CN105803150B - The chrome molybdenum series circular pipe blank slab quality control method of φ 280mm - Google Patents

Abstract

The invention discloses an internal quality control method for producing chrome-molybdenum series round tubular steel billets with a section size of φ280mm in the field of metallurgy. The method includes steps such as converter smelting, LF furnace refining molten steel, RH refining, and continuous casting molten steel. treatment, which improves its high-temperature plasticity and reduces the crack sensitivity of steel grades. Then, in the process of pouring molten steel, the combination of electromagnetic stirring in the mold and electromagnetic stirring at the end of solidification is used to homogenize the composition and temperature of the molten steel, and the billet shell Uniform growth, the solidification structure of the slab is effectively improved, the porosity rating is improved, the area of the equiaxed grain area is enlarged, the compactness of the central area of the round slab is effectively improved, the crack defects of the slab are effectively controlled, the surface non-cleaning rate is improved, and the rolled product The quality control is good and stable.

Description

The quality control method of chrome-molybdenum series round tube billets with a diameter of φ280mm

technical field

The invention relates to the field of iron and steel metallurgy, in particular to a method for controlling the quality of chrome-molybdenum series round tube blanks with a production scale section of φ280mm.

Background technique

Chromium-molybdenum series tube billet steel is mainly used in the production of oil well pipe steel for oil and gas exploitation and transportation. Under the joint action of stress and hydrogen sulfide gas during use, it often occurs suddenly when the force is much lower than its own yield strength. Brittle fracture (called hydrogen sulfide stress corrosion) often causes problems such as production leakage, which has an adverse effect on production and environmental protection. With the development of society and economy, the development trends such as higher requirements for oil and gas transportation efficiency and increased mining difficulty continue to put forward more stringent requirements for the quality of billets. In particular, defects such as slab center looseness and center segregation have adverse effects on the casting slab rolling process and the quality of finished products.

The central segregation and porosity of the continuous casting slab are mainly due to the strong directionality of solidification and heat transfer of the slab during the continuous casting production process, resulting in over-developed columnar crystals, and the redistribution of solute elements between the solid and liquid phases during the solidification of molten steel leads to The concentration of solute alloying elements in the post-solidification part of the slab is relatively high; moreover, the development of columnar crystals leads to uneven nucleation and crystallization during the solidification process in the center of the slab, and a decrease in the compactness of the structure. After solidification, the molten steel solidifies and shrinks to form loose defects. The formed center segregation and center porosity cannot be effectively eliminated in the subsequent heating and rolling process, which affects product quality. For the chrome-molybdenum series tube billet steel, the composition itself determines that the columnar crystals of the cast slab are more developed, the center is more loose, and the center segregation tends to increase. Furthermore, since the slab has a relatively large section and is a round slab, its specific surface area is smaller and the heat transfer efficiency of the slab is lower, making it more difficult to control the internal quality of the slab.

Contents of the invention

In order to overcome the shortcomings of the existing chrome-molybdenum series round tubular steel billets with relatively large cross-sectional sizes, which are generally low in internal quality and prone to cracks, the technical problem to be solved by the present invention is to provide a chrome-molybdenum series with a cross-sectional size of φ280mm A control method for enhancing the internal quality of round tubular steel slabs.

The technical solution adopted by the present invention to solve its technical problems is:

The method for controlling the quality of the chrome-molybdenum series round tube billets of φ280mm is characterized in that it comprises the following steps:

Ⅰ. Converter smelting: The carbon at the end point of the converter is controlled according to the mass percentage of 0.05%~0.15%, the end point temperature is controlled at ≥1670°C, and the small platform is fed with aluminum wire at a feed rate of 1.4~2.0m per ton of steel for deoxidation treatment. The target composition of the platform is controlled by mass percentage: [C]0.20%～0.35%, [Si]0.16%～0.30%, [Mn]1.00%～1.40%, [P]≤0.013%, at least 3min after deoxidation alloying soft argon blowing treatment;

