A 235MPa grade high weathering steel with low cost and long service life for petrochemical platforms and its preparation method

Abstract

The present invention discloses a low-cost and long-life 235 MPa grade high weathering steel for petrochemical platforms and a preparation method thereof. Based on Q235 weathering steel, a higher content of Al element is added optimally and the Ni element is not added, so that it is possible to obtain a low-cost and long-life 235 MPa grade high weathering steel for petrochemical platforms with good corrosion resistance, while significantly reducing the production cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of weathering steel, and particularly relates to a low-cost and long-life 235 MPa grade high weathering steel for petrochemical platforms and a preparation method thereof. Background Art

[0002] Baosteel's weathering steel system is relatively comprehensive, including weathering steels for railway vehicles developed around 2000, mainly 09CuPTiRe, 09CuPCrNi, and 05CuPCrNi series. In addition, there are 700 MPa grade high-strength container plates BS700MC, S350W with high Cr (3%), and S450W weathering steel. In recent years, fire-resistant weathering steel has also been developed. In terms of composition, it belongs to the Corten-A, Cu-P series, and high Cr series.

[0003] Domestic weathering steels for welded structures are mainly based on the Cu-Cr-Ni series, with P content below 0.04%. Representative steel grades include 16CuCr (Q235NH), 12MnCuCr (Q295NH), 15MnCuCr (Q355NH), and 15MnCuCr-QT (Q460NH). Baosteel developed the Cr-Cu-Mo series seawater corrosion-resistant steel grade Q345C-NHY3 by referring to the composition characteristics of Japanese seawater corrosion-resistant steels.

[0004] In recent years, to reduce costs and reduce the use of precious alloying elements such as Ni element, Japanese scholar Nishimura used Al and Si as the main alloying elements in marine atmospheric corrosion-resistant steels to improve corrosion resistance. Its main components are 0.17C-1.5Mn-0.8Si-0.8Al. Five-year atmospheric exposure tests in Chiba Prefecture showed that due to the formation of nano-scale Si n+ (n = 2 or 3) and Al 3+ complex oxides in the inner rust layer, the rust layer resistance and charge transfer resistance were improved, and thus excellent corrosion resistance was obtained.

[0005] The weathering steel in South Korea is mainly RAWS50 produced by POSCO. South Korean scholars Choi et al. studied the influence of Si addition on the corrosion performance of Ca-modified weathering steel. The basic principle is that after adding Ca, inclusions such as CaO and CaS are formed, and after hydrolysis, OH -1 is generated to increase the pH value of the thin liquid film on the material surface; while the addition of Si will form acidic oxides such as CaO x ·SiO2 or CaO·Al2O3·SiO2, which conflicts with the role of Ca. Tests showed that the Si addition amount should be less than 0.62 wt% to obtain good corrosion resistance.

[0006] Yoon-Seak Choi et al. added a small amount of Co and W to weathering steel containing 1 wt% Cr and studied its corrosion in synthetic seawater. The results showed that the addition of Co and W improved the corrosion resistance of the weathering steel, mainly because they could increase the corrosion potential and reduce the corrosion current density. The compounds formed by Co and W could more or less promote the formation of rust layers in aqueous solutions. The application of Al has been attracting increasing attention, mainly including electrical steel with high Si and Al contents, high-strength automotive sheet steel with high Mn and Al contents, and some experiments are also being carried out in some corrosion-resistant steels.

[0007] With the country's increasingly strict environmental protection requirements, the application of traditional galvanized weathering steel is restricted and cannot meet the requirement of a 25-year service life under the condition of no painting in seaside refineries, resulting in a situation where there is no available steel for the construction platform of seaside petrochemical refineries. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a low-cost and long-life 235 MPa grade high-performance weathering steel for petrochemical platforms, and its chemical composition by mass percentage is: C: 0.08 - 0.12%, Si ≤ 0.50%, Mn: 0.5 - 0.8%, S ≤ 0.02%, P ≤ 0.03%, Cu: 0.40 - 0.50%, Cr: 1.0 - 1.25%, Al: 1.3 - 1.5%, and the rest are Fe and inevitable impurities;

[0009] The preparation method of the low-cost and long-life 235 MPa grade high-performance weathering steel for petrochemical platforms includes the following technological steps:

