GB2410503A - High-cleanliness steel and process for producing the same - Google Patents
High-cleanliness steel and process for producing the same Download PDFInfo
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- GB2410503A GB2410503A GB0509771A GB0509771A GB2410503A GB 2410503 A GB2410503 A GB 2410503A GB 0509771 A GB0509771 A GB 0509771A GB 0509771 A GB0509771 A GB 0509771A GB 2410503 A GB2410503 A GB 2410503A
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/10—Handling in a vacuum
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/06—Deoxidising, e.g. killing
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Abstract
A process for producing a high-cleanliness steel, comprising the steps of: transferring a molten steel produced in an arc melting furnace or a converter to a ladle refining furnace to refine the molten steel; subjecting the refined molten steel to circulation-type vacuum degassing; and then casting the degassed molten steel into an ingot, wherein the refining in the ladle furnace is carried out for not more than 60 min, and the degassing in the circulation-type vacuum degassing device is carried out for not less than 25 min under such conditions that the amount of the molten steel circulated in the circulation-type vacuum degassing device is at least 8 times larger than the total amount of the molten steel.
Description
24 1 0503
Description
HIGH-CLEANLINESS STEEL AND PROCESS FOR PRODUCING THE SAME
TECHNICAL FIELD
The present invention relates to a high-cleanliness steel for use as steels for mechanical parts required to possess fatigue strength, fatigue life, and quietness, particularly, for example, as steels for rolling bearings, steels for constant velocity joints, steels for gears, steels for continuously variable transmission of toroidal type, steels for mechanical structures for cold forging, tool steels, and spring steels, and a process for producing the same.
Steels for use in mechanical parts required to possess fatigue strength and fatigue life should be high-cleanliness (low content of non-metallic inclusions in steels) steels.
Conventional production processes of these high-cleanliness steels include: (A) oxidizing refining of a molten steel in an arc melting furnace or a converter; (B) reduction refining in a ladle furnace (LF); (C) circulation vacuum degassing in a circulation-type vacuum degassing device (RH) (PH treatment); (D) casting of steel ingots by continuous casting or conventional ingot casting, and (E) working of steel ingots by press forging and heat treatment of steel products. In the process (A), scrap is melted by arc heating, or alternatively, a molten steel is introduced into a converter where oxidizing refining is performed, followed by the transfer of the molten steel to a ladle furnace.
The temperature, at which the molten steel is transferred, is generally a high temperature of about 30 C above to less than 100 C above the melting point of the steel. In the process (B), a deoxidizer alloy of aluminium, manganese, silicon, etc. is introduced into the ladle furnace, to which the molten steel has been transferred, where reduction refining is carried out by deoxidation and desulphurisation with a desulphuriser to regulate the alloying constituents. A generally accepted knowledge is such that the effect increased with increasing the treatment time.
In this process, a long time of more than 60 min is adopted, and t the treatment temperature is generally 50 C above the melting point of the steel. In the RH treatment in the process (C), vacuum degassing is carried out in a circulation vacuum degassing tank while circulating the molten steel through the circulation vacuum degassing tank to perform deoxidation and dehydrogenation. In this case, the amount of the molten steel circulated is about to 6 times the total amount of the molten steel. In the process (D), the RH treated molten steel is transferred to a tundish where the molten steel is continuously cast into a bloom, a billet, a slab or the like. Alternatively, the molten steel from the ladle is poured directly into a steel ingot mold to cast a steel ingot.
In the process (E), for example, a bloom, a billet, a slab, or a steel ingot is rolled or forged, followed by heat treatment to prepare a steel product which is then shipped.
When steels having a particularly high level of cleanliness are required, in the above process, the cast steel ingot is provided as a raw material which is then subjected to vacuum re- melting or electroslag re-melting to prepare such steels.
In recent years, mechanical parts have become used under more and more severe conditions. This has lead to more and more severe requirements for properties of steel products, and steel products having a higher level of cleanliness have been required in the art. The above-described conventional production processes (A) to (E), however, are difficult to meet this demand.
In order to meet this demand, steel products have been produced by the vacuum resmelting or the electroslag ore-melting. These methods, however, pose a problem of significantly increased production cost.