Ⅱ. Refining molten steel in LF furnace: Refining the molten steel obtained in step Ⅰ in LF furnace, blowing argon and stirring throughout the refining process, adding titanium alloy material at the end of refining, and adding the amount according to 0.018% to 0.038% of the weight of molten steel. Afterwards, 200-500m of feeding amount is fed to silicon-calcium line for calcium treatment, and the first batch of tundish is fed according to the upper limit. After refining, continue to blow argon and stir for at least 5 minutes, and the exit temperature is controlled at 1560-1620°C;

Ⅲ. RH refining: RH refining is carried out on the molten steel obtained in step Ⅱ. RH refining requires that the argon flow rate is controlled at 1150-1450NL/min, the vacuum degree is less than 3mbar, the processing time is ≥10min, and the mass percentage of RH outbound Alt is 0.030%～ 0.055% control, continue to blow argon for at least 5 minutes after the vacuum treatment, and control the exit temperature at 1550-1580°C;

Ⅳ. Continuous casting of molten steel: The continuous casting process adopts the method of combining electromagnetic stirring of the mold and electromagnetic stirring at the end of solidification. Among them, the parameters of electromagnetic stirring of the mold are: stirring current 200-350A, 2-4Hz; parameters of electromagnetic stirring at the end of solidification For: stirring current 100-150A, frequency 4.0-7.0Hz.

Further, in the smelting process of steps I, II and III, medium-intensity argon blowing and agitation are used, subject to no large boiling on the surface of the molten steel; soft argon blowing and agitation is used after smelting to make the molten steel slightly agitated.

Furthermore, the following parameters need to be controlled during pouring of molten steel in step IV: the degree of superheat is controlled at 25-38°C, the casting speed is controlled at 0.60-1.20m/min, and the cooling of the crystallizer is controlled at 2400-2500L/min , The specific water volume of the secondary cooling is controlled at 0.21-0.33/kg _steel .

The beneficial effects of the present invention are as follows: firstly, the purity of molten steel is improved according to reasonable parameter control in the molten steel smelting stage, and by micro-titanium treatment of molten steel, its high-temperature plasticity is improved, and the crack sensitivity of steel grades is reduced. In the process of molten steel pouring, the combination of mold electromagnetic stirring and solidification end electromagnetic stirring is adopted, so that the composition and temperature of molten steel are homogenized, the slab shell grows uniformly, the solidification structure of the slab is effectively improved, the porosity rating is improved, and the equiaxed grain The area area is expanded, the compactness of the central area of the round billet is effectively improved, the crack defects of the cast billet are effectively controlled, the surface no-cleaning rate is improved, and the quality control of the rolled product is good and stable.

Detailed ways

The present invention will be further described below through specific embodiments, but it should not be construed as a limitation of the present invention. Those skilled in the art may also make various modifications, replacements, and changes based on the above solutions. All modifications, replacements and changes made based on the above-mentioned technical ideas belong to the scope of the present invention.

The method for controlling the quality of the chrome-molybdenum series round tube billets of φ280mm is characterized in that it comprises the following steps:

Ⅰ. Converter smelting: The carbon at the end point of the converter is controlled according to the mass percentage of 0.05%~0.15%, the end point temperature is controlled at ≥1670°C, and the small platform is fed with aluminum wire at a feed rate of 1.4~2.0m per ton of steel for deoxidation treatment. Platform target composition is controlled by mass percentage: [C]0.20%～0.35%, [Si]0.16%～0.30%, [Mn]1.00%～1.40%, [P]≤0.013%, at least 3min after deoxidation alloying soft argon blowing treatment;

