Weathering steel and its preparation method

Abstract

This invention relates to a weathering steel and its preparation method. The chemical composition of the steel is: C: 0.070%–0.10%, Si: 0.40%–0.60%, Mn: 0.4%–0.6%, P: 0.07%–0.15%, S≤0.006%, Al: 1.2%–3.2%, Cr: 0.20%–0.40%, Ni: 0.03%–0.05%, Cu: 0.20%–0.40%, B: 0.002%–0.005%, N: 0.008%–0.012%, TO: 0.0005%–0.0020%, with the remainder being Fe and impurities. The steel achieves fine-grain strengthening by forming AlN, improving mechanical properties and corrosion resistance. The addition of aluminum provides solid solution strengthening, forming a dense alumina film on the surface of the weathering steel, which, together with chromium oxide, constitutes a dense protective layer, thereby improving the corrosion resistance of the steel.

Description

Technical Field

[0001] This invention relates to the field of weathering steel production technology, and in particular to a weathering steel and its preparation method. Background Technology

[0002] Steel corrosion is a widespread and serious problem that harms all sectors of the national economy and national defense. Statistics show that in some industrialized countries, economic losses due to corrosion account for 2% to 4% of the GDP, with atmospheric corrosion being the primary form of steel structure corrosion, accounting for about half of all corrosion losses. Therefore, the development of highly corrosion-resistant weathering steel is of great significance. It possesses excellent atmospheric corrosion resistance and can be used in the manufacture of equipment such as containers, demonstrating its wide range of applications.

[0003] Weathering steel is a low-alloy steel with excellent corrosion resistance in the atmosphere. Current research, both domestically and internationally, generally agrees that after prolonged exposure to the atmosphere, a dense and well-adhesive oxide layer forms on the surface of high-corrosion-resistant weathering steel, isolating the steel substrate from external corrosive substances and significantly improving its corrosion resistance. In China, weathering steel is mainly used in railway vehicles and containers, while abroad it is more widely used in exposed steel structures and municipal facilities, with bridge construction being its most significant application.

[0004] Chinese patent application No. 202210897106.0 discloses "a weathering steel with high corrosion resistance and its preparation method." The chemical composition and mass percentage of the weathering steel are as follows: C: 0.05-0.12%, Si: 0.20-0.60%, Mn: 0.30-0.60%, P: 0.07-0.15%, S≤0.015%, Cu: 0.25-0.55%, Ni≤0.20%, Cr: 0.30-0.60%, with the balance being Fe and unavoidable impurities. The relative corrosion rate of this weathering steel against atmospheric corrosion is less than 40% of that of Q345B. It adopts a conventional Cu-P-Cr-Ni composition design, requiring only optimization of the heating process to avoid the impact of increased Cu and Cr elements on the steel's hot brittleness and the increase in smelting costs. The production process is relatively easy to control; no increase in alloy costs or production costs is required, allowing for direct industrial production. However, due to the lack of elements that refine the crystallinity in this weathering steel, its yield strength, tensile strength, elongation, and relative corrosion resistance are all relatively low.

[0005] Chinese invention patent application number 202111041519.0 discloses "a high-alumina weathering steel and its preparation method". The high-alumina weathering steel comprises the following elements by mass percentage: Al: 4-11%, Si: 0.2-2%, P: 0.01-0.35%, Cu: 0.15-0.5%, Cr: 0.2-1.5%, Ni: 0.1-5.5%, and the balance Fe. Al, as a strengthening element in weathering steel, is dissolved into the matrix of the weathering steel through solid solution strengthening. During tensile testing, it increases the resistance to dislocation movement by hindering dislocation movement, effectively improving tensile strength. Simultaneously, Al has a strong solid solution strengthening effect in weathering steel, which is beneficial for significantly improving the strength of the weathering steel while effectively refining the rust layer products on the surface of the weathering steel. However, its high aluminum content requires repeated smelting to achieve uniform composition, and it can only be produced by die casting, resulting in low production efficiency and significantly increased production costs. In addition, it fails to effectively utilize the grain-refining effects of nitrogen and aluminum, directly affecting the performance of the steel plate.