Under these circumstances, the present invention has been made, and it is an object of the present invention to provide steel products having a high level of cleanliness without relying upon the re-melting process.
D I S CLOSURE OF THE I NVENT I ON
The present inventors have made extensive and intensive studies on the production process of high-cleanliness steels with a view to attaining the above object. As a result, they have found the cleanliness of steels can be significantly improved by the following processes.
The invention will be described. In the conventional process using a refining furnace, such as an arc melting furnace or a converter, melting and oxidizing refining are mainly carried out, for example, in the arc melting furnace or the converter, and the reduction period (deoxidation) is carried out in ladle furnace. On the other hand, the present invention is directed to a process for producing a high-cleanliness steel, comprising the steps of: transferring a molten steel produced in an arc melting furnace or a converter to a ladle furnace to refine the molten steel; subjecting the refined molten steel to circulation-type vacuum degassing; and then casting the degassed molten steel into an ingot, wherein the refining in the ladle furnace is carried out for not more than 60 min. preferably not more than 45 min. more preferably 45 to 25 min. and, while the degassing subsequently to this step is generally carried out for less than 25 min in a circulation-type vacuum degassing device in such a manner that the amount of the molten steel circulated is brought to not less than times the total amount of the molten steel, in the present invention, in the circulation-type vacuum degassing device, the amount of the molten steel circulated in the degassing is brought to at least 8 times, preferably at least 10 times, particularly preferably at least 15 times, larger than the total amount of the molten steel, and the degassing time is at least 25 min. According to a preferred embodiment of the present invention, the molten steel is transferred to the ladle furnace in such a manner that the tapping temperature of the molten steel is at least 100 C above, preferably at least 120 C above, more preferably 150 C above, the melting point of the steel.
The present invention embraces the high-cleanliness steel produced by the above production process.
According to the present invention, preferably, the content of oxygen in the steel is not more than 10 ppm. Preferably, when the content of carbon in the steel is less than 0.6t by mass, the content of oxygen in the steel is not more than 8 ppm.
Particularly preferably, in the case of C > 0.6% by mass, the oxygen content is not more than 6 ppm.
Preferably, in the steel according to the present invention, the number of oxide inclusions having a size of not less than Em as detected by dissolving the steel product in an acid, for example, oxide inclusions having an A12O3 content of not less than 50%, is not more than 40, preferably not more than 30, more preferably not more than 20, per 100 g of the steel product.
7] In the steel of the present invention, for example, when the maximum inclusion diameter in 100 mm2 of the surface of the steel product is measured in 30 sites, the predicted value of the maximum inclusion diameter in 30000 mm2 as calculated according to statistics of extreme values is not more than 60 am, preferably not more than 40 am, more preferably not more than m.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the oxygen content of products in 10 (heats) according to the process of the present invention using short-time LF treatment and long-time RH treatment in treatment of a molten steel of steel SUJ 2, and the oxygen content of products in 10 (heats) according to the conventional process using long-time LF treatment and short-time RH treatment; FIG. 2 is a diagram showing the oxygen content of products in 10 (heats) according to the process of the present invention using short-time LF treatment and long-time RH treatment in the treatment of a molten steel of steel SCM 435, and the oxygen content of products in 10 (heats) according to the conventional process using long-time LF treatment and short-time RH treatment; FIG. 3 is a diagram showing the maximum predicted inclusion diameter according to statistics of extreme values in products in (heats) according to the process of the present invention using short- time LF treatment and long-time RH treatment in treatment of a molten steel of steel SUJ 2, and the maximum predicted inclusion diameter in products in 10 (heats) according to the conventional process using long- time LF treatment and short-time RH treatment; FIG. 4 is a diagram showing the maximum predicted inclusion diameter according to statistics of extreme values in products in (heats) according to the process of the present invention using short- time LF treatment and long-time RH treatment in the treatment of a molten steel of steel SCM 435, and the maximum predicted inclusion diameter in products in 10 (heats) according to the conventional process using long- time LF treatment and short-time RH treatment; FIG. 5 is a diagram showing the Llo life as determined by the thrust rolling service life test of products in 10 (heats) according to the process of the present invention using short-time LF treatment and long-time RH treatment in treatment of a molten steel of steel SUJ 2, and the Llo life of products in 10 (heats) according to the conventional process using long-time LF treatment and short-time RH treatment; and FIG. 6 is a diagram showing the Llo life as determined by the thrust rolling service life test of products in 10 (heats) according to the process of the present invention using short-time LF treatment and long-time RH treatment in treatment of a molten steel of steel SCM 435, and the L1o life of products in 10 (heats) according to the conventional process using long-time LF treatment and short-time RH treatment.