Ⅱ. Refining molten steel in LF furnace: Refining the molten steel obtained in step Ⅰ in LF furnace, blowing argon and stirring throughout the refining process, adding titanium alloy material at the end of refining, and adding the amount according to 0.018% to 0.038% of the weight of molten steel. Afterwards, 200-500m of feeding amount is fed to silicon-calcium line for calcium treatment, and the first batch of tundish is fed according to the upper limit. After refining, continue to blow argon and stir for at least 5 minutes, and the exit temperature is controlled at 1560-1620°C;

Ⅲ. RH refining: carry out RH refining on the molten steel obtained in step Ⅱ. RH refining requires that the argon flow rate is controlled at 1150-1450NL/min, the vacuum degree is less than 3mbar, and the processing time is ≥10min. 0.055% control, continue to blow argon for at least 5 minutes after the vacuum treatment, and control the exit temperature at 1550-1580°C;

Ⅳ. Continuous casting of molten steel: The continuous casting process adopts the method of combining electromagnetic stirring of the mold and electromagnetic stirring at the end of solidification. Among them, the parameters of electromagnetic stirring of the mold are: stirring current 200-350A, 2-4Hz; parameters of electromagnetic stirring at the end of solidification For: stirring current 100-150A, frequency 4.0-7.0Hz.

Steps I, II and III are mainly to control the smelting of molten steel to ensure that the molten steel has stable characteristics, uniform composition and proper pouring temperature before pouring. Among them, the converter smelting process is mainly to carry out deoxidation treatment, and add and control some alloy elements to ensure the internal quality of molten steel and improve its stability. Purity, and make the composition of molten steel more stable and uniform.

The key of the present invention lies in the stage of continuous casting of molten steel in step IV. Usually, in the continuous casting process, the solidification structure of the cast slab presents the fine-grained zone of the chilling layer, the columnar crystal zone, and the central equiaxed grain successively from the surface of the cast slab to the center. area of the crystallizer tissue. In order to improve the quality of the slab and improve the internal quality of the slab, the current metallurgical industry generally adopts the electromagnetic stirring technology and the solidification end pressing technology, and the effects achieved by the two technologies are not ideal. The improvement of the present invention lies in the combination of electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification. The working mechanism of this technology is: the electromagnetic stirring of the mold generates electromagnetic torque through the magnetic field, changes the flow field and temperature field distribution of the molten steel inside the mold, promotes the uniformity of the composition and temperature of the molten steel in the mold, and weakens the directionality of solidification heat transfer , to promote the uniform growth of the billet shell; furthermore, the electromagnetic stirring at the end of solidification drives the flow of molten steel in the mushy region by force, scours and fuses the columnar grain structure, improves the nucleation rate of equiaxed crystals, inhibits the growth of columnar crystals, and expands the equiaxed crystal region , Improve the internal quality of the slab.

During the process of converter smelting, LF furnace refining and RH vacuum treatment, argon blowing is used for stirring. In the process of smelting, argon blowing with medium intensity is generally used for stirring, as long as there is no large boiling on the surface of molten steel; Stir with argon to slightly stir the molten steel. The main purpose of argon blowing and stirring is to make the molten steel fully react, to mix the components in the molten steel evenly, and to facilitate the subsequent formation of a stable solidification structure during the molten steel pouring process.

Furthermore, in the stage of continuous casting molten steel, if the electromagnetic stirring intensity at the solidification end is not well controlled, it is easy to cause severe erosion of the dendrite tip, resulting in ring-shaped aggregation of low-melting-point and high-fluidity low-solute-concentration molten steel, forming a bright white band. Therefore, it is necessary to precisely control the stirring intensity. In addition, in order to ensure the quality of the final product, the following parameters need to be controlled during molten steel pouring: the degree of superheat is controlled at 25-38°C, and the casting speed is controlled at 0.60-1.20m/min. The crystallizer cooling is controlled at 2400-2500L/min, and the specific water volume of the secondary cooling is controlled at 0.21-0.33/kg _steel . Reasonable superheating, casting speed matching control and secondary cooling strength formulation are the key to the control of heat transfer during solidification of the slab, that is, the key to the quality control of the slab center.

