A method for improving macroscopic and microscopic segregation of large steel ingots

Abstract

The present invention discloses a method for improving the macroscopic and microscopic segregation of large steel ingots, and relates to the technical field of steel ingot casting. The present invention adopts the hanging method of rare earth-molybdenum-magnesium metal rods, strictly controls the ratio of three elements, and the rare earth-molybdenum-magnesium is mutually carrier. The rising speed of the molten steel in the mold is stably controlled during the pouring process, ensuring that the rare earth-molybdenum-magnesium elements are stably and evenly collected and enter the molten steel, effectively playing the role of rare earth in improving the microscopic segregation of grain boundaries and microscopic elements, and dynamically controlling the distance between the pouring position and the metal steel liquid surface within a stable range during the pouring process, controlling the pouring speed to stabilize the liquid level rising speed, and performing bag-changing pouring at a reasonable time node, realizing the hierarchical pouring of large steel ingots, reducing the hot melt convection space, and avoiding the formation of macroscopic segregation due to the flushing of molten steel so that the enriched solute molten steel is brought out of the paste zone.

Description

Technical Field

[0001] The invention relates to the technical field of steel ingot casting, in particular to a method for improving macroscopic and microscopic segregation of large steel ingots. Background Art

[0002] Segregation is mainly divided into macrosegregation and microsegregation. Macrosegregation refers to the uneven distribution of chemical composition on a macro scale caused by factors such as solute redistribution, fluid flow, and free equiaxed crystal movement during the solidification process. It is generally larger than the range of the grain size, and mainly includes positive segregation, negative segregation, A segregation, and V segregation. Macrosegregation will seriously reduce the mechanical properties, corrosion resistance, fatigue resistance, and processing characteristics of the material, and even cause cracks during forging, resulting in the scrapping of the steel ingot, causing huge economic losses. Once formed, macrosegregation is not easy to eliminate. Microsegregation refers to the phenomenon of uneven chemical composition on a micro scale, which generally occurs within a grain range. It is mainly caused by the solidification of solute-rich steel liquid between microscopic spaces. Microsegregation will greatly affect the performance of the steel. Although the subsequent treatment process can alleviate microsegregation to a certain extent, only by solving microsegregation from the source can the quality of the ingot be better improved. Summary of the invention

[0003] According to the above technical problem of the influence of macro-segregation and micro-segregation of large castings and forgings on steel performance, a method for improving the macro- and micro-segregation of large steel ingots is provided. The present invention mainly adopts the hanging method of rare earth-molybdenum-magnesium metal rods, strictly controls the proportion of the three elements, and the rare earth-molybdenum-magnesium three are mutually carriers, and stably controls the rising speed of the molten steel in the casting mold during the pouring process, ensuring that the rare earth-molybdenum-magnesium elements are stably and evenly collected and enter the molten steel, and effectively plays the role of rare earth in improving the micro-segregation of grain boundaries and microscopic elements.

[0004] The technical means adopted by the present invention are as follows:

[0005] A method for improving macroscopic and microscopic segregation of large steel ingots, before the large steel ingot is cast, a rare earth metal rod is hung in the steel ingot mold, and the molten steel is divided into multiple packages under vacuum and cast one by one until solidification is completed, specifically comprising the following steps:

[0006] Step 1: Control the superheat of molten steel before pouring to 70℃-90℃, vacuum degree ≤10Pa, and a[O] in steel ≤0.0004%;

[0007] Step 2: Before pouring, hang the rare earth metal rod to the center of the ingot mold, with the bottom of the metal rod 50-100mm away from the bottom of the ingot mold;

[0008] Step 3: Divide the molten steel into 3 packages of unequal mass, wherein the mass ratio of package 1: package 2: package 3 is 3-4:2:2;

[0009] Step 4: Pour the first bag of molten steel into the ingot mold. After the first bag is poured, wait 3-5 minutes before pouring the second bag.

[0010] Step 5: After the second bag is poured, wait 6-7 minutes before pouring the third bag;

[0011] Step 6: pour the third package until it is completed;

[0012] During the pouring process, the distance between the pouring position and the molten steel surface is always kept between 500mm and 650mm, and the rising speed of the molten steel in the mold during pouring is 10 to 12mm / s;

[0013] The diameter of the rare earth metal rod is 5-10 mm, and its composition by mass percentage is: RE: 45%~60%, Mg: 20%~30%, Mo: 20%~25%.

[0014] Furthermore, the RE content in the steel is controlled to be in the range of 0.010% to 0.040% by mass.

[0015] Furthermore, the rare earth RE is composed of one or more of Y, Sc, and Ce.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. The present invention provides a method for improving the macro and micro segregation of large steel ingots, which adopts the hanging process of rare earth-molybdenum-magnesium metal rods, strictly controls the ratio of the three elements, and the rare earth-molybdenum-magnesium is a carrier of each other. The rising speed of the molten steel in the mold is stably controlled during the pouring process, ensuring that the rare earth-molybdenum-magnesium elements are stably and evenly collected and enter the molten steel, and effectively plays the role of rare earth in improving the micro segregation of grain boundaries and microscopic elements.

[0018] 2. The method for improving the macro and micro segregation of large steel ingots provided by the present invention has a reasonable process design, a low temperature of the ingot mold in the initial state, and the molten steel is divided into three packages with reasonable unequal masses, so that the masses of the three packages of molten steel to be poured are gradually reduced in sequence, and finally the solidification time of the three packages of molten steel tends to be the same.

[0019] 3. The present invention provides a method for improving the macro and micro segregation of large steel ingots. The method dynamically controls the distance between the pouring position and the metal steel liquid surface within a stable range during the pouring process, controls the pouring speed to stabilize the liquid surface rising speed, and performs ladle changing pouring at a reasonable time node, thereby realizing the layered pouring of large steel ingots, reducing the hot melt convection space, and avoiding the formation of macro segregation due to the flushing of molten steel, which causes the enriched solute molten steel to be brought out of the pasty zone.

[0020] Based on the above reasons, the present invention can be widely promoted in the field of steel ingot casting technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 It is a flow chart of a method for improving macroscopic and microscopic segregation of large steel ingots in an embodiment of the present invention;

[0023] Figure 2 Schematic diagram of typical analysis points of a method for improving macro and micro segregation of large steel ingots in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0025] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0027] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, the numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0028] In the description of the present invention, it is necessary to understand that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, perpendicular, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention: the directional words "inside and outside" refer to the inside and outside relative to the contours of each component itself.

[0029] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below their position devices or structures". Thus, the exemplary term "above" may include both "above" and "below". The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0030] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0031] Example 1

[0032] A 500kg vacuum induction furnace was used for smelting. The steel grade was 40Cr4MoV, and its specific composition was C 0.41%, Si 0.60%, Mn 0.65%, P 0.006%, S 0.004%, Cr 3.51%, Ni 0.12%, Mo 0.63%, V0.21%. The composition of the rare earth metal rod for smelting was RE 60% (including Y 20%, Sc 20%, Ce 20%), Mg 20%, Mo 20%, and its total mass was 0.085kg.

[0033] Before pouring, the superheat of the molten steel is 70℃, the vacuum is 9Pa, a[O]=0.0004%, and a rare earth rod with a diameter of Φ=5mm is hung at a position 50mm away from the bottom of the ingot mold. The molten steel is divided into 3 packages, the mass of the first package is 216kg, and the mass of the second and third packages is 142kg. Pour the first package of molten steel along the ingot mold, wait for 3min, and pour the second package. After the second package is poured, wait for 7min, and pour the third package until it is completed. During the entire pouring process, the distance between the pouring position and the molten steel surface is always controlled to be kept at 500mm. The rising speed of the molten steel in the mold is 12mm / s. After pouring, take it out after cooling down.

[0034] Example 2

[0035] A 500kg vacuum induction furnace was used for smelting. The steel grade was 40Cr4MoV, and its specific composition was C 0.45%, Si 0.70%, Mn 0.49%, P 0.005%, S 0.005%, Cr 3.35%, Ni 0.15%, Mo 0.66%, V0.26%. The composition of the rare earth metal rod for smelting was RE 45% (including Y 15%, Sc 30%), Mg 30%, Mo25% by mass percentage, and its total mass was 0.43kg.

[0036] Before pouring, the superheat of the molten steel is 90℃, the vacuum degree is 7Pa, a[O]=0.0003%, and the rare earth rod with a diameter of Φ=10mm is hung at a position 100mm away from the bottom of the ingot mold. The molten steel is divided into 3 packages, the mass of the first package is 250kg, and the mass of the second and third packages is 125kg. Pour the first package of molten steel along the ingot mold, wait for 5 minutes, and pour the second package. After the second package is poured, wait for 6 minutes, and pour the third package until it is completed. During the entire pouring process, the distance between the pouring position and the molten steel surface is always controlled to be kept at 650mm. The rising speed of the molten steel in the mold is 10mm / s. After pouring, take it out after cooling down.

[0037] Example 3

[0038] A 500kg vacuum induction furnace was used for smelting. The steel grade was 40Cr4MoV, and its specific composition was C 0.40%, Si 0.65%, Mn 0.63%, P 0.006%, S 0.006%, Cr 3.52%, Ni 0.16%, Mo 0.62%, V0.24% by mass percentage. The composition of the rare earth metal rod for smelting was RE 50% (Ce 50%), Mg 25%, Mo 25% by mass percentage, and its total mass was 0.2kg.

[0039] Before pouring, the superheat of the molten steel is 80℃, the vacuum degree is 6Pa, a[O]=0.0004%, and the rare earth rod with a diameter of Φ=5mm is hung at a position 75mm away from the bottom of the ingot mold. The molten steel is divided into 3 packages, the mass of the first package is 230kg, and the mass of the second and third packages is 135kg. Pour the first package of molten steel along the ingot mold, wait for 5min, and pour the second package. After the second package is poured, wait for 7min, and pour the third package until it is completed. During the entire pouring process, the distance between the pouring position and the molten steel surface is always controlled to be kept at 550mm. The rising speed of the molten steel in the mold is 11mm / s. After pouring, take it out after cooling down.