A preferred production process of a high-cleanliness steel according to the invention comprises the following steps (1) to (5).
(1) A molten steel is subjected to oxidizing refining in an arc melting furnace or a converter to prepare a molten steel having a predetermined chemical composition and a predetermined temperature which is then transferred to a ladle furnace.
(2) The molten steel transferred to the ladle furnace is subjected to reduction refining in a ladle furnace and the chemical composition of the molten steel is regulated. At that time, in the ladle furnace, it is a general knowledge that an stirring gas is blown through the bottom of the ladle at 1.5 to 5.0 Nl/min/t to forcibly agitate the molten steel and, in this case, an stirring time longer than 60 min provides better effect.
On the other hand, in the present invention, the refining time in the ladle refining is brought to not more than 60 min. preferably not more than 45 min. more preferably 25 to 45 min. (3) The molten steel, which has been subjected to reduction refining and regulation of chemical composition in step (2), is degassed by circulating the molten steel through a circulation- type vacuum degassing device, and, in addition, the chemical composition of the steel is finally regulated. In this case, it is a general knowledge that the degassing time is less than 25 min and, in a circulation-type vacuum degassing device, satisfactory results are obtained by bringing the amount of the molten steel circulated to about 5 times the total amount of the molten steel.
On the other hand, in the present invention, the amount of the molten steel circulated is brought to at least 8 times, preferably at least 10 times, more preferably at least 15 times the total amount of the molten steel, and the degassing is carried out for a longer period of time, that is, for not less than 25 min. The steps (2) and (3) are most important to the present invention. The ladle refining time for refining while heating in step (2) is brought to a necessary minimum time, and the degassing not involving heating in step (3), particularly circulation-type vacuum degassing is carried out in such a manner that a nozzle is dipped in the molten steel and only the molten steel is circulated. Therefore, the slag on the upper surface of the molten steel is in a satisfactorily quiet state, and, thus, the number of oxide inclusions from slag into the molten steel is fewer than that during the reduction period process in the ladle furnace. In this system, when the floating separation time for oxide inclusions is satisfactorily ensured, an increase in oxygen content caused by contamination from refractories or slag on the inner side of the ladle furnace can be prevented and, in addition, the formation of large inclusions having a size of not less than about 30 Am can be prevented. This can realize the production of high-cleanliness steels.
(4) The molten steel, which has been subjected to final regulation of the chemical composition in step (3), is cast into an ingot.
(5) The ingot is press forged into a product shape which is then optionally heat treated to provide a steel product.
In the production process of a high-cleanliness steel, according to a preferred embodiment, in the steps (1) to (5), in transferring the molten steel after step (1) to the ladle refining furnace, while the molten steel is generally tapped at a temperature of about 50 C above the melting point of the steel, in the present invention, the molten steel is tapped at a temperature of at least 100 C above, preferably at least 120 C above, more preferably 150 C above, the melting point of the steel. By virtue of this constitution, the metal deposited around the ladle furnace can be fully dissolved in the molten steel, and the slag can be fully floated, whereby the separation and dropping of the metal and slag into the molten steel in an advanced refining state during the ladle refining, thereby increasing the oxygen content, can be prevented.
The present invention embraces a high-cleanliness steel produced by the above means.
According to a preferred embodiment, the high-cleanliness steel according to the present invention is a high-cleanliness steel, excellent particularly in rolling fatigue life, which is characterized in that the content of oxygen in the steel is not more than 10 ppm; preferably, when the content of carbon in the steel is less than 0.6\ by mass, the content of oxygen in the steel is not more than 8 ppm; and, Particularly preferably, in the case of C > 0.6% by mass, the oxygen content is not more than 6 ppm. It is generally known that lowering the oxygen content can contribute to improved rolling fatigue life. Among the steels produced by the production process according to the present invention, highcleanliness steels having an oxygen content of not more than 10 ppm, preferably not more than 8 ppm in the case of C > 0.6% by mass in the steel, particularly preferably not more than 6ppm in the case of C > 0.6% by mass, stably exhibit excellent rolling fatigue life.
