Method for improving low-temperature toughness of reinforcing steel bar used in special environment

Abstract

The invention discloses a method for improving the low-temperature toughness of a steel bar used in a special environment. The method comprises the steps of molten iron desulfurization, converter smelting, LF refining, VD vacuum degassing and small square billet continuous casting. The steel bar comprises the following chemical components in percentage by mass: 0.09 to 0.12 percent of C, 0.20 to 0.30 percent of Si, 1.10 to 1.35 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, 0.30 to 0.35 percent of Cu, 0.20 to 0.30 percent of Cr, 0.40 to 0.50 percent of Ni, 0.03 to 0.05 percent of V, 0.03 to 0.05 percent of Nb, 0.0010 to 0.0050 percent of La or Ce and the balance of iron and impurities. The yield strength Rp0.2 of the rolled steel bar is larger than or equal to 425 MPa, the tensile strength Rm is larger than or equal to 560 MPa, the ductility A is larger than or equal to 20%, and the uniform ductility Agt is larger than or equal to 10%.

Description

Technical Field

[0001] The invention belongs to the field of smelting and rolling technology, and in particular relates to a method for improving the low-temperature toughness of steel bars used in special environments. Background Art

[0002] Reinforced concrete structures are often used in extreme conditions. Their working conditions are not only low temperature and corrosive environment, but also have to withstand dynamic loads and caused by natural or technical reasons. Lower temperature and corrosive environment increase the probability of brittle failure of reinforced structures, causing accidents in building structures. There is little research and development of building steel bars for extremely cold and complex corrosive environments in China, and there is no application. This poses a great hidden danger to buildings in this area. Therefore, a special environment steel bar production is developed, but it is found during the trial production process that the low-temperature impact performance requirements cannot be met in the hot-rolled state. In order to improve its toughness, a method for improving the low-temperature toughness of steel bars for special environments is proposed. Summary of the invention

[0003] The object of the present invention is to provide a method for improving the low temperature toughness of steel bars for special environments, wherein the yield strength R p0.2 ≥425MPa, tensile strength R m ≥560MPa, elongation A≥20%, uniform elongation A gt ≥10%, strength-to-yield ratio R m 0 / R eL 0 ≥1.25, flexural ratio R eL 0 / R eL ≤1.30.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The present invention discloses a method for improving the low-temperature toughness of steel bars for special environments, comprising molten iron desulfurization - converter smelting - LF refining - VD vacuum degassing - billet continuous casting; wherein:

[0006] The KR method is used for desulfurization of molten iron. The desulfurizer is mainly limestone, with a small amount of fluorite and aluminum as flux. The converter adopts double-blowing converter smelting, and the final slag basicity is controlled at 3.0. In order to ensure low phosphorus, suitable tapping temperature and relatively stable tapping carbon, the converter adopts low carbon drawing process, and the converter endpoint control target C is not more than 0.06%, and the tapping temperature is controlled to be not less than 1630℃. Refining uses ferromanganese, metallic manganese, low-carbon ferrochrome, nickel plate or nickel-iron alloy for fine-tuning of composition, and aluminum deoxidation is used for final deoxidation. After VD vacuum degassing, some denaturants need to be added to the molten steel. Agent, adding rare earth alloy to steel to make non-metallic inclusions denatured and spheroidized, forming CeOS type non-metallic inclusions, eliminating or reducing their adverse effects on steel properties, thereby purifying steel, reducing the total amount of inclusions, and reducing corrosion between the contact surface of inclusions and steel matrix; adding vanadium nitrogen alloy and niobium iron alloy before steeling; continuous casting molten steel is cast under protective gas, and the drawing speed is controlled at 1.9±0.2m / min. Due to the addition of alloy, weak cooling is adopted for the secondary cooling water distribution of continuous casting to prevent surface cracks and internal defects of the ingot, and at the same time, the constant drawing speed is guaranteed as much as possible to obtain the continuous casting ingot;

[0007] The heating temperature of the ingot is controlled at 1100-1250℃, the temperature of the ingot out of the furnace is 1100-1200℃, the starting rolling temperature is 1000-1050℃, and the final rolling temperature is 850-910℃; water penetration is not allowed during the rolling cooling process; heat treatment is performed after rolling;

[0008] The chemical composition of the steel bar by mass percentage is: C: 0.09-0.12%, Si: 0.20-0.30%, Mn: 1.10-1.35%, P: ≤0.015%, S: ≤0.010%, Cu: 0.30-0.35%, Cr: 0.20-0.30%, Ni: 0.40-0.50%, V: 0.03-0.05%, Nb: 0.03-0.05%, La or Ce: 0.0010-0.0050%, and the balance is Fe and inevitable impurity elements.

