Production method for improving flaw detection qualification rate of marine crankshaft steel

Abstract

The invention relates to a production method for improving the flaw detection qualification rate of marine crankshaft steel. The production method comprises the following steps: S1, a converter smelting process; s2, an LF refining procedure; s3, VD process: bottom nitrogen blowing is carried out in the whole VD process, the nitrogen mass content is controlled to be 0.010% or below, and no alloy or bract core wire is fed after VD vacuum breaking; s4, a continuous casting process: crystallizer stirring, secondary cooling stirring and tail end stirring are adopted, the superheat degree of a continuous casting tundish is controlled at 24-25 DEG C, the offline temperature is 650-680 DEG C, slow cooling is performed at 500-600 DEG C, and heat preservation is performed for 2-4 days; and S5, a continuous rolling procedure is conducted, specifically, the single-pass rolling reduction of a cogging mill is increased by 110-115 mm, the roller way speed is increased by 0.5-1.0 m / min, and the rolling temperature is controlled. The problem of insufficient impact performance is solved, and the product quality is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and specifically relates to a production method for improving the flaw detection qualification rate of marine crankshaft steel. Background Art

[0002] The steel used for crankshafts plays an important role in marine engines and is known as the "heart" of marine engines, playing a crucial role in the safe operation of ships. The steel for large crankshafts has requirements equivalent to the service life of ships, and the cost of crankshaft steel accounts for half of the total cost of the ship. Due to its large size, high smelting technical difficulty, high molten steel purity, and high flaw detection grade, during production, the purity of molten steel is mainly controlled, the internal quality of continuous casting billets is improved, and the flaw detection qualification rate is increased.

[0003] Currently, when producing this type of steel, Al wire is generally added after VD breaking the vacuum to control Al. The main problems of this type of process are as follows: (1) When feeding Al wire after breaking the vacuum, part of the molten steel where the Al wire is inserted is exposed, which is likely to cause secondary oxidation of the molten steel and affect the purity of the molten steel. (2) Aluminum oxide inclusions are formed, blocking the continuous casting nozzle and affecting production.

[0004] Many special steel mills do not add Ni and V when producing marine crankshaft steel. The problems are as follows: (1) Customers require high-temperature tempering, with a tempering temperature of 620 - 640 °C and high tensile strength requirements, and the present invention can meet the requirements. (2) After high-temperature tempering, the low-temperature impact requirement is AKV2(-20 °C) ≥ 34 J, and the present invention can meet the requirements.

[0005] Currently, N element is not added when producing marine crankshaft steel. The problem is that nitride inclusions are formed, especially TiN, which affects the impact of the product. Summary of the Invention

[0006] The purpose of the present invention is to provide a production method for improving the flaw detection qualification rate of marine crankshaft steel, which has a high flaw detection qualification rate, excellent mechanical properties, solves the problem of insufficient impact performance, and significantly improves the product quality. The technical solution of the present invention is as follows:

[0007] A production method for improving the flaw detection qualification rate of marine crankshaft steel includes the following processes:

[0008] S1. Converter smelting process: During converter smelting, light burned dolomite and quicklime are added, the oxygen supply time is 14 - 16 minutes, high-carbon tapping is carried out, tapping with a slide gate to prevent slag from flowing into the ladle, slagging during tapping is strictly prohibited, the tapping temperature is 1600 - 1612 °C, during the tapping process, steel core aluminum is added in sequence. To avoid the mutual diffusion of molten steel and slag, alloys and slag materials are added, and the mass content of P in the tapped molten steel ≤ 0.008%. To ensure diffusion deoxidation, aluminum wire is fed after tapping ends;

[0009] S2. LF refining process: During refining, quicklime and refining slag are added. The white slag holding time is 19 - 21 min; no ferrosilicon alloy is added in LF, but aluminum pellets are added; before the ladle is lifted after refining, aluminum wire is fed in.

