A 550MPa grade nickel-saving rare earth weathering steel and its production process

Abstract

The invention discloses a 550MPa grade nickel-saving rare earth weathering steel and a production process thereof. The chemical composition of the steel by mass percentage comprises: C: 0.05-0.16%, Si: 0.10-0.40%, Mn: 1.10-1.20%, P: 0.008-0.016%, S≤0.005%, Cr: 0.55-0.65%, Cu: 0.10-0.15%, Nb: 0.03-0.05%, Ti: 0.015-0.025%, Al: 0.025-0.050%, O: 0.0005-0.0030%, La: 0.005-0.015%, La / S≥5, and the remainder is Fe and unavoidable impurities. The invention aims to reduce the amount of precious alloys such as nickel and copper in the weathering steel, significantly reduce the alloy cost, and achieve excellent performance.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and in particular to a 550MPa grade nickel-saving rare earth weathering steel and a production process thereof. Background Art

[0002] Rare earth elements in steel can purify molten steel, modify inclusions, and form microalloys, improving the steel's resistance to atmospheric corrosion. The development and application of high-strength, corrosion-resistant, and lightweight steels is a trend in the development of conventional carbon steel and a key measure for the steel industry to achieve its "dual carbon" goals. my country possesses abundant resources of the rare earth elements lanthanum and cerium, providing the prerequisites for their large-scale application in steel manufacturing. Leveraging the role of rare earth elements in improving steel's weather resistance, optimizing alloy composition design, and developing production processes are key research areas in the steel industry aimed at reducing production costs and improving steel performance.

[0003] Document 1 discloses a rare earth-containing cold-rolled automotive weathering steel with a yield strength of 280 MPa. Its chemical composition (by weight): C ≤ 0.09%, Si 0.10-0.40%, Mn 0.10-0.30%, P 0.05-0.11%, S ≤ 0.015%, Al 0.010-0.03%, Cu 0.2-0.35%, and RE 0.03-0.08%. When the RE content is 0.045-0.065%, the relative corrosion rate is 42-54% compared to Q235 steel, with the rare earth-added steel exhibiting a 10% reduction in corrosion rate compared to the unadded steel. This steel does not contain precious alloys such as Ni and Cr, reducing alloy costs. However, due to the high S content and lack of oxygen content control, the steel contains a high number of rare earth inclusions, resulting in a high rare earth content, which adversely affects continuous casting production.

[0004] Document 2 discloses a 400 MPa-grade high-silicon weathering steel, its preparation method, and its application. Its chemical composition includes: C ≤ 0.12%, Si: 1.20-2.00%, Mn ≤ 1.50%, P: 0.005-0.030%, S ≤ 0.015%, Cr: 0.20-0.80%, Ni: 0.10-0.40%, Cu: 0.20-0.60%, and Als ≥ 0.010%. Its I value is as high as 8.05-9.16, its relative corrosion rate is ≤ 40% compared to Q355B, and its -40°C impact strength is ≥ 60 J. However, the cost of adding Cr, Ni, and Cu alloys is high, and the high Si content adversely affects weldability.

[0005] Document 3 discloses a method and process for producing economical weathering steel using RE-P reinforcement. This weathering steel contains the following chemical composition: C: 0.01-0.08%, Si: 0.10-0.30%, Mn: 1.30-1.70%, P: 0.06-0.10%, S: 0.001-0.005%, Ca: 0.0010-0.0050%, Cu: 0.08-0.20%, Al: 0.015-0.06%, Nb: 0.01-0.07%, O: 0.0010-0.0030%, and RE: 0.02-0.06%. Compared to Q355B, its 72-hour relative corrosion rate is 56-59%. This steel uses less alloying and is relatively inexpensive, but its atmospheric corrosion resistance is average. Summary of the Invention

[0006] The purpose of the present invention is to provide a 550MPa grade nickel-saving rare earth weathering steel and its production process, which can reduce the amount of precious alloys such as nickel and copper in weathering steel, significantly reduce alloy costs, and have excellent performance.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The invention discloses a 550MPa grade nickel-saving rare earth weathering steel. The chemical composition of the steel comprises, by mass percentage, C: 0.05-0.16%, Si: 0.10-0.40%, Mn: 1.10-1.20%, P: 0.008-0.016%, S≤0.005%, Cr: 0.55-0.65%, Cu: 0.10-0.15%, Nb: 0.03-0.05%, Ti: 0.015-0.025%, Al: 0.025-0.050%, O: 0.0005-0.0030%, La: 0.005-0.015%, La / S≥5, and the remainder is Fe and unavoidable impurities.