[0009] Furthermore, the chemical composition of the steel bar in mass percentage is: C: 0.11%, Si: 0.25%, Mn: 1.25%, P: 0.012%, S: 0.003%, Cu: 0.32%, Cr: 0.24%, Ni: 0.43%, V: 0.039%, Nb: 0.032%, Ce: 0.0021%, and the remainder is Fe and inevitable impurity elements.

[0010] Furthermore, a tempering heat treatment process was selected: 350°C, keeping warm for 2 hours and then air cooling.

[0011] Furthermore, the yield strength R of the steel bar after rolling p0.2 ≥425MPa, tensile strength R m≥560MPa, elongation A≥20%, uniform elongation A gt ≥10%, strength-to-yield ratio R m 0 / R eL 0 ≥1.25, flexural ratio R eL 0 / R eL ≤1.30.

[0012] 5. The method for improving the low temperature toughness of steel bars for special environments according to claim 3, characterized in that after tempering: the -50°C impact performance meets the requirement of ≥34J.

[0013] Compared with the prior art, the beneficial technical effects of the present invention are:

[0014] The object of the present invention is to provide a method for improving the low temperature toughness of steel bars for special environments, wherein the yield strength R p0.2 ≥425MPa, tensile strength R m ≥560MPa, elongation A≥20%, uniform elongation A gt ≥10%, strength-to-yield ratio R m 0 / R eL 0 ≥1.25, flexural ratio R eL 0 / R eL ≤1.30.

[0015] After tempering: -50℃ impact performance meets the requirement of ≥34J. DETAILED DESCRIPTION

[0016] The chemical composition of the steel bars involved in this example is shown in Table 1. Take the iron ore melt to make molten iron, and the molten iron is desulfurized by the KR method, that is, the molten iron is stirred by a stirring paddle with a speed of 90r / min, and after stirring for 2min, a desulfurizer is added, and the desulfurizer is a mixture of lime powder and fluorite in a mass ratio of 9:1, and the reaction is stirred for 10min and allowed to stand for 5min. After the molten iron is desulfurized and allowed to stand, the desulfurization slag is removed to stabilize the desulfurization effect, prevent the desulfurization slag from entering the converter and causing the converter to return to sulfur, and ensure that the sulfur content in the steel is controlled below 0.01%. Top and bottom combined blowing is used for decarburization and dephosphorization, and steel is tapped at 1650℃ until the carbon content in the molten steel is less than 0.05% and the phosphorus content is less than 0.01%. During the tapping process, protective gas is used to stir the molten steel at a pressure of 0.5MPa. When the tapping is 1 / 4, ferrosilicon and silicon-manganese alloy are added for deoxidation, and carburizer, coke and slag-making materials are added. A large amount of slag is prevented during tapping. At the same time, the protective gas pressure gradually decreases with the amount of tapping. Adopt LF furnace refining, deoxidize at 1575℃ until the oxygen content in the molten steel is 0.002%, add ferromanganese, nickel plate, ferrochrome, vanadium nitrogen alloy, ferroniobium alloy, copper plate, feed calcium iron alloy wire after refining, and ensure that the soft blowing time is not less than 15min. After VD degassing, add rare earth alloy at a yield rate of 30%. Continuous casting tries to ensure constant pulling speed for steel drawing, the pulling speed is controlled at 1.9m / min, the cross-section of the ingot is 150mm×150mm, the length of the ingot is 11.5m, and the protective slag uses a special protective slag for peritectic steel with a basicity of 1.2-1.4. The continuous casting billet is heated to 1250±50℃, and the temperature measured after taking out of the furnace is 1160±30℃. After descaling with high-pressure water, it is rolled at a temperature of 1030±20℃. The final rolling specification is Φ20mm. After rough rolling and intermediate rolling, the final rolling temperature is controlled at 890±20℃. The steel bars cannot be penetrated by water during the rolling process. Then they are put on the cooling bed and enter the finishing and bundling process.

[0017] Table 1 Chemical composition / %

[0018] C And min P S With Cr us Nb V What 0.11 0.25 1.25 0.012 0.003 0.32 0.24 0.43 0.032 0.039 0.0021

[0019] After rolling, the hot rolled steel bars were taken for testing of mechanical properties and low temperature impact properties, as shown in Tables 2 and 3.