[0040] Comparative Example

[0041] A 500kg vacuum induction furnace is used for smelting. The smelting steel grade is 40Cr4MoV, and its specific composition by mass percentage is C 0.42%, Si 0.62%, Mn 0.63%, P 0.009%, S 0.006%, Cr 3.55%, Ni 0.11%, Mo 0.61%, and V0.20%.

[0042] Before pouring, the superheat of the molten steel is 75°C, the vacuum degree is 10Pa, a[O]=0.0010%, there is no rare earth metal rod hanging, the molten steel is not sub-packaged, and it is poured directly along the ingot mold. The distance between the pouring position and the bottom of the ingot mold is 1500mm. The rising speed of the molten steel in the mold is 13mm / s. After pouring, it is taken out after cooling.

[0043] The above four groups of steel ingots were cut longitudinally along the center line using a sawing machine, and half of them were ground and polished. The element distribution of the cross section was analyzed using an in-situ analyzer. According to the segregation index = ( - ) / Calculate, where The element content is measured by the in-situ instrument. is the element content in the steel standard sample, and the typical position diagram is as follows Figure 2As shown, 7 sampling points were taken along the vertical center line direction, and the segregation index along the center line direction was taken as the average value of the 9 points. The element segregation conditions at typical positions are listed in Tables 1 to 6. The other half of the ingot was taken for microscopic segregation analysis. Samples were taken from the midpoint of the center line of the comparative example and the ingots of Examples 1 to 3, and after polishing and etching, the microscopic segregation degree was analyzed by electron probe. The segregation index calculation formula is: = Maximum concentration of microscopic solutes ÷Minimum concentration of microscopic dry solute , the results are shown in Table 7.

[0044] The segregation results of C, Si, Mn, Cr, and Mo at typical positions along the vertical center line of the ingot (Table 1-5) show that the positive and negative segregation degrees of the elements in Examples 1-3 are significantly improved compared to the segregation degrees of the elements in the comparative example, and the segregation index is reduced by about 50%. From the results obtained by taking samples along the center line of the ingot as the average value (Table 6), it can be seen that compared to the comparative example, the segregation degrees of C, Si, Mn, Cr, and Mo elements in Examples 1-3 are significantly improved, especially the segregation degrees of C, Cr, and Mo elements are more significantly improved. In general, the present invention effectively improves the macroscopic segregation problem of the ingot. From the results of microscopic segregation detection, the present invention reduces the segregation of the microscopic element components of the ingot, and the microscopic segregation index of C, Si, Mn, Cr, and Mo is reduced by about 10%-20%.

[0045] Table 1 C segregation index of ingot along the vertical center line

[0046]

[0047] Table 2 Si segregation index along the vertical center line of the ingot

[0048]

[0049] Table 3 Mn segregation index along the vertical center line of the ingot

[0050]

[0051] Table 4 Cr segregation index along the vertical center line of the ingot

[0052]

[0053] Table 5 Mo segregation index along the vertical center line of the ingot

[0054]

[0055] Table 6 Element segregation index along the center line of the ingot

[0056]

[0057] Table 7 Microsegregation index of ingot

[0058]

[0059] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for improving macroscopic and microscopic segregation of large steel ingots, characterized in that: Before the large ingot casting process, a rare earth metal rod is hung in the ingot mold, and the molten steel is divided into multiple packages under vacuum and poured one by one until solidification is completed. The specific steps include: Step 1: Control the superheat of molten steel before pouring to 70℃-90℃, vacuum degree ≤10Pa, and a[O] in steel ≤0.0004%; Step 2: Before pouring, hang the rare earth metal rod to the center of the ingot mold, with the bottom of the metal rod 50-100mm away from the bottom of the ingot mold; Step 3: Divide the molten steel into 3 packages of unequal mass, wherein the mass ratio of package 1: package 2: package 3 is 3-4:2:2; Step 4: Pour the first bag of molten steel into the ingot mold. After the first bag is poured, wait 3-5 minutes before pouring the second bag. Step 5: After the second bag is poured, wait 6-7 minutes before pouring the third bag; Step 6: Carry out pouring of the third package until completion; During the pouring process, the distance between the pouring position and the molten steel surface is always kept between 500mm and 650mm, and the rising speed of the molten steel in the mold during pouring is 10 to 12mm / s; The diameter of the rare earth metal rod is 5-10 mm, and its composition by mass percentage is: RE: 45%~60%, Mg: 20%~30%, Mo: 20%~25%.

2. A method for improving macroscopic and microscopic segregation of large steel ingots according to claim 1, characterized in that: The RE content in the controlled steel is in the range of 0.010% to 0.040% by mass.

3. A method for improving macroscopic and microscopic segregation of large steel ingots according to claim 1, characterized in that: The rare earth RE is composed of one or more of Y, Sc and Ce.

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