CN113088625B - Method for modifying austenitic heat-resistant steel carbide - Google Patents
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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Abstract
The invention discloses a method for modifying austenite heat-resistant steel carbide, which comprises the steps of sequentially carrying out electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium core-spun wire feeding, continuous casting or die casting, primary rolling, cogging or die casting, rolling, full solid solution and aging solid solution on a magnesium-containing austenite heat-resistant steel raw material to obtain the modified austenite heat-resistant steel; the method comprises the following steps of feeding a nickel-magnesium cored wire at the last refining stage of an LF furnace, wherein the nickel-magnesium cored wire comprises the following components in percentage by mass: 7-25% of Mg, 5-45% of Ni, 20-40% of Cr, 0.1-5.5% of N, and the balance of Fe and inevitable impurity elements. The method for modifying the austenitic heat-resistant steel has the advantages of stable wire feeding process, mild reaction, no violent splashing, high yield of magnesium element up to 20-35 percent and low production cost.
Description
Technical Field
The invention relates to the technical field of ferrous metallurgy, in particular to a method for modifying austenitic heat-resistant steel carbide.
Background
The austenitic heat-resistant steel is widely applied to industrial production such as metallurgy, mine, petrochemical industry, electric power and the like, and is used for preparing heat-resistant components such as a dry quenching furnace lining plate, a thermal power plant boiler burner nozzle, a circulating fluidized bed boiler hood, a cyclone cylinder inner cylinder hanging piece and the like. The high-temperature stability and the high-temperature strength are two major performance indexes of the heat-resistant steel and are also important research subjects of the heat-resistant steel in engineering application.
21-4N steel is a typical heat resistant steel, also called as valve steel, which is commonly used at home and abroad as a steel for exhaust valves, and comprises the following components: 0.48-0.58% of C, less than 0.35% of Si, 8.00-10.00% of Mn, less than 0.04% of P, less than 0.03% of S, 20.00-22.00% of Cr, 3.25-4.50% of Ni, 0.3% of Cu, 0.35-0.5% of N and 0.02-0.03% of Al. The stability of the high-temperature operation process of the heat-resistant steel is always a hot point concerned by manufacturers and users at home and abroad. Many manufacturers or users try to add proper alloy elements and research and select proper heat treatment process to achieve a relatively stable structural state, so as to avoid serious change of the structure during use, which leads to the failure of the exhaust valve in operation.
One of the main problems affecting the high-temperature stable service of the heat-resistant steel is the coarseness of carbides in the steel. High-end users require the carbide size in steel to be below 10 microns, and common users require the carbide size in steel to be below 20 microns, but in the actual production process, the carbide size is mostly between 30 and 50 microns because the continuous casting heat-resistant steel is large, and the large-size carbide causes the phenomena of macrosegregation of carbon elements on the cross section, uneven grain size distribution and the like, so that the high-temperature stability of steel is reduced, and the performance uniformity is poor.
The austenitic heat-resistant steel is high alloy steel, the contents of carbon and chromium in the steel are very high, serious dendritic segregation can occur during solidification, the concentration of carbon and other metal elements in local areas is increased, and when the condition of forming eutectic components is reached, the segregation can be formed into ledeburite eutectic carbide which is directly separated out from a liquid phase. Such liquid carbide is also called primary carbide, which has a high Cr content, a high hardness and size, and is most harmful among carbide defects. However, if the concentration of carbon and other metal elements in a local area during solidification does not reach the precipitation condition, these areas are elongated into a segregation zone of high concentration after hot working deformation, and a large amount of carbides are precipitated during subsequent cooling to form a band-shaped structure, and hence are also called band-shaped carbides. Furthermore, in the rolling process, the austenitic heat-resistant steel is affected by liquated carbides and band-like carbides, which precipitate proeutectoid carbides along grain boundaries during austenite cooling, and these carbides surround the austenite grains and appear as a network under a microscope, and thus are also called network carbides. The band-shaped carbides and the network-shaped carbides are essentially carbides precipitated from austenite and are also collectively referred to as secondary carbides, and the presence of the secondary carbides weakens intermetallic bonding force, lowers mechanical properties of the steel, particularly lowers impact toughness, and easily causes grain boundary cracking, thereby lowering wear resistance of the steel.
