A 1670mpa grade steel bar for prestressed concrete

Abstract

This invention relates to a 1670MPa grade prestressed concrete steel bar, belonging to the field of prestressed concrete steel bars. It was primarily invented to address the problem of delayed fracture in existing prestressed concrete steel bars. Wire rod is used as the raw material, with the following chemical composition by mass percentage: C: 0.35–0.40%, Si: 1.60–1.80%, Mn: 0.5–0.8%, P: ≤0.020%, S: ≤0.015%, Cr: 0.20–0.30%, Nb: 0.03–0.05%, V: 0.04–0.07%, Ti: 0.04–0.07%, with the balance being Fe and impurities. The wire rod production process includes smelting, continuous casting, heating, rolling, and cooling. The wire rod undergoes mechanical descaling, drawing, forming, induction heating quenching, induction heating tempering, holding and secondary tempering, and water cooling after holding to obtain this product. Its advantage is resistance to delayed fracture.

Description

Technical fields:

[0001] This invention belongs to the field of steel bars for prestressed concrete, and in particular relates to a steel bar for prestressed concrete that can meet the strength level of 1670MPa. Background technology:

[0002] Steel bars for prestressed concrete are mainly used in prestressed concrete pipe piles. They are generally made of medium-carbon low-alloy steel wire rod, such as 30MnSi, which is drawn and spiral-grooved, induction heated to above 900℃ for quenching, and tempered above 400℃ to obtain a tempered troostite structure, which has good comprehensive mechanical properties and is widely used in the market. The strength grade of the steel bars is generally 1470MPa. With the development of large-diameter pipe piles and industries such as seaports and docks, the use of 1570MPa grade steel bars is constantly increasing. In order to further improve the construction efficiency of prestressed concrete projects and save steel bars and other materials, the market demand for 1670MPa grade prestressed concrete steel bars has emerged. To improve hardenability, alloying elements such as Si and Mn are added to the steel of prestressed concrete steel bars to improve hardenability. Induction heating is used in the steel bar production process, which heats up very quickly. Alloying elements and impurities in the steel cannot be completely dissolved during the austenitization process and accumulate at the original austenite grain boundaries, becoming hydrogen accumulation points. When the steel bar has surface defects or central segregation that produces core martensite, it will cause delayed fracture of the steel bar during subsequent use.

[0003] Currently, the steel grade used for 1470MPa grade prestressed concrete steel bars is 30MnSi, with a C content of 0.28-0.33%, a Si content of 0.60-0.90%, and a Mn content of 0.90-1.30%, without other alloying elements. The strength of this steel bar can only reach the 1470MPa level, and it is prone to delayed fracture in winter. The commonly used steel grade for 1570MPa grade steel bars is 35Si2Mn, with a C content of 0.34-0.38%, a Si content of 1.55-1.85%, and a Mn content of 0.60-0.90%, without other alloying elements. Due to the low Si content of 35Si2Mn steel and the lack of added microalloying elements to inhibit delayed fracture, delayed fracture problems frequently occur in the steel bars. Therefore, the development of 1670MPa grade PC steel bars, in addition to improving the strength of the steel bars, also needs to reduce their delayed fracture sensitivity, lower their yield strength ratio, improve uniform elongation performance, and prevent rapid fracture after yielding. Summary of the Invention:

[0004] The technical problem to be solved by the present invention is to provide a 1670MPa grade prestressed concrete steel bar, which uses wire rod as the raw material for the steel bar. The steel bar produced has the characteristics of high tensile strength, good plasticity, resistance to delayed fracture, and stable product performance, and can well meet the requirements of industries such as large-size pipe piles.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The steel bar raw material is wire rod, and the chemical composition and mass percentage of the wire rod are as follows: C: 0.35-0.40%, Si: 1.60-1.80%, Mn: 0.5-0.8%, P: ≤0.020%, S: ≤0.015%, Cr: 0.20-0.30%, Nb: 0.03-0.05%, V: 0.04-0.07%, Ti: 0.04-0.07%, with the balance being Fe and unavoidable impurities.