[0006] Therefore, there is an urgent need to develop a low-cost weathering steel and its preparation method that can effectively refine grains, produce uniform composition, has a simple process, is safe and reliable, and can improve product quality. Summary of the Invention

[0007] This invention provides a weathering steel and its preparation method. By forming fine and dispersed AlN in the steel, a fine grain strengthening effect is achieved, improving the mechanical properties of the steel. Due to the fine grain effect, the degree of segregation at grain boundaries can also be reduced, thereby improving the corrosion resistance of the steel. By adding metallic aluminum to the steel to achieve solid solution strengthening, a dense alumina film is formed on the surface of the weathering steel, which together with the chromium oxide formed by chromium elements constitutes a dense protective layer, thereby improving the corrosion resistance of the steel.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A weathering steel, wherein the chemical composition of the steel by weight percentage is: C: 0.070%–0.10%, Si: 0.40%–0.60%, Mn: 0.4%–0.6%, P: 0.07%–0.15%, S≤0.006%, Al: 1.2%–3.2%, Cr: 0.20%–0.40%, Ni: 0.03%–0.05%, Cu: 0.20%–0.40%, B: 0.002%–0.005%, N: 0.008%–0.012%, TO: 0.0005%–0.0020%, with the remainder being Fe and unavoidable impurities.

[0010] A method for preparing weathering steel, wherein the preparation of the steel plate includes converter smelting, LF refining, continuous casting and hot rolling processes, wherein the following processes are controlled:

[0011] (1) Converter smelting: The final carbon content of the converter is controlled at 0.04% to 0.06%, the final slag basicity is controlled at 2.7 to 3.5, the final phosphorus content is controlled at 0.04% to 0.08%, and the final active oxygen content is controlled at 0.03% to 0.05%; the tapping temperature of molten steel is controlled at 1645 to 1665℃;

[0012] (2) LF refining: The binary basicity of the refining slag is controlled at 2.4 to 3.0, and the active oxygen in the molten steel is controlled at less than 3 ppm;

[0013] (3) Continuous casting: The superheat of molten steel is controlled at 20-25℃;

[0014] (4) Hot rolling: The continuous casting billet is heated to 1160~1190℃ and held for 1.5~2.5h, and then rolled; the final rolling temperature is controlled at 870~890℃ and the coiling temperature is 560~580℃. The finished steel plate is obtained after rolling.

[0015] Furthermore, during the converter smelting process, scrap steel is added to the converter and mixed with molten iron before oxygen blowing smelting is carried out, followed by the addition of a nickel-copper alloy.

[0016] Furthermore, during the converter smelting process, aluminum deoxidation is carried out during the tapping process, silicon-manganese alloy is added for alloying, and lime particles are used to cover the surface of the molten steel.

[0017] Furthermore, during the LF refining process, lime, fluorite, and aluminum segments are added to the molten steel.

[0018] Furthermore, the continuous casting process is protected throughout the entire casting process.

[0019] Furthermore, the thickness of the finished steel plate is 3 to 20 mm.

[0020] Furthermore, the finished steel plate has a yield strength of 720–830 MPa, a tensile strength of 770–890 MPa, and an elongation of 37%–44%; its relative corrosion rate for atmospheric corrosion resistance is less than 30% of that of Q345B.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1) The increased content of aluminum, nitrogen, and boron in the steel achieves a fine grain effect, improves the mechanical properties and corrosion resistance of the steel, and reduces the content of expensive nickel alloys; it also reduces the heating temperature of the continuous casting billet in the heating furnace, thus reducing energy consumption.

[0023] 2) No alloys such as niobium, vanadium, and titanium are added to the steel, which reduces the amount of precious alloys added to weathering steel, saves resources, and reduces production costs.

[0024] 3) The production process is simple and the product performance is safe and reliable. Detailed Implementation

[0025] The weathering steel described in this invention has the following chemical composition by weight percentage: C: 0.070%–0.10%, Si: 0.40%–0.60%, Mn: 0.4%–0.6%, P: 0.07%–0.15%, S≤0.006%, Al: 1.2%–3.2%, Cr: 0.20%–0.40%, Ni: 0.03%–0.05%, Cu: 0.20%–0.40%, B: 0.002%–0.005%, N: 0.008%–0.012%, TO: 0.0005%–0.0020%, with the remainder being Fe and unavoidable impurities.