In order to improve the distribution form of carbide in steel, reduce the carbide proportion, particularly the proportion of reticular carbide and improve the performance of austenitic gas valve steel, elements such as rare earth and the like can be added into the steel to regulate and control the carbide form, but the rare earth has higher cost and complex detection, and the nozzle is easy to accumulate tumors in the pouring process.
Because the melting point of magnesium is 649 ℃, the boiling point is 1090 ℃, the vapor pressure is 2.038MPa at 1600 ℃, namely the vapor pressure is more than 20 atmospheric pressures, the boiling point of magnesium is low, and the magnesium is volatilized to cause low yield in the process of adding the magnesium cored wire into molten steel; on the other hand, when the oxygen level of the molten steel is higher, the magnesium-fed cored wire can generate deoxidation reaction, and the problems of remarkable reduction of yield, serious splashing in the process and violent reaction can also be caused.
Disclosure of Invention
To this end, the present invention provides a method for modifying carbides of austenitic heat-resistant steels.
In order to achieve the above purpose, the invention provides the following technical scheme:
the embodiment of the invention provides a method for modifying austenite heat-resistant steel carbide, which comprises the steps of sequentially carrying out electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium cored wire feeding, continuous casting or die casting, rolling, full solid solution and aging solid solution on magnesium-containing austenite heat-resistant steel to obtain the modified austenite heat-resistant steel;
the method comprises the following steps of feeding nickel-magnesium cored wires at the last refining stage of the LF furnace, wherein the nickel-magnesium cored wires comprise the following components in percentage by mass: 7-25% of Mg, 5-45% of Ni, 20-40% of Cr, 0.1-5.5% of N, and the balance of Fe and inevitable impurity elements.
Preferably, the modified austenitic heat-resistant steel comprises the following components in percentage by mass: 0.48 to 0.58 percent of C, less than or equal to 0.35 percent of Si, 8 to 10 percent of Mn, less than or equal to 0.04 percent of P, less than or equal to 0.01 percent of S, 20 to 22 percent of Cr, 3.25 to 4.25 percent of Ni, 0.35 to 0.50 percent of N, less than or equal to 0.30 percent of Cu, 0.0005 to 0.0015 percent of Mg, 0.020 to 0.030 percent of Al, and the balance of iron and inevitable impurities.
Preferably, in the last stage of refining in the LF furnace, when the nickel-magnesium cored wire is fed, the alkalinity value of the molten steel slag layer is 3.5-6.0, the molten steel slag amount is more than 4% of the molten steel amount, the temperature of the molten steel is controlled at 1425-1470 ℃, the mass percent of Al in the molten steel is 0.02-0.03%, and the oxygen activity in the molten steel is controlled at 3-6 ppm.
Preferably, during electric furnace smelting, the oxygen content of the molten steel is controlled below 300 ppm;
in the early stage of AOD furnace smelting, the alkalinity is controlled to be more than 5, and the oxygen content of molten steel in the last stage of smelting is controlled to be less than 100 ppm.
Preferably, the superheat degree of the molten steel is controlled to be 40-75 ℃ at the final stage of refining in the LF furnace.
Preferably, the nickel-magnesium cored wire consists of a core material and an iron sheet surrounding the core material, the thickness of the iron sheet is 0.3mm, the outer diameter of the cored wire is 13mm, and the weight of each cored wire is 550-750 g.
Preferably, after the nickel-magnesium core-spun yarn is fed in at the last refining stage of the LF furnace, covering carbonized rice hulls on the surface of the molten steel, and stirring by soft argon blowing, wherein the soft argon blowing is based on the condition that the slag surface is not blown broken and the molten steel is not exposed.
Preferably, the feeding speed of the nickel-magnesium cored wire is 180-240 m/min.
In the process of molten steel solidification, deoxidation products of magnesium are fine and dispersed, are easy to precipitate at a crystal boundary, and can block the growth of carbides and disperse the carbides; magnesium is a strong deoxidizer, reduces the contents of oxygen and sulfur in steel, and can change the forms and distribution of sulfide and aluminum oxide in the steel, thereby improving the cleanliness of the austenitic valve steel.