[0007] The selection of the amount (by weight percentage) of each of the above elements and their functions are explained below:

[0008] C: In this invention, carbon (C) is used to improve the strength of the steel bar. C can effectively increase the strength of steel through the formation of carbides. However, when the C content is too high, it will affect the weldability of the steel bar and reduce its plasticity. Therefore, in this invention, the C content is controlled at 0.35-0.40%.

[0009] Silicon (Si) is used to improve the hardenability of steel, enhance strength through solid solution strengthening, increase tempering resistance, and improve the tempering stability of the microstructure, thereby reducing the degree to which strength decreases with increasing tempering temperature. Higher Si content results in stronger resistance to delayed fracture. However, excessive Si content affects roll welding performance; based on tensile strength calculations, the silicon content should be controlled between 1.60% and 1.80%.

[0010] Mn: Mn combines with S to form MnS. MnS can act as a hydrogen embrittlement initiation point to induce delayed fracture and increase the delayed fracture sensitivity of high-strength steel. Mn can strengthen through solid solution and improve hardenability, but its effect is relatively small. Therefore, the Mn content is controlled at 0.5% to 0.8%.

[0011] S and P: S forms MnS inclusions with Mn in steel, while P tends to segregate at grain boundaries, reducing the toughness of the steel and increasing the tendency for intergranular fracture. Therefore, the P content should be controlled below 0.020%, and the S content should be controlled below 0.015%.

[0012] Cr: It can improve hardenability and has a strengthening effect, increasing the strength of steel and its corrosion resistance. It also has a certain inhibitory effect on hydrogen atoms entering the steel matrix. In this invention, the Cr content is controlled at 0.20-0.30%.

[0013] Nb, V, and Ti: Nb, V, and Ti can refine grains and reduce grain boundary element segregation. They form compounds with C and N that precipitate during tempering. The precipitated niobium carbonitride and titanium vanadium carbonitride are effective hydrogen traps. These transgranular hydrogen traps capture hydrogen absorbed by the steel matrix, preventing its diffusion, reducing the hydrogen concentration at grain boundaries, and preventing grain boundary cracking. Therefore, the addition of Nb, V, and Ti can prevent delayed fracture of steel bars.

[0014] The above-mentioned method for producing wire rod and steel bars for prestressed steel bars includes the production of wire rod as raw material for steel bars and the production of steel bars. The specific steps are as follows:

[0015] 1. Wire rod production:

[0016] (1) Smelting: The process employs converter smelting followed by ladle refining. Molten iron is desulfurized and slag-removed before being poured into the converter, with approximately 10% of self-produced scrap steel added to reduce residual element content. The carbon content of the steel tapped from the converter is 0.20-0.30%. A carbon raiser is added after the furnace, and ferrosilicon and ferromanganese are used as deoxidizers. Alloying elements such as Nb, V, and Ti are added and their content adjusted to ensure the chemical composition and temperature of the molten steel meet process requirements. After refining, the steel is transferred to the continuous casting process, where it is continuously cast into 180*180mm billets via a tundish and crystallizer. A long nozzle protection system is used throughout the casting process. The tundish superheat is controlled at 20-30℃, the casting speed is 1.5m / s, and electromagnetic stirring is used in the crystallizer. Light pressure is applied at the end of solidification to prevent center segregation.

[0017] (2) A six-machine, six-strand billet continuous casting machine is used to continuously cast 180mm×180mm billets. An immersion-type long nozzle is used from the molten steel ladle to the tundish and from the tundish to the crystallizer. Argon gas is introduced for full-process protection during casting. The crystallizer is electromagnetically stirred. At the end of solidification, the billet is lightly pressed.

[0018] (3) Heating: The billet is heated at 1000-1100℃ for 1.5-2 hours. To ensure the surface quality of the wire rod, the descaling pressure is 18-22 MPa.

