A method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe

Abstract

The invention discloses a preparation method of a low-temperature annealed water-cooled centrifugal ductile iron pipe, and relates to the technical field of ductile iron pipe casting. The method comprises the following steps: S1 molten iron smelting and tempering; S2 spheroidization and inoculation of molten iron: pouring the qualified molten iron smelted in step S1 into a spheroidizing bag, adding ferrosilicon alloy for inoculation, and spraying passivated granular magnesium into the spheroidizing bag to complete the spheroidization and inoculation of the molten iron; S3 molten iron pouring and cooling: the molten iron after spheroidization and inoculation is poured into a pipe mold with insulation, an inoculant is added simultaneously, the pipe mold is rotated, and the outer wall of the pipe mold is cooled by cooling water, and when the pipe mold is cooled to 500-600°C, it is pulled out of the pipe mold and transported to the front of the annealing furnace; S4 residual sand spraying and heat treatment: the residual sand in the cast iron pipe is cleaned up, sent to the annealing furnace, and annealed in turn through a heating section, a heat preservation section and a slow cooling section. The invention solves the problems of high energy consumption and large deformation and dents of cast iron pipes in the annealing furnace.

Description

Technical Field

[0001] The invention relates to the technical field of ductile iron pipe casting, and in particular to a method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe. Background Art

[0002] Water-cooled centrifugal ductile iron pipe refers to a ductile iron pipe that is cast by pouring spheroidized molten iron into a metal mold (also known as a pipe mold), forming the spheroidized molten iron under the action of high-speed centrifugal force and rapidly cooling it in the pipe mold. At the same time, the outer wall of the pipe mold is cast by spraying or immersion water cooling.

[0003] Due to the fast cooling rate of water-cooled metal mold casting, a large amount of cementite is produced in the pipe during the solidification process. Cementite is a hard and brittle metal compound, which greatly reduces the plasticity of ductile iron and does not meet the standard requirements in the cast state.

[0004] At present, in the existing technology, in order to obtain high-strength and high-elongation ductile iron pipes, the ductile iron pipes must be annealed at a high temperature of 930-960℃. The high-temperature annealing curve of the water-cooled centrifugal ductile iron pipe is as follows: Figure 1 As shown, annealing is performed through a total of 4 curves, namely, through a heating section 1, a holding section 2, a quick cooling section 3, and a slow cooling section 4. The required heat treatment temperature is too high, and the consumption of coal gas or natural gas is large, which is not conducive to energy conservation. At the same time, the annealing furnace used for the water-cooled centrifugal ductile iron pipe is a high-temperature continuous annealing furnace. Because the wall thickness of the centrifugal cast iron pipe of larger specifications (≥DN800 specifications) is high, the required annealing temperature is high and the annealing time is long. It is ensured that the cast iron pipe reaches the holding section 2 after 10-20 minutes in the heating section 1. After maintaining the holding section 2 for 15-20 minutes, a large amount of cementite and pearlite in the cast iron pipe structure are transformed into austenite, and then enter the quick cooling section 3. A fan is used to quickly reduce the pipe body temperature to 750-850°C, so that the austenite in the structure is transformed into pearlite. After 5-10 minutes at the temperature, it enters the slow cooling section 4 in the furnace. After maintaining the pearlite in the structure for 20-30 minutes, the cast pipe is taken out of the annealing furnace.

[0005] In the prior art, large-sized centrifugal cast iron pipes will cause elliptical deformation of the pipe body under the action of high temperature and self-weight in the annealing furnace. If they are not effectively calibrated after leaving the furnace, it will affect the subsequent centrifugal cement mortar lining, resulting in uneven thickness of the mortar lining layer. Cast iron pipes with severe elliptical deformation will be scrapped.

[0006] In addition, in commissioned processing and production, customers are currently demanding more and more differentiated cast pipe products, that is, the normal wall thickness of centrifugal ductile iron pipes is K9 grade, while the demand for thin-walled pipes of grade C, K8, and K7 is increasing. For larger-sized cast iron pipes, the thinner the wall thickness, the greater the elliptical deformation in the high-temperature annealing furnace. At the same time, when the cast iron pipe is in the insulation section 2, dents are very likely to occur on the annealing furnace track, which has a greater impact on the appearance and internal anti-corrosion coating lining. Summary of the invention

[0007] The present invention provides a method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe, aiming to solve the problems of high source consumption and large deformation and dents of the cast iron pipe in the annealing furnace in the above-mentioned background technology.

