and rolling process is as follows, which includes the following steps: (1) Weathering steel billets are obtained by smelting according to the chemical composition, and then forged into square billets; (2) Heat the forged billet to the austenitizing temperature, hold it at the temperature to make the billet completely austenitized; forge and roll it into a forging billet or plate. (3) Use a rapid cooling system to cool the forging billet or plate obtained in step (2) to room temperature, then heat and hold the forging billet or plate to room temperature; (4) Heat the forging billet or plate obtained in step (3) and keep it at a certain temperature, then cool it to room temperature in the furnace to obtain low-cost 590MPa high-strength weathering steel for marine environments; Step (4) The final low-cost 590MPa high-strength weathering steel obtained in the marine environment has a granular bainitic structure + martensite structure + a small amount of retained austenite structure.

2. The low-cost 590MPa high-strength weathering steel for marine environments according to claim 1, characterized in that, Step (2) The heating temperature is 1180℃~1220℃, and the holding time is 0.5h~1h.

3. The low-cost 590MPa high-strength weathering steel for marine environments according to claim 1, characterized in that, Step (3) The heating temperature is 700-750℃ and the holding time is 60-90min.

4. The low-cost 590MPa high-strength weathering steel for marine environments according to claim 1, characterized in that, Step (3) Rapid cooling system: The cooling rate of the plate is 40-60℃ / s and the cooling rate of the forging billet is 50-70℃ / s. The rapid cooling system adopts a water cooling system.

5. The low-cost 590MPa high-strength weathering steel for marine environments according to claim 1, characterized in that, Step (4) Heat the forged billet or plate obtained in step (3) at a temperature of 680℃±20℃ and hold for 1 hour.

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