KR100206353B1 - Bearing steel for high-clean carburizing and manufacturing method thereof - Google Patents

Abstract

The present invention discloses a method for producing a high-carburization bearing steel requiring high fatigue resistance. First, the steel sheet contains, by weight%, 0.20 to 0.60% of C, 0.001 to 0.05% of Si, 0.01 to 0.20% of Mn, 0.01 to 0.05% of Cr, 0.001 to 0.02% of P and 0.001 to 0.0040% of S (CaO / SiO2) in the range of 2.0 to 2.5; adding a reducing agent in a range of 7 to 10 Kg per ton of molten steel to control the basicity (CaO / SiO2) of the slag; Introducing oxygen into the molten steel in the range of 15 to 25 Nm < 3 > per molten steel; A lecturing step of lecturing and inputting a deoxidizing agent during lecture; Stirring the inert gas at a rate of 150 to 250 L / min in the lower part of the ladle, adding a coagulant to bring the basicity into the range of 5 to 7; Performing a vacuum degassing process at a vacuum degree of 3 mbar or less for 15 minutes or more; Casting or continuous casting; Heating at a temperature of 1,050 to 1,250 占 폚 to perform ordinary hot rolling at a total shaping annealing ratio of 8S or more; Followed by normalizing treatment at a temperature of 850 to 950 캜 in a heat treatment furnace, followed by air cooling.

Description

Manufacturing Method of High-Carbon Bearing Steel for Carburizing

FIG. 1 is a graph comparing the amounts of nonmetal inclusions of the present invention steel and conventional steel.

FIG. 2 is a graph comparing rotational contact fatigue life according to the oxygen content of the present invention steels and conventional steels.

[0001]

In general, carburized bearing steels are structural alloy steels widely used as parts of ruler bearings of automobiles and various industrial machines. When these are used at the driving parts of an automobile wheel or machine, these parts are operated under a constant load, It is important to secure endurance and abrasion resistance.

Although the fatigue resistance and the wear resistance of such carburizing bearing steel can be improved by the carburizing heat treatment, even after the carburizing treatment under the same conditions, the characteristics of the carburizing part largely depend on the original steel material, The manufacturing process of bearing steel for carburizing is very important.

Bearing steels for carburizing have different rotational contact fatigue life depending on factors such as the cleanliness of the steel determined during steelmaking, chemical composition and mechanical properties such as chemical composition and heat treatment. Among the factors, cleanliness, ie, The fatigue life of a bearing made of bearing steel for carburization depends largely on the impurities contained and nonmetallic inclusions.

[PROBLEMS TO BE SOLVED BY THE INVENTION]

However, as shown in Table 1, the conventional conventional bearing steel for carburization contains 14.8 ppm of O and 8.3 ppm of O, and 0.015 wt% of O, 0.020 wt% of S, %, P contains 0.018 wt%, 0.020 wt%, Ti contains 0.0027 wt%, and 0.0054 wt%. Bearings made of bearing steel for carburizing containing such impurities are operated to continuously receive a high load at the rotating part of the machine, so that the repetitive contact stress applied between the inner and outer raceways of the bearing and the balls or rolling elements The non-metallic inclusions or micro-cracks generated at the defective portions are developed into spalling due to the stress concentration phenomenon and finally reach the fatigue limit point So that it is destroyed.

Accordingly, an object of the present invention is to provide a method for manufacturing a bearing steel for high-clean carburization, in which stress concentration is prevented from concentrating on nonmetallic inclusions or defective parts, thereby improving fatigue life.

[Structure and operation of the invention]

In order to achieve the above object, the present invention provides a method of manufacturing a bearing steel for high-clean carburization, comprising the steps of: 0.1 to 0.3% of C, 0.1 to 0.5% of Si, 0.5 to 1.0% of Mn, Of Al, 0.005 to 0.05% of Al, 0.01% or less of S, P, 0.02% or less of Ti, 0.003% or less of Ti and 0.0012% or less of O as impurities and the balance being Fe and unavoidable impurities The formed steel is hot-rolled by cast ingot or continuous casting method. Then, normalizing and air-cooling.

Hereinafter, the reasons for limiting the chemical components and components added to the inventive bearing steel for carburizing carburization will be described.

When C is added in an amount of 0.1% or less, the required strength and curability required after carburization can not be obtained. When 0.3% or more is added, the toughness is lowered, and therefore the addition range is limited to 0.1 to 0.3%.

Si is a deoxidizing element and 0.1% or more is added for the purpose of deoxidation in the production of molten steel. However, when 0.5% or more is added, SiO is deteriorated due to degradation of toughness and reaction with O in steel, The addition range is limited to 0.1 to 0.5%.

Mn is an element of deoxidation and desulfurization which is added at a content of 0.5% or more for the purpose of deoxidation and desulfurization of molten steel while securing the hardenability and surface hardness of the steel, and when it is added in excess of 1%, hardening control is difficult, Since the nonmetallic inclusions are over-distributed in the steel, the fatigue life is lowered, so the addition range is limited to 0.5 to 1.0%.

Cr is an element which precipitates a deoxidizer to increase the hardness of the carburized layer after carburizing. If it is 0.5% or less, it is impossible to obtain a Singe diameter. When 1.5% or more is added, a crumbly carbide is formed at grain boundaries, The Cr content is limited to 0.5 to 1.5%.

Al is a deoxidizing element which is reacted with O in the refining process and is made of Al2O3, which is a carbonic acid product, and is added to the AlN to react with N of the steel type and to precipitate out of the system, thereby adding 0.005% or more for the purpose of refining the crystal grains of the steel . However, if it is added in an amount of 0.05% or more, it can not be lifted off from the steel in the steel and remains in the molten steel, and acts as the most harmful Al ₂ O ₃ inclusion in the bearing steel, thereby remarkably lowering the fatigue life. Is limited to 0.005 to 0.05%.

O is an impurity, which is present in the molten steel and reacts with Mn, Si and Al, which are carbonic acid elements, and is formed of MnO, SiO ₂ and Al ₂ O ₃ , respectively, Is minimized.

S is an impurity which mainly binds to Mn to form MnS which is a noxious inclusion, which causes fatigue life degradation, and when present in the form of center segregation, it causes cracks due to stress concentration during operation of the bearing, ≪ / RTI >

P is an impurity, which is apt to cause segregation, and it is minimized to 0.02% or less since unhealthy cast structure is likely to be produced.

Ti is an impurity, which reacts with N in the steel to form TiN inclusions that are deteriorated in fatigue life, so that the TiN content is minimized to 0.003% or less.

[Example]

Hereinafter, the present invention will be described in detail by way of examples.

The bearing steel for high-carburnt carburization according to the present invention is characterized in that it contains 0.1 to 0.3% of C, 0.1 to 0.5% of Si, 0.5 to 1.0% of Mn, 0.5 to 1.5% of Cr, 0.005 to 0.05% of Al, , S: not more than 0.01%, P: not more than 0.02%, Ti: not more than 0.003%, and O: not more than 0.0012%, and the balance includes Fe and unavoidable impurities.

A method of manufacturing a bearing steel for high-clean carburization of the present invention is as follows.

First, scrap iron is charged to an electric furnace (60 tons) by a certain standard, and molten steel is manufactured by completely dissolving it by electric arc. In other words, an electrolytic step in which the scrap iron is completely dissolved is carried out. Here, the molten steel in the molten state in which the scrap iron is completely dissolved contains, by weight, 0.20 to 0.60% of C, 0.001 to 0.05% of Si, 0.01 to 0.20% of Mn, 0.01 to 0.05% of Cr, 0.001 to 0.02% of P, , 0.001 to 0.0040% of S, and 0.20% or less of Cu, 0.025% or less of Sn, 0.02% or less of As, 0.02% or less of Sb, 0.002% or less of Pb and 0.002% or less of Ti as harmful impurities, Is made of Fe and a trace amount of impurities normally contained.

Subsequently, in order to absorb and remove impurities such as phosphorus and non-metallic inclusions by slag in the upper layer of the electric furnace, the quick lime as a conditioning agent is added in a range of 7 to 10 Kg per ton of molten steel to form slag. At this time, the basicity of the slag, that is, the ratio of CaO / SiO2 is controlled in the range of 2.0 to 2.5.

Thereafter, the oxidation refining step is carried out. That is, oxygen is blown into the molten steel in the range of 15 to 25 Nm³ per ton of molten steel, the impurities are converted into oxides by the oxidation reaction, and then they are flashed by slag.

Then, perform the lecture step. That is, in order to remove remaining oxygen in molten steel at the time of steel making, alloying iron Mn and Si having a degree of carbonate lower than Al are firstly introduced at the beginning of the ladle according to the target alloy composition, and then Al, which is the main carbonic acid element, Kg is injected at the end of the line and the nozzle stopper under the electric furnace is used to prevent leakage of the electric furnace slag to the ladle which adversely affects the ladle refining.

After the lecture stage, the lathes are subjected to reduction refinement. That is, in order to sufficiently float and remove the carbonated product formed by the addition of Al at the time of excavation, the inert gas such as Ar gas is stirred at a rate of 150 to 250 L / min under the ladle, In the case of quicklime at a rate of 4 to 5: 1, 6 to 8 kg per ton of molten steel is charged to carry out a reduction refining step for controlling the slag of high-salt air with a basicity ranging from 5 to 7 and having a low melting point of the slag composition.

After the reduction refining step, a degassing step is performed. That is, vacuum degassing is performed for 15 minutes or more at a vacuum level of 3 mbar or less after completion of reduction and refining. Through this vacuum degassing process, impurity components and nonmetallic inclusions can be minimized and high cleanliness can be ensured.

When the composition of the alloy subjected to the above-described processes is analyzed, it is preferable that the composition contains 0.10 to 0.30% of C, 0.1 to 0.5% of Si, 0.5 to 1.0% of Mn, 0.5 to 1.5% of Cr, 0.005 to 0.05% , P: not more than 0.02%, S: not more than 0.01%, Ti: not more than 0.003%, and O: not more than 0.0012%, and the balance being Fe and unavoidable impurities.

After the amount of the non-metal intervening material is minimized through the above steps, the steel ingot casting or continuous casting is performed.

Then, it is heated at a temperature of 1,050 to 1,250 ° C., which is more economical, easy to manage, and excellent in workability, and is subjected to ordinary hot rolling at a total forming hardening ratio of 8 S or more.

After hot rolling, normalizing treatment is carried out at a temperature of 850 to 950 캜 in a continuous heat treatment furnace with a relatively low production cost and excellent productivity, followed by air cooling to produce a bearing steel for high-clean carburization.

Table 1 below shows the chemical composition and O content of the steels of the present invention and conventional steels, respectively. Table 2 shows the composition of the slag after ladle refining. Table 3 shows the number of surface cracks as a measure of the number of base cracks, and Table 4 shows the amount of nonmetal inclusions measured by ASTM E 45 A method and the rotational contact fatigue life (L ₁₀ life) of the bearings.

First, in Table 1, A, B, C, and D steel are inventive steel, E steel is a carburizing steel developed in our manufacturing process, and F steel is a foreign bearing steel that has been used in domestic .

The steel of the present invention adopts the high-clean steelmaking process for the steels A, B, C, and D, and the high-clean steelmaking method is a steelmaking method for extremely reducing impurities based on the existing hypoxic steelmaking method. The results are summarized as follows.

In order to reduce the impurities P and Ti, the impurities contained in the scrap and the ferroalloy itself were first suppressed, and after the electric furnace was crushed, a basic agent was added to make a basic slag having a basicity (CaO / SiO2) of 2 to 2.5, After oxidizing and refining efficiently in the state where the temperature of the molten steel was lowered, the electro-acidic slag was removed by 100% after excavation to minimize P and Ti contents.

In the case of O, which is an impurity, the existing deoxidation method which only Al is injected at the time of excavation is reduced by applying a combination of Mn, Si and Al in the order of weak affinity with O. Further, in order to reduce the content of O and S, after the vacuum degassing was sufficiently performed in the state where the stirring strength of the molten steel was increased, the reoxidization of the molten steel was suppressed to the maximum, and the slag composition during the ladle refining was evaluated as shown in Table 2 below. O and S contents were minimized by optimizing the deoxidation and desulfurization reaction efficiency by controlling the composition of the high basicity coarse melting point.

In the same manner as above, the O, S, P and Ti impurity elements and the nonmetal inclusions due to these elements are minimized, and then the steel ingot is cast or continuously cast, and the heating is performed at 1,050 to 1,250 ° C., Rolled and rolled products were subjected to a normalizing treatment at a temperature of 850 to 950 占 폚 in a continuous heat treatment furnace and then air-cooled to produce a bearing steel for high-cleaned carburization.

For the quality evaluation test of the bearing steel manufactured according to the present invention, the normalized steel material was made into a specimen and eight specimens were taken from the 1 / 2R region to analyze chemical components containing impurities and to measure the content and amount of O Respectively.