[0008] In order to achieve the above technical objectives, the present invention mainly adopts the following technical solutions:

[0009] The present invention provides a method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe, comprising the following steps:

[0010] S1 Hot metal smelting and tempering: Select appropriate hot metal from blast furnace, pour it into electric furnace, add scrap steel for tempering, or melt qualified pig iron in electric furnace to obtain qualified hot metal composition before spheroidization;

[0011] S2 spheroidization and inoculation of molten iron: pour the qualified molten iron smelted in step S1 into a spheroidizing bag, add ferrosilicon alloy with a silicon content of ≥72% for a first inoculation treatment, and use a blowing process to blow passivated granular magnesium with an active magnesium content of ≥92% into the spheroidizing bag to complete the spheroidization and inoculation treatment of the molten iron;

[0012] S3 molten iron pouring and cooling: the molten iron after spheroidization and inoculation treatment in step S2 is poured into the pipe mold while maintaining its temperature at 1310-1420°C. During the pouring process, the inoculant is added synchronously with the molten iron flowing into the pipe mold. Then the pipe mold is rotated at a speed of 750-1000r / min for 0.3-4min, and the outer wall of the pipe mold is cooled by cooling water. When the cast iron pipe is cooled to 500-600°C in the pipe mold, it is pulled out of the pipe mold and transported to the front of the annealing furnace;

[0013] S4 residual sand blasting and heat treatment: clean the residual sand in the cast iron pipe in the as-cast state in step S3, and then send the cast iron pipe into a continuous annealing furnace. The cast iron pipe passes through a heating section, a heat preservation section and a slow cooling section in the annealing furnace in sequence for annealing to obtain a ductile iron pipe.

[0014] In a preferred embodiment of the present invention, the blast furnace molten iron includes the following components, by mass percentage: silicon 0.6-1.6%, manganese ≤0.25%, phosphorus ≤0.08%, sulfur ≤0.03%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.035%, aluminum ≤0.035%, molybdenum ≤0.01%, vanadium (V) ≤0.04%, boron ≤0.008%, and the balance is Fe and impurity elements.

[0015] In another preferred embodiment of the present invention, in step S1, the qualified molten iron composition before spheroidization includes the following components, by mass percentage: carbon 3.5-3.9%, silicon 0.6-1.6%, manganese ≤0.3%, phosphorus ≤0.08%; sulfur ≤0.03%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.04%, aluminum ≤0.04%, molybdenum ≤0.01%, vanadium ≤0.05%, boron ≤0.008%, and the remainder is Fe and impurity elements.

[0016] In another preferred embodiment of the present invention, in step S2, the composition of the molten iron after spheroidization and inoculation, by mass percentage, includes the following components: carbon 3.3-3.7%, silicon 3.0-3.3%, manganese ≤0.3%, phosphorus ≤0.08%, sulfur ≤0.03%, magnesium 0.05-0.08%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.04%, aluminum ≤0.04%, molybdenum ≤0.01%, vanadium ≤0.05%, boron ≤0.008%, wherein the carbon equivalent is 4.4-4.8%, and the balance is Fe and impurity elements.

[0017] In another preferred embodiment of the present invention, in step S3, the mass of the inoculant added synchronously with the inflow of molten iron during the pouring process is 0.18-0.25% of the amount of the molten iron.

[0018] Furthermore, the inoculant is 75Si-Te ferrosilicon or silicon-barium-calcium alloy with a particle size of 0.3-1.5 mm.