[0010] S3. VD process: Nitrogen is blown from the bottom throughout the VD process, and the nitrogen mass content is controlled at 0.010% or less. After the VD breaks the vacuum, no alloy or cored wire is fed in to ensure that inclusions float up sufficiently.

[0011] S4. Continuous casting process: The continuous casting drawing speed is controlled at 0.35 - 0.38 m / min, the water volume in the mold is 4300 - 4500 L / min, the specific water volume is 0.11 - 0.12 L / kg. Mold stirring, secondary cooling stirring, and final stirring are adopted. The superheat in the tundish during continuous casting is controlled at 24 - 25 °C, the offline temperature is 650 - 680 °C, and slow cooling is carried out at 500 - 600 °C for 2 - 4 days.

[0012] S5. Continuous rolling process: Increase the single - pass reduction of the blooming mill by 110 - 115 mm, the roller table speed is 0.5 - 1.0 m / min, and the rolling temperature is controlled.

[0013] Preferably, during the converter smelting process in S1, the addition amount of light - burned dolomite is 20 - 25 kg / t of steel, and the addition amount of quicklime is 36 - 40 kg / t of steel.

[0014] Preferably, in S1, the C mass content of high - carbon tapping is ≥0.07%.

[0015] Preferably, in S1, the addition amount of aluminum - cored wire is 1.8 - 2.0 kg / t of steel.

[0016] Preferably, after the tapping is completed in S1, aluminum wire is fed in at 0.4 - 0.6 kg / t of steel.

[0017] Preferably, in S1, the alloy and slag materials include ferrosilicon with an addition amount of 1.3 - 1.9 kg / t of steel, low - carbon ferrochrome with 18 - 21 kg / t of steel, ferromanganese with 12 - 17 kg / t of steel, quicklime with 4.7 - 6 kg / t of steel, and refining slag with 5.7 - 6.7 kg / t of steel.

[0018] Preferably, in S2, during the refining process, the consumption of quicklime is 4.8 - 5.3 kg / t of steel, and the refining slag is 1 - 1.5 kg / t of steel;

[0019] During the LF refining process, the consumption of aluminum pellets is 0.4 - 0.6 kg / t of steel;

[0020] Before the ladle is lifted, aluminum wire is fed in according to a mass of 0.055 - 0.060%.

[0021] Preferably, in S3, the VD holding time is 17 - 19 min.

[0022] Preferably, in S4, the electric stirring current is 90 - 100 A and the electric stirring frequency is 4.0 - 5.0 HZ.

[0023] Preferably, in S5, the rolling temperature is 930 - 990 °C.

[0024] Preferably, the chemical composition and mass percentage of the marine crankshaft steel are as follows: C: 0.38 - 0.45%, Si: 0.15 - 0.37%, S ≤ 0.010%, P ≤ 0.015%, Cr: 1.00 - 1.20%, Mn: 0.70 - 0.80%, Mo: 0.15 - 0.30%, Ni: 0.30 - 0.50%, V: 0.05 - 0.20%, Al: 0.020 - 0.040%, N: 0.008 - 0.01%. Compared with the existing process methods, the present invention has the following advantages:

[0025] 1. In the present invention, the Si element alloy in the converter tapping is directly adjusted to the internal control composition, and aluminum grains are used for deoxidation in refining without using silicon carbide. The Ca element in ferrosilicon affects inclusions.

[0026] 2. The Ni and V elements can improve hardenability. V combines with N to form nitrides to increase the strength of the steel, and Ni can improve low-temperature impact. However, N combines with Ti to form titanium nitride, which affects impact. Therefore, during production, Ti ≤ 0.0030% is controlled. More sufficient vanadium nitride precipitates, the casting blank offline temperature is controlled, and the slow cooling time is increased.

[0027] 3. The present invention optimizes the wire feeding sequence, feeds aluminum wire before VD, reduces the secondary oxidation of molten steel. Bottom blowing nitrogen is carried out throughout the process, and no cored wire is added after VD, improving the purity of molten steel. The inclusions are fine.