[0026] The present invention discloses a method for preparing weathering steel, wherein the preparation of the steel plate includes converter smelting, LF refining, continuous casting, and hot rolling processes, wherein the following processes are controlled:

[0027] (1) Converter smelting: The final carbon content of the converter is controlled at 0.04% to 0.06%, the final slag basicity is controlled at 2.7 to 3.5, the final phosphorus content is controlled at 0.04% to 0.08%, and the final active oxygen content is controlled at 0.03% to 0.05%; the tapping temperature of molten steel is controlled at 1645 to 1665℃;

[0028] (2) LF refining: The binary basicity of the refining slag is controlled at 2.4 to 3.0, and the active oxygen in the molten steel is controlled at less than 3 ppm;

[0029] (3) Continuous casting: The superheat of molten steel is controlled at 20-25℃;

[0030] (4) Hot rolling: The continuous casting billet is heated to 1160~1190℃ and held for 1.5~2.5h, and then rolled; the final rolling temperature is controlled at 870~890℃ and the coiling temperature is 560~580℃. The finished steel plate is obtained after rolling.

[0031] Furthermore, during the converter smelting process, scrap steel is added to the converter and mixed with molten iron before oxygen blowing smelting is carried out, followed by the addition of a nickel-copper alloy.

[0032] Furthermore, during the converter smelting process, aluminum deoxidation is carried out during the tapping process, silicon-manganese alloy is added for alloying, and lime particles are used to cover the surface of the molten steel.

[0033] Furthermore, during the LF refining process, lime, fluorite, and aluminum segments are added to the molten steel.

[0034] Furthermore, the continuous casting process is protected throughout the entire casting process.

[0035] Furthermore, the thickness of the finished steel plate is 3 to 20 mm.

[0036] Furthermore, the finished steel plate has a yield strength of 720–830 MPa, a tensile strength of 770–890 MPa, and an elongation of 37%–44%; its relative corrosion rate for atmospheric corrosion resistance is less than 30% of that of Q345B.

[0037] The rationale for setting the chemical composition and content range of the weathering steel described in this invention is as follows:

[0038] Carbon: Carbon can promote the formation of austenite and stabilize the austenite structure. Increasing the carbon content can improve the strength of steel. However, when the carbon content is too high, chromium-rich carbides are easily formed, leading to intergranular corrosion. Therefore, this invention controls the carbon content to 0.070% to 0.10%.

[0039] Silicon: Silicon is a ferrite-forming and stabilizing element, used for deoxidation during smelting to improve the cleanliness of molten steel. Increasing the silicon content can improve the corrosion resistance of steel, but excessive silicon content will accelerate the precipitation of intermetallic phases. Therefore, this invention controls the silicon content to be between 0.40% and 0.60%.

[0040] Manganese: Manganese is an austenite forming and stabilizing element, which is beneficial to obtaining an austenite structure and improving the stability of austenite; adding manganese can also significantly improve the strength of steel and reduce the damage of sulfur to steel; however, manganese will reduce the corrosion resistance of steel. Therefore, the manganese content is controlled at 0.4% to 0.6% in this invention.

[0041] Phosphorus: Phosphorus can improve the strength of corrosion-resistant steel and enhance its resistance to atmospheric corrosion, promoting the formation of a protective rust layer on the steel surface; the effect is even better when phosphorus is used in combination with copper; however, phosphorus can easily cause cold brittleness in steel, and phosphorus segregation at grain boundaries or phase boundaries can lead to a decrease in toughness; therefore, the phosphorus content in this invention is controlled at 0.07% to 0.15%.

[0042] Sulfur: Sulfur is a harmful element in materials. Sulfur can easily cause hot brittleness in steel, easily deteriorate the hot workability of weathering steel, and reduce the corrosion resistance of steel due to the formation of manganese sulfide. Therefore, the sulfur content should be strictly controlled. This invention controls the sulfur content to below 0.006%.