[0009] Furthermore, its chemical composition in mass percentage includes: C: 0.10%, Si: 0.19%, Mn: 1.15%, P: 0.016%, S: 0.003%, Cr: 0.603%, Cu: 0.122%, Nb: 0.043%, Ti: 0.016%; Al: 0.040%, O: 0.0030%, La: 0.0049%, and the rest is Fe and unavoidable impurities.

[0010] Furthermore, its chemical composition in mass percentage includes: C: 0.16%, Si: 0.35%, Mn: 1.17%, P: 0.009%, S: 0.002%, Cr: 0.621%, Cu: 0.119%, Nb: 0.040%, Ti: 0.031%; Al: 0.029%, O: 0.0020%, La: 0.0085%, and the rest is Fe and unavoidable impurities.

[0011] Furthermore, its chemical composition in mass percentage includes: C: 0.16%, Si: 0.36%, Mn: 1.17%, P: 0.010%, S: 0.002%, Cr: 0.626%, Cu: 0.118%, Nb: 0.034%, Ti: 0.028%; Al: 0.030%, O: 0.0023%, La: 0.0096%, and the rest is Fe and unavoidable impurities.

[0012] Furthermore, its chemical composition by mass percentage includes: C: 0.12%, Si: 0.35%, Mn: 1.16%, P: 0.009%, S: 0.002%, Cr: 0.637%, Cu: 0.111%, Nb: 0.036%, Ti: 0.025%; Al: 0.039%, O: 0.0016%, La: 0.0148%, and the rest is Fe and unavoidable impurities.

[0013] In the above technical solution, the chemical composition content range of the alloying elements of weathering steel is limited. The main reasons are as follows:

[0014] Carbon is an important strengthening element that strongly influences the mechanical properties of steel. High carbon content adversely affects weldability. When the carbon content is between 0.10% and 0.15%, the crack sensitivity of the steel increases due to the peritectic reaction during solidification. Therefore, the present invention controls the carbon content within the range of 0.05% to 0.16%, preferably within the range of 0.05% to 0.10%.

[0015] Si: A deoxidizing element in the steelmaking process and a solid solution strengthening element in steel, increasing Si content increases steel strength. Increasing Si content increases the corrosion resistance index (I) of steel, improving corrosion resistance, but also has a negative impact on weldability. Therefore, the present invention controls the Si content within the range of 0.10-0.40%, preferably within the range of 0.10-0.20%.

[0016] Mn is an important strengthening element in steel, helping to reduce hot brittleness and improve its plasticity and toughness. However, a Mn content above 1.2% can easily cause component segregation, affecting the uniformity of steel properties. Therefore, the present invention limits the Mn content to 1.10-1.20%.

[0017] Phosphorus (P) has a strong solid solution strengthening effect, which can improve the corrosion resistance of steel. However, P is a strong segregating element. Excessive P content can easily lead to component segregation, increase the ductile-brittle transition temperature of steel, and deteriorate the steel's weldability. Excessively low P content can increase steelmaking costs. Therefore, the present invention controls the P content within a relatively low range of 0.008% to 0.016%.

[0018] S: S is a harmful element in steel. It can easily cause crack defects caused by hot brittleness, reduce steel toughness, deteriorate steel weldability, and be detrimental to steel corrosion resistance. Therefore, the present invention controls the S content within 0.005%, preferably within 0.002%.

[0019] Cr: is the main corrosion-resistant element in steel. Increasing the Cr content can improve the strength and hardenability of steel without causing significant component segregation in the steel. However, high Cr content can adversely affect weldability. Therefore, the present invention controls the Cr content within the range of 0.55-0.65%, and preferably controls the Si content within the range of 0.60-0.65%.

[0020] Cu: An important corrosion-resistant element in steel, Cu promotes the compactness and stability of the inner rust layer, thereby improving atmospheric corrosion resistance. However, high Cu content can easily cause "copper brittle" crack defects, adversely affecting weldability. Cu is a precious alloying element, and an increase in its content can significantly increase the cost of steel. Therefore, the present invention controls the Cu content within the range of 0.10-0.15%, preferably within the range of 0.10-0.13%.