According to the method provided by the embodiment of the invention, production conditions are controlled before wire feeding, the molten steel [ O ] before wire feeding is required to be controlled to be 3-6ppm, and under the condition of a lower oxygen level, the reaction of adding magnesium into the steel is mainly a modification reaction, so that the effect of dispersing carbide can be achieved; if the oxygen level is too high, the main reaction of the magnesium cored wire added into the steel is deoxidation reaction, magnesium reacts with oxygen to generate a large amount of deoxidation products, deoxidation slag floats upwards, and the effect of modifying inclusions cannot be achieved.
The principle is as follows.
Mg(g)+[O]＝MgO(s) △G1 θ＝-614000+208.28T (1)
2[Al]+3[O]＝Al2O3(s) △G2 θ＝-1202070+386.28T (2)
1873 when the K reaches the equilibrium state,
get the
The activity coefficients of all elements are substituted to obtain the activity coefficient, and when 1873K and aluminum deoxidation are independently adopted, the activity coefficients meet the following requirements:
[％O]3·[％Al]2＝1.22×10-12 (4)
1873K, [ Al]When the deoxidation reaches the equilibrium, Al in the molten steel]And [ O ]]The relationship therebetween satisfies formula (4). It can be seen that when deoxygenated with aluminum alone, [ Al ]]At a content of 200ppm, [ O ] in equilibrium therewith]The content is 12.5 ppm; when in steel, [ Al ]]At a content of 300ppm, [ O ] in equilibrium therewith]The content was 11.1 ppm. 1873K magnesium-aluminum complex, except that the metal reacts with dissolved oxygen alone, according to MgO-Al2O3Phase diagram, the following reactions also occur:
MgO(s)+Al2O3(s)＝MgO·Al2O3(s)△G3 θ＝-35600-2.09T (5)
3[Mg]+Al2O3(s)＝3MgO(s)+2[Al]△G4 θ＝-992130+332.76T (6)
calculated from equations (1), (2) and (5):
[Mg]+4[O]+2[Al]＝MgO·Al2O3(s)△G5 θ＝-1969070+623.87T (7)
1873K when the reaction reaches equilibrium, i.e.:
taking aMgO·Al2O3Substituting 1 into the above equation yields:
and (4) and (9) are combined, and the activity coefficient of each element is substituted to obtain:
[ Mg ] obtained from the formula (10)]And [ Al]The relationship between them is that the component of the inclusion in the molten steel is Al2O3With MgO. Al2O3As shown in fig. 1. When in molten steel [% Mg]3/[％Al]2>At 1.5X 10-14, the inclusion component is MgO. Al2O3(ii) a When in molten steel [% Mg]3/[％Al]2<At 1.5X 10-14, the inclusion component is Al2O3。
Combining (1) and (9), and substituting the activity coefficients of the elements to obtain:
[ Mg ] derived from formula (11)]And [ Al]The relationship between the components of the inclusion in the molten steel is MgO and MgO & Al2O3As shown in fig. 1. When in molten steel [% Al [% ]]2/[％Mg]3>At 2.64X 109, the inclusion component in the molten steel is MgO. Al2O3(ii) a When in molten steel [% Al [% ]]2/[％Mg]3<At 2.64X 109, the inclusion content in the molten steel is MgO. Thus, the [ Mg ] in the molten steel can be obtained when the steel is at 1873K (1600 ℃), in the molten steel]-[Al]-[O]The reaction sequence of (1) is shown in the attached figureAs shown. Wherein curve 1 in the figure is [ Al ] in molten steel at 1873K]-[O]Balance line, curve 2 is 1873K, [ Mg ] in molten steel]-[O]And (6) balancing the line. The red line indicates the inclusion component in the steel and [ O ] in the molten steel when the content of dissolved aluminum in the molten steel is 0.025%]The relationship between the contents.
As shown in FIG. 1, 1873K is that the content of dissolved aluminum in steel is in the range of 0.02% to 0.03%, the Al content is 0.025% on average, and when the magnesium reforming reaction occurs (region B between line 1 and line 2), the [ O ] may be in the range of 3 to 6ppm, and the reforming reaction may occur if a trace amount of dissolved magnesium is present under the equilibrium condition. If [ O ] is higher than 6ppm (C region), then the deoxidation reaction is dominant; if [ O ] is less than 3ppm (region A), neither the deoxidation reaction nor the upgrading reaction will occur.