Embodiment one:

A steelmaking plant adopts 120t converter smelting → LF furnace refining → RH refining → six-machine six-strand casting machine φ280mm section continuous casting to produce 30CrMo series round tube blanks. At the end of converter blowing, the end point carbon is controlled at 0.10%, and the end point temperature is controlled at 1675°C; the small platform is fed with 1.7m/t steel into the aluminum wire for deoxidation; the mass percentage of the outbound composition of the small platform is controlled as: [C]0.28 %, [Si] 0.21%, [Mn] 1.25%, [P] 0.011%; after deoxidation and alloying, carry out soft blowing argon stirring, stirring time 3.5min, stirring does not appear bare molten steel.

When the molten steel arrives at the LF refining furnace, the molten steel fluctuates slightly after argon blowing, and there is no major turning phenomenon. After refining, Ti is added at 0.026% of the weight of the molten steel; 7.5 minutes of soft blowing and stirring were carried out after the line feeding, and the temperature at the exit of LF was 1585°C.

After the molten steel arrives at the station, the RH refining argon blowing argon flow rate is controlled at 1300NL/min, the processing time is 10 minutes when the vacuum degree is less than 3mbar, the RH outbound Alt is 0.045%, after the vacuum treatment is completed, the argon blowing is ≥ 6.5min, and the outbound temperature is controlled at 1550°C.

After the RH refining process is completed, it is transferred to a six-machine six-strand casting machine with a φ280mm section for molten steel casting. The actual control of the key technical parameters related to the requirements of the present invention is: electromagnetic stirring of the crystallizer: stirring current 200A, stirring frequency 3.0Hz; solidification end electromagnetic Stirring: Stirring current 105A, stirring frequency 7.0Hz; the actual continuous casting process of molten steel is controlled at 25-32°C; casting speed is 1.0m/min; crystallizer cooling is controlled at 2500L/min; secondary cooling specific water volume is controlled at 0.33 /kg steel.

After the stable application of the above technical inventions, the quality of the slab has been well controlled. The low-magnification detection results of the slab are: the porosity of the slab is 0.5 to 1.0, and the diameter ratio of the loose area is 40.2%. There are no defects such as cracks on the surface of the slab, and the surface of the slab is comprehensive. The no-cleanup rate reaches 99.2%. The in-situ analysis and density test of the slab metal shows that the central area is 0.82, and the length ratio of the equiaxed crystal region of the slab is 50.1% to 54.2%; the segregation test result of the slab drill sample is 0.96 to 1.03.

Embodiment two:

A steelmaking plant adopts 120t converter smelting → LF furnace refining → RH refining → six-machine six-strand casting machine φ280mm section continuous casting to produce 35CrMo series round tube blanks. At the end of converter blowing, the end point carbon is controlled at 0.05%, and the end point temperature is controlled at 1680°C; the small platform is fed with 2.0m/t _steel into the aluminum wire for deoxidation; the outbound composition of the small platform is controlled as: [C]0.30%, [Si] 0.18%, [Mn] 1.30%, [P] 0.012%; after deoxidation and alloying, soft argon blowing and stirring were carried out, the stirring time was 4.0min, and the molten steel did not appear exposed during stirring.

When the molten steel reaches the LF refining furnace, the molten steel fluctuates slightly after argon blowing, and there is no big turning phenomenon. After refining, Ti is added at 0.035% of the weight of the molten steel; 7.5 minutes of soft blowing and stirring were carried out after the wire feeding, and the temperature of LF exiting the station was 1600°C.

After the molten steel arrives at the station, the RH refining argon blowing argon flow rate is controlled at 1400NL/min, the processing time is 12 minutes when the vacuum degree is less than 3mbar, the Alt of the RH outbound station is 0.050%, and the argon is blown for 7.5min after the vacuum treatment is completed, and the outbound temperature is controlled at 1570 ℃.