[0010] 1) Use an intermediate frequency induction furnace for melting. In order to control the burning loss of Al, add Al after all other alloys are melted, and continue melting for 10 - 15 minutes before starting casting to cast into round ingots;

[0011] 2) Cut off the head and tail of the round ingot, keep it warm at 1190°C - 1200°C for 2 - 2.5 h, perform one upsetting and one drawing, and then forge it into a square billet;

[0012] 3) Keep the square billet warm at 1080°C - 1120°C for 1 - 1.5 h, hot roll it for 6 - 8 passes, and the final rolling temperature ≥ 850°C;

[0013] 4) Air cool to room temperature after rolling.

[0014] In some embodiments, the chemical composition of the low-cost and long-life 235 MPa grade high-performance weathering steel for petrochemical platforms by mass percentage is: C: 0.08%, Si ≤ 0.50%, Mn: 0.5%, S ≤ 0.02%, P ≤ 0.03%, Cu: 0.40%, Cr: 1.0%, Al: 1.3%, and the rest are Fe and inevitable impurities.

[0015] In some embodiments, the chemical composition of the low-cost long-life 235 MPa grade high weathering steel for petrochemical platforms is by mass percentage: C: 0.12%, Si ≤ 0.50%, Mn: 0.8%, S ≤ 0.02%, P ≤ 0.03%, Cu: 0.50%, Cr: 1.25%, Al: 1.5%, and the balance is Fe and unavoidable impurities.

[0016] In some embodiments, the chemical composition of the low-cost long-life 235 MPa grade high weathering steel for petrochemical platforms is by mass percentage: C: 0.1%, Si ≤ 0.50%, Mn: 0.6%, S ≤ 0.02%, P ≤ 0.03%, Cu: 0.48%, Cr: 1.2%, Al: 1.38%, and the balance is Fe and unavoidable impurities.

[0017] In some embodiments, in step 1), a 50 kg intermediate frequency induction furnace is used for melting, and finally cast into a 50 kg round ingot.

[0018] In some embodiments, in step 2), the size of the square billet is 75 mm thick and 100 mm wide.

[0019] In some embodiments, in step 3), after hot rolling for 6 - 8 passes, the thickness of the steel plate is controlled to be 3 mm.

[0020] Under the existing national standard system, the present invention provides a low-cost long-life 235 MPa grade high weathering steel for petrochemical platforms and its preparation method. Based on the Q235 weathering steel, by optimizing the addition of a high content of Al, without adding Ni element, adding an appropriate amount of Cr element, and adjusting the contents of other elements, the production cost can be significantly reduced while ensuring good corrosion resistance. Description of the Drawings

[0021] Figure 1 Metallographic structure photograph of the Q235CNH weathering steel produced for the example.

[0022] Figure 2 For the results of cyclic immersion corrosion.

[0023] Figure 3 For the results of coupon corrosion in Qingdao.

[0024] Figure 4 Scanning pictures of the rust layer on the surface of Q235B weathering steel, where (a-1)-(a-3) are scanning pictures of the rust layer dense area at different magnifications (50X, 500X, 100KX) respectively, and (b) is the energy spectrum diagram of the surface rust layer.

[0025] Figure 5It is a scanning composition diagram of the rust layer on the surface of Q235B weathering steel.

[0026] Figure 6 It is a scanning picture of the rust layer on the surface of Q235CNH weathering steel. Among them, (a-1)-(a-3) are the scanning pictures of the dense area of the rust layer at different magnification multiples (50X, 500X, 100KX) respectively, and (b) is the energy spectrum diagram of the surface rust layer.

[0027] Figure 7 It is a scanning composition diagram of the rust layer on the surface of Q235CNH weathering steel.

[0028] Figure 8 It is the XRD diagram of the Q235CNH rust layer.

[0029] Figure 9 It is the Raman spectroscopy analysis result of the weathering steel after 30 days of salt spray corrosion. Specific implementation manners

[0030] The present invention aims to provide a low-cost and long-life 235MPa grade high-performance weathering steel for petrochemical platforms and its preparation method. Based on Q235 weathering steel, by optimizing the addition of a high content of Al and without adding Ni element, it can significantly reduce the production cost and extend the service life while ensuring good corrosion resistance. The design principle of the Al component is as follows:

[0031] Al element: Al can strengthen the ferrite structure, promote the uniform corrosion of the steel; inhibit the crystallization of corrosion products, reduce the influence of internal volume change on the rust layer structure; preferentially oxidize itself to generate fine-grained oxides Al2O3 and FeAl2O4 protective films, enhance the compactness of the rust layer, and increase the corrosion potential of the steel, all of which are beneficial to alleviating the corrosion of the steel matrix. In the present invention, a high content (1.3-1.5%) of Al element is optimized and added.

[0032] The content of the present invention is described in detail through specific embodiments below. The embodiments are intended to help understand the present invention, rather than limiting the content of the present invention.

[0033] Embodiment: Production process of low-cost and long-life 235MPa grade high-performance weathering steel for petrochemical platforms

[0034] (1) Use a 50Kg intermediate frequency induction furnace for melting. In order to control the burning loss of Al, Al is added after all other alloys are melted, and melting is continued for 10-15 minutes before casting, and cast into a 50kg round ingot. Control the chemical composition and content of the round ingot as follows: C: 0.1%, Si: 0.50%, Mn: 0.6%, S: 0.02%, P: 0.03%, Cu: 0.48%, Cr: 1.2%, Al: 1.38%, and the rest are Fe and inevitable impurities.

[0035] In this step, round ingots with the following chemical compositions were also produced:

[0036] Q235B: C: 0.1%, Si: 0.3%, Mn: 0.3%, S: 0.02%, P: 0.14%, Cu: 0.30%, the rest being Fe and inevitable impurities;

[0037] Comparative Example 1: C: 0.1%, Si: 0.50%, Mn: 0.6%, S: 0.02%, P: 0.03%, Cu: 0.48%, Cr: 1.2%, Al: 1.05%, the rest being Fe and inevitable impurities.

[0038] (2) Cut off the heads and tails of the round ingots, hold them at 1190 °C - 1200 °C for 2 h, perform one upsetting and one drawing, and then forge them into square billets with a thickness of 75 mm and a width of 100 mm.

[0039] (3) Hold at 1080 °C - 1120 °C for 1 h, hot roll, and after 6 - 8 passes of rolling, control the thickness of the steel plate at 3 mm, and the final rolling temperature ≥ 850 °C.

[0040] (4) Air cool to room temperature after rolling.

[0041] This example produced a weathering steel with a yield strength of 235 MPa grade, named Q235CNH weathering steel. Its metallographic structure photo is as Figure 1 shown. The structure of this steel consists of ferrite plus a small amount of pearlite. And weathering steels of Q235B and Comparative Example 1 were obtained respectively.

[0042] Perform cyclic immersion corrosion experiments on the obtained Q235CNH weathering steel, Q235B and weathering steel of Comparative Example 1. Use a mixed solution of NaHSO3 (sodium bisulfite) and NaCl, with an initial concentration of 1.0×10 -2 mol / L (NaHSO3) + 1% NaCl; conduct cyclic immersion experiments for 72 h (3 days), 144 h (6 days), 240 h (10 days), 480 h (20 days). The results are as Figure 2 shown. It can be seen that the weight loss rate of Q235CNH weathering steel is only half of that of traditional Q235B weathering steel and is significantly lower than that of the weathering steel of Comparative Example 1. The above results show that Q235CNH weathering steel has good corrosion resistance.

[0043] Also conduct Qingdao coupon corrosion experiments and salt spray corrosion experiments on this Q235CNH weathering steel, both of which show that this steel has good corrosion resistance.

[0044] As Figure 3As shown, the surface pictures of Q235B (a) and Q235CNH (b) weathering steels after 29 days of coupon exposure in Qingdao are presented. It can be seen that both weathering steels have undergone uniform corrosion, but the compactness of the rust layer of Q235CNH weathering steel is significantly lower than that of Q235B weathering steel.

[0045] Figure 4 For Figure 3 Figure (a) shows the scanning pictures of the surface rust layer of Q235B, where (a - 1)-(a - 3) are the scanning pictures of the rust layer dense areas at different magnification levels (50X, 500X, 100KX) respectively. It can be seen that in the areas completely covered by the rust layer, the rust layer forms circular patches, and the patchy rust layer has started to form a dense rust layer structure, but the surface of the rust layer is uneven. (b) is the energy spectrum diagram of the surface rust layer, and it can be seen that the rust layer is mainly composed of Fe and O. Figure 5 Figure shows the surface scanning composition diagram of the rust layer of Q235B weathering steel.

[0046] Figure 6 For Figure 3 Figure (b) shows the scanning pictures of the surface rust layer of Q235CNH, where (a - 1)-(a - 3) are the scanning pictures of the rust layer dense areas at different magnification levels (50X, 500X, 100KX) respectively, and (b) is the energy spectrum diagram of the surface rust layer. It can be seen that the growth pattern of the Q235CNH rust layer is similar to that of Q235B steel. Figure 7 Figure shows the surface scanning composition diagram of the rust layer of Q235CNH weathering steel. It can be seen from the figure that Al has entered the rust layer. Existing data shows that the addition of Al will increase the stability of the rust layer structure, forming FeAl2O4. Al is beneficial to the densification of the rust layer, thereby blocking the intrusion of Cl - . XRD analysis was also carried out on the Q235CNH rust layer, and the results are as Figure 8 shown. It can be seen that the rust layer is mainly composed of Fe3O4 and Fe(OH).

[0047] The rust layer after 30 days of salt spray corrosion was analyzed. The salt spray corrosion medium was 3.5% NaCl aqueous solution, and the results are as Figure 9 shown. From Figure 9 it can be known that the amount of α-Fe(OH) in Q235CNH after the addition of Al is significantly more than that of the traditional Q235B weathering steel, indicating that this rust layer has a more stable structure and corrosion resistance.

[0048] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms, characterized in that: The chemical composition of the low-cost, long-life 235MPa grade high-grade weathering steel for the petrochemical platform is as follows by mass percentage: C: 0.08-0.12%, Si≤0.50%, Mn: 0.5-0.8%, S≤0.02%, P≤0.03%, Cu: 0.40-0.50%, Cr: 1.0-1.25%, Al: 1.3-1.5%, and the rest is Fe and unavoidable impurities; The method for preparing the low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms comprises the following process steps: 1) Use a medium frequency induction furnace for smelting. In order to control the burning loss of Al, add Al after all other alloys are melted, and continue smelting for 10-15 minutes before casting to cast into round ingots; 2) Remove the head and tail of the round ingot, keep it at 1190℃-1200℃ for 2-2.5h, perform one-up and one-down, and then forge it into a square billet; 3) keeping the billet at 1080° C.-1120° C. for 1-1.5 h, hot rolling for 6-8 passes, and the final rolling temperature is ≥850° C.; 4) Air cool to room temperature after rolling.

2. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: The chemical composition of the low-cost, long-life 235MPa grade high-grade weathering steel for the petrochemical platform is as follows by mass percentage: C: 0.08%, Si≤0.50%, Mn: 0.5%, S≤0.02%, P≤0.03%, Cu: 0.40%, Cr: 1.0%, Al: 1.3%, and the rest is Fe and unavoidable impurities.

3. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: The chemical composition of the low-cost, long-life 235MPa grade high-grade weathering steel for the petrochemical platform is as follows by mass percentage: C: 0.12%, Si≤0.50%, Mn: 0.8%, S≤0.02%, P≤0.03%, Cu: 0.50%, Cr: 1.25%, Al: 1.5%, and the rest is Fe and unavoidable impurities.

4. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: The chemical composition of the low-cost, long-life 235MPa grade high-grade weathering steel for the petrochemical platform is as follows by mass percentage: C: 0.1%, Si≤0.50%, Mn: 0.6%, S≤0.02%, P≤0.03%, Cu: 0.48%, Cr: 1.2%, Al: 1.38%, and the rest is Fe and unavoidable impurities.

5. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: In step 1), a 50 kg medium frequency induction furnace is used for smelting, and finally a 50 kg round ingot is cast.

6. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: The size of the billet in step 2) is 75 mm thick and 100 mm wide.

7. The low-cost, long-life 235MPa grade high-grade weathering steel for petrochemical platforms according to claim 1 is characterized in that: In step 3), after 6-8 passes of hot rolling, the thickness of the steel plate is controlled to be 3 mm.

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