Further, according to a preferred embodiment, the steels produced according to the process of the present invention include highcleanliness steels possessing excellent rolling fatigue life and fatigue strength, which are characterized in that the number of oxide inclusions having a size of not less than 20 Em as detected by dissolving the steel product in an acid, for example, oxide inclusions having an Al2O3 content of not less than 50%, is not more than 40, preferably not more than 30, more preferably not more than 20, per 100 g of the steel product. This evaluation method for steel products reflects both the oxygen content and the maximum inclusion diameter in a predetermined volume. Regarding the fatigue strength, fatigue life, and quietness, in the case of steels having the same oxygen content, oxide inclusions having a certain large size are harmful, and, in particular, oxide inclusions having a size of not less than 20 Am are harmful.
Therefore, among the steels produced by the process according to the present invention, steels, wherein the number of oxide inclusions having a size of not less than 20 PM (for example, having an A12O3 content of not less than 50%) as detected by dissolving the steel product in an acid is not more than 40, preferably not more than 30, more preferably not more than 20, per g of the steel product, are high-cleanliness steels having both excellent rolling fatigue life and excellent fatigue strength and, in addition, excellent quietness.
According to a preferred embodiment, the steels according to the present invention further include high-cleanliness steels, which are excellent particularly in rotating bending fatigue strength and cyclic stress fatigue strength and are characterized in that, when the maximum inclusion diameter in 100 mm2 of the cross-section of the steel product is measured in 30 sites, the predicted value of the maximum inclusion diameter in 30000 mm2 as calculated according to statistics of extreme values is not more than 60 am, preferably not more than 40 m, more preferably not more than 25 am. The cyclic stress fatigue strength and the fatigue limit are known to greatly depend upon the maximum inclusion diameter in a predetermined volume. This is disclosed in Japanese Patent Laid-Open No. 194121/1999 of which the applicant is identical to that in the application of the present invention.
High-cleanliness steels, wherein, for example, typically when the maximum inclusion diameter in 100 mm2 of the cross-section of the steel product is measured in 30 sites, the predicted value of the maximum inclusion diameter in 30000 mm2 as calculated according to \ 9 statistics of extreme values is not more than 60 am, preferably not more than 40 m, more preferably not more than 25 am, stably exhibit excellent fatigue strength. In this case, the high- cleanliness steels have an oxygen content of not more than 10 ppm, preferably not more than 8 ppm in the case of C > 0.6% by mass, and a predicted value of maximum inclusion diameter of not more than 60 am, preferably not more than 40 am, more preferably not more than 25 am. The steels produced by the process according to the present invention are high-cleanliness steels possessing both excellent rolling fatigue life and excellent fatigue strength.
While acid dissolution is a very time-consuming, troublesome work, the above method, which, without steel product dissolution work, can observe a certain area under a microscope to statistically predict the maximum inclusion diameter, is advantageously simple.
Further, particularly in fatigue created by cyclic stress of tensile compression, it is known that the maximum diameter of inclusions present at a site susceptible to failure is a great factor which governs the strength. This method, which can statistically predict this maximum diameter, is advantageous.
EXAMPLE
A molten steel, which had been subjected to oxidizing smelting and produced by a melting process in an arc melting furnace was then transferred to a ladle furnace where the molten steel was subjected to ladle refining for a short period of time of not more than 60 min. Next, degassing was carried out for not less than 25 min. In particular, degassing was carried out in a circulation-type vacuum degas sing device in such a manner that the amount of the molten steel circulated was not less than 8 times the total amount of the molten steel, followed by an ingot production process using casting. Steel products of JIS SUJ 2 and SCM 435 in 10 heats thus obtained were examined for the oxygen content of the products, the predicted value of the maximum inclusion diameter according to statistics of extreme values, and Llo service life by a thrust-type rolling service life test. In the measurement of the predicted value of the maximum inclusion diameter, a test piece was taken off from a 60 forged material, the observation of 100 mm2 was carried out for 30 test pieces, and the maximum inclusion diameter in 30000 mm2 was predicted according to statistics of extreme values. In the thrust-type rolling service life test, a test piece having a size of 60 X 20 x 8.3T, which had been subjected to carburizing, quench hardening and tempering, was tested at a maximum hertz stress Pmax: 4900 MPa, followed by calculation to determine the L1o service life.