[0019] In the present invention, in step S4, the temperature of the heating section rises from no more than 500°C to 820-850°C within 5-15 minutes, the temperature of the insulation section is 820-850°C, and is kept warm for 15-25 minutes, and the temperature of the slow cooling section drops from 820-850°C to below 700°C within 15-20 minutes.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention adjusts the smelting composition, increases the proportion of graphitized elements (the present invention mainly increases the Si element content), reduces the relative content of carbide elements, changes the nucleation mechanism during the solidification molding process of the cast pipe, makes a large amount of graphite nucleate into balls in the cast state, and reduces the content of carbide structure. Compared with the existing process, the cast structure of the ductile iron pipe is mainly ferrite, containing a small amount of carbide structure.

[0022] 2. The present invention obtains ductile iron with a pure ferrite matrix through a heat treatment process. Due to the large increase in silicon content, the stability of carbides is reduced. On the basis of not reducing the heat treatment efficiency, carbides can be decomposed at low temperatures to obtain pure ferrite matrix ductile iron.

[0023] 3. The annealing temperature of the insulation section of the water-cooled centrifugal ductile iron pipe is reduced, and the total annealing time of the cast iron pipe in and out of the annealing furnace is shortened by 15-25 minutes, greatly improving production efficiency;

[0024] 4. Since the temperature of the insulation section 2 is low, the consumption of coal gas or natural gas is low, and the use of the fan in the quick cooling section is reduced, which greatly saves energy;

[0025] 5. When producing K7 or C grade thin-walled cast iron pipes, the deformation is reduced during annealing to avoid the concave defects caused by the track;

[0026] 6. The strength of the cast pipe has been improved from the original 420MPa to 520MPa;

[0027] 7. Reduce the expansion of graphitization during the annealing process of the cast pipe and maintain the consistency of the size of the cast pipe product. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is an annealing curve diagram of ductile iron pipe in the prior art; wherein, 1: heating section 2: insulation section 3: fast cooling section 4: slow cooling section;

[0029] Figure 2 A production process flow chart of a low temperature annealed water-cooled centrifugal ductile iron pipe provided in an embodiment of the present invention;

[0030] Figure 3 The annealing curve diagram of the water-cooled centrifugal ductile iron pipe for low-temperature annealing provided by the embodiment of the present invention, wherein 1: heating section 2: insulation section 3: slow cooling section. DETAILED DESCRIPTION

[0031] In order to make the technical solution of the present invention clearer and more specific, the present invention is further described below. Any equivalent replacement of the technical features of the technical solution of the present invention and any solution derived by conventional reasoning shall fall within the protection scope of the present invention.

[0032] Example

[0033] like Figure 2 As shown, the present invention discloses a method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe, comprising the following steps:

[0034] S1 Hot metal smelting and tempering: Select appropriate blast furnace hot metal, silicon (SI): 0.6-1.6%; manganese (Mn): ≤0.25%, phosphorus (P): ≤0.08%; sulfur (S) ≤0.03%; chromium (Cr) ≤0.03%; copper (Cu) ≤0.03%; titanium (Ti) ≤0.11%; lead (Pb) ≤0.01%; antimony and tin (Sb+Sn) ≤0.025%; nickel (Ni) ≤0.035%, aluminum (Al) ≤0.035%, molybdenum (Mo) ≤0.01%; vanadium (V) ≤0.04%; boron (B) ≤0.008%, and pour it into the electric furnace, add a certain proportion of scrap steel for tempering, or put it into the electric furnace. The mixed pig iron blocks are melted in an electric furnace to obtain the following molten iron composition before spheroidization: carbon (C): 3.5-3.9%, silicon (Si): 0.6-1.6%; manganese (Mn): ≤0.3%, phosphorus (P): ≤0.08%; sulfur (S) ≤0.03%; chromium (Cr) ≤0.03%; copper (Cu) ≤0.03%; titanium (Ti) ≤0.11%; lead (Pb) ≤0.01%; antimony and tin (Sb+Sn) ≤0.025%; nickel (Ni) ≤0.04%, aluminum (Al) ≤0.04%, molybdenum (Mo) ≤0.01%; vanadium (V) ≤0.05%; boron (B) ≤0.008%, and the balance is Fe and impurity elements.

[0035] S2 Spheroidization and inoculation of molten iron: Pour the qualified molten iron smelted in step S1 into the spheroidizing bag, add ferrosilicon alloy with a silicon content ≥ 72% for inoculation, and quickly use high-pressure nitrogen blowing technology to blow passivated granular magnesium with an active magnesium content of ≥ 92% into the spheroidizing bag to complete the spheroidization and inoculation of the molten iron.

[0036] The composition of the molten iron after spheroidization is as follows: carbon (C): 3.3-3.7%, silicon (SI): 3.0-3.3%; manganese (Mn): ≤0.3%, phosphorus (P): ≤0.08%; sulfur (S) ≤0.03%; magnesium (Mg): 0.05-0.08%; chromium (Cr) ≤0.03%; copper (Cu) ≤0.03%; titanium (Ti) ≤0.11%; lead (Pb) ≤0.01%; antimony and tin (Sb+Sn) ≤0.025%; nickel (Ni) ≤0.04%, aluminum (Al) ≤0.04%, molybdenum (Mo) ≤0.01%; vanadium (V) ≤0.05%; boron (B) ≤0.008%, carbon equivalent (CE): 4.4-4.8%, and the balance is Fe and impurity elements.

[0037] The present invention adjusts the smelting composition, increases the proportion of graphitized elements (the present invention mainly increases the Si element content), reduces the relative content of carbide elements, changes the nucleation mechanism during the solidification molding process of the cast pipe, makes a large amount of graphite nucleated into balls in the cast state, and reduces the content of carbide structure. Compared with the existing process, the cast structure of the ductile iron pipe is mainly ferrite, containing a small amount of carbide structure.

[0038] S3: pouring and cooling molten iron: the molten iron after spheroidization and inoculation treatment in step S2 is poured into the pipe mold while maintaining its temperature at 1310-1420°C. During the pouring process, an inoculant of 0.18-0.25% of the molten iron is added synchronously with the molten iron flowing into the pipe mold. The inoculant is 75Si-Fe alloy or silicon-barium-calcium alloy with a particle size of 0.3-1.5mm. The pipe mold is then rotated at a speed of 750-1000r / min for 0.3-4min. The molten iron is rapidly cooled after being poured into the pipe mold. In order to improve production efficiency and refine the matrix structure, cooling water is used on the outer wall of the pipe mold. When the cast iron pipe is cooled to 500-600°C in the pipe mold, it is pulled out of the pipe mold and transported to the front of the annealing furnace by a chain.

[0039] S4 Residual sand blasting and heat treatment: There will be some residual sand in the sand core of the socket of the cast iron pipe before the annealing furnace. In order to prevent the residual sand from covering and affecting its metallographic structure transformation, a device with high pressure blasting at one end and residual sand collection at the other end is used to clean the residual sand in the cast iron pipe in the as-cast state in step S3, and then the cast iron pipe is transported into the continuous annealing furnace through a chain. Figure 3 As shown, after the cast iron pipe is kept in the annealing furnace for 5-15 minutes through the heating section 1, the temperature is heated from no more than 500°C to the required holding section temperature of 820-850°C, which can be shortened by at least 5 minutes compared with the current heating section of 10-20 minutes. Due to the increase in Si element content, a small amount of carbides existing in the cast iron pipe structure in the holding section 2 are decomposed. Under the temperature of 820-850°C, it is kept for 15-25 minutes before entering the slow cooling section 3. The temperature of the slow cooling section drops from 820-850°C to below 700°C within 15-20 minutes. At this time, the pearlite existing in the metallographic structure is transformed into ferrite after 15-20 minutes. The total annealing time is saved by 15-25 minutes compared with the current annealing time, which greatly improves the production efficiency. At the same time, due to the low temperature of the holding section 2, the consumption of coal gas or natural gas is less, and the use of the fan in the fast cooling section is reduced, which greatly saves energy.

[0040] Test example

[0041] The performance of the ductile iron pipe prepared by the embodiment of the present invention is tested

[0042] Experimental method: Standard test bars are processed according to the national standard of ductile iron pipes, and tensile tests are carried out on a universal testing machine for measurement.

[0043] Results: The performance indicators of the ductile iron pipe prepared by the embodiment of the present invention are as follows: tensile strength is 520-550 MPa, yield strength is 380-420 MPa, and elongation is 15-20%.

[0044] The above embodiments are merely descriptions for the purpose of clearly illustrating the present invention, and are not limitations on the implementation methods. For those skilled in the art, other different forms of changes or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all the implementation methods here, and the obvious changes or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing a low-temperature annealed water-cooled centrifugal ductile iron pipe, characterized in that: The steps include: S1 Hot metal smelting and tempering: Select appropriate hot metal from blast furnace, pour it into electric furnace, add scrap steel for tempering, or melt qualified pig iron in electric furnace to obtain qualified hot metal composition before spheroidization; S2 spheroidization and inoculation of molten iron: pour the qualified molten iron smelted in step S1 into a spheroidizing bag, add ferrosilicon alloy with a silicon content of ≥72% for a first inoculation treatment, and use a blowing process to blow passivated granular magnesium with an active magnesium content of ≥92% into the spheroidizing bag to complete the spheroidization and inoculation treatment of the molten iron; S3 molten iron pouring and cooling: the molten iron after spheroidization and inoculation treatment in step S2 is poured into the pipe mold while maintaining its temperature at 1310-1420°C. During the pouring process, the inoculant is added synchronously with the molten iron flowing into the pipe mold. Then the pipe mold is rotated at a speed of 750-1000r / min for 0.3-4min, and the outer wall of the pipe mold is cooled by cooling water. When the cast iron pipe is cooled to 500-600°C in the pipe mold, it is pulled out of the pipe mold and transported to the front of the annealing furnace; S4 residual sand blasting and heat treatment: clean the residual sand in the cast iron pipe in the as-cast state in step S3, and then send the cast iron pipe into a continuous annealing furnace, where the cast iron pipe passes through a heating section, a heat preservation section and a slow cooling section in sequence for annealing, so as to obtain a ductile iron pipe; The molten iron in the blast furnace includes the following components by mass percentage: silicon 0.6-1.6%, manganese ≤0.25%, phosphorus ≤0.08%, sulfur ≤0.03%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.035%, aluminum ≤0.035%, molybdenum ≤0.01%, vanadium (V) ≤0.04%, boron ≤0.008%, the balance is Fe and impurity elements; In step S1, the qualified molten iron composition before spheroidization includes the following components by mass percentage: carbon 3.5-3.9%, silicon 0.6-1.6%, manganese ≤0.3%, phosphorus ≤0.08%; sulfur ≤0.03%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.04%, aluminum ≤0.04%, molybdenum ≤0.01%, vanadium ≤0.05%, boron ≤0.008%, the balance is Fe and impurity elements; In step S2, the composition of the molten iron after spheroidization and inoculation includes the following components by mass percentage: carbon 3.3-3.7%, silicon 3.0-3.3%, manganese ≤0.3%, phosphorus ≤0.08%, sulfur ≤0.03%, magnesium 0.05-0.08%, chromium ≤0.03%, copper ≤0.03%, titanium ≤0.11%, lead ≤0.01%, antimony and tin ≤0.025%, nickel ≤0.04%, aluminum ≤0.04%, molybdenum ≤0.01%, vanadium ≤0.05%, boron ≤0.008%, wherein the carbon equivalent is 4.4-4.8%, and the balance is Fe and impurity elements; In step S4, the temperature of the heating section rises from no more than 500°C to 820-850°C within 5-15 minutes, the temperature of the insulation section is 820-850°C for 15-25 minutes, and the temperature of the slow cooling section drops from 820-850°C to below 700°C within 15-20 minutes.

2. The method for preparing the low temperature annealed water-cooled centrifugal ductile iron pipe according to claim 1, characterized in that: In step S3, during the pouring process, the mass of the inoculant added synchronously with the inflow of molten iron is 0.18-0.25% of the amount of the molten iron.

3. The method for preparing the low temperature annealed water-cooled centrifugal ductile iron pipe according to claim 2, characterized in that: The inoculant is 75Si-Te ferrosilicon or silicon-barium-calcium alloy with a particle size of 0.3-1.5 mm.

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