[0028] 4. When blooming in the present invention, the single-pass reduction amount is increased, and at the same time, the speed of the blooming roller table is reduced, allowing the external force to penetrate the inside of the steel, improving the flaw detection qualification rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Appendix Figure 1 : Morphology diagram of inclusions in the steel of Example 1 (showing fine and uniform distribution).

[0030] Appendix Figure 2 : Macrostructure diagram of the casting blank of Example 1 (center porosity ≤ 0.5 grade).

[0031] Appendix Figure 3 : X-ray energy spectrum analysis diagram (EDS) of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0032] Example 1:

[0033] Taking the marine crankshaft steel produced by a certain steel plant as an example, the production route is: converter - LF - VD - CC. The specific production process is as follows:

[0034] 1. Converter steelmaking process: During the converter steelmaking process, 25 kg of light burned dolomite and 38 kg of quicklime are added per ton of steel. The oxygen supply time is 14 minutes. The tapping carbon content is 0.09%. The tapping is carried out with a slide gate slag stopper to prevent slag from entering the ladle during tapping. The tapping temperature is 1607 °C. During the tapping process, 1.8 kg of steel core aluminum, 1.3 kg of ferrosilicon, 19 kg of low-carbon ferrochrome, 12 kg of ferromanganese, 6 kg of quicklime, and 6 kg of refining slag are added per ton of steel in sequence. To avoid the mutual diffusion of molten steel and slag, the phosphorus content of the tapped molten steel should be ≤0.008%. To ensure diffusion deoxidation, 0.6 kg of aluminum wire is fed per ton of steel after tapping is completed.

[0035] 2. LF refining process: During the refining process, 5 kg of quicklime and 1 kg of refining slag are used per ton of steel. The white slag holding time is 21 minutes. No ferrosilicon alloy is added in LF. During the LF refining process, 0.5 kg of aluminum pellets are used per ton of steel. After the refining is completed and before the ladle is lifted, 0.055% of aluminum wire is fed.

[0036] 3. VD process: The VD holding time is 19 minutes. Nitrogen is blown from the bottom throughout the VD process, and the nitrogen content is controlled at 0.008%. No alloy or cored wire is fed after the VD breaks the vacuum to ensure that inclusions float up sufficiently.

[0037] 4. Continuous casting process: The continuous casting casting speed is controlled at 0.36 m / min. The water volume in the mold is 4300 L / min, and the specific water volume is 0.11 L / kg. Mold stirring, secondary cooling stirring, and final stirring are adopted. The electric stirring current is 100 A, and the electric stirring frequency is 5.0 HZ. The superheat of the tundish during continuous casting is controlled at 24 °C, the off-line temperature is 680 °C, and it is slowly cooled and insulated at 500 - 600 °C for 3 days.

[0038] 5. Continuous rolling process: Increase the single-pass reduction of the blooming mill by 115 mm, the roller table speed is 0.8 m / min, and the rolling temperature is controlled. The rolling temperature is 930 °C.

[0039] The morphology of inclusions in the steel is shown in the appendix Figure 1 and the macrostructure diagram of the continuous casting billet is shown in the appendix Figure 2 and the X-ray energy spectrum analysis is shown in the appendix Figure 3 .

[0040] Example 2:

[0041] Taking the marine crankshaft steel produced by a certain steel plant as an example, the production route is: converter - LF - VD - CC. The specific production process is as follows:

[0042] 1. Converter smelting process: During converter smelting, 23 kg / t of light burned dolomite and 40 Kg / t of quicklime are added. The oxygen supply time is 16 minutes. The tapping carbon content is 0.08%. The tapping is carried out with a slide gate slag stopper to strictly prevent slag from flowing into the ladle. The tapping temperature is 1610 °C. During tapping, 1.9 kg / t of steel core aluminum, 1.5 kg / t of ferrosilicon, 21 kg / t of low-carbon ferrochrome, 13 kg / t of ferromanganese, 5 kg / t of quicklime, and 6.7 kg / t of refining slag are added in sequence. The phosphorus content of the tapped molten steel is ≤0.007%. To ensure diffusion deoxidation, 0.5 kg / t of aluminum wire is fed into the molten steel after tapping.

