JPS616212A - Manufacture of case hardening steel for bearing - Google Patents

Abstract

PURPOSE:To improve remarkably the rolling fatigue characteristics of a steel contg. prescribed percentages of C, Si, Mn, S, Cr, etc. by heating the steel to a prescribed temp. and hot rolling it. CONSTITUTION:A steel contg., by weight, 0.05-0.35% C, <=0.4% Si, 0.5-2% Mn, <=20ppm Al2O3, Al and N satisfying Al/N=1.9-3.5 and Al+2N=0.045-0.065%, <=0.01% S and 0.3-1.5% Cr is refined. The steel is heated to 3,750(Al%+2N%) +950 deg.C and hot rolled so that AlN is precipitated by <=40ppm after rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing case hardened steel for bearings, and more specifically, to a method for manufacturing case hardened steel for bearings, and more specifically, a case hardened steel that has a high austenite grain coarsening temperature and is suitable for manufacturing bearings that have excellent rolling fatigue characteristics. Concerning a method of manufacturing steel.

Bearings made of case-hardened steel have excellent toughness and impact resistance, and are therefore suitably used in rolling bearings that are subject to impact loads, such as in vehicles, automobiles, construction machinery, rolling mills, and the like. However, conventionally known bearings made of case-hardened steel do not have sufficient transfer characteristics. Furthermore, in general, when case hardening steel requires cold working, it is cold worked as is when the working rate is low, and when the working rate is high,
Good workability is achieved through annealing treatment, and cold processing is performed.
This is often followed by carburizing, but in such cases,
It is already well known that depending on the processing rate, austenite crystal grains in non-carburized parts become coarse during carburizing, increasing internal hardness, and causing quenching distortion and a decrease in toughness.

As a result of intensive research by the present inventors to solve the above-mentioned problems, the amount of S in steel and ^1□0. In addition to reducing the amount of inclusions, the content of Al and N is defined by the weight ratio of Al7N and (, l + 2N), that is, the sum of the amount of Al and twice the amount of N, and such steel is heated to a specified level before hot rolling. By heating to a high temperature and suppressing the amount of AIN after hot rolling to a predetermined value or less, a steel material with a high austenite grain coarsening temperature can be obtained. It is possible to obtain a case hardened steel in which coarsening of austenite crystal grains is suppressed and a fine grain structure is maintained even after subsequent carburizing treatment, and the transfer characteristics, which are the most important characteristics of bearings, are significantly improved. Find out,
This led to the present invention.

Accordingly, an object of the present invention is to provide a method for manufacturing case hardening steel that can obtain a bearing having a high austenite grain coarsening temperature and excellent rolling fatigue characteristics.

The method for producing case-hardened steel for bearings according to the present invention is characterized in that the weight%
C 0.05-0.35%, Si 0.4% or less, Mn 0.5-2.0%, AIzOs 201) 11111 or less, Al and Nt-
A7! /N weight ratio in the range of 1.9 to 3.5 (Aj2+2
N) 0.045-0.065%, S 0.0
1% or less and 0.3 to 1.5% of Cr is heated before hot rolling at a temperature defined by the following formula: T (°C) (means the weight percent of the element in the steel) and then hot rolling to reduce the amount of ^IN precipitation to 40 ppm or less after rolling.

The chemical composition of the steel used in the method of the present invention will be explained.

C must be contained in an amount of at least 0.05% by weight in order to maintain high core toughness when the steel material is quenched and tempered after carburizing. However, if it is contained in an excessively large amount, the toughness of the steel after the tempering treatment will be deteriorated, so the upper limit is set to 0.35% by weight.

Sl is added as a deoxidizing agent, but if it is too much,
Since the amount of Sho□-based inclusions increases and significantly impairs cold workability, the upper limit is set at 0.4% by weight.

Mn is an essential element to increase hardenability and increase strength after tempering treatment, but when added in excess, it reduces toughness and machinability, so its content should be 0.5~ 2.
The range is 0% by weight.

In the method of the present invention, the At'/N weight ratio of AI and N is 1.9 to 3.5, and (A j2 +2N
) is required to be controlled within the range of 0.045 to 0.065% by weight. (Am!+2N) amount is less than the above range, or when the kl/N weight ratio is out of the above range, heat to a predetermined temperature according to the present invention before hot rolling. Also, when cold working is performed after annealing treatment and carburizing treatment is performed after this, AIN is not precipitated in an amount sufficient to prevent austenite crystal grains from becoming enlarged, so coarsening of austenite crystal grains is unavoidable. On the other hand, when the amount of (Az+2N) is greater than the above range, Ai'N cannot be sufficiently infused into the steel by heating before rolling, and austenite becomes coarse during carburization. That is, in the present invention, the steel contains Al and N (A
j! +2N) and Al/N are both within predetermined ranges. It requires that.

