Microalloying slab production process capable of reducing hydrogen evolution

Abstract

The invention belongs to the technical field of microalloyed slab production processes, and particularly discloses a microalloyed slab production process capable of reducing hydrogen evolution, which comprises the following steps: drying molten iron, scrap steel and alloy entering a converter, controlling the water content of the scrap steel, putting the molten iron and the scrap steel into the converter for smelting, the method comprises the following steps: carrying out combined blowing by combining bottom argon blowing and top oxygen blowing, controlling the flow rate of top oxygen blowing according to the silicon content in molten iron, tapping by using a slag-stopping tapping technology, carrying out vacuum degassing and feeding a calcium line into molten steel during refining, adjusting the cooling water amount and keeping the temperature of the molten steel in a crystallizer according to the casting speed during continuous casting, and simultaneously stirring by using an electromagnetic stirring technology; by controlling key links of production of the microalloyed plate blank, hydrogen fused into molten steel is controlled in different procedures, and the content of hydrogen in the microalloyed plate blank is reduced, so that the precipitation amount of hydrogen is reduced; the problems of hydrogen embrittlement and white spots of steel caused by hydrogen precipitation in the production process of microalloyed slabs are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-alloyed slab production process, and particularly relates to a micro-alloyed slab production process for reducing hydrogen precipitation. BACKGROUND

[0002] The micro-alloyed slab is high-performance steel material made by adding trace alloying elements (such as Nb, V, Ti, etc.) in ordinary billets. These elements significantly improve the strength, toughness and weldability of the material through mechanisms such as grain refinement and precipitation strengthening, while maintaining a low carbon equivalent. The production process of the micro-alloyed slab mainly includes: hot metal pretreatment, converter / electric furnace smelting, LF / RH / VD refining, continuous casting, slab slow cooling, surface cleaning and detection, finally obtaining a micro-alloyed slab with high strength, high toughness and uniform internal quality. The micro-alloyed slab is widely used in oil and gas pipelines, automobile beams, engineering machinery and other fields with high requirements for material performance, and is usually combined with controlled rolling and controlled cooling process to fully develop its performance potential. In the smelting process of molten steel, due to factors such as raw materials, refractory materials and moisture in the atmosphere, hydrogen dissolves in the high-temperature liquid steel in atomic state, dissolving a certain amount of hydrogen in the molten steel. In the subsequent solidification and cooling process, hydrogen atoms are difficult to fully diffuse and discharge. When the hydrogen content in the steel is high, hydrogen molecules will be formed under certain conditions, causing huge internal stress, resulting in hydrogen embrittlement and white spot defects in the steel, which seriously reduces the strength, toughness and fatigue performance of the steel, affects the reliability of the steel in the subsequent processing and use process, and further causes hydrogen precipitation problems. If the precipitation rate is too fast, hydrogen will produce "white spot" defects or micro-cracks in the slab, especially in the high-strength matrix of micro-alloyed steel, which has higher hydrogen embrittlement sensitivity and may cause delayed fracture in subsequent rolling or service. SUMMARY

[0003] The purpose of the present application is to provide a micro-alloyed slab production process for reducing hydrogen precipitation, to solve the problem of hydrogen embrittlement and white spot caused by hydrogen precipitation in the production process of micro-alloyed slab.

[0004] To achieve the above purpose, the basic scheme provided by the present application is: a micro-alloyed slab production process for reducing hydrogen precipitation, comprising the following steps: S1. Dry the molten iron, scrap steel and alloy entering the converter, control the water content of the scrap steel to be less than 0.1%, and put the dried molten iron and scrap steel into the converter for smelting; S2. In the converter smelting process, use a combination of bottom argon blowing and top oxygen blowing, control the top oxygen flow according to the silicon content of the molten iron, control the blowing time to be 14-16 min, and control the bottom argon flow to be 0.05-0.15 m 3When the molten steel is tapped into the ladle, the slag-tapping tapping technology is used; S3. After the ladle enters the refining furnace, vacuum degassing is first performed, the vacuum degree is reduced to below 67 Pa within 3-8 min, calcium wire is fed into the molten steel, and then the refined ladle is sent to continuous casting; S4. During continuous casting, the cooling water amount is adjusted according to the casting speed, the temperature of the molten steel in the crystallizer is controlled to be maintained at 1510-1530 DEG C, and at the same time, the molten steel is stirred by applying an alternating magnetic field in the crystallizer by using electromagnetic stirring technology.

[0005] The principle and beneficial effects of the present application are that: before smelting, the raw materials for smelting are first dried, which can reduce the hydrogen dissolved in the molten steel during smelting, in the smelting stage, the bottom argon blowing and the top oxygen blowing are combined to strengthen the molten pool stirring, and the hydrogen diffusion efficiency is improved, the composite blowing creates more uniform molten steel conditions for subsequent vacuum degassing, in the tapping stage, the slag-tapping tapping technology can reduce the amount of slag brought in from 5-8 kg / t of steel in the traditional method to 1-2 kg / t of steel, reducing the amount of slag brought in can directly reduce the hydrogen absorption amount of the molten steel, after the slag-tapping tapping, the slag layer thickness of the ladle is reduced from 80 mm to below 30 mm, the hydrogen content of the molten steel is reduced by 0.3-0.5 ppm, the slag-tapping tapping reduces the contact area of the slag and the molten steel, in the refining stage, rapidly reducing the vacuum degree can quickly remove the hydrogen in the molten steel, reduce the gas hole defect, shorten the refining time, in the continuous casting stage, the temperature of the billet is controlled by distributing the water amount in sections, further reducing the hydrogen content in the billet, by controlling the key links of micro-alloyed plate billet production, the hydrogen dissolved in the molten steel is controlled in different processes, the hydrogen content in the micro-alloyed plate billet is effectively reduced, thereby reducing the precipitation amount of hydrogen and improving the quality of the plate billet.

[0006] Scheme two, this is a preferred embodiment of the basic scheme, in step S1, the temperature of the molten iron is raised to 1300-1350 DEG C before entering the converter, and maintained for 10-15 minutes; the scrap steel is dried in a drying kiln before entering the furnace, the drying temperature is controlled at 200-250 DEG C, and the drying time is 2-3 hours; the alloy is dried by vacuum drying before entering the furnace, the drying temperature is controlled at 150-200 DEG C, and the drying time is 1-2 hours; can reduce the water content and hydrogen source in the raw materials, reduce the decomposition of water during subsequent high-temperature smelting to dissolve hydrogen into the molten steel, thereby significantly reducing the hydrogen content in the steel, reducing the burden of subsequent hydrogen removal process, improving production efficiency and product qualification rate.

[0007] Scheme three, this is a preferred embodiment of the basic scheme, in step S2, when 0.3%≤ silicon element mass percentage of molten iron≤0.8%, the top oxygen blowing flow is 2.5-3.5 m 3 / (t·min), and the blowing time is controlled at 14-16 min; when the silicon element mass percentage of molten iron is >0.8%, the short-time strong oxygen supply flow of top oxygen blowing is 3.5-3.8 m 33.2-3.5 m 3 3.2-3.5 m, blowing time is 13-15 min; the specific top blowing oxygen flow is calculated according to the silicon content of the molten iron; the mixing time of the molten steel under the composite blowing condition is shortened by 30%-50% compared with the simple top blowing, which shortens the diffusion path of hydrogen atoms, improves the degassing efficiency, and reduces the hydrogen content at the end of the converter by 0.5-0.8 ppm compared with the simple top blowing.

[0008] Scheme four, which is a preferred scheme of scheme three, the calculation formula of the top blowing oxygen flow according to the silicon content of the molten iron is: , wherein QO2(Si) refers to the oxygen amount required for silicon oxidation, w(Si) refers to the mass fraction of silicon in the molten iron, G refers to the weight of the molten iron, Q refers to the oxygen amount required, t refers to the smelting time, and QO2 refers to the oxygen amount required for carbon and manganese oxidation and temperature rise; the gas entrainment caused by over-oxidation or spitting can be reduced, the risk of hydrogen absorption by the molten steel is indirectly reduced, the molten pool temperature can be stabilized by precisely controlling the oxygen, water decomposition can be avoided, the hydrogen content can be effectively controlled, and the steel purity can be improved.