The content of active oxygen in austenitic heat-resistant steel is too high, magnesium element added in the steel is easy to generate deoxidation reaction and cannot play a role in modifying, therefore, only dissolved oxygen in the steel is controlled in a B zone, the thermodynamic condition of the reaction for generating magnesium aluminate spinel in the steel is met, and Mg and Al in the steel2O3React to form MgO, wherein the MgO is contained in Al2O3Bonding and producing MgO2O3High melting point MgO. Al2O3The carbide is stably dispersed and distributed in the molten steel and can be further used as a mass point for carbide precipitation so as to achieve the effects of reducing carbide precipitation and refining carbide.
The invention has the following advantages:
the method for modifying the austenitic heat-resistant steel has the advantages of stable wire feeding process, mild reaction, no violent splashing, high yield of magnesium element up to 20-35 percent and low production cost; the carbide in the magnesium austenitic heat-resistant steel tends to be ellipsoidal, the size is smaller, the distribution is more uniform, and massive carbides larger than 30 mu m basically disappear; the plasticity and the strength of the magnesium-containing austenitic heat-resistant steel product are improved, the production cost is reduced, the quality of the austenitic heat-resistant steel is improved, and the magnesium-containing austenitic heat-resistant steel is suitable for industrial popularization and application.
The method of the invention obtains the control condition of the oxygen position before adding magnesium, the magnesium is added under the low oxygen position, mainly the modification reaction, and the addition amount of magnesium is obviously reduced to 5-20 ppm.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It should be apparent that the drawings in the following description are merely exemplary, and that other embodiments can be derived from the drawings provided by those of ordinary skill in the art without inventive effort.
The structures, ratios, sizes, and the like shown in the present specification are only used for matching with the contents disclosed in the specification, so as to be understood and read by those skilled in the art, and are not used to limit the conditions that the present invention can be implemented, so that the present invention has no technical significance, and any structural modifications, changes in the ratio relationship, or adjustments of the sizes, without affecting the effects and the achievable by the present invention, should still fall within the range that the technical contents disclosed in the present invention can cover.
FIG. 1 shows the value of [ Mg ] at 1873K in the method for modifying austenitic heat-resistant steel carbide according to the embodiment of the present invention]And [ O ]]And [ Mg)]Reduction of the formed Al2O3The inclusion reaction sequence chart is that the curve 1 is [ Al ] in molten steel under 1873K]-[O]A balance line; curve 2 is [ Mg ] in molten steel at 1873K]-[O]And (6) balancing the line. The red line indicates the inclusion component in the steel and [ O ] in the molten steel when the content of dissolved aluminum in the molten steel is 0.025%]The relationship between the contents;
FIG. 2 is a metallographic representation of carbides in austenitic heat-resistant steel without magnesium modification according to an embodiment of the present invention;
fig. 3 is a three-dimensional morphology of carbides in the modified austenitic heat-resistant steel prepared in the embodiment of the present invention.
Detailed Description
The present invention is described in terms of particular embodiments, other advantages and features of the invention will become apparent to those skilled in the art from the following disclosure, and it is to be understood that the described embodiments are merely exemplary of the invention and that it is not intended to limit the invention to the particular embodiments disclosed. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
The embodiment of the invention provides a method for modifying austenite heat-resistant steel carbide, which comprises the steps of sequentially carrying out electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium core-spun wire feeding, die casting or continuous casting, rolling, full solid solution and aging solid solution on magnesium-containing austenite heat-resistant steel to obtain modified austenite heat-resistant steel;
wherein, nickel-magnesium core-spun yarn is fed in the last stage of LF furnace refining, when the nickel-magnesium core-spun yarn is fed, the alkalinity value of a molten steel slag layer in the LF furnace is 3.5-6.0, the molten steel slag amount is more than 4% of the molten steel amount, the white slag refining time is more than 25min, the molten steel temperature is controlled at 1425-1470 ℃, the too low wire feeding temperature can possibly cause insufficient superheat degree pouring, the too high temperature can cause serious carbon segregation of heat-resistant steel, and the volatilization of magnesium is serious. The mass percent of Al in the molten steel is 0.02-0.03%, the [ O ] (active oxygen) in the molten steel is controlled at 3-6ppm, and the superheat degree of the molten steel is controlled at 40-75 ℃ at the last refining stage of the LF furnace. In the embodiment, the oxygen level is controlled to ensure that the magnesium feeding has the function of modifying carbide instead of deoxidation by controlling the oxygen level of the molten steel so as to ensure the effect of modifying carbide by magnesium.
In this embodiment, the nickel-magnesium cored wire includes the following components by mass percent: 7-25% of Mg, 5-45% of Ni, 20-40% of Cr, 0.1-5.5% of N, and the balance of Fe and inevitable impurity elements. In the method, in the process of modifying the austenitic heat-resistant steel, the fed nickel-magnesium core-spun yarn components are accurately designed, wherein magnesium is the most key element of the core-spun yarn, the most important function is a modifier, nickel and chromium elements and magnesium can form an alloy compound, the nickel-magnesium element cannot cause the change of the components of molten steel, the nickel and chromium have the effect of diluting the magnesium element and slowing down the intensity of reaction, the nitrogen element in the core-spun yarn can also supplement nitrogen in the molten steel to play a role in increasing nitrogen, and in the process of feeding the yarn, because the affinity of the magnesium to oxygen is far greater than that of the magnesium to nitrogen, the magnesium is more prone to be combined with the oxygen to generate fine and dispersed magnesium oxide particles, and the nitrogen element is released to make up for the loss of nitrogen, thereby reducing the nitrogen blowing cost.
In the electric furnace smelting process, the austenitic heat-resistant steel needs to strictly prevent over-blowing, reduce the oxygen level and reduce the oxygen content to below 300 ppm.
In the early stage of AOD furnace smelting, the alkalinity is controlled to be more than 5, the aluminum adding amount in the deoxidation period is increased, and the oxygen content of molten steel in the last stage of smelting is controlled to be less than 100 ppm. So as to ensure that the oxygen content at the end of smelting is less than 100 ppm; controlling the oxygen content of the molten steel below 100 ppm;
the nickel-magnesium cored wire consists of a core material and an iron sheet wrapping the core material, wherein the iron sheet is made of low-carbon steel, the thickness of the iron sheet is 0.3-0.4mm, the outer diameter of the cored wire is 13mm, the wire feeding speed is 180-plus-one (240 m/min), the weight of each cored wire is 450-plus-one (600 g), and the cored wire is suitable for various wire feeding machines in factories. In the invention, in order to melt and release the core material at the position close to the bottom of the molten steel as much as possible, reasonable iron sheet thickness and wire feeding speed are required to be set. If the iron sheet is too thin or the wire feeding speed is too slow, the fed cored wire is melted at the upper part of the molten steel, so that core materials are released in advance, magnesium steam cannot be dissolved into the steel in time in the floating process and overflows the molten steel, the yield is greatly reduced, and the liquid level is stirred and splashed more violently. If the iron sheet is too thick or the wire feeding speed is too high, the cored wire can directly pierce the bottom of the steel ladle, the refractory material of the steel ladle is damaged, and the molten steel is polluted.
After feeding a nickel-magnesium cored wire in the last refining stage of an LF furnace, covering carbonized rice hulls on the surface of molten steel, soft-blowing a steel ladle by argon flow of 200 plus 350NL/min, wherein the soft-blowing argon flow is based on not blowing to break a slag surface and not exposing the molten steel, the soft-blowing time is 7-12min, measuring the temperature, sampling, measuring the components and the temperature, and after the technological requirements of a finished product of the target magnesium-containing free-cutting molten steel are met, hoisting and tapping.
The magnesium-containing austenitic heat-resistant steel prepared by the method comprises the following components in percentage by mass: 0.48 to 0.58 percent of C, less than or equal to 0.35 percent of Si, 8 to 10 percent of Mn, less than or equal to 0.04 percent of P, less than or equal to 0.01 percent of S, 20 to 22 percent of Cr, 3.25 to 4.25 percent of Ni, 0.35 to 0.50 percent of N, less than or equal to 0.30 percent of Cu, 0.0005 to 0.0015 percent of Mg, 0.020 to 0.030 percent of Al, and the balance of iron and inevitable impurities.
The magnesium content of the magnesium-containing austenitic heat-resistant steel prepared by the method of the embodiment is measured by ICP, and the yield of the magnesium is calculated to be 20-35% according to the addition amount. The carbides in the magnesium-containing austenitic heat-resistant steel prepared in the embodiment are observed and counted by adopting a metallographic microscope, the carbides are obviously refined, the layered structure is thinned, the carbides are uniformly distributed, and the massive carbides larger than 30 microns basically disappear.