Furthermore, in the method of the present invention, A+, O in the steel used
3 The amount of inclusions is 20 ppm or less, and the amount of S is 0.0
It is necessary to keep it below 1%. Al□0° and S are case hardened steel that is carburized, quenched and tempered! It has a significant effect on the transfer fatigue properties of steel when subjected to jlIt treatment, and
l. 0. When the amount exceeds 20 pppH, or when the amount of S is 0.
If the amount exceeds 0.1%, the transfer fatigue characteristics deteriorate in any case.

It is necessary to add 0.3% by weight or more of Cr to ensure carburizability and hardenability. However, when adding too much, the hardness decreases due to residual austenite in the surface layer, and Since carbides are generated and deteriorate the toughness, the upper limit of the amount added is 1.5% by weight.

In the method of the present invention, in addition to the above-mentioned alloying elements, the steel used includes, for example, MO to improve the hardenability of the steel.
0.05 to 0.50% by weight of N to suppress coarsening of austenite grains during carburizing and to reduce firing distortion.
i can be contained in a range of 0.01 to 1.50% by weight.

Furthermore, in the method of the present invention, AJN in the steel after hot rolling the steel having the above chemical composition is 40pp.
m or less. In this way, A after hot rolling
To reduce IN to 40 pp+m or less, a predetermined amount of Aj! according to the present invention as described above. This is achieved by heating the steel containing N and N to a temperature T (° C.) or higher defined by the following formula depending on the amount of (Al+2N) before hot rolling.

T≧3750(,l!%+2N%)+950 (However, element % means the weight % of the element in the steel.) As a result of extensive experiments by the present inventors, the The amount of AIN has a clear correlation with the heating temperature of the steel before hot rolling, and as the amount of (Al!+2N) in the steel increases, the amount of AIN in the steel after hot rolling is reduced to 40 ppm or less. It was discovered that the lower limit temperature becomes higher, and that this lower limit temperature is defined as a linear function of the amount (/112N) approximately as shown above.

According to the method of the present invention, as described above, CAI+2N)
[1 and Aj! /N weight ratio is heated to a temperature higher than the temperature specified by the above formula according to the amount of (Az + 2N) before hot rolling, and the AN/N after rolling is 4 oppm or less. Since the austenite grain coarsening temperature has been raised, the coarsening of the austenite grains is suppressed in the obtained steel even after annealing, cold working, and carburizing, and thus the fine grain structure is maintained. In addition, by regulating the amount of Al□031 and S in the steel to below predetermined values, case hardening steel with significantly improved rolling contact fatigue properties can be obtained.

The present invention will be specifically explained in more detail with reference to Reference Examples and Examples below.

Reference Example The steel shown in Table 1 was heated to each temperature shown in Table 2 before hot rolling, then rolled into a wire rod with a diameter of 15 m, and annealed at a temperature of 740°C. 5
Cold worked at 0% working rate, then average heating rate 1°C
After increasing the temperature to 950° C./min and maintaining this temperature for 3 hours, the coarsening rate of the austenite grains was measured. Second result
As shown in the table and FIGS. 1, 2, and 3, according to the present invention, the steel material is (Aj!+2N) before hot rolling. All steels obtained by heating above the temperature specified by the above formula and then hot rolling have an AIN of 40pp.
m or less, and the structure maintains a fine-grained structure even after cold working after annealing and further high-temperature heat treatment. However, when the temperature before hot rolling is below the temperature specified by the above formula, the amount of Al/N after rolling is 40 ppm.
, the austenite crystal grains are significantly enlarged during the carburizing process.

When the amount of Al1N after rolling is 40 ppm or less, coarsening of austenite crystal grains is not observed, but when the amount of Al1N after rolling is 40 ppm or more, coarsening is noticeable when the processing rate is 10% and 40% or more. It is clear that

Figure 3 shows that the amount of Al after rolling is regulated by the amount of (A l + 2 N) and the heating temperature before hot rolling. The lower limit temperature below is defined as a linear function of (A l + 2 N> quantity.

Example 1 Case hardened steels having chemical compositions shown in Table 3 Steel No. 1 to 5 were heated in a heating furnace at 1250° C. and then rolled into wire rods with a diameter of 151 m. Table 3 shows the amount of Al1N precipitated in these rolled materials. Next, a cylindrical transfer fatigue test piece was prepared from this rolled material.