[0019] (4) Rolling temperature: The temperature of entering the finishing mill is 880℃~930℃, and the wire rod output temperature is controlled at 880℃~900℃;

[0020] (5) Wire rod cooling: After spinning, the wire rod is air-cooled on the air-cooling roller conveyor at a cooling rate of 5-6℃ / s.

[0021] 2. Steel bar production;

[0022] Process: Wire rod — mechanical descaling — drawing — spiral groove forming — induction heating quenching — induction heating tempering — heat preservation and secondary tempering — water cooling after heat preservation — winding — packaging.

[0023] The quenching temperature is 920-950℃, the tempering temperature is 420-450℃, and the secondary tempering temperature is 400-430℃. Secondary tempering increases the residence time of the steel bar at the tempering temperature, increases carbide precipitation, and the secondary precipitation makes the precipitated phase distribution more dispersed, effectively improving the comprehensive mechanical properties and stress corrosion resistance of the steel bar.

[0024] Cable winding and packaging. Large coils are used for winding and packaging to prevent scratches.

[0025] The steel bar for 1670MPa prestressed concrete described in this invention uses wire rod as the raw material. The steel bar produced has the characteristics of high tensile strength, good plasticity, resistance to delayed fracture, and stable product performance, which can well meet the requirements of industries such as large-size pipe piles.

[0026] This invention improves the tensile strength of steel bars by modifying the chemical composition design of the wire rod and by adopting a secondary tempering process during the steel bar production process. This not only meets the strength requirement of 1670MPa, but also improves the uniform elongation performance and resistance to delayed fracture of the steel bars.

[0027] Through the analysis of the mechanism of improving the strength and plasticity of high-strength prestressed concrete steel bars and enhancing their resistance to delayed fracture, the influence of various chemical elements on the tensile strength and plasticity of steel bars, and the role of alloying elements in strengthening the original austenite grain boundaries and reducing the sensitivity to delayed fracture, it was determined that medium-carbon, high-silicon, and low-alloy steel should be used, the Si content should be increased, and the tempering temperature of steel bars should be increased by adding Cr, Nb, V, and Ti elements. A secondary tempering technology was also developed to improve the plasticity and resistance to delayed fracture while ensuring the tensile strength of PC steel bars.

[0028] The advantages of this invention are:

[0029] This invention uses wire rod as the raw material for steel bars. Compared with ordinary strength steel bars, the 1670MPa grade produced has the characteristics of high tensile strength, good plasticity, resistance to delayed fracture, and stable product performance.

[0030] This invention, by adding certain amounts of elements such as C, Si, Cr, Nb, V, and Ti, produces steel bars with excellent mechanical properties and resistance to delayed fracture, meeting the requirements of industries such as large-diameter pipe piles.

[0031] This invention employs a secondary tempering process, which significantly improves the overall mechanical properties of the steel bar. Attached image description:

[0032] Figure 1 The microstructure of the controlled-rolling and controlled-cooling wire rod in this invention shows that the wire rod has fine grains and very little proeutectoid ferrite content, which is beneficial for quenching and induction heating austenitization.

[0033] Figure 2 This is the tempered martensite structure of the steel bar in this invention.

[0034] Figure 3 These are transmission electron microscope (TEM) images of the precipitated phases such as titanium vanadium carbonitride in this invention. Detailed implementation method:

[0035] The following is combined Figure 1-3 The present invention will be further described;

[0036] This invention uses wire rod as the raw material for steel bars. The chemical composition and mass percentage of the wire rod are as follows: C: 0.35-0.40%, Si: 1.60-1.80%, Mn: 0.5-0.8%, P: ≤0.020%, S: ≤0.015%, Cr: 0.20-0.30%, Nb: 0.03-0.05%, V: 0.04-0.07%, Ti: 0.04-0.07%, with the balance being Fe and unavoidable impurities.