[0021] Nickel is an important strengthening element in steel, helping to improve its corrosion resistance. Maintaining a certain nickel content can prevent "copper brittle" cracks. However, nickel is an expensive alloying element in steel, significantly increasing the cost of the steel. Therefore, the present invention, while maintaining a low Cu content, does not add additional nickel to reduce alloy costs.

[0022] Nb: It significantly refines grains, improving the strength and toughness of steel. It also precipitates as carbonitrides, reducing the steel's high-temperature plasticity and causing crack defects in the ingot. However, it has no significant adverse effect on the steel's weldability. Therefore, the present invention controls the Nb content within the range of 0.03-0.05%.

[0023] Ti: It can refine grains and easily form carbonitrides during solidification, thereby reducing the formation of Nb carbonitrides and improving the high-temperature plasticity of steel. Therefore, the present invention controls the Ti content within the range of 0.015-0.025%.

[0024] Al: is an important deoxidizing element in steel, which helps to increase the intrinsic grain size and improve the performance of steel. Increasing its content helps to reduce the active oxygen content and S content in steel. Therefore, the present invention controls the Al content within the range of 0.025-0.050%.

[0025] Oxygen is a harmful element in steel, primarily present as oxide inclusions. It adversely affects the mechanical and fatigue properties of steel and is easily oxidized with rare earth elements to form inclusions. Therefore, the present invention controls the O content within a range of 0.0005% to 0.0030%, preferably within 0.0015%.

[0026] RE: Rare earth elements (RE) can purify molten steel, modify inclusions, and significantly improve the steel's corrosion resistance. However, excessive RE content can easily form inclusions that are difficult to remove. Therefore, the present invention limits the RE content to 0.005-0.015%. Preferably, the La content is controlled to 0.010-0.015%. Furthermore, the La / S ratio is controlled to be greater than 5.

[0027] A production process for 550MPa grade nickel-saving rare earth weathering steel comprises the following steps:

[0028] 1) Hot metal desulfurization: Use KR hot metal desulfurization pretreatment, the sulfur content of hot metal entering the converter is ≤0.002%, and the desulfurization slag cleaning area is ≥90%;

[0029] 2) Converter: Final oxygen content is not more than 0.07%, tapping temperature is greater than 1620°C, synthetic slag is added during tapping, and aluminum content is not less than 0.05%;

[0030] 3).LF refining: white slag time is more than 15 minutes, lanthanum ferroalloy is added 5 minutes after calcium treatment, and soft blowing is carried out after 5 minutes;

[0031] 4) Continuous casting: Protective casting is carried out throughout the entire process, and a high-alkalinity tundish covering agent is used. The nitrogen content of the continuous casting molten steel is controlled to be less than 4ppm, and the nitrogen content is controlled to be no more than 2ppm;

[0032] 5). Hot rolling: TMCP process is used for rolling, with heating temperature of 1180-1200℃, starting rolling temperature of 960-1010℃, finishing rolling temperature of 840-880℃, and coiling temperature of 530-570℃.

[0033] Furthermore, the nickel-saving rare earth weathering steel produced has a tensile strength of 700-750MPa, a yield strength of 570-620MPa, an elongation of 20-26%, and an impact energy greater than 50J; compared with Q355B, the relative corrosion rate of weathering steel is less than 50%.

[0034] Compared with the prior art, the present invention has the following beneficial technical effects:

[0035] By adopting the 550MPa grade nickel-saving rare earth weathering steel and its production process of the present invention and the rare earth lanthanum microalloying method, the amount of precious alloys such as copper and nickel is reduced while improving the weather resistance of the steel, thereby avoiding quality defects caused by copper and significantly reducing the production cost of 550MPa grade weathering steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 is the relationship between carbon content and tensile strength and yield strength;

[0038] Figure 2 This is the relationship between La content and relative corrosion rate. DETAILED DESCRIPTION

[0039] The present invention is described in more detail below using examples. These examples are merely descriptions of the best mode for carrying out the present invention and are not intended to limit the scope of the present invention in any way.

[0040] The 550MPa grade nickel-saving rare earth weathering steel and its production process of the present invention are: molten iron pre-desulfurization - converter smelting - LF refining - continuous casting - hot rolling, specifically comprising the following steps:

[0041] 1. Hot metal desulfurization: Use KR hot metal desulfurization pretreatment, the sulfur content of hot metal entering the converter is ≤0.002%, and the desulfurization slag cleaning area is ≥90%.