Comparative steels 2 and 4 have S contents outside the range specified by the present invention, and comparative steels 3 and 5 have an AI! 03 IF is outside the range defined by the present invention. Each specimen was carburized at a temperature of 975°C for 2 hours, then quenched, then tempered by heating at a temperature of 160°C for 2 hours, and then finished by lapping and subjected to a Hertzian stress of 600 kg/112. Tested.

Figure 4 shows the relationship between the number of stress repetitions and the failure rate. It is clear that the case hardened steel according to the present invention has significantly better transfer fatigue properties than conventional steel.

Based on these results, the life span is defined as the time when 10% of the specimen breaks in the transfer test, and the relationship between this life span and the S content is shown in Figure 5. 0. The quantities and relationships are shown in Figure 6. When the S content is 0.01% or less, and when the Al. 0. It is clear that when the content is below 20 ppn+, the transfer fatigue properties are significantly improved.

Next, the above invention steel 1 was heated under the same conditions as above, hot rolled, pickled, descaled, and then 97%
Carburizing was carried out at a temperature of 5° C. for 3 hours under the condition of a carbon potential of 0.87, and after quenching, the material was tempered at a temperature of 160° C. for 2 hours. The hardness distribution curve of this carburized product is shown in FIG.

Further, FIG. 8 shows the relationship between carburizing temperature (carburizing time: 3 hours) and austenite grain size for the steel 1 of the present invention.

According to the method of the present invention, a fine grain structure is maintained even after carburizing treatment. On the other hand, the present invention till is heated to 1100°C,
It was hot rolled to have an AIN content of 1102 pp. FIG. 8 shows the relationship between carburizing temperature and austenite grain size when this was similarly carburized. It is clear that as the carburizing temperature increases, the austenite becomes coarser.

Example 2 Regarding the invention steel 6 having the chemical composition shown in Table 3,
After heating to a temperature of 1250°C in the same manner as in Example 1, and for comparative steel 7, after heating it to a temperature of 1100°C lower than the temperature specified by the above formula (1160°C), Each was hot rolled into a rolled material with a diameter of 15 mm. After pickling and descaling, the steel is used as it is or subjected to 10% cold working and heated to a temperature of 975°C for 7 hours to undergo pseudo-carburizing treatment. The austenite grain size was measured.

As a result, according to the invention steel, the austenite coarsening rate was 0% in both the non-cold worked part and the 10% worked part, but according to the comparison steel, the austenite coarsening rate was 50% in the unworked part and 10% worked part. It reached 100% in the department. Note that the austenite coarsening rate here indicates the proportion of crystal grains with a grain size larger than 6.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between Al7N weight ratio and crystal grain coarsening rate, Fig. 2 is a graph showing the relationship between (Az 1,2N) I and crystal grain coarsening rate, and Fig. 3 is a graph showing the relationship between (Az 1,2N)I and crystal grain coarsening rate. + 2 N
) amount, heating temperature before hot rolling, and Al' in steel after hot rolling
FIG. 4 is a graph showing the relationship between stress repetition rate and failure rate in the transfer fatigue test of Steel 1 and Comparative Steels 2.3.4 and 5 according to the method of the present invention. FIG. Figure 6 is a graph showing the relationship between the life span and the S content, with the life span defined as the time when 10% of the specimen breaks in the transfer fatigue test.
FIG. 7 is a graph showing the relationship between the amount and the depth from the surface and Hv when the steel according to the present invention is carburized, quenched and tempered.
FIG. 8 is a graph showing the relationship between carburizing temperature and austenite grain size when Inventive Steel 1 is treated according to the present invention, along with a comparative method. Figure 1 AQ/N Natsume Do Ratio Figure 2 0.050 0. +00 A0. +2NC +i%) Figure 4 Condolence 5Q Figure 6 0 20 40 6O A&'203 Cpprn)

Claims (1)

[Claims] (1) C0.05-0.35% by weight, Si 0.4% or less, Mn 0.5-2.0%, Al_2O_3 20ppm or less, Al and N at an Al/N weight ratio of 1.9-3.5 In the range of (
A steel containing 0.045 to 0.065% as Al+2N), 0.01% or less of S, and 0.3 to 1.5% of Cr is heated to a temperature T (℃) T defined by the following formula before hot rolling. ≧3750 (Al% + 2N%) + 950 (However, element % means the weight % of the element in the steel.) After that, hot rolling is performed to reduce the amount of AlN precipitation after rolling to 40 ppm or less. A method for manufacturing case-hardened steel for bearings, characterized by: