GB2410253A

GB2410253A - High-cleanliness steel and process for producing the same

Info

Publication number: GB2410253A
Application number: GB0509772A
Authority: GB
Inventors: Ichiro Sato; Kaichiro Ishido; Tomomi Mori; Toshihiro Irie; Kazuya Kodama; Kiyoshi Kawakami; Shuhei Kitano
Original assignee: Sanyo Special Steel Co Ltd
Current assignee: Sanyo Special Steel Co Ltd
Priority date: 2000-06-05
Filing date: 2001-06-05
Publication date: 2005-07-27
Anticipated expiration: 2021-06-05
Also published as: GB2410253B; GB2410503B; GB2410252B; GB2410252A; GB0509771D0; GB0509770D0; GB0509772D0; GB2410503A

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 where the molten steel is refined by gas stirring; subjecting the molten steel to circulation-type vacuum degassing; and then casting the degassed molten steel into an ingot, wherein an electromagnetic induction stirrer is provided in the ladle and, in addition to the gas stirring, electromagnetic stirring is carried out for 50 to 80 min, thereby performing ladle refining.

Description

24 1 0253

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, steeds 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 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 re-melting or the electroslag re-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.

DISCLOSURE OF THE INVENTION

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 refining.

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 as an out-furnace refining furnace to perform refining; subjecting the molten steel to circulation-type ladle degassing; and then casting the degassed molten steel into an ingot, wherein the refining in the ladle is carried out in such a manner that, in addition to stirring by gas introduced from the bottom of the ladle, stirring is carried out by electromagnetic induction, and this ladle refining is carried out for 50 to 80 min. preferably 70 to 80 min. According to the present invention, preferably, the ladle refining by the gas stirring and the electromagnetic stirring in the ladle is carried out in an inert atmosphere.

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 0.6% 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 of the present invention, the number of oxide inclusions having a size of not less than 20 Am 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.

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 25 m.

A preferred production process of a high-cleanliness steel according to the fifth 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 refining furnace is subjected to reduction refining in the ladle furnace and the chemical composition of the molten steel is regulated. At that time, in the ladle furnace, 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 addition, electromagnetic stirring is carried out. Thus, ladle refining is carried out for 50 to 80 min. preferably 70 to 80 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 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 fifth invention. In the ladle refining time for refining while gas stirring and electromagnetic stirring in step (2), even when the refining is not short-time refining, that is, even refining for a long period of time, i.e., 50 to 80 min. preferably 70 to 80 min. can also satisfactorily enhance the cleanliness. The stirring energy of the electromagnetic stirring is brought to to 700 w per ton of the molten steel. As described above, the electromagnetic stirring does not agitate slag itself.

Therefore, it is possible to prevent breaking of the slag equilibrium system caused by melt loss of refractories of the furnace and the inclusion of slag. Further, since degassing, particularly circulationtype 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, the slag on the upper surface of the molten steel is in a satisfactorily quiet state, and the number of oxide inclusions from slag into the molten steel is fewer than that during the reduction period process in the ladle.

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 can be prevented and, in addition, the formation of large inclusions having a size of not less than about Em 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, 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 ladle refining in step (2) among the steps (1) to (5), particularly the ladle is brought to an inert atmosphere and thus is blocked from the air, and, in this state, ladle refining is carried out (step 6). In this preferred embodiment of the present invention, step (6) is most important to the present invention.

The practice of the ladle refining in an inert atmosphere while blocking from the air in step (6), in combination of the ladle refining wherein refining is carried out by gas stirring in combination with electromagnetic stirring in step (2), permits, even when the refining is not short-time refining, that is, even refining for a long period of time, i.e., 50 to 80 min. preferably to 80 min. to satisfactorily enhance the cleanliness.

Specifically, the ladle is covered. The space defined by the cover is filled with an inert gas, for example, an argon gas, a nitrogen gas, or a mixed gas composed of an argon gas and a nitrogen gas to seal the molten steel in the ladle from the air.

Thus, the equilibrium system of the slag is maintained.

Preferably, the pressure of the inert gas within the cover is reduced to not more than 10 Torr. This can further enhance the effect. According to this constitution, the slag can be fully floated, and 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 sealing gas is a gas of not less than 50 Nm/H, and, in the case of refining under reduced pressure, a gas flow rate below this range is also possible.

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, high-cleanliness 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, 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 Am as detected by dissolving the steel product in an acid, for example, oxide inclusions having an Al203 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 Am (for example, having an Al2O3 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 m, 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 statistics of extreme values is not more than 60 m, preferably not more than 40 am, more preferably not more than 25 m, 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 in the steel, particularly preferably not more than 6ppm 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 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 of JIS SCM 435, which had been subjected to oxidizing refining and produced by a melt process in an arc furnace, was transferred to a ladle furnace provided with an electromagnetic induction stirrer where 50 to 80 min in total of ladle refining (stirring by gas for a short time in an inert atmosphere + electromagnetic stirring) was carried out. Next, degassing was carried out for 20 to 30 min. In particular, degassing was carried out in a circulation-type degassing device in such a manner that the amount of the molten steel circulated was not less than 12 times the total amount of the molten steel, followed by an ingot production process using casting to produce steel products of SCM 435 in 10 heats. For comparison, a molten steel of JIS SCM 435, which had been subjected to oxidizing refining and produced by a melt process in the same manner as described above in an arc furnace through the conventional operation, was transferred to a ladle furnace where the molten steel was stirred by gas for 35 to 50 min to perform ladle refining. Next, circulation-type degassing was carried out for not more than 25 min. followed by an ingot production process using casting to produce steel products of SCM 435 in 10 heats. These products 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 L1o 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 65 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 the present invention and test results are shown in Table 1, and a comparative example of the conventional operation and test resultsare shown in Table 2.

As is apparent from Table 1, for SCM 435 steel products of heats produced according to the process of the present invention, wherein a molten steel of JIS SCM 435, which has been subjected to oxidizing refining and produced by a melt process in an arc furnace, is transferred to a ladle furnace provided with an electromagnetic induction stirrer, where 50 to 80 min in total of ladle refining (stirring by gas for a short time in an inert atmosphere + electromagnetic stirring) is carried out, and the molten steel is degassed for 20 to 30 min. in particular, degassing is carried out in a circulation-type degassing device in such a manner that the amount of the molten steel circulated is not less than 12 times the total amount of the molten steel, followed by an ingot production process using casting, that is, steel Nos. 1 to 10, the oxygen content of the product is 5.4 to 6.6 ppm, the number of inclusions having a size of not less than Am per 100 g of the steel product is 5 to 14, and the maximum predicted inclusion diameter is 30.6 am. That is, these products are very clean steels. Further, these products have very highly improved Llo life. For the overall evaluation, all of these products are evaluated as very good (O).

By contrast, as can be seen in Table 2, for SCM 435 steel products of 10 heats produced according to the comparative conventional process, wherein a molten steel of JIS SCM 435, which has been subjected to oxidizing refining and produced by a melt process in an arc furnace, is transferred to a ladle furnace where the molten steel is stirred by gas for 35 to 50 min to perform ladle refining, and the molten steel is subjected to circulation-type degassing for not more than 25 min. followed by an ingot production process using casting, the oxygen content of the product is slightly larger than that in the present invention although the oxygen content is relatively low. Further, the number of inclusions having a size of not less than 20 Am per 100 g of the steel product is much larger than that in the present invention and is 42 to 59, and the maximum predicted inclusion diameter is also larger than that in the present invention and is 55.2 to 91.0 m. Further, the Llo life is also lower than that in the present invention and is one-tenth to one-fifth of that in the present invention. All the comparative steels are evaluated as failure (X).

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 re-melting process which incurs very high cost. This can realize the provision of high-cleanliness steels for use as steels for mechanical parts i 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

CLAIMS: l. 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 where the molten steel is refined by gas stirring; subjecting the molten steel to circulation-type vacuum degassing; and then casting the degassed molten steel into an ingot, wherein an electromagnetic induction stirrer is provided in the ladle and, in addition to the gas stirring, electromagnetic stirring is carried out for 50 to 80 min. thereby performing ladle refining.
2. The process according to claim l, wherein the ladle refining by the gas stirring and the electromagnetic stirring in the ladle is carried out in an inert atmosphere.
3. A high-cleanliness steel produced by the process according to claim l or 2.
4. The high-cleanliness steel according to claim 3, wherein the content of oxygen in the steel is not more than lO 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 lOO 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.