[0009] Scheme five, which is a preferred scheme of the basic scheme, in step S3, the vacuum degree is maintained at 67 Pa or below for 15-20 min, and the calcium wire feeding amount is 0.8-1.2 kg / t; the driving force for hydrogen precipitation is rapidly expanded in the high vacuum environment, so that the dissolved hydrogen efficiently escapes in the form of bubbles, the opportunity for secondary hydrogen absorption by the molten steel can be reduced, and short-time efficient degassing can ensure the hydrogen removal rate and avoid excessive temperature drop affecting the continuous casting process.

[0010] Scheme six, which is a preferred scheme of the basic scheme, in step S4, when the casting speed fluctuates in the range of 0.6-1.0 m / min, the molten steel temperature in the crystallizer is stabilized by combining the fixed water amount of the crystallizer and the dynamic water amount of the secondary cooling, the fixed water amount is set to a fixed value according to the demand when producing different specifications of slabs, and the temperature difference between the inlet and outlet water is controlled to be 8-12℃; the dynamic water amount of the secondary cooling is linearly adjusted according to the casting speed, when the casting speed is 0.6 m / min, the total water amount of the secondary cooling is 65 m 3 / h, and the water amount of the secondary cooling increases by 10-12 m 3 / h for each increase of 0.1 m / min of the casting speed; the alternating magnetic field strength is controlled to be 50-100 mT, and the stirring frequency is 3-5 Hz; the molten steel temperature in the crystallizer is stabilized by combining the fixed water amount of the crystallizer and the dynamic water amount of the secondary cooling, the dynamic water amount of the secondary cooling is linearly adjusted according to the casting speed of the billet, the surface cracks and internal defects caused by uneven cooling of the cast billet are reduced, the risk of hydrogen enrichment at the cracks is reduced, and the temperature gradient of the solidification front is stabilized, which indirectly improves the hydrogen embrittlement resistance of the billet.

[0011] Scheme seven, which is preferred for scheme six, when the superheat of the molten steel in the crystallizer is > 35℃, the secondary cooling water quantity is increased by 5%-8% under the same casting speed; the solidification of the surface layer of the casting blank can be accelerated to form a dense blank shell, the enrichment of hydrogen to the liquid phase area is inhibited, the hydrogen diffusion capacity of the solidification front is reduced through the high solubility temperature range of hydrogen, and more hydrogen is dissolved in the matrix rather than precipitated as molecular hydrogen, thereby reducing the risk of hydrogen-induced cracking. DETAILED DESCRIPTION

[0012] The application will be further described in detail by specific embodiments: EMBODIMENT A microalloyed slab production process for reducing hydrogen precipitation, comprising the following steps: S1. Dry the molten iron, scrap steel and alloy entering the converter to raise the temperature to 1300-1350℃ and maintain for 10-15 minutes; before the scrap steel enters the furnace, it is dried in a drying kiln, the drying temperature is controlled at 200-250℃, the drying time is 2-3 hours, and the water content of the scrap steel is controlled below 0.1%, the dried molten iron and scrap steel are put into the converter for smelting; before the alloy enters the furnace, it is dried in a vacuum drying manner, the drying temperature is controlled at 150-200℃, and the drying time is 1-2 hours; S2. During the converter smelting process, composite blowing is combined by bottom argon blowing and top oxygen blowing, the top oxygen flow is controlled according to the silicon content of the molten iron, and the blowing time is controlled within 14-16 min; when 0.3%≤ mass percentage of silicon element of the molten iron≤0.8%, the top oxygen flow is 2.5-3.5m 3 / (t·min), and the blowing time is controlled at 14-16 min; when the mass percentage of silicon element of the molten iron is >0.8%, the short-time strong oxygen flow is 3.5-3.8m 3 / (t·min), and is reduced to 3.2-3.5m 3 / (t·min) after 6-8 min, the blowing time is 13-15 min, and the calculation formula of the top oxygen flow according to the silicon content of the molten iron is: , wherein QO2(Si) refers to the oxygen quantity required for silicon oxidation, w(Si) refers to the mass fraction of silicon in the molten iron, G refers to the weight of the molten iron, Q refers to the oxygen quantity required, t refers to the smelting time, and QO2 refers to the oxygen quantity required for carbon and manganese oxidation and temperature rise; the flow of bottom argon blowing is controlled at 0.05-0.15m 3 / (t·min), and the molten steel is tapped into the ladle by adopting the slag-blocking tapping technology; S3. After the ladle enters the refining furnace, vacuum degassing is first performed, the vacuum degree is reduced to below 67 Pa within 3-8 min, the duration is 15-20 min, calcium wire is fed into the molten steel, the calcium wire feeding amount is 0.8-1.2 kg / t, and then the refined ladle is sent to continuous casting; S4. During continuous casting, when the casting speed fluctuates in the range of 0.6-1.0 m / min, the molten steel temperature in the crystallizer is stabilized by combining the fixed water amount of the crystallizer and the dynamic water amount of the secondary cooling, when different specifications of slabs are produced, the fixed water amount is set as a fixed value according to the requirements, and the temperature difference between the inlet and outlet water is controlled to be 8-12℃; the dynamic water amount of the secondary cooling is linearly adjusted according to the casting speed, when the casting speed is 0.6 m / min, the total water amount of the secondary cooling is 65 m 3 / h, when the casting speed increases by 0.1 m / min, the water amount of the secondary cooling increases by 10-12 m 3 / h, the temperature of the molten steel in the crystallizer is controlled to be 1510-1530℃, when the superheat of the molten steel in the crystallizer is >35℃, the water amount of the secondary cooling is increased by 5%-8% under the same casting speed, at the same time, the electromagnetic stirring technology is used to apply an alternating magnetic field to the molten steel in the crystallizer for stirring, the alternating magnetic field strength is controlled to be 50-100 mT, and the stirring frequency is 3-5 Hz.

[0013] The implementation mode of the embodiment is: when producing a micro-alloyed slab with a specification of 250×1800 mm, the raw materials are pretreated first, the temperature of the molten iron is increased to 1300-1350℃ before entering the converter, and is maintained for 10-15 min; the scrap steel is dried in a drying kiln before entering the converter, the drying temperature is controlled to be 200-250℃, and the drying time is 2-3 hours; the alloy is dried in a vacuum drying manner before entering the converter, the drying temperature is controlled to be 150-200℃, and the drying time is 1-2 hours, and then the molten iron and the scrap steel are put into the converter for smelting according to the conventional smelting process.

[0014] During the converter smelting process, the combined blowing of bottom argon blowing and top oxygen blowing is used, the top oxygen flow is controlled according to the silicon content of the molten iron, and the blowing time is controlled to be 14-16 min; when 0.3%≤ the mass percentage of silicon element in the molten iron≤0.8%, the top oxygen flow is 2.5-3.5 m 3 / (t·min), the blowing time is controlled to be 14-16 min; when the mass percentage of silicon element in the molten iron is >0.8%, the short-time strong oxygen flow is 3.5-3.8 m 3 / (t·min), it is reduced to 3.2-3.5 m 3 / (t·min) after 6-8 min, the blowing time is 13-15 min, and the specific top oxygen flow is calculated according to the silicon content of the molten iron, and the calculation formula is: , wherein QO2(Si) refers to the oxygen amount required for silicon oxidation, w(Si) refers to the mass fraction of silicon in the molten iron, G refers to the weight of the molten iron, Q refers to the oxygen amount required, t refers to the smelting time, QO2 refers to the oxygen amount required for carbon and manganese element oxidation and temperature rise, the argon flow rate of the bottom argon blowing is controlled to be 0.05-0.15 m 3 / (t·min), when the smelted molten steel is tapped into a ladle, the slag-blocking tapping technology is used for tapping, and the top oxygen flow rate and oxygen blowing time corresponding to different silicon contents in the molten iron when producing 250×1800 mm micro-alloyed slabs are shown in Table 1: Table 1 Top oxygen flow rate and oxygen blowing time corresponding to different silicon contents in the molten iron Molten iron silicon content w[Si] (%) Top-blown oxygen flow range (m3 / (t·min)) Oxygen blowing time reference (min) ≤0.30 2.5-2.8 14-16 0.30-0.50 2.8-3.2 13-15 0.50-0.80 3.2-3.5 12-14 >0.80 3.5-3.8 (short term) 15-17 After the ladle enters the refining furnace, vacuum degassing is first performed, the vacuum degree is reduced to below 67 Pa within 3-8 min, the vacuum degree is maintained below 67 Pa for 15-20 min, then the micro-alloying elements niobium and vanadium are added, the dried alloy is accurately controlled according to the target composition, the refining slag system adopts high-alkalinity slag, after the refining is completed, calcium wire is fed into the molten steel, the calcium wire feeding amount is 0.8-1.2 kg / t, the inclusions are fully modified, and then the refined ladle is sent to continuous casting.