[0037] Example 1;

[0038] The chemical composition and mass percentage of the wire rod are: C: 0.37%, Si: 1.72%, Mn: 0.72%, P: 0.014%, S: 0.006%, Cr: 0.25%, Nb: 0.04%, V: 0.05%, Ti: 0.05%, with the balance being Fe and unavoidable impurities.

[0039] Example 2;

[0040] The chemical composition and mass percentage of the wire rod are as follows: C: 0.36%, Si: 1.70%, Mn: 0.68%, P: 0.013%, S: 0.005%, Cr: 0.26%, Nb: 0.04%, V: 0.06%, Ti: 0.07%, with the balance being Fe and unavoidable impurities.

[0041] Example 3;

[0042] The chemical composition and mass percentage of the wire rod are as follows: C: 0.38%, Si: 1.71%, Mn: 0.71%, P: 0.015%, S: 0.006%, Cr: 0.25%, Nb: 0.05%, V: 0.06%, Ti: 0.06%, with the balance being Fe and unavoidable impurities.

[0043] Comparative example;

[0044] The mass percentages of each chemical component are: C: 0.36%, Si: 1.70%, Mn: 0.70%, P: 0.015%, S: 0.007%; the balance is Fe and unavoidable impurities.

[0045] All embodiments of the present invention are manufactured according to the following process, which is described in detail below with reference to the process of this patent:

[0046] 1. Wire rod production;

[0047] (1) Smelting: The process involves converter smelting and ladle refining. The molten iron used in the converter is desulfurized and slag removed. Ferrosilicon and ferromanganese are used as deoxidizers. Alloying elements such as Nb, V, and Ti are added and their contents are adjusted to ensure that the chemical composition and temperature of the molten steel meet the process requirements.

[0048] (2) Continuous casting: After refining, the billet is hoisted to the continuous casting process and continuously cast into a 180*180mm billet through the tundish and crystallizer. The entire process is protected by a long nozzle during casting. The superheat of the tundish is controlled at 20-30℃, the casting speed is 1.5m / s, and electromagnetic stirring is used in the crystallizer. At the end of solidification, the pressure is lightly applied to prevent center segregation.

[0049] (3) Heating: billet heating temperature and time: 1000-1100℃, heating time 1.5-2h. To ensure the surface quality of the wire rod, the descaling pressure is 18-22MPa.

[0050] (4) Rolling temperature: The temperature of entering the finishing mill is 880℃~930℃, and the temperature of wire rod extrusion is controlled at 880℃~900℃.

[0051] (5) Wire rod cooling: After spinning, the wire rod is air-cooled on the air-cooling roller conveyor at a cooling rate of 5-6℃ / s.

[0052] 2. Steel bar production;

[0053] Process: Wire rod — mechanical descaling — drawing — spiral groove forming — induction heating quenching — induction heating tempering — heat preservation and secondary tempering — water cooling after heat preservation — winding — packaging.

[0054] The quenching temperature is 920-950℃, the tempering temperature is 420-450℃, and the secondary tempering temperature is 400-430℃. Secondary tempering increases the residence time of the steel bar at the tempering temperature, increases carbide precipitation, and the secondary precipitation makes the precipitated phase distribution more dispersed, effectively improving the comprehensive mechanical properties and stress corrosion resistance of the steel bar.

[0055] Cable winding and packaging. Large coils are used for winding and packaging to prevent scratches.

[0056] The chemical composition and production process implementation comparison are shown in Table 1-4.

[0057] Table 1 shows a comparison of chemical composition (Wt%).

[0058]

[0059] Table 2 shows the specific process parameters for the wire rod embodiments and comparative examples of the present invention.

[0060]

[0061] Table 3 Steel Bar Manufacturing Process

[0062]

[0063] Table 4. Test Results of Steel Bar Performance

[0064]

[0065] As can be seen from Table 1-4, compared with ordinary wire rods and steel bars, the wire rods and steel bars have excellent performance and meet the various performance requirements of 1670MPa level by adding C, Si, Cr, Nb, V and Ti alloying elements and advanced steel bar production process.