[0043] Aluminum: Adding aluminum will form a dense alumina film on the surface of weathering steel, which together with the chromium oxide formed by a small amount of chromium will form a dense protective layer, thereby improving the corrosion resistance of the steel; however, excessive aluminum will reduce the toughness of the steel and cause difficulties in continuous casting production. Therefore, this invention controls the aluminum content to 1.2% to 3.2%.

[0044] Chromium: Adding a certain amount of chromium to steel can improve its heat resistance and corrosion resistance. To ensure that the weathering steel described in this invention has good corrosion resistance, a certain amount of chromium needs to be added. Chromium has a significant effect on improving the passivation ability of steel, promoting the formation of a dense passivation film or protective rust layer on the steel surface. At the same time, the addition of chromium can effectively increase the self-corrosion potential of steel and improve its atmospheric corrosion resistance. However, the addition of chromium will increase manufacturing costs. Considering the overall cost, this invention controls the chromium content at 0.20% to 0.40%.

[0045] Nickel significantly improves the corrosion resistance of steel. It also forms a nickel-rich copper phase with copper, which remains solid in the outer oxide layer, reducing the copper enrichment in the matrix and decreasing the chance of liquid nickel-rich phase formation, thus preventing hot brittleness defects. However, excessive nickel increases the adhesion of the oxide scale, which, if pressed into the steel, can create hot-rolling defects on the surface. Furthermore, nickel is a precious metal, and increasing its content significantly increases the alloying cost of the steel. Therefore, this invention controls the nickel content to 0.03%–0.05%.

[0046] Copper: It is beneficial for forming a dense, well-adhesive amorphous oxide (hydrocarbon oxide) protective layer on the surface of steel, resulting in significant corrosion resistance. Copper reacts with sulfur to form insoluble sulfides, thus counteracting the harmful effects of sulfur on the corrosion resistance of steel. However, when the copper content is too high, due to its low melting point (below the billet heating temperature), the precipitated copper accumulates in a liquid state at the austenite grain boundaries, easily causing cracks during heating or hot rolling. Therefore, this invention controls the copper content to 0.20%–0.40%.

[0047] Boron: Boron is distributed at grain boundaries, which can reduce the enrichment of phosphorus at grain boundaries, reduce phosphorus segregation in steel, and improve corrosion resistance; however, excessive addition will increase the brittleness of steel. Therefore, the boron content is controlled at 0.002% to 0.005% in this invention.

[0048] Nitrogen can improve the strength of steel and its corrosion resistance; however, excessive nitrogen content has an adverse effect on the toughness of steel. Therefore, this invention controls the nitrogen content to be between 0.008% and 0.012%.

[0049] Oxygen: Oxygen in steel exists in the form of oxides, which are equivalent to nucleating agents distributed in steel. They can improve the nucleation efficiency during the solidification process of molten steel and refine the grains. However, too much oxide will deteriorate the performance of steel. This invention controls the total oxygen content to 0.0005% to 0.0020%.

[0050] The method for preparing weathering steel according to the present invention includes processes such as converter smelting, LF refining, continuous casting, and hot rolling, which are described in detail below:

[0051] (1) Converter smelting: Scrap steel is added to the converter, and molten iron is added for oxygen blowing smelting. Nickel-copper alloy is added to the converter. The final carbon content of the converter is controlled at 0.04% to 0.06%, the final slag basicity is controlled at 2.7 to 3.5, the final phosphorus content is controlled at 0.04% to 0.08%, and the final active oxygen content is controlled at 0.03% to 0.05%. The tapping temperature of the molten steel is controlled at 1645 to 1665℃. Aluminum deoxidation is carried out during the tapping process, silicon-manganese alloy is added for alloying, and lime particles are used to cover the surface of the molten steel.

[0052] (2) LF refining: Lime, fluorite and aluminum wire segments are added during LF refining; the purpose of adding fluorite is to lower the melting point of the refining slag and improve the fluidity of the slag; the purpose of adding lime is to increase the basicity of the slag and ensure the desulfurization effect of the steel slag; the binary basicity of the refining slag is controlled at 2.4 to 3.0; the amount of aluminum wire segments added is adjusted to control the active oxygen of the molten steel to within 3 ppm, and the composition and temperature of the molten steel are adjusted.