[0015] In the continuous casting process, the cooling water amount is adjusted according to the casting speed, when the casting speed fluctuates in the range of 0.6-1.0 m / min, the molten steel temperature in the mold is stabilized by combining the fixed water amount of the mold and the dynamic water amount of the secondary cooling, the molten steel temperature in the mold is controlled to be maintained at 1510-1530℃, when producing different specifications of slabs, the fixed water amount is set to a fixed value according to the demand, and the temperature difference between the inlet and outlet water is controlled to be 8-12℃; the mold cooling of the 250×1800 mm slab needs to ensure that the outlet slab shell thickness is ≥25 mm, the fixed water amount is designed to be 380-420 m 3 / h, wherein the wide face accounts for 85%-90%, the wide face cooling water amount is 325-370 m 3 / h, double circulation is adopted, the single-side water amount is 160-185 m 3 / h, the narrow face cooling water amount is 55-60 m 3 / h, double-side symmetry is adopted, the slab corner temperature is controlled to avoid corner cracks, when the inlet water temperature is 30-35℃, the outlet water temperature is 40-47℃; The dynamic water amount of the secondary cooling is linearly adjusted according to the casting speed, when the casting speed is 0.6 m / min, the total water amount of the secondary cooling is 65 m 3 / h, the secondary cooling water amount increases by 10-12 m 3When the superheat of the liquid steel in the crystallizer is > 35℃, the secondary cooling water quantity is increased by 5%-8% at the same casting speed, and the electromagnetic stirring technology is used to apply an alternating magnetic field to the liquid steel in the crystallizer for stirring, the alternating magnetic field strength is controlled at 50-100mT, the stirring frequency is 3-5Hz, and when the micro-alloyed slab of 250x1800mm specification is produced, the total secondary cooling water quantity and the liquid steel temperature at the crystallizer outlet corresponding to different casting speeds are adjusted as shown in Table 2: Table 2 The total secondary cooling water quantity and the liquid steel temperature at the crystallizer outlet corresponding to different casting speeds Withdrawal speed (m / min) Total secondary cooling water amount (m3 / h) Mold exit molten steel temperature (℃) 0.6-0.7 65-75 1525-1530 0.7-0.8 75-85 1520-1525 0.8-0.9 85-95 1515-1520 0.9-1.0 95-105 1510-1515 Through production practice, the comparison results before and after the improvement of the micro-alloyed slab production process of 250x1800mm are shown in Table 3: Table 3 The comparison results before and after the improvement of the micro-alloyed slab production process of 250x1800mm Process Before improvement After improvement Hydrogen content in composite blowing process during smelting (ppm) 2.8 2.1 Hydrogen increment in slag blocking process at tapping (ppm) 0.5 0.2. Hydrogen content in controlled cooling water process during continuous casting (ppm) 2.5 1.8 Total process hydrogen content (ppm) 3.0-4.5 1.0-1.5 Ultrasonic flaw detection qualification rate (%) 85-90 96 or more Yield strength fluctuation range (MPa) ±20 ±10 Hydrogen precipitation defect occurrence rate (%) 1.5-2.0 0.1 or less Layered defect occurrence rate (%) 0.8-1.2 0.05 or less The above-mentioned is only the embodiment of the present application, and the well-known specific structure and characteristics and other common knowledge in the scheme are not described too much. It should be pointed out that for those skilled in the art, without departing from the structure of the present application, a number of modifications and improvements can be made, which should also be regarded as the protection scope of the present application, which will not affect the effect and practicality of the present application. The protection scope of the present application should be subject to the content of its claims, and the specific implementation mode and the like in the specification can be used to explain the content of the claims.