Claims

1. A steel bar for 1670MPa grade prestressed concrete, characterized in that: The steel bar raw material is wire rod, and the chemical composition and mass percentage of the wire rod are as follows: C: 0.35-0.40%, Si: 1.60-1.80%, Mn: 0.5-0.8%, P: ≤0.020%, S: ≤0.015%, Cr: 0.20-0.30%, Nb: 0.03-0.05%, V: 0.04-0.07%, Ti: 0.04-0.07%, with the balance being Fe and unavoidable impurities; The specific steps for producing steel bars from raw material wire rod are as follows: Wire rod production: (1) Smelting: The converter smelting + ladle refining is adopted. The molten iron in the converter is desulfurized and slag is removed. 10% of self-produced scrap steel is added in the converter to reduce the residual element content of the steel. The carbon content of the steel produced from the converter is 0.20-0.30%. A carbon raiser is added after the furnace. Ferrosilicon and ferromanganese are used as deoxidizers. Nb, V and Ti alloying elements are added and the content of alloying elements is adjusted so that the chemical composition and temperature of the molten steel meet the process requirements. After refining, it is hoisted to the continuous casting process and continuously cast into 180*180mm billets through the tundish and crystallizer. The casting process employs a long nozzle protection throughout, with the superheat of the tundish controlled at 20-30℃, a casting speed of 1.5m / s, and electromagnetic stirring in the crystallizer. Light pressure is applied at the end of solidification to prevent center segregation. (2) A six-machine, six-strand billet continuous casting machine is used to continuously cast 180mm×180mm billets. An immersion-type long nozzle is used from the molten steel ladle to the tundish and from the tundish to the crystallizer. Argon gas is introduced for full-process protection during casting. The crystallizer is electromagnetically stirred. At the end of solidification, the billet is lightly pressed. (3) Heating: The billet heating temperature is 1000-1100℃, and the heating time is 1.5-2h. To ensure the surface quality of the wire rod, the descaling pressure is 18-22MPa. (4) Rolling temperature: The temperature of entering the finishing mill is 880℃~930℃, and the wire rod extrusion temperature is controlled at 880℃~900℃; (5) Wire rod cooling: After spinning, the wire rod is air-cooled on the air-cooling roller conveyor at a cooling rate of 5-6℃ / s; Steel bar production; Process: Wire rod — mechanical descaling — drawing — spiral groove forming — induction heating quenching — induction heating tempering — heat preservation and secondary tempering — water cooling after heat preservation; The quenching temperature is 920-950℃, the tempering temperature is 420-450℃, and the secondary tempering temperature is 400-430℃.

2. The 1670MPa grade prestressed concrete steel bar according to claim 1, characterized in that: The chemical composition and mass percentage of the wire rod are: C: 0.37%, Si: 1.72%, Mn: 0.72%, P: 0.014%, S: 0.006%, Cr: 0.25%, Nb: 0.04%, V: 0.05%, Ti: 0.05%, with the balance being Fe and unavoidable impurities.

3. The 1670MPa grade prestressed concrete steel bar according to claim 1, characterized in that: The chemical composition and mass percentage of the wire rod are as follows: C: 0.36%, Si: 1.70%, Mn: 0.68%, P: 0.013%, S: 0.005%, Cr: 0.26%, Nb: 0.04%, V: 0.06%, Ti: 0.07%, with the balance being Fe and unavoidable impurities.

4. The 1670MPa grade prestressed concrete steel bar according to claim 1, characterized in that: The chemical composition and mass percentage of the wire rod are as follows: C: 0.38%, Si: 1.71%, Mn: 0.71%, P: 0.015%, S: 0.006%, Cr: 0.25%, Nb: 0.05%, V: 0.06%, Ti: 0.06%, with the balance being Fe and unavoidable impurities.

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