An example of the operation of oxidizing refining in an arc melting furnace or a converter followed by the transfer of the molten steel to a ladle furnace where the ladle refining-was carried out for not more than 60 min and degassing was then carried our in a circulation-t-ype vacuum degassing device for not less than 25 min (here this being referred to as 'short-time LF, long-time RH or short LF or long RH"), that is, shorttime LF, long-time RH, for lO heats of steel SUJ 2 is shown in Table 1.
An example of the operation of oxidizing smelting in an arc melting furnace or a converter followed by the transfer of the molten steel to a ladle furnace where the ladle refining was carried out for not more than 60 min and degassing was then carried out in a circulation-type vacuum degassing device for not less than 25 min. that is, short-time LF, longtime RH treatment, for lO heats of steel SCM 435 is shown in Table 2.
An example of the operation of oxidizing refining in an arc melting furnace or a converter followed by tapping at a high temperature of at least 100 C. above the melting point of the molten steel (in this specification, this being referred to as "high-temperature tapping") to a ladle furnace where the ladle refining was carried out for not more than 60 min and degassing was then carried out in a circulation-type vacuum degassing device for not less than 25 min. that is, short-time LF, long-time RH treatment + high-temperature tapping, for lo heats of steel SUJ 2
is shown in Table 3.
An example of the operation of oxidizing refining in an arc melting furnace or a converter followed by tapping at a high temperature of at least 100 C. above the melting point of the molten steel to a ladle furnace where the ladle refining was carried out for not more than 60 min and degassing was then carried out in a circulation-type vacuum degassing device for not t less than 25 min. that is, short-time LF, long-time RH treatment + high-temperature tapping, for 10 heats of steel SCM 435 is shown
in Table 4.
For comparison with the present invention, an example of the operation according to a prior art technique for steel SUJ 2 is shown in Table 5, and an example of the operation according to a prior art technique for steel SCM 435 is shown in Table 6.
As is apparent from Tables 1 to 4, for steel products produced using short LF, long RH treatment according to the present invention wherein a molten steel produced in an arc melting furnace or a converter is transferred to a ladle furnace to perform ladle refining for a short period of time, i.e., not more than about 60 min. and is then circulated through a circulation-type vacuum degassing device to increase the RH circulation quantity (that is, amount of molten metal circulated/total amount of molten metal) and to perform degassing for a long period of time, i.e., not less than 25 min and for steels producing using a combination of short LF, long RH treatment + high-temperature tapping at a temperature above the conventional operation, i.e., melting point + at least 100 C., for both steel types, SUJ 2 and SCM 435, the oxygen content of the products is small and, in addition, the number of inclusions having a size of not less than 20 Am is significantly decreased.
As can be seen from Tables D1 to D4, for the examples of the present invention, all the steel products are evaluated as good (O) or excellent (I), that is, are excellent high-cleanliness steels. By contrast, as can be seen from Tables 5 and 6, for all the conventional examples, the cleanliness is evaluated as failure (X), and the conventional steel products cannot be said to be clean steels.
For the heats wherein a molten steel is subjected to oxidizing smelting in an arc melting furnace or a converter, both the oxygen content, and the predicted value of the maximum inclusion diameter are reduced by increasing TSH [ (temperature at which molten steel is transferred to ladle furnace)-(melting point of molten steel) =TSH) ] to improve the cleanliness. For the heats, regarding the relationship of the refining time in the ladle furnace with the oxygen content and the predicted value of the maximum inclusion diameter, when the refining time is not more than 60 min. for example, is short and about 25 min. the oxygen content and the predicted value of the maximum inclusion diameter are satisfactorily lowered. The predicted value of the maximum inclusion diameter, however, increases with increasing the refining time. The reason for this is considered as follows.
With the elapse of time, the melt loss of refractories in the ladle furnace is increased, the equilibrium of the slag system is broken, for example, as a result of oxidation due to the contact with the air, and the level of the dissolved oxygen goes beyond the minimum level of dissolved oxygen. Further, the relationship of the amount of molten steel circulated/total amount of molten steel in the circulation-type vacuum degassing device with the oxygen content and the predicted value of the maximum inclusion diameter, the effect of enhancing the cleanliness increases with increasing the amount of molten steel circulated, that is, with increasing the degassing time, and is substantially saturated when the amount of molten steel circulated/total amount of molten steel is not less than 15 times.
It was confirmed that reducing the oxygen content and the predicted value of the maximum inclusion diameter results in improved L1o life. This indicates that steels produced by the process according to the present invention, which can reduce the oxygen content and the predicted value of the maximum inclusion diameter, have excellent fatigue strength properties such as excellent rolling fatigue life.
FIG. l is a diagram showing the oxygen content of products in lO heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SUJ 2, a molten steel, which had been subjected to oxidizing refining and produced by a melting process in an arc melting furnace or a converter, is transferred to a ladle furnace to perform ladle refining for a short period of time and is then subjected to circulation-type vacuum degassing for a long period of time, and the oxygen content of products in lO heats in the conventional process wherein a molten steel, which had been subjected to oxidizing refining and produced by a melting process in an arc melting furnace or a converter, is transferred to a ladle furnace to perform ladle refining for a long period of time and is then subjected to circulation-type vacuum degassing for a short period of time. In FIGS. l, 3, and 5, A1 shows data on the adoption of short-time LF, long-time RH treatment according to the present invention defined in claim l, A2 data on the adoption of a combination of high-temperature tapping + short-time LF, long- time RH treatment according to the present invention defined in claim 2, and conventional data on the conventional process.
FIG. 2 is a diagram showing the oxygen content of products in lO heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SCM 435, a molten steel, which had been subjected to oxidizing refining and produced by a melting process in an arc melting furnace or a converter, is transferred to a ladle furnace to perform ladle refining for a short period of time and is then subjected to circulation-type vacuum degassing for a long period of time, and the oxygen content of products in lo heats in the conventional process wherein a molten steel, which had been subjected to oxidizing refining and produced by a melting process in an arc melting furnace or a converter, is transferred to a ladle furnace to perform ladle refining for a long period of time and is then subjected to circulation-type vacuum degassing for a short period of time. In FIGS. l, 3, and 5, A1 shows data on the adoption of short-time LF, long-time RH treatment according to the present invention defined in claim l, A2 data on the adoption of a combination ofhigh-temperature tapping + short-time LF, long-time RH treatment according to the present invention defined in claim 2, and conventional data on the conventional process.
FIG. 3 is a diagram showing the maximum predicted inclusion diameter determined according to statistics of extreme values in products in lO heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SUJ 2, the process according to claim l or 2 of the present invention is carried out, and the maximum predicted inclusion diameter determined according to statistics of extreme values in products in 10 heats in the conventional process wherein, in the treatment of a molten steel for steel SUJ 2, long- time LF, short time RH treatment is carried out.
FIG. 4 is a diagram showing the maximum predicted inclusion diameter determined according to statistics of extreme values in products in 10 heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SCM 435, the process according to claim 1 or 2 of the present invention is carried out, and the maximum predicted inclusion diameter determined according to statistics of extreme values in products in 10 heats in the conventional process wherein, in the treatment of a molten steel for steel SCM 435, long-time LF, short-time RH treatment is carried out.
FIG. 5 shows data on Llo life as determined by a thrust rolling service life test in products in 10 heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SUJ 2, the process according to claim 1 or 2 of the present invention is carried out, and the Llo life as determined by the thrust rolling service life test in products in 10 heats in the conventional process wherein, in the treatment of a molten steel for steel SUJ 2, longtime LF, short-time RH treatment is carried out.
FIG. 6 shows data on Llo life as determined by a thrust rolling service life test in products in 10 heats in the production process according to the present invention wherein, in the treatment of a molten steel for steel SCM 435, the process according to claim 1 or 2 of the present invention is carried out, the Llo life as determined by the thrust rolling service life test in products in 10 heats in the conventional process wherein, in the treatment of a molten steel for steel SCM 435, long-time LF, short-time RH treatment is carried out.
As is apparent from the test results, it was confirmed that, for both steel SUJ 2 and steel SCM 435, the process, in which a molten steel, which had been subjected to oxidizing refining and produced by a melting process in an arc melting furnace or a converter, is transferred to a ladle furnace to perform ladle refining for a short period of time and is then circulated through a circulation-type vacuum degassing device to perform degassing for a long period of time, can significantly reduce the oxygen content of the products, and the predicted value of the maximum inclusion diameter and, according to the process of the present invention, the cleanliness is significantly improved and the Llo life as determined by the thrust rolling service life test is significantly improved. The addition of treatments to the process, that is, the addition of shorttime LF, long-time RH treatment according to the present invention defined in claim 1, and the addition of high-temperature tapping + short- time LF, long-time RH treatment according to the present invention defined in claim 2, can significantly improve all the oxygen content of products, the predicted value of the maximum inclusion diameter, and the L1o life as determined by the thrust rolling service life test.
The above examples demonstrate that the process according to the present invention can lower the oxygen content and the predicted value of the maximum inclusion diameter, and the Llo life is improved. This indicates that steels produced according to the process of the present invention, which can reduce the oxygen content and the predicted value of the maximum inclusion diameter, have excellent fatigue strength properties, such as excellent rolling fatigue service life.
As is apparent from the foregoing description, the present invention can provide a large quantity of steel products having a very high level of cleanliness without use of a resmelting process which incurs very high cost. This can realize the provision of high-cleanliness steels for use as steels for mechanical parts required to possess fatigue strength, fatigue life, and quietness, particularly, for example, as steels for rolling bearings, steels for constant velocity joints, steels for gears, steels for continuously variable transmission of toroidal type, steels for mechanical structures for cold forging, tool steels, and spring steels, and processes for producing the same, that is, can offer unprecedented excellent effect.
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Claims (6)
- CLAINS: 1. A process for producing a high-cleanliness steel, comprisingthe steps of: transferring a molten steel produced in an arc melting furnace or a converter to a ladle refining furnace to refine the molten steel; subjecting the refined molten steel to circulation-type vacuum degassing; and then casting the degassed molten steel into an ingot, wherein the refining in the ladle furnace is carried out for not more than 60 min. and the degassing in the circulation-type vacuum degassing device is carried out for not less than 25 min under such conditions that the amount of the molten steel circulated in the circulation-type vacuum degassing device is at least 8 times larger than the total amount of the molten steel.
- 2. The process according to claim 1, wherein the molten steel is transferred to the ladle furnace in such a manner that the temperature of the molten steel to be transferred is at least 100 C above the melting point of the steel.
- 3. A high-cleanliness steel produced by the process according to claim 15 or 16.
- 4. The high-cleanliness steel according to claim 2, wherein the content of oxygen in the steel is not more than 10 ppm.
- 5. The high-cleanliness steel according to claim 3, wherein the number of oxide inclusions having a size of not less than 20 Am as detected by dissolving the steel product in an acid is not more than 40 per 100 g of the steel product.