[0043] 2. LF refining process: During refining, 4.8 kg / t of quicklime and 1.1 kg / t of refining slag are used. The white slag holding time is 20 min. No ferrosilicon alloy is added in LF. 0.4 kg / t of aluminum pellets are used during LF refining. After refining and before lifting the ladle, 0.060% of aluminum wire is fed into the molten steel.

[0044] 3. VD process: The VD holding time is 21 min. Nitrogen is blown from the bottom throughout the VD process, and the nitrogen content is controlled at 0.009%. No alloy or cored wire is fed after VD breaking vacuum to ensure that inclusions float up sufficiently.

[0045] 4. Continuous casting process: The continuous casting casting speed is controlled at 0.38 m / min. The water volume in the mold is 4400 L / min, and the specific water volume is 0.12 L / kg. Mold stirring, secondary cooling stirring, and final stirring are adopted. The electric stirring current is 90 A and the electric stirring frequency is 4.0 HZ. The superheat of the tundish during continuous casting is controlled at 25 °C, the off-line temperature is 660 °C, and it is slowly cooled and insulated at 500 - 600 °C for 2 days.

[0046] 5. Continuous rolling process: The single-pass reduction of the blooming mill is increased by 112 mm, the roller table speed is 0.5 m / min, and the rolling temperature is controlled. The rolling temperature is 950 °C.

[0047] Example 3:

[0048] Taking the marine crankshaft steel produced by a certain steel plant as an example, the production route is: converter - LF - VD - CC. The specific production process is as follows:

[0049] 1. Converter smelting process: During converter smelting, 20 kg / t of light burned dolomite and 39 Kg / t of quicklime are added. The oxygen supply time is 15 minutes. The tapping carbon content is 0.07%. The tapping is carried out with a slide gate slag stopper to strictly prevent slag from flowing into the ladle. The tapping temperature is 1600 °C. During tapping, 2.0 kg / t of steel core aluminum, 1.6 kg / t of ferrosilicon, 18 kg / t of low-carbon ferrochrome, 15 kg / t of ferromanganese, 4.7 kg / t of quicklime, and 5.7 kg / t of refining slag are added in sequence. The phosphorus content of the tapped molten steel is ≤0.006%. To ensure diffusion deoxidation, 0.4 kg / t of aluminum wire is fed into the molten steel after tapping.

[0050] 2. LF refining process: During the refining process, the consumption of quicklime is 5.1 kg / t of steel, the refining slag is 1.3 kg / t of steel, and the white slag holding time is 19 min; no ferrosilicon alloy is added in LF, and the consumption of aluminum pellets during the LF refining process is 0.6 kg / t of steel; after the refining is completed and before the ladle is lifted, aluminum wire is fed in at 0.058%.

[0051] 3. VD process: The VD holding time is 17 min, nitrogen is blown from the bottom throughout the VD process, the nitrogen content is controlled at 0.010%, and no alloy or cored wire is fed after the VD breaks the vacuum to ensure that inclusions float up sufficiently.

[0052] 4. Continuous casting process: The continuous casting drawing speed is controlled at 0.35 m / min, the water volume in the mold is 4500 L / min, the specific water volume is 0.11 L / kg, mold stirring, secondary cooling stirring, and final stirring are adopted, the electric stirring current is 90 A, the electric stirring frequency is 5.0 HZ, the superheat of the tundish during continuous casting is controlled at 25 °C, the offline temperature is 650 °C, and it is slowly cooled and kept at 500 - 600 °C for 4 days.

[0053] 5. Continuous rolling process: Increase the single-pass reduction of the blooming mill by 110 mm, the roller table speed is 1.0 m / min, and the rolling temperature is controlled. The rolling temperature is 990 °C.