Example 1
The embodiment provides a method for modifying austenite heat-resistant steel carbide, which comprises the steps of sequentially carrying out 40t steel electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium cored wire feeding, continuous casting, rolling, full solid solution and aging solid solution on a magnesium-containing austenite heat-resistant steel raw material to obtain modified austenite heat-resistant steel; wherein, in the final stage of refining in an LF furnace, the nickel-magnesium core-spun yarn is fed for many times in an intermittent way under the condition of bottom blowing argon, when the nickel-magnesium core-spun yarn is fed, the alkalinity value of a molten steel slag layer in the LF furnace is 5.5, the molten steel slag amount is more than 4 percent of the molten steel amount, the refining time of white slag is more than 25min, the slag amount is 1.6 tons, and the slag amount is controlled to be not more than 1.6 tons, thereby realizing that the oxidation of the molten steel is prevented, simultaneously reducing the volatile molten steel temperature of magnesium steam and controlling the molten steel at 1450 +/-5 ℃, the mass percent of Al in the molten steel to be 0.025 percent, controlling the [ O ] (active oxygen) in the molten steel to be 4ppm, and controlling the superheat degree of the molten steel at the final stage of refining in the LF furnace to be 40-75 ℃. In the embodiment, the nickel-magnesium cored wire consists of a core material and an iron sheet wrapping the core material, wherein the iron sheet is made of low-carbon steel, the thickness of the iron sheet is 0.3mm, the outer diameter of the cored wire is 13mm, and the weight of each cored wire is 450-600 g. The nickel-magnesium cored wire comprises the following components in percentage by mass: 10% of Mg, 30% of Ni, 20% of Cr, 2.5% of N, and the balance of Fe and inevitable impurity elements. In the method for modifying the carbide of the austenitic heat-resistant steel, the wire feeding process is stable, the reaction is smooth, severe splashing does not exist, and safety accidents such as molten steel splashing and the like do not occur.
In the electric furnace smelting process, the austenitic heat-resistant steel needs to strictly prevent over-blowing, reduce the oxygen level and reduce the oxygen content to below 300 ppm. In the early stage of AOD furnace smelting, the alkalinity is controlled to be more than 5, the aluminum adding amount in the deoxidation period is increased, and the oxygen content of molten steel in the last stage of smelting is controlled to be less than 100 ppm. So as to ensure that the oxygen content at the final stage of smelting is less than 100 ppm; controlling the oxygen content of the molten steel below 100 ppm; after feeding a nickel-magnesium cored wire in the last refining stage of an LF furnace, covering carbonized rice hulls on the surface of molten steel, soft-blowing a steel ladle by argon flow of 100NL/min, wherein the soft-blowing argon flow is subject to no blowing to break a slag surface and no exposure of the molten steel, the soft-blowing time is 8min, measuring temperature, sampling, measuring components and temperature, and after the technological requirements of a finished product of the target magnesium-containing free-cutting molten steel are met, hanging the steel for tapping.
The magnesium content of the magnesium-containing austenitic heat-resistant steel prepared by the embodiment is 18ppm by ICP (inductively coupled plasma), and the yield of the magnesium is 28 percent by calculation according to the addition amount. The measurement and calculation verification show that the magnesium-containing austenitic heat-resistant steel prepared by the embodiment comprises the following components in percentage by mass: 0.5% of C, 0.31% of Si, 8.75% of Mn, 0.02% of P, 0.005% of S, 21.60% of Cr, 4.0% of Ni, 0.48% of N, 0.0018% of Mg, and the balance of Fe and inevitable trace elements.
The modified magnesium-containing austenitic heat-resistant steel prepared in the embodiment is subjected to sample preparation, the carbide in the modified magnesium-containing austenitic heat-resistant steel prepared in the embodiment is observed by a metallographic microscope under 100 times after a prepared metallographic sample is subjected to polishing corrosion treatment, the carbide is obviously refined, the layered structure is thinned, the carbide is uniformly distributed, the blocky carbide with the size larger than 30 micrometers completely disappears, the average size of the carbide is within the range of 10-18 micrometers, and the morphology of the carbide in the magnesium-containing austenitic heat-resistant steel in the embodiment is shown in fig. 3.
Example 2
The embodiment provides a method for modifying austenite heat-resistant steel carbide, which comprises the steps of sequentially carrying out 40t steel electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium cored wire feeding, die casting, primary rolling cogging, rolling, full solid solution and aging solid solution on a magnesium-containing austenite heat-resistant steel raw material to obtain modified austenite heat-resistant steel; wherein, in the final stage of refining in the LF furnace, the nickel-magnesium core-spun yarn is fed for multiple times in an intermittent way under the condition of bottom blowing argon, compared with the embodiment, when the nickel-magnesium core-spun yarn is fed for 20 percent more than the nickel-magnesium core-spun yarn, the alkalinity value of a molten steel slag layer in the LF furnace is 5.5, the molten steel slag amount is more than 4 percent of the molten steel amount, the white slag refining time is more than 25min, the slag amount is 1.6 tons, the temperature of the molten steel is controlled at 1450 +/-5 ℃, the mass percent of Al in the molten steel is 0.025 percent, the [ O ] (active oxygen) in the molten steel is controlled at 6ppm, and the superheat degree of the molten steel is controlled at 40-75 ℃ in the final stage of refining in the LF furnace. In the embodiment, the nickel-magnesium cored wire consists of a core material and an iron sheet wrapping the core material, wherein the iron sheet is made of low-carbon steel, the thickness of the iron sheet is 0.3mm, the outer diameter of the cored wire is 13mm, and the weight of each cored wire is 450-600 g. The nickel-magnesium cored wire comprises the following components in percentage by mass: 10% of Mg, 30% of Ni, 20% of Cr, 2.5% of N, and the balance of Fe and inevitable impurity elements. In the method for modifying the carbide of the austenitic heat-resistant steel, the wire feeding process is stable, the reaction is smooth, severe splashing does not exist, and safety accidents such as molten steel splashing and the like do not occur.
In the electric furnace smelting process, the austenitic heat-resistant steel needs to strictly prevent over-blowing, reduce the oxygen level and reduce the oxygen content to below 300 ppm. In the early stage of AOD furnace smelting, the alkalinity is controlled to be more than 5, the aluminum adding amount in the deoxidation period is increased, and the oxygen content of molten steel in the last stage of smelting is controlled to be less than 100 ppm. So as to ensure that the oxygen content at the end of smelting is less than 100 ppm; controlling the oxygen content of the molten steel below 100 ppm; after feeding a nickel-magnesium cored wire in the last refining stage of an LF furnace, covering carbonized rice hulls on the surface of molten steel, soft-blowing a steel ladle by argon flow of 100NL/min, wherein the soft-blowing argon flow is subject to no blowing to break a slag surface and no exposure of the molten steel, the soft-blowing time is 8min, measuring temperature, sampling, measuring components and temperature, and after the technological requirements of a finished product of the target magnesium-containing free-cutting molten steel are met, hanging the steel for tapping.
The modified austenitic heat-resistant steel prepared in this example had a magnesium content of 6ppm as measured by ICP and a magnesium yield of 25% as calculated from the amount added. The measurement and calculation prove that the modified magnesium-containing austenitic heat-resistant steel prepared by the embodiment comprises the following components in percentage by mass: 0.5% of C, 0.31% of Si, 8.75% of Mn, 0.02% of P, 0.005% of S, 20.60% of Cr, 4.0% of Ni, 0.48% of N, 0.0006% of Mg, and the balance of Fe and inevitable trace elements.
For the magnesium-containing austenitic heat-resistant steel prepared in this example, carbides in the magnesium-containing austenitic heat-resistant steel prepared in this example are observed by a metallographic microscope at 100 times after a prepared metallographic sample is subjected to polishing corrosion treatment. The carbide is obviously thinned, the layered structure is thinned, the carbide is uniformly distributed, the blocky carbide with the size of more than 30 mu m completely disappears, and the size of the carbide is 10-25 mu m.
Example 3
The method for modifying austenitic heat-resistant steel carbide of the present example is different from that of example 1 in that a magnesium-containing core wire component is fed, and the magnesium-containing core wire component is 15 mass% of Mg, 15 mass% of Ni, 25 mass% of Cr, and 0.6 mass% of N, and the balance is Fe and inevitable impurity elements. The wire feeding process in the field production of the embodiment is stable, the reaction is smooth, and no violent splashing exists.
