JPH0568526B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing case hardened steel for bearings, and more particularly to a method for manufacturing case hardened steel that has a high austenite grain enlarging temperature and is suitable for manufacturing bearings with excellent rolling fatigue characteristics. Bearings made of case-hardened steel have excellent toughness and impact resistance, and are therefore suitably used in rolling bearings that are subjected 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 rolling 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,
In many cases, cold working is performed to improve workability through annealing, followed by carburizing, but in such cases, depending on the working rate, the austenite grains in the non-carburized area become coarse during carburizing, causing internal damage. It is already well known that it increases hardness and also causes quenching, distortion, and a decrease in toughness. As a result of intensive research to solve the above-mentioned problems, the present inventors have succeeded in reducing the amount of S and Al ₂ O ₃ inclusions in steel, as well as increasing the content of Al and N.
It is defined by the Al/N weight ratio and (Al + 2N), that is, the sum of the amount of Al and twice the amount of N, and the steel is heated to a predetermined temperature before hot rolling.
By suppressing the amount of AlN below a predetermined value, a steel material with a high austenite grain coarsening temperature can be obtained. The inventors have discovered that it is possible to obtain a case-hardened steel that suppresses coarsening, retains a fine grain structure, and has significantly improved rolling characteristics, which are the most important characteristics of bearings, and have thus arrived at the present invention. It is something. 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 with excellent rolling fatigue properties according to the present invention includes, in weight percent, C 0.05 to 0.35%, Si 0.4% or less, Mn 0.5 to 2.0%, Al ₂ O ₃ 20 ppm or less, Al and N. at an Al/N weight ratio of 1.9 to 3.5 (Al+
Steel containing 0.045 to 0.065% as 2N), 0.01% or less of S, and 0.3 to 1.5% of Cr is heated to a temperature T (℃) defined by the following formula before hot rolling: T≧3750 (Al% + 2N%) +950 (However, element % means the weight % of the element in the steel.) After this, hot rolling is performed.
It is characterized by an AlN precipitation amount of 40 ppm or less. 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 heat treatment will be deteriorated, so the upper limit is set to 0.35% by weight. Si is added as a deoxidizing agent, but if it is too large, the amount of SiO ₂ inclusions will increase and the cold workability will be significantly inhibited, so the upper limit is set to 0.4% by weight. Mn is an essential element to increase hardenability and increase strength after heat treatment, but when added in excess, it reduces toughness and machinability, so the content should be 0.5 to 2.0% by weight. % range. In the method of the present invention, the Al/N weight ratio of Al and N is 1.9 to 3.5, and (Al+2N) is 0.045.
It is necessary to regulate the content within the range of ~0.065% by weight.
When the amount of (Al+2N) is less than the above range, and when the Al/N weight ratio is out of the above range, even if heated to a predetermined temperature according to the present invention before hot rolling, When cold working is performed after annealing treatment and carburizing treatment is performed after this, a sufficient amount of AlN is not precipitated to prevent coarsening of austenite crystal grains, so coarsening of austenite crystal grains is unavoidable. On the other hand, when the amount of (Al+2N) is greater than the above range, AlN cannot be sufficiently dissolved into the steel by heating before rolling, and austenite becomes coarse during carburization. That is,
In the present invention, the steel is made of Al and N (Al+
2N) and Al/N are both within predetermined ranges. It requires that. Furthermore, in the method of the present invention, in the steel used,
The amount of Al ₂ O ₃ inclusions is 20 ppm or less, and the amount of S is
It is necessary to keep it below 0.01%. Al ₂ O ₃ and S have a significant effect on the rolling fatigue properties of steel when case hardened steel is carburized and subjected to quenching and tempering treatment, and the amount of Al ₂ O ₃ exceeds 20 ppm. When the amount of S exceeds 0.01%, the rolling fatigue properties deteriorate in either case. Cr is used to ensure carburizability and hardenability.
Although it is necessary to add 0.3% by weight or more,
When added in excess, the hardness decreases due to residual austenite in the surface layer, and giant carbides are generated, deteriorating toughness, so 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 contains, for example, 0.05 to 0.50% by weight of Mo to improve the hardenability of the steel, suppressing coarsening of autestenite grains during carburization, Further, Ni can be contained in a range of 0.01 to 1.50% by weight in order to reduce baking distortion. Furthermore, in the method of the present invention, after hot rolling steel having the above-mentioned chemical composition,
AlN must be 40ppm or less. In this way, in order to reduce the AlN content after hot rolling to 40ppm or less,
As described above, according to the present invention, predetermined amounts of Al and N
This is achieved by heating the steel containing (Al + 2N) to a temperature equal to or higher than the temperature T (°C) defined by the following formula depending on the amount of (Al + 2N) before hot rolling. T≧3750 (Al% + 2N%) + 950 (However, element % means the weight % of the element in the steel.) As a result of extensive experiments by the present inventors, the AlN content in the steel after hot rolling was The amount has a clear correlation with the steel heating temperature before hot rolling, and the (Al +
2N) in the steel after hot rolling increases.
The lower limit temperature for keeping the amount of AlN below 40 ppm is high, and this lower limit temperature is approximately as shown above (Al+
2N) was found to be defined as a linear function of quantity. According to the method of the present invention, as described above, (Al+
2N) and Al/N weight ratio are heated above the temperature specified by the above formula according to the above (Al + 2N) amount before hot rolling to reduce the Al/N after rolling.
By setting the austenite grain coarsening temperature to 40 ppm or less, the resulting steel material can be annealed and cold-worked.
Even with carburizing treatment, coarsening of austenite crystal grains is suppressed, thus not only can a fine grain structure be maintained, but also the amount of Al ₂ O ₃ and S amount in the steel can be regulated to below predetermined values. Therefore, case hardening steel with significantly improved rolling contact fatigue properties can be obtained. The present invention will be specifically explained in more detail by referring to Reference Examples and Examples below. Reference example The steel shown in Table 1 is heated to each temperature shown in Table 2 before hot rolling, then rolled into a wire rod with a diameter of 15 mm, annealed at a temperature of 740°C, and then 10% or Cold working was carried out at a processing rate of 50%, then the temperature was raised to 950°C at an average heating rate of 1°C/min, and after this temperature was maintained for 3 hours, the coarsening rate of the austenite grains was measured. The results are shown in Table 2 and Figure 1.
As shown in FIGS. 2 and 3, according to the present invention,
The steel material is heated to a temperature higher than the temperature specified by the above formula depending on the amount of (Al + 2N) before hot rolling, and then the steel obtained by hot rolling has no AlN.
It is 40 ppm 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 Al/N amount after rolling exceeds 40 ppm, so that the austenite crystal grains become significantly coarsened during the carburizing treatment. When the amount of AlN after rolling is 40 ppm or less, coarsening of austenite grains is not observed, but
It is clear that when the amount of AlN after rolling is 40 ppm or more, coarsening is noticeable when the processing rate is 10% or 40% or more. Figure 3 shows that the amount of Al after rolling is regulated by the amount of (Al + 2N) and the heating temperature before hot rolling, and the lower limit is 40 ppm or less for the amount of AlN in the steel after hot rolling. Temperature is defined as a linear function of (Al+2N) quantity. Example 1 After heating case hardening steel having the chemical composition shown in Table 3 steel numbers 1 to 5 in a heating furnace at 1250°C, the diameter
It was rolled into a 15mm wire rod. In these rolled materials
Table 3 shows the amount of AlN precipitated. Next, a cylindrical rolling contact 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 Al ₂ O ₃ amounts outside the range specified by the present invention. Each specimen was carburized at a temperature of 975°C for 2 hours, quenched, then tempered by heating at a temperature of 160°C for 2 hours, and finished by wrapping the specimen to a Hertzian stress of 60 kg/mm ^2. was used for testing. 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 rolling fatigue properties than conventional steel. In addition, based on these results, the life of the test piece is defined as the time when 10% damage occurs in the rolling test, and the relationship between this life and the S content is shown in Figure 5, and the relationship between the amount of Al ₂ O ₃ and the The relationship is shown in FIG. When the S content is 0.01% or less, and when the Al ₂ O ₃ content is
It is clear that rolling fatigue properties are significantly improved when the content is 20 ppm or less. Next, the above-mentioned invention steel 1 was heated under the same conditions as above, hot-rolled, pickled, descaled, and then carburized for 3 hours at a temperature of 975°C and a carbon potential of 0.87. After quenching,
It 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 No. 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 invention steel 1
was heated to 1100℃, hot rolled, and the amount of AlN was reduced.
It was set to 102ppm. FIG. 8 shows the relationship between carburizing temperature and austenite grain size No. when this was similarly carburized. It is clear that as the carburizing temperature increases, the austenite becomes coarser. Example 7 Inventive steel 6 having the chemical composition shown in Table 3 was heated to a temperature of 1250°C in the same manner as in Example 1, and comparative steel 7 was heated to a temperature of 1250°C as specified by the above formula. (1160℃)
After heating to 1100°C, each was hot rolled into a rolled material with a diameter of 15 mm. After pickling and descaling, it is left as is or subjected to 10% cold processing and heated to a temperature of 975℃ for 7 hours.

【table】

【table】

[Table] The austenite grain size of the steel thus obtained by heating and pseudo-carburizing was measured. As a result, according to the inventive steel, the austenite coarsening rate was 0% in both the non-cold worked part and the 10% worked part, but according to the comparative steel, the austenite coarsening rate was 50% in the unworked part, and 0% in the 10% worked part. It reached 100% in the department. still,
Here, the austenite coarsening rate indicates the proportion of crystal grains larger than grain size No. 6.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Al/N weight and crystal grain coarsening rate, Figure 2 is a graph showing the relationship between (Al + 2N) amount and grain coarsening rate, and Figure 3 is a graph showing the relationship between (Al + 2N) amount and grain coarsening rate. A graph showing the relationship between pre-hot rolling heating temperature and AlN content in the steel after hot rolling, Figure 4 shows Steel 1 produced by the method of the present invention and Comparative Steels 2, 3, and 4.
Figure 5 is a graph showing the relationship between the number of stress repetitions and the failure rate in the rolling contact fatigue test. Figure 6 is a graph showing the relationship between test piece life and
A graph showing the relationship between the amount of Al ₂ O ₃ , FIG. 7 is a graph showing the relationship between the depth from the surface and Hv hardness when the steel according to the present invention is carburized, quenched and tempered, and FIG. 2 is a graph showing the relationship between carburizing temperature and austenite particle size No. when treated according to the present invention, together with a comparative method.

Claims (1)

[Claims] 1% by weight: C 0.05-0.35%, Si 0.4% or less, Mn 0.5-2.0%, Al ₂ O ₃ 20ppm or less, Al and N in an Al/N weight ratio of 1.9-3.5 ( Al
+2N) 0.045 to 0.065%, S 0.01% or less, and Cr 0.3 to 1.5% before hot rolling at a temperature T (°C) defined by the following formula: T≧3750 (Al% + 2N%) +950 (However, element % means the weight % of the element in the steel.) After that, it is hot rolled and the rolled
A method for producing case hardened steel for bearings with excellent rolling contact fatigue properties characterized by having an AlN precipitation amount of 40 ppm or less.