High-performance carbon steel capable of inhibiting post-weld heat treatment strength degradation

Abstract

The high-performance carbon steel comprises the following components in percentage by weight: 0.05 to 0.25 percent of C, 0.10 to 0.40 percent of Si, 0.55 to 2.0 percent of Mn, 0.01 to 0.05 percent of N, less than or equal to 0.020 percent of S, less than or equal to 0.018 percent of P, less than or equal to 0.005 percent of As, less than or equal to 0.008 percent of Sb, less than or equal to 0.008 percent of Sn and the balance of Fe and inevitable impurities, and the elements also need to simultaneously meet the following conditions: less than or equal to 0.020 percent of P, less than or equal to 0.020 percent of As, less than or equal to 0.020 percent of Sb and less than or equal to 0.008 percent of Sn; the inevitable impurities comprise Cr, Mo and Mn, and (Cr + Mo + Mn) * N is controlled to be greater than or equal to 0.010%; the microscopic structure of the high-performance carbon steel is ferrite and pearlite, the high-performance carbon steel can be directly used in a hot rolling state, the postweld heat treatment inhibition performance is stable, the room-temperature yield strength is larger than or equal to 240 MPa, the tensile strength is larger than or equal to 415 MPa, the high-temperature instantaneous tensile yield strength at 350 DEG C is larger than or equal to 175 MPa, and the tensile strength is larger than or equal to 350 MPa.

Description

Technical Field

[0001] This invention relates to the field of carbon steel production, and specifically to a high-performance carbon steel that inhibits the deterioration of strength after post-weld heat treatment. Background Technology

[0002] Welding is the main method of joining steel, including carbon steel such as furnace tubes.

[0003] Post-weld heat treatment aims to improve the microstructure and properties of the welded joint, soften the hardened zone, reduce hardness, improve impact toughness and creep limit, and prevent brittle failure of the welded structure.

[0004] Secondly, it can relax residual welding stress, prevent delayed cracking, and improve the reliability and lifespan of the weldment.

[0005] Third, improve the corrosion resistance of the welded joint by performing post-weld heat treatment, namely PWHT.

[0006] Although post-weld heat treatment has many benefits for the weld itself, it is well known that it will inevitably degrade the strength of the base material in the heat-affected zone, especially when welding dissimilar steels such as carbon steel and alloy steel. If the post-weld heat treatment process is implemented according to the alloy steel process, the performance and strength of the carbon steel will deteriorate more severely.

[0007] First, the post-weld heat treatment temperature for carbon steel or carbon manganese steel is 600-650℃, while that for C-Mo, Mn-Mo, C-Mo and other alloy steels with Cr≤0.5% is 600-720℃, and that for Cr-Mo alloy steel with 0.5%<Cr≤2% is even higher at 700-750℃. The welding process control for different steel grades is shown in Table 1.

[0008] Table 1

[0009]

[0010]

[0011] Secondly, post-weld heat treatment requires heat preservation and low cooling rate, similar to furnace cooling, rather than high cooling rate cooling such as air cooling to strengthen the properties of the substrate.

[0012] Furthermore, repairs or additional welding are permitted when welding is defective, meaning that multiple post-weld heat treatments result in a longer cumulative strength degradation period.

[0013] The higher the heat treatment temperature after welding, the longer the heat treatment time, the slower the cooling rate, and the more cycles, the more severe the degradation of the matrix strength.

[0014] In addition, to ensure the overall performance of the equipment, some systems undergo overall modifications such as... Figure 1The simulated post-weld heat treatment shown involves a holding time of up to 8 hours, followed by cooling at a low rate of ≤50℃ / h to 400℃ before air cooling. Therefore, for safety and reliability reasons, downstream industries, especially the chemical and equipment industries represented by refining, have increased the performance requirements for the carbon steel furnace tube substrate itself under simulated post-weld heat treatment conditions, with performance indicators referencing the normalized state.