[0054] Example 4:

[0055] Taking the crankshaft steel produced by a certain steel plant as an example, the production route is: converter - LF - VD - CC. The specific production process is as follows:

[0056] 1. Converter smelting process: During the converter smelting process, 17 kg / t of steel of light-burned dolomite, 36 Kg / t of steel of quicklime are added, the oxygen supply time is 15 minutes, the tapping C is 0.07%, the tapping is carried out with a slide gate to prevent slag from flowing into the ladle, and slagging during tapping is strictly prohibited. The tapping temperature is 1612 °C. During the tapping process, 2.0 kg / t of steel of steel core aluminum, 1.9 kg / t of steel of ferrosilicon, 18 kg / t of steel of low-carbon ferrochrome, 17 kg / t of steel of ferromanganese, 4.7 kg / t of steel of quicklime, and 5.8 kg / t of steel of refining slag are added in sequence. The P content of the tapped molten steel is ≤0.006%. To ensure diffusion deoxidation, aluminum wire is fed in at 0.6 kg / t of steel after the tapping is completed.

[0057] 2. LF refining process: During the refining process, the consumption of quicklime is 5.3 kg / t of steel, the refining slag is 1.5 kg / t of steel, and the white slag holding time is 19 min; no ferrosilicon alloy is added in LF, and the consumption of aluminum pellets during the LF refining process is 0.6 kg / t of steel; after the refining is completed and before the ladle is lifted, aluminum wire is fed in at 0.055%.

[0058] 3. VD process: The VD holding time is 17 min, nitrogen is blown from the bottom throughout the VD process, the nitrogen content is controlled at 0.008%, and no alloy or cored wire is fed after the VD breaks the vacuum to ensure that inclusions float up sufficiently.

[0059] 4. Continuous casting process: The continuous casting drawing speed is controlled at 0.35 m / min, the water volume in the mold is 4500 L / min, the specific water volume is 0.11 L / kg, and mold stirring, secondary cooling stirring, and end stirring are adopted. The electric stirring current is 90 A and the electric stirring frequency is 5.0 HZ. The superheat in the tundish during continuous casting is controlled at 25 °C, the offline temperature is 670 °C, and slow cooling is carried out at 500 - 600 °C for 3 days of heat preservation.

[0060] 5. Continuous rolling process: Increase the single-pass reduction of the blooming mill by 113 mm, the roller table speed is 0.9 m / min, and the rolling temperature is controlled at 980 °C.

[0061] The experimental parameters of Examples 1 - 4 and the properties of the obtained steel are shown in Tables 1 - 5.

[0062] Table 1 Chemical composition and its percentage content

[0063] Example C Si P S Cr Mn Mo Ni V Al N 1 0.38 0.15 0.009 0.010 1.00 0.73 0.16 0.35 0.16 0.026 0.008 2 0.40 0.27 0.010 0.008 1.10 0.75 0.15 0.30 0.05 0.020 0.009 3 0.41 0.26 0.008 0.004 1.20 0.80 0.25 0.47 0.18 0.029 0.010 4 0.45 0.37 0.015 0.005 1.13 0.70 0.30 0.50 0.20 0.040 0.008

[0064] Table 2 Inclusion inspection of Examples 1 - 4

[0065]

[0066] Table 3 Macrostructure quality of Examples 1 - 4

[0067] Example Central porosity General porosity Shrinkage cavity Subsurface crack Central segregation Subsurface blowhole 1 0.5 - - - - - 2 0.5 - - - - - 3 0.5 - 0.5 - - - 4 0.5 - 0.5 - - -

[0068] Table 4 Qualification rate of magnetic particle inspection of crankshaft steel in Examples 1 - 4

[0069] Example Specification Flaw detection standard Whether qualified 1 230 GB / T4162 Grade A Qualified 2 235 GB / T4162 Grade A Qualified 3 220 GB / T4162 Grade A Qualified 4 240 GB / T4162 Grade A Qualified

[0070] Table 5 Mechanical property inspection of Examples 1 - 4

[0071] Example Rm, MPa ReL, MPa A，％ Z，％ KV2(-20℃), J 1 900 650 16 55 39 2 980 680 18 53 34 3 1000 670 17 58 42 4 920 690 19 53 39

[0072] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application belong to the scope protected by the present invention.