- 6. The high-cleanliness steel according to claim 3, wherein the predicted value of the maximum inclusion diameter in 30000 mm2 as calculated according to statistics of extreme values is not more than 60 am.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000167088A JP2001342516A (en) | 2000-06-05 | 2000-06-05 | Highly clean steel and production method |
| JP2000167086A JP4562244B2 (en) | 2000-06-05 | 2000-06-05 | Manufacturing method of high cleanliness steel |
| JP2000167087A JP2001342514A (en) | 2000-06-05 | 2000-06-05 | Highly clean steel and production method |
| JP2000167085A JP2001342512A (en) | 2000-06-05 | 2000-06-05 | Highly clean steel and production method |
| JP2000167089A JP2001342515A (en) | 2000-06-05 | 2000-06-05 | Highly clean steel and production method |
| GB0500783A GB2406580B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB0509771D0 GB0509771D0 (en) | 2005-06-22 |
| GB2410503A true GB2410503A (en) | 2005-08-03 |
| GB2410503B GB2410503B (en) | 2005-09-07 |
Family
ID=34743970
Family Applications (3)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0509771A Expired - Lifetime GB2410503B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
| GB0509772A Expired - Fee Related GB2410253B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
| GB0509770A Expired - Lifetime GB2410252B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
Family Applications After (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0509772A Expired - Fee Related GB2410253B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
| GB0509770A Expired - Lifetime GB2410252B (en) | 2000-06-05 | 2001-06-05 | High-cleanliness steel and process for producing the same |
Country Status (1)
| Country | Link |
|---|---|
| GB (3) | GB2410503B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2028285B1 (en) * | 2006-06-09 | 2016-03-23 | Kabushiki Kaisha Kobe Seiko Sho | High cleanliness spring steel and high cleanliness spring excellent in fatigue characteristics |
| CN102808062B (en) * | 2012-07-19 | 2014-03-05 | 中国科学院金属研究所 | Method for controlling A segregation of steel ingots by purification of molten steel |
| CN114875197B (en) * | 2022-05-10 | 2023-11-14 | 江苏利淮钢铁有限公司 | Technological method for controlling inclusion types in steel to improve purity of molten steel |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0325242A2 (en) * | 1988-01-21 | 1989-07-26 | Nkk Corporation | Method for refining molten steel in a vacuum |
| EP0548868A2 (en) * | 1991-12-24 | 1993-06-30 | Kawasaki Steel Corporation | Method of refining of high purity steel |
| JP2000129335A (en) * | 1998-10-20 | 2000-05-09 | Nkk Corp | Production of extra-low sulfur steel excellent in cleanliness |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3347718C2 (en) * | 1983-09-02 | 1986-05-28 | Ulrich Dipl.-Ing. 5905 Freudenberg Glasmeyer | Process for the production of steels with a high degree of purity and low gas content in steelworks and steel foundries |
| EP0163784B1 (en) * | 1984-05-25 | 1989-11-02 | China Steel Corporation | Two stage deoxidation process in steel-making |
| JPS6396210A (en) * | 1986-10-09 | 1988-04-27 | Sumitomo Metal Ind Ltd | Pre-deoxidizing method in converter interior |
| JPH02179813A (en) * | 1988-12-28 | 1990-07-12 | Nippon Steel Corp | Method for refining molten metal into high purity and high cleanliness |
| JP2847774B2 (en) * | 1989-07-07 | 1999-01-20 | 住友金属工業株式会社 | Refining method of molten metal |
| JPH10237533A (en) * | 1997-02-27 | 1998-09-08 | Sumitomo Metal Ind Ltd | Production of hic resistant steel |
| JPH1192811A (en) * | 1997-09-12 | 1999-04-06 | Sumitomo Metal Ind Ltd | Refinement of molten metal |
-
2001
- 2001-06-05 GB GB0509771A patent/GB2410503B/en not_active Expired - Lifetime
- 2001-06-05 GB GB0509772A patent/GB2410253B/en not_active Expired - Fee Related
- 2001-06-05 GB GB0509770A patent/GB2410252B/en not_active Expired - Lifetime
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0325242A2 (en) * | 1988-01-21 | 1989-07-26 | Nkk Corporation | Method for refining molten steel in a vacuum |
| EP0548868A2 (en) * | 1991-12-24 | 1993-06-30 | Kawasaki Steel Corporation | Method of refining of high purity steel |
| JP2000129335A (en) * | 1998-10-20 | 2000-05-09 | Nkk Corp | Production of extra-low sulfur steel excellent in cleanliness |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0509770D0 (en) | 2005-06-22 |
| GB2410252A (en) | 2005-07-27 |
| GB2410252B (en) | 2005-09-07 |
| GB0509772D0 (en) | 2005-06-22 |
| GB0509771D0 (en) | 2005-06-22 |
| GB2410253A (en) | 2005-07-27 |
| GB2410253B (en) | 2005-09-14 |
| GB2410503B (en) | 2005-09-07 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PE20 | Patent expired after termination of 20 years |
Expiry date: 20210604 |