Patent Application
20140356220
The present disclosure relates to wear resistant austenitic steel having superior machinability and ductility, and a method for producing the wear resistant austenitic steel.
Along with the development of the mining, oil, and gas industries, the wear on steel used for mining, transportation, and refining applications has become problematic. Particularly, as oil sands have been recently developed in earnest as an unconventional source of petroleum, the wear on steel members caused by slurry containing oil, gravel, and sand is one of the main factors increasing the production cost of oil from oil sands, and thus, the development and practical implementation of steel having a high degree of resistance to wear are increasingly required.
In the mining industry, Hadfield steel having high wear resistance has been mainly been used. Hadfield steel is high-strength steel having a high manganese content, and there have been steady efforts to improve the wear resistance of such steel by adding large amounts of carbon and manganese thereto to increase the formation of austenite and wear resistance therein. However, due to a high carbon content in Hadfield steel, carbides may be formed at high temperature in a network manner along austenite grain boundaries of the Hadfield steel, and thus the physical properties of the Hadfield steel (particularly, ductility) are markedly worsened.
To prevent the formation of such network-shaped precipitates of carbides, a method for manufacturing high-manganese steel by rapidly cooling the high-manganese steel to room temperature after a solution heat treatment or a hot working process is performed on the high-manganese steel at a high temperature has been proposed. However, if a relatively thick steel sheet is formed by the proposed method, the effect of preventing the precipitation of carbides may not be sufficiently obtained by rapid cooling. In addition, if a welding process is performed, it is difficult to control the rate of cooling after the welding process and thus difficult to suppress the formation of network-shaped precipitates of carbides. Therefore, physical properties of steel may be markedly worsened. In addition, alloying elements such as manganese or carbon inevitably segregate in a high-manganese ingot or slab during solidification, and such segregation is facilitated in a post processing process such as a hot rolling process. As a result, carbides may partially precipitate in the form of a network along intensive segregation zones of a final product, and thus the microstructure of the final product may be inhomogeneous, resulting in poor physical properties.
Generally, the content of carbon in steel may be increased to improve the wear resistance of steel, and the content of manganese in the steel may be increased to prevent the deterioration of physical properties of the steel caused by the precipitation of carbides. However, this method increases the amounts of alloying elements and thus the manufacturing cost of steel. Furthermore, the addition of manganese to steel decreases the corrosion resistance of the steel as compared with general carbon steel. Thus, such steel may not be used in fields requiring corrosion resistant steel.
Furthermore, since the machinability of austenitic high-manganese steel is poor due to a high degree of work hardenability, the lifespans of cutting tools may be decreased, and thus costs for cutting tools may be increased. In addition, process suspension times may be increased due to the need for the frequent replacement of cutting tools. Eventually, manufacturing costs may be increased.
Aspects of the present disclosure may provide austenitic steel having improved machinability, ductility, and wear resistance through suppressing the formation of carbides, and a method for producing the austenitic steel.
However, aspects of the present disclosure are not limited thereto. Additional aspects will be set forth in part in the description which follows, and will be apparent from the description to those having ordinary skill in the art to which the present disclosure pertains.
According to an aspect of the present disclosure, wear resistant austenitic steel having superior machinability and ductility may include, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) and inevitable impurities.
According to another aspect of the present disclosure, a method for producing wear resistant austenitic steel having superior machinability and ductility may include: reheating a steel slab to a temperature of 1050° C. to 1250° C., the steel slab including, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) and inevitable impurities; performing a finish hot rolling process on the steel slab within a temperature range of 800° C. to 1050° C. to form a steel sheet; and cooling the hot-rolled steel sheet to a temperature of 600° C. or lower at a cooling rate of 10° C./s to 100° C./s,
According to the present disclosure, the formation of carbides in the austenitic steel may be suppressed to prevent the deterioration of the austenitic steel, and the wear resistance of the austenitic steel may be sufficiently improved. Therefore, the austenitic steel may be used for an extended period of time, even in corrosive environments.
Hereafter, wear resistant austenitic steel having superior machinability and ductility, and a method for producing the wear resistant austenitic steel will be described in detail according to embodiments of the present disclosure, so that those of ordinary skill in the related art may clearly understand the scope and spirit of the embodiments of the present disclosure.
The inventors found that if the composition of steel is properly adjusted, the steel has a high degree of wear resistance without a decrease in ductility caused by carbides and a high degree of machinability. Based on this knowledge, the inventors invented wear resistant austenitic steel and a method of producing the wear resistant austenitic steel.
That is, manganese and carbon are added to the steel of the embodiments of the present disclosure to improve the wear resistance of the steel while controlling the content of the carbon relative to the content of the manganese to minimize the formation of carbides. Furthermore, additional elements are added to the steel to further suppress the formation of carbides and thus to sufficiently improve the toughness of the steel in addition to improving the wear resistance of the steel, and in conjunction therewith, the contents of calcium and sulfur in the steel are adjusted to markedly improve the machinability of the steel (austenitic high-manganese steel).
According to the embodiments of the present disclosure, the steel may include, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) and inevitable impurities.
The numerical ranges of the contents of the elements are set because of reasons described below. In the following description, the content of each element is given in weight % unless otherwise specified.
Manganese (Mn): 8% to 15%
Manganese is a main element for stabilizing austenite in high manganese steel like the steel of the embodiments of the present disclosure. In the embodiments of the present disclosure, it may be preferable that the content of manganese be 8% or greater for forming austenite as a main component of the microstructure of the steel. If the content of manganese is less than 8%, ferrite may be formed, and thus austenite may not be sufficiently formed. On the other hand, if the content of manganese is greater than 15%, problems such as decrease in a corrosion resistance of the steel, increase in difficulties in the manufacturing process and increase in manufacturing costs may occur. Also, the work hardenability of the steel may be decreased due to a decreased in tensile strength.
Carbon (C) 23%<33.5C—Mn≦37%
Carbon is an element for stabilizing austenite and forming austenite at room temperature. Carbon increases the strength of the steel. Particularly, carbon dissolved in austenite of the steel increases the work hardenability of the steel and thus increases the wear resistance of the steel. However, as described above, if the content of carbon in the steel is insufficient, the stability of austenite is low, and the wear resistance of the steel may be insufficient due to the formation of martensite or a low degree of work hardenability of austenite. On the other hand, if the content of carbon in the steel is excessive, it is difficult to suppress the formation of carbides.
Therefore, in the embodiments of the present disclosure, the content of carbon in the steel may be determined according to the contents of other elements in the steel. The inventors found a relationship between carbon and manganese in the formation of carbides, and the relationship is illustrated in
If it is assumed that the contents of the other elements of the steel are within the ranges of the embodiments of the present disclosure, it may be preferable that the value of 33.5C—Mn be adjusted to be 37 or less (where C and Mn refer to the content of carbon and the content of manganese in weight %), so as to prevent the formation of carbides. This corresponds to the right boundary of the parallelogram region in
Copper (Cu): 1.6C-1.4(%)≦Cu≦5%
Due to a low solid solubility of copper in carbides and a low diffusion rate of copper in austenite, copper tends to concentrate in interfaces between austenite and carbides. Therefore, if fine carbide nuclei are formed, copper may surround the fine carbide nuclei, and thus additional diffusion of carbon and growth of carbides may be retarded. That is, copper suppresses the formation and growth of carbides. Therefore, in the embodiments of the present disclosure, copper is added to the steel. The content of copper in the steel is not independently determined but may be determined according to the formation behavior of carbides. For example, the content of copper may be set to be equal to or greater than 1.6C-1.4 weight % so as to effectively suppress the formation of carbides if the content of copper in the steel is less than 1.6C-1.4 weight %, the conversion of carbon into carbides may not be suppressed. In addition, if the content of copper in the steel is greater than 5 weight %, the hot workability of the steel may be lowered. Therefore, it may be preferable that the upper limit of the content of copper be set to 5 weight %. Particularly, in the embodiments of the present disclosure, when the content of carbon added to the steel for improving wear resistance is considered, the content of copper may preferably be 0.3 weight or greater, more preferably, 2 weight % or greater, so as to obtain a sufficient effect of suppressing the formation of carbides.
In the embodiments of the present disclosure, the other component of the steel is iron (Fe). However, impurities in raw materials or manufacturing environments may be inevitably included in the steel, and such impurities may not be able to be removed from the steel, Such impurities are well-known to those of ordinary skill in the art to which the present disclosure pertains, and thus descriptions thereof will not be given in the present disclosure.
In the embodiments of the present disclosure, sulfur (S) and calcium (Ca) may be further included in the steel in addition to the above-described elements, so as to improve the machinability of the steel.
Sulfur (S) 0.03% to 0.1%
In general, it is known that sulfur added together with manganese forms manganese sulfide which is easily cut and separated during a cutting process. That is, sulfur is known as an element improving the machinability of steel. Sulfur is melted by heat generated during a cutting process, and thus reduces friction between chips and cutting tools. That is, sulfur increases the lifespan of cutting tools by lubricating the surface of the cutting tools, reducing the wear on the cutting tools, and preventing accumulation of cutting chips on the cutting tool. However, if the content of sulfur in the steel is excessive, mechanical characteristics of the steel may deteriorate due to a large amount of coarse manganese sulfide elongated during a hot working process, and the hot workability of the steel may deteriorate due to the formation of iron sulfide. Therefore, it may be preferable that the upper limit of the content of sulfur in the steel be 0.1%. If the content of sulfur in the steel is less than 0.03%, the machinability of the steel may not be improved, and thus it may be preferable that the lower limit of the content of sulfur in the steel be 0.03%.
Calcium (Ca) 0.001% to 0.01%
Calcium is usually used to control the formation of manganese sulfide. Since calcium has a high affinity for sulfur, calcium forms calcium sulfide together with sulfur, and along with this, calcium is dissolved in manganese sulfide. Since manganese sulfide crystallizes around calcium sulfide functioning as crystallization nuclei, during a hot working process, manganese sulfide may be less elongated and may be maintained in a spherical shape. Therefore, the machinability of the steel may be improved. However, if the content of calcium is greater than 0.01%, the above-described effect is saturated. In addition, since the percentage recovery of calcium is low, a large amount of calcium raw material may have to be used, and thus the manufacturing cost of the steel may be increased. On the other hand, if the content of calcium in the steel is less than 0.001%, the above-described effect is insignificant. Thus, it may be preferable that the lower limit, of the content of calcium be 0.001%.
In the embodiments of the present disclosure, chromium (Cr) may be included in the steel in addition to the above-described elements so as to further improve the corrosion resistance of the steel.
Cr: 8% or Less (Excluding 0%)
Generally, manganese lowers the corrosion resistance of steel. That is, in the embodiments of the present disclosure, manganese included in the steel in the above-described content range may lower the corrosion resistance of the steel, and thus chromium is added to the steel to improve the corrosion resistance of the steel in addition, if chromium is added to the steel in an amount within the range, the strength of the steel may also be improved. However, if the content of chromium in the steel is greater than 8 weight %, the manufacturing cost of the steel is increased, and carbon dissolved in the steel may be converted into carbides along grain boundaries to lower the ductility of the steel and particularly resistance of the steel to sulfide stress cracking. In addition, ferrite may be formed in the steel, and thus austenite may not be formed as a main microstructure in the steel. Therefore, it may be preferable that the upper limit of the content of chromium be 8 weight %. Particularly, to maximize the effect of improving the corrosion resistance of the steel, it may be preferable that the content of chromium in the steel be set to be 2 weight % or greater. Since the corrosion resistance of the steel is improved by the addition of chromium, the steel may be used for forming slurry pipes or as an anti sour gas material.
The steel having the above-described composition is austenitic steel having 90 area % or more of austenite. In a later processing process, austenite of the steel may be markedly hardened, and thus the steel may have a high degree of hardness. In addition to austenite, some other microstructures such as martensite, bainite, pearlite, and ferrite may be inevitably formed in the steel as impurity microstructures. In the present disclosure, the sum of the amounts of the phases of the steel is put as 100%, and the content of each microstructure is denoted as a proportion of the sum without considering the amounts of precipitates such as a carbide precipitate.
Furthermore, in the embodiments of the present disclosure, it may be preferable that the steel include 10 area % or less of carbides (based on the total area of the steel). Since carbides lower the ductility of the steel, the amounts of carbides in the steel may be adjusted to be low. For example, in the embodiments of the present disclosure, since the area fraction of carbides in the steel is 10% or less, when the steel is used as wear resistant steel, problems caused by low ductility such as premature fracturing and a decrease in impact toughness may not arise.
Hereinafter, a method for producing the wear resistant austenitic steel will be described according to an embodiment of the present disclosure. The steel may be produced by a manufacturing method commonly known in the related art, and the manufacturing method of the related art may include a conventional hot rolling process in which a slab is reheated, roughly-rolled, and finish-rolled. After the hot rolling process, the steel may be cooled by a conventional cooling method. For example, in an embodiment of the present disclosure, the steel may be produced by an exemplary method proposed by the inventors as follows.
A steel slab is prepared, which includes, by weight %, 8% to 15% of manganese (Mn), carbon (C) satisfying 23%<33.5C—Mn≦37%, copper (Cu) satisfying 1.6C-1.4(%)≦Cu≦5%, and the balance of iron (Fe) and inevitable impurities.
As described above, the steel slab may further include sulfur (S) and calcium (Ca).
Furthermore, as described above, the steel slab may further include chromium (Cr).
The steel slab is reheated to a temperature of 1050° C. to 1250° C.
The steel slab (or ingot) may be reheated in a reheating furnace for a hot rolling process. If the steel slab is reheated to a temperature lower than 1050° C., the load acting on a rolling mill may be markedly increased, and alloying elements may not be sufficiently dissolved in the steel slab. On the other hand, if the reheating temperature of the steel slab is too high, crystal grains may excessively grow, and thus the strength of the steel slab may be lowered. Particularly, in the above-described composition range of the steel of the present disclosure, carbides may melt in grain boundaries, and if the steel slab is reheated to a temperature equal to or higher than the solidus line of the steel slab, hot-rolling characteristics of the steel slab may deteriorate. Therefore, the upper limit of the reheating temperature may be set to be 1250° C.
Thereafter, the steel slab is finish-rolled at a temperature of 800° C. to 1050° C. to form a steel sheet.
As described above, the steel slab is rolled within the temperature range of 800° C. to 1050° C. If the steel slab is roiled at a temperature lower than 800° C., the load of rolling may be large, and carbides may precipitate and grow coarsely. Thus, desired ductility may not be obtained. The upper limit of the rolling temperature is set to be 1050° C.
The steel sheet formed by hot rolling is cooled to a temperature of 600° C. or lower at a cooling rate of 10° C./s to 100° C./s.
After the finish rolling, the steel sheet may be cooled at a sufficiently high cooling rate to suppress the formation of carbides in grain boundaries. If the cooling rate is less than 10° C./s, the formation of carbides may not be sufficiently suppressed, and thus carbides may precipitate in grain boundaries during cooling. This may cause problems such as premature fracture, a ductility decrease, and a wear resistance decrease. Therefore, the cooling rate may be adjusted to be high, and the upper limit, of the cooling rate may not be limited to a particular value as long as the cooling rate is within an accelerated cooling rate range. However, it may be difficult to increase the cooling rate to a value greater than 100° C./s by a conventional accelerated cooling technique.
Although the steel sheet is cooled at a high cooling rate, if the cooling of the steel sheet is terminated at a high temperature, carbides may be formed and grow in the steel sheet. Therefore, in the embodiment of the present disclosure, the steel sheet may be cooled to a temperature of 600° C. or lower.
Hereinafter, the embodiments of the present disclosure will be described more specifically through examples. However, the examples are for clearly explaining the embodiments of the present disclosure and are not intended to limit the spirit and scope of the present disclosure.
Slab samples having elements and compositions illustrated in Table 1 were reheated, hot-rolled, and cooled under the conditions illustrated in Table 2. Then, properties of the samples such the microstructure, elongation, strength, and carbide fraction were measured as illustrated in Table 3. In Table 1, the content of each element is given in weight.
In addition, a wear test (ASTM G65) and an immersion test (ASTM G31) for evaluating corrosion rates were performed on comparative samples and inventive samples, and the results are illustrated in Table 4 below.
33.5C—Mn of Comparative Sample A1 was 6.8 which was outside of the range of the embodiments of the present disclosure. Thus, due to a lack of carbon stabilizing austenite, a large amount of martensite was formed in Comparative Sample A1, and a desired austenitic microstructure was not formed in Comparative Sample A1.
Comparative Sample A2 had manganese and carbon within the content ranges of the embodiments of the present disclosure. However, copper was not added to Comparative Sample A2, and thus the formation of carbides was not suppressed. That is, large amounts of carbides were formed along grain boundaries of Comparative Sample A2, and thus a desired microstructure and elongation were not obtained. In Comparative Sample A2, a sufficient degree of work hardenabiliy was not obtained due to premature fracture and a decreased amount of dissolved carbon caused by the formation of carbides. Therefore, the wear amount of Comparative Sample A2 was relatively large.
Comparative Samples A3 and A4 had manganese and carbon within the content ranges of the embodiments of the present disclosure. However, the content of copper in each of Comparative Samples A3 and A4 was outside of the range of the embodiments of the present disclosure. Therefore, like in Comparative Sample A2, large amounts of carbides were formed in Comparative Samples A3 and A4, and thus a desired microstructure and elongation were not obtained. Since the contents of copper in Comparative Samples A3 and A4 were outside of the range of the embodiments of the present disclosure, the formation of carbides was not effectively suppressed, and thus the amounts of dissolved carbon and elongation of Comparative Samples A3 and A4 were reduced to cause premature fracture. Thus, a sufficient degree of work hardenability was not obtained in Comparative Sample A3 and A4, and thus the wear resistance of Comparative Samples A3 and A4 was reduced.
Although the composition of Comparative Sample A5 satisfied the conditions of the embodiments of the present disclosure, the cooling rate of Comparative Sample A5 after rolling process was outside of the range of the embodiments of the present disclosure. That is, due to a low cooling rate, the formation of carbides was not effectively suppressed, and thus the ductility of Comparative Sample A5 was decreased.
However, in Inventive Samples A1 to A6 having elements and compositions according to the embodiments of the present disclosure, the formation of carbides in grain boundaries was effectively suppressed owing to the addition of copper, and thus physical properties of Inventive Samples A1 to A6 were not worsened. In detail, although Inventive Samples A1 to A6 had high carbon contents, the formation of carbides was effectively suppressed owing to the addition of copper, and thus Inventive Samples A1 and A6 had desired microstructures and properties. Since carbon was sufficiently dissolved in austenite and the formation of carbides in grain boundaries was effectively suppressed, the elongation of Inventive Samples A1 to A6 was stably maintained, and the tensile strength of inventive Samples A1 to A6 was high. Therefore, the work hardenability of Inventive Samples A1 to A6 was sufficient, and thus the wear amounts of Inventive Samples A1 to A6 were small.
Particularly, according to results of a corrosion test, the corrosion rates of inventive Samples A5 and A6 to which chromium was additionally added were low. That is, the corrosion resistance of Inventive Samples A5 and A6 was improved. The effect of improving corrosion resistance by the addition of chromium may be clearly understood by comparison with corrosion rates of Inventive Samples A1 to A4. In addition, the strength of Inventive Samples A5 and A6 was improved by solid-solution strengthening induced by the addition of chromium.
Steel slabs (Inventive Samples and Comparative Samples) having compositions illustrated in Table 5 were manufactured by a continuous casting process. In Table 5, the content of each element is given in weight %.
The steel slabs were reheated, finish-rolled, and cooled under the conditions illustrated in Table 6 so as to form steel sheets.
The austenite fraction, carbide fraction, elongation, yield strength, and tensile strength of each of the steel sheets were measured as illustrated in Table 7. Holes were repeatedly formed in each of the steel sheets by using a drill having a diameter of 10 mm and formed of high speed tool steel in conditions of a drill speed of 130 rpm and a drill movement rate of 0.08 mm/rev. The number of holes formed in each steel sheet until the drill was worn down to the end of its lifespan was counted as illustrated in Table 3.
In addition, a wear test (ASTM G65) and an immersion test (ASTM G31) for evaluating corrosion rates were performed on each of the steel sheets (comparative samples and inventive samples), and the results are illustrated in Table 8 below.
In the inventive samples having carbon and manganese within the content ranges of the embodiments of the present disclosure, the formation of carbides in grain boundaries was effectively suppressed owing to the addition of copper, and thus physical properties of the inventive samples were not worsened. In detail, although the inventive samples had high carbon contents, the formation of carbides was effectively suppressed owing to the addition of copper, and thus the inventive samples had desired microstructures and properties. Since carbon was sufficiently dissolved in austenite and the formation of carbides in grain boundaries was effectively suppressed, the elongation of the inventive samples was stably maintained, and the tensile strength of the inventive samples was high. Therefore, the work hardenability of the inventive samples was sufficient, and thus the wear amounts of the inventive samples were small.
The machinability of Comparative Samples B1 to B5 was poor because sulfur and calcium were not added to Comparative Samples B1 to B5 or the contents of sulfur and calcium in Comparative Samples B1 to B5 were outside of the ranges of the embodiments of the present disclosure.
However, Inventive Samples B1 to B5 including sulfur and calcium within the content ranges of the embodiments of the present disclosure had superior machinability as compared with the comparative samples. Particularly, in Inventive Samples B2 to B4 having different sulfur contents, the machinability thereof was improved in proportion to the content of sulfur.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2011-0145213 | Dec 2011 | KR | national |
| 10-2012-0151507 | Dec 2012 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR2012/011536 | 12/27/2012 | WO | 00 | 6/26/2014 |