Claims

1. A production method for improving the flaw detection qualification rate of marine crankshaft steel, characterized in that, It includes the following processes: S1. Converter smelting process: During converter smelting, light-burned dolomite and quicklime are added. The oxygen supply time is 14 - 16 minutes. Steel is tapped at high carbon drawing. The tapping is carried out with a slide gate slag stopper to prevent slag from flowing into the ladle. The tapping temperature is 1600 - 1612°C. During tapping, steel core aluminum is added successively, followed by alloys and slag materials. The mass content of P in the tapped molten steel is ≤0.008%. After tapping, aluminum wire is fed; S2. LF refining process: During refining, quicklime and refining slag are added. The white slag holding time is 19 - 21 min; No ferrosilicon alloy is added in LF, but aluminum pellets are added; After refining and before lifting the ladle, aluminum wire is fed; S3. VD process: Nitrogen is blown from the bottom throughout the VD process, and the mass content of nitrogen is controlled at 0.010% and below. No alloys and cored wire are fed after the VD breaks the vacuum; S4. Continuous casting process: The continuous casting casting speed is controlled at 0.35 - 0.38 m / min, the water volume in the mold is 4300 - 4500 L / min, the specific water volume is 0.11 - 0.12 L / kg. Mold stirring, secondary cooling stirring, and final stirring are adopted. The superheat in the tundish during continuous casting is controlled at 24 - 25°C, the off-line temperature is 650 - 680°C, and slow cooling is carried out at 500 - 600°C for 2 - 4 days; S5. Continuous rolling process: Increase the single-pass reduction of the blooming mill by 110 - 115 mm, the roller table speed is 0.5 - 1.0 m / min, and the rolling temperature is controlled; 2. The production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S1, the addition amount of light-burned dolomite during converter smelting is 20 - 25 kg / t of steel, and the addition amount of quicklime is 36 - 40 kg / t of steel.

3. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S1, the mass content of C during high carbon drawing tapping is ≥0.07%.

4. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S1, the addition amount of steel core aluminum is 1.8 - 2.0 kg / t of steel; In S1, after tapping, aluminum wire is fed at 0.4 - 0.6 kg / t of steel.

5. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S1, the alloys and slag materials include ferrosilicon with an addition amount of 1.3 - 1.9 kg / t of steel, low-carbon ferrochrome with 18 - 21 kg / t of steel, ferromanganese with 12 - 17 kg / t of steel, quicklime with 4.7 - 6 kg / t of steel, and refining slag with 5.7 - 6.7 kg / t of steel.

6. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S2, the consumption of quicklime during refining is 4.8 - 5.3 kg / t of steel, and the refining slag is 1 - 1.5 kg / t of steel; The consumption of aluminum pellets during LF refining is 0.4 - 0.6 kg / t of steel; Before lifting the ladle, aluminum wire is fed at a mass of 0.055 - 0.060%.

7. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S3, the VD holding time is 17 - 19 min.

8. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that In S4, the electric stirring current is 90 - 100 A, and the electric stirring frequency is 4.0 - 5.0 HZ.

9. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, In S5, the rolling temperature is 930 - 990°C.

10. A production method for improving the flaw detection qualification rate of marine crankshaft steel according to claim 1, characterized in that, The chemical composition and mass percentage of the marine crankshaft steel are C: 0.38 - 0.45%, Si: 0.15 - 0.37%, S ≤ 0.010%, P ≤ 0.015%, Cr: 1.00 - 1.20%, Mn: 0.70 - 0.80%, Mo: 0.15 - 0.30%, Ni: 0.30 - 0.50%, V: 0.05 - 0.20%, Al: 0.020 - 0.040%, N: 0.008 - 0.01%.

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