[0042] 2. Converter: The final oxygen content is not more than 0.07%, the tapping temperature is greater than 1620℃, synthetic slag is added during the tapping process, and the aluminum content is not less than 0.05%

[0043] 3.LF refining: White slag time is greater than 15 minutes. Lanthanum-iron alloy is added 5 minutes after calcium treatment, and the product is discharged after 5 minutes of soft blowing. High-purity lanthanum-iron alloy is used, with the main components being lanthanum: 10-30%, sulfur <0.002%, oxygen <0.005%, and the rest being iron and a small amount of other elements.

[0044] 4. Continuous casting: Protective casting is carried out throughout the entire process. High-alkalinity tundish covering agent is used. The nitrogen addition amount in the continuous casting molten steel is controlled to be less than 4ppm, and the preferred nitrogen addition amount is controlled to be no more than 2ppm.

[0045] 5. Hot rolling: TMCP process is used for rolling, with heating temperature of 1180-1200℃, starting rolling temperature of 960-1010℃, finishing rolling temperature of 840-880℃, and coiling temperature of 530-570℃.

[0046] The La content of Examples 1 to 4 ranges from 0.005% to 0.015%. Except for the fact that Comparative Example 1 does not contain rare earth, the contents of other components are all within the scope of the present invention. Comparative Example 2 is Q355B (a comparative sample used for the cyclic leaching test). The chemical composition contents of the Examples and Comparative Examples are shown in Table 1.

[0047] Table 1 Chemical composition of Examples and Comparative Examples (%)

[0048]

[0049] Note: The Q550NH standard states that the lower limit of Ni content may not be required after negotiation between the supply and demand parties.

[0050] The mechanical properties of the examples and comparative examples are shown in Table 2. When the carbon content is between 0.05% and 0.10%, the mechanical properties fully meet the requirements of the Q550NH standard. As the carbon content increases, the tensile strength and yield strength of the steel increase (e.g. Figure 1 Therefore, it is preferable to control the carbon content in the range of 0.05 to 0.10%.

[0051] Table 2 Mechanical properties of the embodiments and comparative examples

[0052]

[0053] The atmospheric corrosion resistance index I and the relative corrosion rate of the circulatory immersion corrosion test of the examples and comparative examples are shown in Table 3, where:

[0054] I=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 .

[0055] Table 3 Relative corrosion rates of the examples and comparative examples

[0056] performance La / S I value Relative corrosion rate (%) Example 1 1.6 4.0 53.4 Example 2 4.3 4.1 47.1 Example 3 4.8 4.1 45.5 Example 4 7.4 4.0 43.8 Comparative Example 1 0 3.8 55.8 Comparative Example 2 0 0.7 100％

[0057] like Figure 2 The relative corrosion rate decreases with the increase of the rare earth element La content. When La / S is greater than 4.8, the relative corrosion rate is less than 45.5%. Therefore, it is preferred to control the La content in the range of 0.010-0.015%, and further La / S should be greater than 5.

[0058] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A 550MPa grade nickel-saving rare earth weathering steel, characterized in that: Its chemical composition by mass percentage includes: C: 0.12%, Si: 0.35%, Mn: 1.16%, P: 0.009%, S: 0.002%, Cr: 0.637%, Cu: 0.111%, Nb: 0.036%, Ti: 0.025%; Al: 0.039%, O: 0.0016%, La: 0.0148%, and the rest is Fe and inevitable impurities; the production process includes the following steps: 1) Hot metal desulfurization: Use KR hot metal desulfurization pretreatment, the sulfur content of hot metal entering the converter is ≤0.002%, and the desulfurization slag cleaning area is ≥90%; 2) Converter: The final oxygen content is not more than 0.07%, the tapping temperature is greater than 1620℃, synthetic slag is added during the tapping process, and the aluminum content is not less than 0.05%; 3) LF refining: white slag time is more than 15 minutes, lanthanum-ferroalloy is added 5 minutes after calcium treatment, and soft blowing is carried out after 5 minutes; 4) Continuous casting: Protective casting is carried out throughout the entire process, and a high-alkalinity tundish covering agent is used. The nitrogen content of the continuous casting molten steel is controlled to be less than 4ppm, and the nitrogen content is controlled to be no more than 2ppm; 5). Hot rolling: TMCP process is used for rolling, with heating temperature of 1180~1200℃, starting rolling temperature of 960~1010℃, finishing rolling temperature of 840~880℃, and coiling temperature of 530~570℃.

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