[0020] Table 2 Mechanical properties results

[0021] <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A％ <![CDATA[A gt / %]]> <![CDATA[R m 0 / R eL 0 ]]> <![CDATA[R eL 0 / R eL ]]> 427 589 25.0 14 1.38 1.07

[0022] Table 3 Low temperature impact performance results

[0023]

[0024] From Table 3, we can see that the low-temperature impact performance of the steel bar is low in the hot-rolled state. In order to improve the low-temperature toughness, the rolled steel bar is subjected to tempering heat treatment. According to the above composition, the tempering heat treatment process is selected as follows: 350°C, air cooling after keeping warm for 2 hours, and low-temperature impact performance test after cooling to room temperature. The test results are shown in Table 4.

[0025] Table 4 Low temperature impact performance results after tempering heat treatment

[0026]

[0027]

[0028] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving the low temperature toughness of steel bars for special environments, characterized in that: Including hot metal desulfurization - converter smelting - LF refining - VD vacuum degassing - billet continuous casting; among which: The KR method is used for desulfurization of molten iron. The desulfurizer is mainly limestone, with a small amount of fluorite and aluminum as flux. The converter adopts double-blowing converter smelting, and the final slag basicity is controlled at 3.

0. In order to ensure low phosphorus, suitable tapping temperature and relatively stable tapping carbon, the converter adopts low carbon drawing process, and the converter endpoint control target C is not more than 0.06%, and the tapping temperature is controlled to be not less than 1630℃. Refining uses ferromanganese, metallic manganese, low-carbon ferrochrome, nickel plate or nickel-iron alloy for fine-tuning of composition, and aluminum deoxidation is used for final deoxidation. After VD vacuum degassing, some denaturants need to be added to the molten steel. Agent, adding rare earth alloy to steel to make non-metallic inclusions denatured and spheroidized, forming CeOS type non-metallic inclusions, eliminating or reducing their adverse effects on steel properties, thereby purifying steel, reducing the total amount of inclusions, and reducing corrosion between the contact surface of inclusions and steel matrix; adding vanadium nitrogen alloy and niobium iron alloy before steeling; continuous casting molten steel is cast under protective gas, and the drawing speed is controlled at 1.9±0.2m / min. Due to the addition of alloy, weak cooling is adopted for the secondary cooling water distribution of continuous casting to prevent surface cracks and internal defects of the ingot, and at the same time, the constant drawing speed is guaranteed as much as possible to obtain the continuous casting ingot; The heating temperature of the ingot is controlled at 1100-1250℃, the temperature of the ingot out of the furnace is 1100-1200℃, the starting rolling temperature is 1000-1050℃, and the final rolling temperature is 850-910℃; water penetration is not allowed during the rolling cooling process; heat treatment is performed after rolling; The chemical composition of the steel bar by mass percentage is: C: 0.09-0.12%, Si: 0.20-0.30%, Mn: 1.10-1.35%, P: ≤0.015%, S: ≤0.010%, Cu: 0.30-0.35%, Cr: 0.20-0.30%, Ni: 0.40-0.50%, V: 0.03-0.05%, Nb: 0.03-0.05%, La or Ce: 0.0010-0.0050%, and the balance is Fe and inevitable impurity elements.

2. The method for improving the low temperature toughness of steel bars for special environments according to claim 1, characterized in that: The chemical composition of the steel bar by mass percentage is: C: 0.11%, Si: 0.25%, Mn: 1.25%, P: 0.012%, S: 0.003%, Cu: 0.32%, Cr: 0.24%, Ni: 0.43%, V: 0.039%, Nb: 0.032%, Ce: 0.0021%, and the remainder is Fe and unavoidable impurity elements.

3. The method for improving the low temperature toughness of steel bars for special environments according to claim 1, characterized in that: Select tempering heat treatment process: 350℃, keep warm for 2 hours and then air cool.

4. The method for improving the low temperature toughness of steel bars for special environments according to claim 1, characterized in that: The yield strength R of the steel bar after rolling p0.2 ≥425MPa, tensile strength R m ≥560MPa, elongation A≥20%, uniform elongation A gt ≥10%, strength-to-yield ratio R m 0 / R eL 0 ≥1.25, flexural ratio R eL 0 / R eL ≤1.

30.

5. The method for improving the low temperature toughness of steel bars for special environments according to claim 3, characterized in that: After tempering: -50℃ impact performance meets the requirement of ≥34J.

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