The modified austenitic heat-resistant steel prepared in the example has 16ppm of magnesium content measured by ICP, and the yield of magnesium calculated according to the addition amount is 29%. The measurement and calculation verification show that the magnesium-containing austenitic heat-resistant steel prepared by the embodiment comprises the following components in percentage by mass: 0.5% of C, 0.31% of Si, 8.75% of Mn, 0.02% of P, 0.01% of S, 20.60% of Cr, 4.0% of Ni, 0.48% of N, 0.0016% of Mg, and the balance of Fe and inevitable trace elements.
The samples of the rolled magnesium-containing austenitic heat-resistant steel material prepared in the embodiment are prepared, the prepared metallographic sample is subjected to grinding, polishing and corrosion treatment, and then under the condition of 100 times of a metallographic microscope, carbides in the magnesium-containing austenitic heat-resistant steel prepared by the process in the embodiment are observed, the carbides are obviously refined, the layered structure is thinned, the carbides are uniformly distributed, massive carbides with the size of more than 30 mu m basically disappear, and the size of the carbides is 10-25 mu m.
Although the invention has been described in detail above with reference to a general description and specific examples, it will be apparent to one skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (6)
1. The method for modifying the carbide of the austenitic heat-resistant steel is characterized by comprising the steps of sequentially carrying out electric furnace smelting, AOD furnace smelting, LF furnace refining, nickel-magnesium cored wire feeding, continuous casting or die casting, rolling, full solid solution and aging solid solution on a magnesium-containing austenitic heat-resistant steel raw material to obtain the modified austenitic heat-resistant steel;
the method comprises the following steps of feeding a nickel-magnesium cored wire at the last refining stage of an LF furnace, wherein the nickel-magnesium cored wire comprises the following components in percentage by mass: 7-25% of Mg, 5-45% of Ni, 20-40% of Cr, 0.1-5.5% of N, and the balance of Fe and inevitable impurity elements;
the modified austenitic heat-resistant steel comprises the following components in percentage by mass: 0.48 to 0.58 percent of C, less than or equal to 0.35 percent of Si, 8 to 10 percent of Mn, less than or equal to 0.04 percent of P, less than or equal to 0.01 percent of S, 20 to 22 percent of Cr, 3.25 to 4.25 percent of Ni, 0.35 to 0.50 percent of N, less than or equal to 0.30 percent of Cu, 0.0005 to 0.0015 percent of Mg, 0.020 to 0.030 percent of Al, and the balance of iron and inevitable impurities;
and in the last refining stage of the LF furnace, when the nickel-magnesium cored wire is fed, the alkalinity value of a molten steel slag layer is 3.5-6.0, the molten steel slag amount is more than 4% of the molten steel amount, the molten steel temperature is controlled at 1425-1470 ℃, the mass percent of Al in the molten steel is 0.02-0.03%, and the oxygen activity in the molten steel is controlled at 3-6 ppm.
2. The method for upgrading austenitic heat-resistant steel carbide according to claim 1, wherein,
during electric furnace smelting, the oxygen content of the molten steel is controlled below 300 ppm;
in the early stage of AOD furnace smelting, the alkalinity is controlled to be more than 5, and the oxygen content of molten steel in the last stage of smelting is controlled to be less than 100 ppm.
3. The method for upgrading austenitic heat-resistant steel carbide according to claim 1, wherein,
and in the final stage of refining in the LF furnace, the superheat degree of molten steel is controlled to be 40-75 ℃.
4. The method for upgrading austenitic heat-resistant steel carbide according to claim 1, wherein,
the nickel-magnesium cored wire consists of a core material and an iron sheet wrapping the core material, wherein the thickness of the iron sheet is 0.3mm, the outer diameter of the cored wire is 13mm, and the weight of the cored wire per meter is 550-750 g.
5. The method for upgrading austenitic heat-resistant steel carbide according to claim 1, wherein,
and after feeding the nickel-magnesium core-spun yarn in the last refining stage of the LF furnace, covering carbonized rice hulls on the surface of the molten steel, and stirring by soft argon blowing, wherein the soft argon blowing is based on that the slag surface is not blown open and the molten steel is not exposed.
6. The method for upgrading austenitic heat-resistant steel carbide according to claim 1, wherein,
the wire feeding speed of the nickel-magnesium cored wire is 180-240 m/min.
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