[0053] (3) Continuous casting: The molten steel after LF refining is transported to the continuous casting platform for casting. During the casting process, protection is provided to prevent air intake from causing oxygen and nitrogen to increase in the molten steel. The casting temperature of the molten steel is controlled to be higher than the liquidus temperature, that is, the superheat is controlled to be 20-25℃.

[0054] (4) Hot rolling: The continuously cast billet is heated to 1160-1190℃ in a heating furnace and held for 1.5-2.5 hours, and then rolled out of the furnace; the final rolling temperature is controlled at 870-890℃ and the coiling temperature is 560-580℃; the finished steel plate is obtained after rolling.

[0055] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0056]

Example

[0057] The smelting composition of the steel in each embodiment and comparative example is shown in Table 1; the smelting process parameters of the steel in each embodiment and comparative example are shown in Table 2; the refining and continuous casting process parameters of the steel in each embodiment and comparative example are shown in Table 3; and the hot rolling process parameters of the steel plates in each embodiment and comparative example are shown in Table 4. The properties of the finished steel plates in each embodiment and comparative example are shown in Table 5.

[0058] Table 1. Smelting composition of steel, wt%.

[0059] Example C Si Mn P S Al Cr Ni Cu B N TO Example 1 0.070 0.60 0.50 0.07 0.005 1.2 0.40 0.030 0.20 0.0020 0.0080 0.0020 Example 2 0.093 0.44 0.44 0.134 0.004 2.7 0.24 0.046 0.36 0.0043 0.0109 0.0016 Example 3 0.076 0.56 0.54 0.086 0.005 1.6 0.36 0.034 0.24 0.0026 0.0088 0.0016 Example 4 0.079 0.54 0.56 0.094 0.004 1.8 0.34 0.037 0.26 0.0029 0.0093 0.0015 Example 5 0.082 0.52 0.60 0.102 0.005 2.0 0.20 0.050 0.40 0.0050 0.0120 0.0015 Example 6 0.085 0.50 0.40 0.114 0.006 2.2 0.30 0.042 0.31 0.0035 0.0103 0.0005 Example 7 0.088 0.47 0.42 0.125 0.003 2.4 0.27 0.044 0.34 0.0039 0.0105 0.0009 Example 8 0.100 0.40 0.48 0.150 0.006 3.2 0.32 0.040 0.28 0.0032 0.0097 0.0013 Example 9 0.096 0.42 0.46 0.142 0.005 3.0 0.22 0.048 0.38 0.0048 0.0115 0.0017 Example 10 0.073 0.58 0.52 0.078 0.006 1.4 0.38 0.032 0.22 0.0023 0.0084 0.0018 Comparative Example 1 0.080 0.48 0.51 0.086 0.004 0.034 0.38 0.042 0.28 / 0.0033 0.0016 Comparative Example 2 0.092 0.54 0.46 0.092 0.005 0.028 0.36 0.052 0.34 / 0.0038 0.0018

[0060] Table 2 Steel smelting process parameters

[0061]

[0062]

[0063] Table 3 Refining and Continuous Casting Process Parameters for Steel

[0064] Example Refining slag alkalinity Oxygen content of molten steel (ppm) Casting temperature, °C Example 1 3.0 2 1526 Example 2 2.8 3 1536 Example 3 2.6 3 1528 Example 4 2.4 2 1527 Example 5 2.5 2 1531 Example 6 2.7 3 1534 Example 7 2.5 1 1528 Example 8 2.8 2 1529 Example 9 2.9 3 1533 Example 10 2.7 3 1535 Comparative Example 1 2.5 2 1540 Comparative Example 2 2.6 3 1546

[0065] Table 4 Hot rolling process parameters for steel plates

[0066] Example Billet heating temperature, °C Insulation time, h Final rolling temperature, ℃ Winding temperature, ℃ Steel plate thickness, mm Example 1 1160 2.0 870 560 20 Example 2 1185 2.5 882 580 10 Example 3 1190 1.5 888 565 3 Example 4 1165 2.3 890 562 15 Example 5 1185 2.1 875 576 8 Example 6 1170 2.4 885 580 12 Example 7 1175 2.2 883 572 14 Example 8 1178 1.9 878 564 10 Example 9 1185 1.7 885 568 8 Example 10 1190 1.8 876 573 12 Comparative Example 1 1205 2.0 901 582 12 Comparative Example 2 1210 2.1 895 586 10