After the LF refining process is completed, it is transferred to a six-machine six-strand casting machine with a φ280mm section for molten steel casting. The actual control of the key technical parameters related to the requirements of the present invention is: electromagnetic stirring of the crystallizer: stirring current 260A, stirring frequency 4.0Hz; solidification end electromagnetic Stirring: Stirring current 110A, stirring frequency 6.5Hz; the actual continuous casting process of molten steel is controlled at 31-38°C; casting speed is 0.7m/min; crystallizer cooling is controlled at 2400L/min; /kg _steel .

After the stable application of the above technical inventions, the quality of the slab has been well controlled. The low-magnification detection results of the slab are: the porosity of the slab is 0.5 to 1.0, and the diameter ratio of the loose area is 41.1%; the surface of the slab has no defects such as cracks, and the surface of the slab is comprehensive The no-cleanup rate reached 97.8%. The in-situ analysis and density test of the billet metal shows that the central area is 0.83, and the length ratio of the equiaxed grain region of the billet is 50.3% to 52.2%; the segregation test result of the billet drill sample is 0.95 to 1.04.

Claims (3)

1. the chrome molybdenum series circular pipe blank slab quality control method of φ 280mm, which is characterized in that comprise the following steps：

Ith, converter smelting：Converter terminal carbon is controlled according to mass percent 0.05% ~ 0.15%, outlet temperature >=1670 DEG C control System, chain-wales are fed by the feed quantity of 1.4 ~ 2.0m of steel per ton plus aluminum steel, carry out deoxidation treatment, and chain-wales target component presses quality hundred Divide ratio：[C] 0.20% ~ 0.35%, [Si] 0.16% ~ 0.30%, [Mn] 1.00% ~ 1.40%, [P]≤0.013% control, deoxygenated alloy The soft blow argon processing of at least 3min is carried out after change；

IIth, LF stoves refined molten steel：The molten steel obtained in step I is subjected to LF stove refinings, whole Argon stirring is refined, refines latter stage Add in titanium alloy material, addition by molten steel weight 0.018% ~ 0.038% carry out with addition of, afterwards carry out 200 ~ 500m feed quantity It feeds and silicon-calcium wire is added to carry out Calcium treatment, add wherein the first heat of middle bag is fed by the upper limit, continue Argon stirring after refining at least 5min, out-station temperature is by 1560 ~ 1620 DEG C of controls；

IIIth, RH is refined：The molten steel obtained in step II is subjected to RH refinings, RH refinings require argon flow amount by 1150 ~ 1450NL/ Min is controlled, vacuum degree ＜ 3mbar, and processing time >=10min, the outbound Alt of RH are controlled according to mass percent 0.030% ~ 0.055% System continues Argon at least 5min after application of vacuum, out-station temperature is by 1550 ~ 1580 DEG C of controls；

IVth, continuous casting steel：Continuous casting link using M-EMS in a manner that solidification end electromagnetic agitation is combined, In, the parameter of M-EMS is：200 ~ 350A of stir current, 2 ~ 4Hz of frequency；Solidification end stirring parameter is： 100 ~ 150A of stir current, 4.0 ~ 7.0Hz of frequency.

2. the chrome molybdenum series circular pipe blank slab quality control method of φ 280mm as described in claim 1, it is characterized in that：Step Ith, intensity Argon stirs during smelting process uses in II and III, is subject to molten steel surface and does not occur turning over boiling greatly；After smelting It is stirred using soft blow argon, molten steel is made to have faint agitation.

3. the chrome molybdenum series circular pipe blank slab quality control method of φ 280mm as described in claim 1, it is characterized in that：In step It carries out also needing to control following parameter when molten steel cast in rapid III：The degree of superheat is controlled at 25 ~ 38 DEG C, and casting speed control is 0.60 ~ 1.20m/min, crystallizer cooling control are controlled in 2400 ~ 2500L/min, Water Rate for Secondary Cooling Water in 0.21 ~ 0.33/kg.