Claims

1. A process for producing microalloyed slabs that reduces hydrogen evolution, characterized in that, Includes the following steps: S1. First, dry the molten iron, scrap steel and alloy entering the converter, control the moisture content of the scrap steel to be less than 0.1%, and put the dried molten iron and scrap steel into the converter for smelting; S2. During the converter smelting process, a combined blowing method using bottom-blown argon and top-blown oxygen is employed. The top-blown oxygen flow rate is controlled according to the silicon content of the molten iron, and the blowing time is controlled at 14-16 minutes. The bottom-blown argon flow rate is controlled at 0.05-0.15 m³ / min. 3 / (t·min), when tapping the molten steel into the ladle, the slag-blocking tapping technology is used; S3. After the ladle enters the refining furnace, it is first degassed under vacuum. Within 3-8 minutes, the vacuum level is reduced to below 67 Pa. Calcium wire is then fed into the molten steel, and the refined ladle is then sent to continuous casting. S4. During continuous casting, the cooling water volume is adjusted according to the billet pulling speed to control the temperature of the molten steel in the crystallizer to be maintained at 1510-1530℃. At the same time, electromagnetic stirring technology is used to apply an alternating magnetic field to stir the molten steel in the crystallizer.

2. The microalloyed slab production process for reducing hydrogen evolution according to claim 1, characterized in that, In step S1, before the molten iron enters the converter, the temperature is raised to 1300-1350℃ and maintained for 10-15 minutes; before the scrap steel enters the furnace, it is dried in a drying kiln with the drying temperature controlled at 200-250℃ and the drying time being 2-3 hours; before the alloy enters the furnace, it is dried in a vacuum with the drying temperature controlled at 150-200℃ and the drying time being 1-2 hours.

3. The microalloyed slab production process for reducing hydrogen evolution according to claim 1, characterized in that, In step S2, when the mass percentage of silicon in the molten iron is 0.3% ≤ 0.8%, the top-blown oxygen flow rate is 2.5-3.5 m³ / h. 3 / (t·min), the blowing time is controlled at 14-16min; when the mass percentage of silicon in the molten iron is >0.8%, the short-term strong oxygen supply flow rate of the top-blown oxygen is 3.5-3.8m³. 3 / (t·min), decreasing to 3.2-3.5m after 6-8 minutes. 3 / (t·min), the blowing time is 13-15min; the specific top-blown oxygen flow rate is calculated based on the silicon content of the molten iron.

4. The microalloyed slab production process for reducing hydrogen evolution according to claim 3, characterized in that, The formula for calculating the top-blown oxygen flow rate based on the silicon content of molten iron is as follows: In this context, QO2(Si) refers to the amount of oxygen required for silicon oxidation, w(Si) refers to the mass fraction of silicon in molten iron, G refers to the weight of molten iron, Q refers to the amount of oxygen required, t refers to the smelting time, and QO2 refers to the amount of oxygen required for the oxidation of carbon and manganese and for heating.

5. The microalloyed slab production process for reducing hydrogen evolution according to claim 1, characterized in that, In step S3, maintain the vacuum level below 67 Pa for 15-20 minutes, and feed calcium wire at a rate of 0.8-1.2 kg / t.

6. The microalloyed slab production process for reducing hydrogen evolution according to claim 1, characterized in that, In step S4, when the drawing speed fluctuates within the range of 0.6-1.0 m / min, the temperature of the molten steel in the crystallizer is stabilized by combining the fixed water volume of the crystallizer and the dynamic water volume of the secondary cooling system. When producing slabs of different specifications, the fixed water volume is set to a fixed value according to the requirements, and the temperature difference between the inlet and outlet water is controlled to be 8-12℃. The secondary cooling water flow is linearly adjusted according to the pumping speed. When the pumping speed is 0.6 m / min, the total secondary cooling water flow is 65 m³ / min. 3 / h, for every 0.1m / min increase in pulling speed, the secondary cooling water flow rate increases by 10-12m³ / h. 3 / h; the alternating magnetic field strength is controlled at 50-100mT, and the stirring frequency is 3-5Hz.

7. The microalloyed slab production process for reducing hydrogen evolution according to claim 6, characterized in that, When the superheat of the molten steel in the crystallizer is greater than 35°C, the amount of secondary cooling water increases by 5%-8% at the same casting speed.

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