In order to evaluate the rolling contact fatigue life, which is the application characteristic of the bearing steel, the normalized steel is carburized at a temperature of 900 to 950 ° C., then calcined at a temperature of 800 to 900 ° C., The specimens were processed to 60 mm in diameter and 10 mm in thickness to make fatigue test specimens. The fatigue test of the bearings was carried out using a thrusttype rotary contact fatigue tester, while spinel # 60 lubricating oil was injected, while rotating a constant orbit on the fatigue test piece through three steel spheres having a diameter of 3/8 inch, An axial load was applied, and a fatigue test was conducted while rotating the shaft at a rotation speed of 1,000 rpm.

The fatigue life criterion was defined as the fatigue life value by calculating the number of revolutions until fatigue was reached by repeated contact pressure between the raceway wheel and rolling elements during the fatigue test.

In this test, it is possible to represent the life of the bearing. It is more reliable than the average lifetime, and the widely used L life (fatigue life when 10% of test number causes fatigue fracture) is taken.

As shown in Table 1, S, P, and Ti, which are impurities in the chemical components, and the O content, which are gas components, were significantly decreased as compared with the conventional steel E, and impurities Respectively.

In addition, in the case of the steel of the present invention, since the alumina (AlO) inclusions and macro oxide inclusions in the steel are sufficiently lifted and removed in the steelmaking refining process by applying the high-clean steelmaking method, As shown in Fig. 3, unlike the conventional steel E, no scratches were generated.

※ Nonmetallic inclusion test method of steel (ASTM E 45 A)

FIG. 1 shows the average nonmetal inclusions in the steels according to the present invention shown in Table 4 together with existing steels and comparative steels. It is assumed that the nonmetallic inclusions are sulfide, silicate and C, and globular oxide is D Respectively, and for each of them, Thin means a thin state and Heay means a heavy state.

In particular, sulfide-based inclusions and oxide inclusions of the present invention were remarkably reduced compared to the conventional steel E, and the sulfide content was lower than that of foreign F. This is a result that the amount of interstitials in the steel was also largely reduced due to the decrease of the impurities O and S, and overall the present invention was superior to the foreign acid.

As shown in Table 4 and FIG. 2, the fatigue life, which is the required characteristic of the bearing, is minimized by minimizing the amounts of O, S, P, and Ti impurities in the steel, It showed a much better fatigue life than steel.

[Effects of the Invention]

As described above, the steel according to the present invention is manufactured by a high-clean steelmaking process to reduce the amount of impurities and non-metallic materials to a minimum, to ensure cleanliness, to cast steel produced by the high-clean steelmaking process, The rolling contact fatigue life of the bearing steel after the ordinary carburizing heat treatment is not less than 7.5 x 10 < 6 > cycles by performing the normalizing treatment in the continuous heat treatment furnace with low manufacturing cost after the ordinary hot rolling, It is possible to obtain a high-quality carburizing steel for carburization having a high life-span that is extremely suppressed from surface defects due to the above-mentioned problems.

Claims (2)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.20 to 0.60% of C, 0.001 to 0.05% of Si, 0.01 to 0.20% of Mn, 0.01 to 0.05% of Cr, 0.01 to 0.05% of Cr, 0.001 to 0.02% of P and 0.001 to 0.0040% Cu, at most 0.02% of Cu, at most 0.02% of Sn, at most 0.02% of As, at most 0.02% of Sb, at most 0.002% of Pb and at most 0.002% of Ti and the balance of Fe and trace amounts of impurities (CaO / SiO2) in the range of 2.0 to 2.5 by adding the crude agent in the range of 7 to 10 Kg per ton of molten steel; Introducing oxygen into the molten steel in the range of 15 to 25 Nm < 3 > per molten steel; A lecturing step of lecturing and inputting a deoxidizing agent during lecture; Stirring the inert gas at a rate of 150 to 250 L / min in the lower part of the ladle, adding a coagulant to bring the basicity into the range of 5 to 7; Performing a vacuum degassing process at a vacuum degree of 3 mbar or less for 15 minutes or more; Casting or continuous casting; Heating at a temperature of 1,050 to 1,250 占 폚 to perform ordinary hot rolling at a total shaping annealing ratio of 8S or more; A step of subjecting the steel sheet to a normalizing treatment at a temperature of 850 to 950 占 폚 in a heat treatment furnace followed by air cooling. The method according to claim 1, wherein in said ladle step, deoxidizing agents of Mn and Si are introduced at the beginning of the ladle and Al is introduced at the end of the ladle.