[0015] Chinese patent CN113166900A discloses a medium-high temperature alloy steel with excellent high-temperature strength and post-weld heat treatment resistance. Its composition, by mass percentage, includes Cr: 1-3%, Mo: 0.5-1.5%, Ni: 0.05-0.5%, and Al: 0.005-0.1%, and includes one or more of Ir: 0.005-0.10% and Rh: 0.005-0.10%, with the balance being Fe and unavoidable impurities.

[0016] Japanese Patent JP2023553169A discloses a pressure vessel steel plate whose mechanical properties do not decrease even after high-temperature post-weld heat treatment. Its composition, by mass percentage, includes: C: 0.10-0.16%, Si: 0.20-0.35%, Mn: 0.4-0.6%, Cr: 7.5-8.5%, Mo: 0.7-1.0%, Al: 0.005-0.05%, P≤0.015%, S≤0.002%, Nb: 0.001-0.025%, V: 0.25-0.35%, with the balance being Fe and unavoidable impurities.

[0017] Korean Patent SA518391749A discloses a pressure vessel steel plate with excellent PWHT resistance. Its composition, by mass percentage, includes: C: 0.05-0.17wt%, Si: 0.50-1.00wt%, Mn: 0.3-0.8wt%, Cr: 1.0-1.5wt%, Mo: 0.3-1.0wt%, Ni: 0.003-0.30%, Cu: 0.003-0.30%, Sol.Al: 0.005-0.06%, P≤0.015%, S≤0.020%, and two or more elements selected from Nb: 0.002-0.025wt%, V: 0.002-0.03wt%, and Co: 0.002-0.15wt%; the remainder is Fe and residues.

[0018] Korean patent KR102280641B1 discloses a high-temperature weld heat-treatable steel plate that exhibits minimal strength and toughness degradation even after prolonged application of PWHT at high temperatures. Its composition, by mass percentage, includes: C: 0.10–0.16%, Si: 0.20–0.35%, Mn: 0.4–0.6%, Cr: 6.5–7.5%, Mo: 0.7–0.9%, Al: 0.005–0.05%, P≤0.015%, S≤0.020%, Nb: 0.002–0.025%, and V: 0.25–0.35%.

[0019] However, the above patents CN113166900A, JP2023553169A, SA518391749A, KR102280641B1, etc. are all alloy steels, which are inherently resistant to long-term high-temperature aging and strength degradation.

[0020] Chinese patent CN104109801B discloses a steel plate resistant to PWHT softening and embrittlement. Its composition, by mass percentage, includes: C: 0.15–0.19%, Si ≤ 0.30%, Mn: 1.4–1.7%, P ≤ 0.013%, S ≤ 0.003%, Ni: 0.05–0.3%, V: 0.09–0.15%, Al ≤ 0.01%, N: 0.015–0.03%, Ca: 0.001–0.004%, with the balance being Fe and unavoidable impurities. This invention employs a low-alloy steel composition system of high C, high Mn, and extremely low Al, combined with high V and high N alloying. However, it still requires controlled rolling and subsequent heat treatment processes to achieve resistance to PWHT softening and embrittlement.

[0021] European patent EP3372702B1 discloses a steel with excellent strength and toughness in the central part of the plate thickness direction even after long-term PWHT, but it requires quenching and tempering treatment, and the microstructure is at least one of tempered bainite and tempered martensite.

[0022] The steel plates disclosed in the above patents CN104109801B, EP3372702B1 and CN102782169B need to be used in a heat-treated state, rather than a hot-rolled state.