[0067] Table 5 Performance of Finished Steel Plates

[0068] Example Yield strength, MPa Tensile strength, MPa Elongation, % Relative corrosion rate, % Lateral cold bending 0a, 180° Example 1 720 770 44 26 qualified Example 2 738 785 41 25 qualified Example 3 732 781 43 29 qualified Example 4 784 810 38 26 qualified Example 5 740 793 42 24 qualified Example 6 830 890 37 27 qualified Example 7 812 864 39 29 qualified Example 8 824 876 38 28 qualified Example 9 803 865 40 27 qualified Example 10 764 847 42 29 qualified Comparative Example 1 562 637 34 45 qualified Comparative Example 2 584 682 31 48 qualified

[0069] As shown in Table 5, this invention refines the grain size by increasing the aluminum and nitrogen content in the steel. Simultaneously, aluminum oxidizes to form aluminum oxide, which, together with chromium oxide in the steel, forms a dense protective layer. The addition of boron reduces phosphorus segregation at grain boundaries, thereby improving the steel's yield strength, tensile strength, elongation, and atmospheric corrosion resistance. The relative corrosion rates of the finished steel plates in each embodiment are all below 30% of those of Q345B, and significantly lower than those in the comparative example.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A weathering steel, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.070%–0.10%, Si: 0.40%–0.60%, Mn: 0.4%–0.6%, P: 0.07%–0.15%, S≤0.006%, Al: 2.2%–3.2%, Cr: 0.20%–0.40%, Ni: 0.03%–0.05%, Cu: 0.20%–0.40%, B: 0.002%–0.005%, N: 0.0103%–0.012%, TO: 0.0005%–0.0020%, with the remainder being Fe and unavoidable impurities. The yield strength of the finished steel plate is 720–830 MPa, the tensile strength is 770–890 MPa, and the elongation is 37%–44%. The relative corrosion rate of atmospheric corrosion resistance is less than 30% of that of Q345B.

2. The method for preparing weathering steel as described in claim 1, characterized in that, The preparation of steel plates includes converter smelting, LF refining, continuous casting, and hot rolling processes, among which the following processes are controlled: (1) Converter smelting: The final carbon content of the converter is controlled at 0.04% to 0.06%, the final slag basicity is controlled at 2.7 to 3.5, the final phosphorus content is controlled at 0.04% to 0.08%, and the final active oxygen content is controlled at 0.03% to 0.05%; the tapping temperature of molten steel is controlled at 1645 to 1665℃; (2) LF refining: The binary basicity of the refining slag is controlled at 2.4 to 3.0, and the active oxygen in the molten steel is controlled at less than 3 ppm; (3) Continuous casting: The superheat of molten steel is controlled at 20-25℃; (4) Hot rolling: The continuous casting billet is heated to 1160-1190℃ and held for 1.5-2.5h, and then rolled; the final rolling temperature is controlled at 870-890℃ and the coiling temperature is 560-580℃. The finished steel plate is obtained after rolling.

3. The method for preparing weathering steel according to claim 2, characterized in that, During the converter smelting process, scrap steel is added to the converter and mixed with molten iron before oxygen blowing smelting is carried out, followed by the addition of a nickel-copper alloy.

4. The method for preparing weathering steel according to claim 2, characterized in that, During the converter smelting process, aluminum deoxidation is carried out during the tapping process, silicon-manganese alloy is added for alloying, and lime particles are used to cover the surface of the molten steel.

5. The method for preparing weathering steel according to claim 2, characterized in that, During the LF refining process, lime, fluorite, and aluminum wire segments are added to the molten steel.

6. The method for preparing weathering steel according to claim 2, characterized in that, The continuous casting process is protected throughout the entire casting process.

7. The method for preparing weathering steel according to claim 2, characterized in that, The thickness of the finished steel plate is 3 to 20 mm.

Images