[0023] Chinese patent CN102782169B discloses a steel plate with excellent resistance to post-weld heat treatment (PWHT). Its composition, by mass percentage, includes one or more of the following: C: 0.1-0.3%, Si: 0.15-0.50%, Mn: 0.6-1.2%, P≤0.035%, S≤0.020%, Al: 0.001-0.05%, Cr: 0.01-0.35%, Mo: 0.005-0.2%, V: 0.005-0.05%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Ca: 0.0005-0.005%, Ni: 0.05-0.5%, Cu: 0.005-0.5%, Co: 0.005-0.2%, and W: 0.005-0.2%, with the balance being Fe and unavoidable impurities. This patented steel plate not only requires the addition of multiple elements such as Cr, Mo, V, Nb, Ti, Cu, Co, and W, with Cu+Ni+Cr+Mo ≤ 1.5%, Cr+Mo ≤ 0.4%, V+Nb ≤ 0.1%, and Ca / S ≤ 1.0, but also requires at least one of Cu, Co, and W. Co and W are not common associated elements in iron ore, and their maximum content is as high as 0.2%. Furthermore, after hot rolling, it needs to undergo heat treatment at 850-950℃, which is essentially normalizing heat treatment.

[0024] Chinese patent CN107109589A discloses a high-strength steel with excellent low-temperature toughness after PWHT treatment. Its composition, by mass percentage, includes: C: 0.02–0.15%, Si: 0.05–0.50%, Mn: 1.0–2.0%, Al: 0.005–0.1%, P≤0.015%, S≤0.0015%, Nb: 0.01–0.03%, and V: 0.01–0.03%. %, Ti: 0.01~0.03%, Cr≤0.005%, Mo≤0.005%, Cu: 0.02~0.50%, Ni: 0.05~0.60%, B: 0.0002~0.0010%, N: 0.0035~0.0065%, with the balance being Fe and other unavoidable impurities, but its fine structure consists of a composite structure of ferrite with an area fraction of 35~40% and the balance being bainite.

[0025] Chinese patent CN110088339A discloses a pressure vessel steel plate and its manufacturing method. The plate's composition, by mass percentage, includes: C: 0.10–0.20%, Si: 0.15–0.40%, Mn: 1.15–1.50%, Mo: 0.45–0.60%, Cu: 0.03–0.30%, P≤0.025%, S≤0.025%, Al: 0.005–0.06%; C r: 0.03-0.30%, Nb: 0.002-0.025%, Zr: 0.002-0.025% or more; the balance being Fe and unavoidable impurities, wherein the microstructure after post-weld heat treatment (PWHT) at 600°C to 660°C for 60 hours comprises a mixed microstructure of ferrite, pearlite and tempered bainite, and the area fraction of tempered bainite is at least 10% (excluding 100%).

[0026] The patents CN107109589A, CN110088339A, EP3372702B1, etc., are not the conventional ferrite and pearlite structures of carbon steel.

[0027] The ferritic + bainitic steel disclosed in US Patent 11624101B2 exhibits excellent strength and toughness even after long-term post-weld heat treatment. Its composition, by mass percentage, includes: C: 0.1–0.15%, Si: 0.15–0.5%, Mn: 1.2–1.8%, P ≤ 0.01%, S ≤ 0.01%, Al: 0.01–0.05%, Nb: 0.01–0.05%, Ni: 0.01–0.05%. 25%, Cu≤0.01%, Mo:0.01~0.1%, V:0.01~0.05%, Ti≤0.003%, B≤5ppm, N:20~100ppm, 0.5<[(Ti+Nb+B) / 3.5N+(Cu / Ni)]<2.5, Nb / (Ti+Nb+V)≥20% in M(C,N), but air cooling is still required after PWHT, and it is not suitable for post-weld heat treatment processes with low cooling rates such as furnace cooling.

[0028] In summary, the main conventional measures to address the degradation of the strength of carbon steel after welding heat treatment are to change the composition or heat treatment. However, no carbon steel with an excellent microstructure of ferrite + pearlite has been found that can be used directly in the hot-rolled state without necessarily being in a normalized or other heat-treated state to suppress the degradation of the strength after welding heat treatment. Summary of the Invention

[0029] The purpose of this invention is to provide a high-performance carbon steel that inhibits the deterioration of strength after post-weld heat treatment. Its microstructure is still the conventional ferrite + pearlite of carbon steel, which can be used directly in the hot-rolled state. It inhibits the stability of post-weld heat treatment performance, with a yield strength ≥240MPa and tensile strength ≥415MPa at room temperature, and an instantaneous tensile yield strength ≥175MPa and tensile strength ≥350MPa at 350℃.

[0030] To achieve the above objectives, the technical solution of the present invention is as follows:

[0031] A high-performance carbon steel for inhibiting post-weld heat treatment strength degradation, comprising the following weight percentages: C: 0.05–0.25%, Si: 0.10–0.40%, Mn: 0.55–2.0%, N: 0.01–0.05%, S≤0.020%, P≤0.018%, As≤0.005%, Sb≤0.008%, Sn≤0.008%, with the balance including Fe and unavoidable impurities. Furthermore, the above elements must simultaneously satisfy the following conditions:

[0032] P + As + Sb + Sn ≤ 0.020%;

[0033] The unavoidable impurities include Cr and Mo, and the control is (Cr+Mo+Mn)×N≥0.010%.

[0034] Preferably, the carbon steel composition contains 0.08% to 0.20% C.

[0035] Preferably, the carbon steel composition contains 0.15-0.35% Si.

[0036] Preferably, the carbon steel composition contains 0.80–1.75% Mn.

[0037] Preferably, in the carbon steel composition, N: 0.015-0.045%.

[0038] Preferably, in the carbon steel composition, S≤0.010% and / or P≤0.015% and / or As≤0.003% and / or Sb≤0.005% and / or Sn≤0.005%.

[0039] Furthermore, the balance consists of Fe and unavoidable impurities.

[0040] The microstructure of the carbon steel described in this invention is ferrite + pearlite.

[0041] The carbon steel described in this invention has a yield strength ≥240MPa at room temperature, a tensile strength ≥415MPa, an instantaneous tensile yield strength ≥175MPa at 350℃, and a tensile strength ≥350MPa.

[0042] In the carbon steel composition design described in this invention:

[0043] The oxidation potential of element C is close to that of element Fe, and it is partially retained in molten steel. Element C is not only one of the most common inherent elements in steel, but also one of the most economical elements to ensure strength, especially to inhibit the deterioration of strength after post-weld heat treatment. However, excessive carbon content, especially high carbon equivalent, is not conducive to welding. Therefore, this invention controls the C content to be 0.05-0.25%, preferably 0.08-0.20%.

[0044] The oxidation potential of element Si is higher than that of element Fe. It is rarely retained in molten steel, but it is also one of the most common inherent elements in steel. It is beneficial to improve strength, especially to inhibit the deterioration of strength after post-weld heat treatment. However, too high a Si content is not conducive to rolling. In this invention, the Si content is controlled at 0.10-0.40%, preferably Si: 0.15-0.35%.

[0045] The oxidation potential of element Mn is close to that of element Fe, and it is partially retained in molten steel. It is also one of the most common inherent elements in steel. It is one of the most economical elements that is beneficial to strength, especially in inhibiting the deterioration of strength after post-weld heat treatment. However, too high a content is not conducive to welding. This invention controls the Mn content to be between 0.55% and 2.0%, preferably between 0.80% and 1.75%.

[0046] Element nitrogen (N) also contributes to strength, but is not required in conventional carbon steel. This invention limits the use of N to replace carbon (C) to improve strength, reduce carbon equivalent, and facilitate welding. As an element that inhibits the deterioration of strength after weld heat treatment, this invention limits its additional addition, but excessive amounts are detrimental to rolling. This invention controls the N content to 0.01–0.05%, preferably 0.015–0.045%. The microstructure of the steel of this invention is ferrite + pearlite, with a carbon content of 0.05–0.25%, predominantly ferrite. Ferrite has a body-centered cubic structure. The N content is highest at 590℃, approximately 0.1%, and decreases with decreasing temperature, reaching as low as 0.001% at 100℃, indicating that the N content in ferrite itself is relatively low. Cr, Mo, and Mn are nitride-forming elements, which help increase the nitrogen content, thereby avoiding excessive reduction in strength after weld heat treatment.

[0047] The oxidation potential of element S is close to that of element Fe. It is partially retained in molten steel and is also one of the inherent harmful residual elements in conventional steel. It deteriorates and inhibits the strength of post-weld heat treatment. Therefore, the lower the value, the better. This invention limits S to ≤ 0.020%, preferably S ≤ 0.010%.

[0048] The oxidation potential of element P is close to that of element Fe. It is partially retained in molten steel and is also one of the inherent harmful residual elements in conventional steel. It degrades and inhibits the strength of post-weld heat treatment. Therefore, the lower the value, the better. This invention limits P to ≤ 0.018%, preferably P ≤ 0.015%.

[0049] As has a lower oxidation potential than Fe and is completely retained in molten steel. It is one of the inherent harmful residual elements in steel, which degrades and inhibits the strength of post-weld heat treatment. Therefore, the lower the value, the better. This invention limits As to ≤0.005%, preferably As ≤0.003%.

[0050] Element Sb has a lower oxidation potential than element Fe and is completely retained in molten steel. It is one of the inherent harmful residual elements in steel, which degrades and inhibits the strength of post-weld heat treatment. Therefore, the lower the value, the better. This invention limits Sb to ≤ 0.008%, and preferably Sb ≤ 0.005%.

[0051] Sn has a lower oxidation potential than Fe and is completely retained in molten steel. It is also one of the inherent harmful residual elements in steel, which degrades and inhibits the strength of post-weld heat treatment. Therefore, the lower the value, the better. This invention limits Sn to ≤ 0.008%, preferably Sn ≤ 0.005%.

[0052] In addition to the fact that elements such as P, As, Sb, and Sn can degrade the strength of the post-weld heat-treated state, they can also accumulate or even couple with each other, forming low-melting-point agglomerates or products that further degrade the strength of the post-weld heat-treated state. Therefore, in order to prevent the coupling side effects of harmful residual elements such as P, As, Sb, and Sn, this invention limits P+As+Sb+Sn≤0.020%.

[0053] As beneficial residual elements, the content of alloying elements such as Cr and Mo is not limited in principle. However, nitrogen can interact with metallic elements such as Cr, Mo, and Mn to form common single nitrides, such as Mn. x N z (In addition to M representing a metallic element, x, z, etc. representing numbers) it can also generate coupled nitrides M of two or more elements. x M y N z The present invention limits (Cr+Mo+Mn)×N≥0.01%, and through the coupling effect between beneficial elements Cr, Mo, Mn and N, precipitation strengthening is achieved to ensure strength, especially resistance to the deterioration of strength after welding heat treatment, effectively improving the room temperature / high temperature strength of carbon steel, the service temperature above 350℃, and resistance to more stringent post-weld heat treatment processes.

[0054] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0055] In terms of composition design, this invention rationally utilizes elements such as Mn, C, and N, which are beneficial to improving strength, and their contents. It discovers and fully utilizes the coupling effect between these elements, controlling (Cr+Mo+Mn)×N≥0.01% to generate a single nitride M. x N z Or generate a coupled nitride M of two or more elements. x M y Nz Through precipitation strengthening, the strength is guaranteed, especially to prevent the deterioration of the strength after welding heat treatment, effectively improving the room temperature / high temperature strength of carbon steel; and strictly limiting elements such as P and Sn that cause the deterioration of the strength after welding heat treatment, limiting P+As+Sb+Sn≤0.020%, to prevent the coupling side effects of harmful residual elements such as P, As, Sb, and Sn from forming low melting point agglomerates or products, thus ensuring the strength of carbon steel, especially its resistance to the deterioration of the strength after welding heat treatment.

[0056] Under the premise of meeting the above composition requirements, the steel of this invention still has the conventional ferrite + pearlite structure of carbon steel, requiring no heat treatment and can be used directly in the hot-rolled state. The room temperature yield strength of the hot-rolled state and the post-weld heat-treated state are ≥240MPa, the tensile strength is ≥415MPa, the instantaneous tensile yield strength at 350℃ is ≥175MPa, and the tensile strength is ≥350MPa. The obtained carbon steel can be used directly in the hot-rolled state, unlike existing technologies which require normalizing or other heat treatment processes. Attached Figure Description

[0057] Figure 1 This is a schematic diagram simulating the post-weld heat treatment process. Detailed Implementation

[0058] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0059] Table 1 shows the steel composition of the embodiments and comparative examples of the present invention, with the balance including Fe and unavoidable impurities. The properties of the steels of the embodiments and comparative examples of the present invention are shown in Table 2.

[0060] See Figure 1 The hot-rolled plates obtained in the embodiments and comparative examples of the present invention are directly subjected to simulated post-weld heat treatment, the process being (690℃×2h-furnace cooling to 400℃ air cooling and exiting the furnace), 3 times.

[0061] As can be seen from Table 2, the room temperature yield strength of the hot-rolled steel and the post-weld heat-treated steel (PWHT state) of the embodiments of the present invention is ≥240MPa, the tensile strength is ≥415MPa, the instantaneous tensile yield strength at 350℃ is ≥175MPa, and the tensile strength is ≥350MPa.

[0062] In Comparative Examples 1 and 2, even if the individual elemental composition meets the requirements of this invention, the tensile properties of the steel obtained in the end do not meet the strength index required by this invention because P+Sn+As+Sb≥0.02 in Comparative Example 1 and (Cr+Mn+Mo)×N≤0.01 in Comparative Example 2.

[0063]

[0064]

[0065]

Claims

1. A high-performance carbon steel for inhibiting post-weld heat treatment strength degradation, comprising the following composition by weight percentage: C: 0.05–0.25%, Si: 0.10–0.40%, Mn: 0.55–2.0%, N: 0.01–0.05%, S≤0.020%, P≤0.018%, As≤0.005%, Sb≤0.008%, Sn≤0.008%, with the balance including Fe and unavoidable impurities, and the above elements must simultaneously satisfy the following: P + As + Sb + Sn ≤ 0.020%; The unavoidable impurities include Cr and Mo, and the control is (Cr+Mo+Mn)×N≥0.010%.

2. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in claim 1, characterized in that, The carbon steel composition contains C: 0.08–0.20 wt%.

3. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in claim 1 or 2, characterized in that, The carbon steel composition contains Si: 0.15–0.35 wt%.

4. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in claim 1, 2, or 3, characterized in that, The carbon steel composition contains Mn: 0.80–1.75 wt%.

5. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in claim 1, 2, 3, or 4, characterized in that, The carbon steel composition contains N: 0.015–0.045 wt%.

6. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in claim 1, 2, 3, 4, or 5, characterized in that, In the carbon steel composition, S≤0.010wt% and / or P≤0.015wt% and / or As≤0.003wt% and / or Sb≤0.005wt% and / or Sn≤0.005wt%.

7. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in any one of claims 1 to 6, characterized in that, The balance consists of Fe and unavoidable impurities.

8. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in any one of claims 1 to 7, characterized in that, The microstructure of the carbon steel is ferrite + pearlite.

9. The high-performance carbon steel for suppressing post-weld heat treatment strength degradation as described in any one of claims 1 to 8, characterized in that, The carbon steel has a yield strength ≥240MPa at room temperature, a tensile strength ≥415MPa, and an instantaneous tensile yield strength ≥175MPa and a tensile strength ≥350MPa at 350℃.

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