JP2523516B2 - Method for manufacturing case-hardening steel with excellent rolling fatigue - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing case-hardening steel having excellent rolling fatigue, and more specifically, ultracleanness, that is, high fatigue life (rolling fatigue). The present invention relates to a method for manufacturing case hardening steel.

[Prior Art] In recent years, there is an increasing demand for bearing steels having excellent rolling fatigue properties. In order to improve the characteristics, reduction of various inclusions causing characteristic deterioration, that is, O ₂ , P, S, Ti
Is important.

Therefore, in the prior art, the ultra-clean case-hardening steel with low impurities is manufactured by a vacuum melting method, and after the electric furnace refining and converter refining, the outside furnace refining is performed, and continuous casting and ingot casting are performed. However, it was impossible to produce stable ultra-clean steel by this method. For example, according to this conventional method, [O] 8 to 15 ppm,
[P] [S] was 0.010 to 0.020 wt% and [Ti] was 20 to 40 ppm, and rolling fatigue resistance was B ₁₀ 2 × 10 ⁷ at most.

Also, in the past, adding Cr was in the process of molten steel treatment, and the problem of Ti pickup from the Cr source and the problem that reoxidation is very likely due to the long heating time and the case of continuous casting There was a problem that the center segregation was very large.

[Problems to be Solved by the Invention] The present invention has been made in view of the various problems as described above, and as a result of intensive research by the present inventors, it has an adverse effect on the fatigue life of case hardening steel. P, S, T that give harmful substances
We have developed a method for producing an ultra-clean case-hardening steel with a high fatigue life that can reduce the amounts of i and O ₂ extremely and also reduce the center segregation significantly.

[Means for Solving Problems] A feature of the method for producing case-hardening steel having excellent rolling contact fatigue resistance according to the present invention is that P is 0.01 wt% or less and S 0.007 wt% in hot metal pretreatment.
After reducing the amount to below, add all the product Cr content in the furnace under the slag minimum blowing in the converter and add Ti 10ppm.
The molten metal reduced to less than O2 and less than 7 ppm by O ₂ refining was subjected to electromagnetic stirring in the mold during continuous casting and at the end of solidification to reduce the center segregation of the slab and finally
C 0.05 to 0.35 wt% steel P 0.005 wt% or less, S 0.005 wt%
Hereinafter, it is to be a clean steel having Ti of 10 ppm or less and O _{2 of} 7 ppm or less.

The method for producing case-hardening steel having excellent rolling fatigue properties according to the present invention will be described in detail below.

That is, the method of manufacturing a case-hardening steel having excellent rolling contact fatigue resistance according to the present invention is a hot metal pretreatment-> converter refining-> outside furnace refining-> continuous casting, in a step of performing electromagnetic stirring in the mold / solidifying end stage stirring. It is a method for producing case-hardening steel having ultra-clean, high-fatigue life and excellent rolling fatigue property, which is capable of removing substances harmful to case-hardening steel in each step and enabling mass production with reduced center segregation.

Each step of the method for manufacturing case-hardening steel having excellent rolling fatigue property according to the present invention will be described below. In the hot metal pretreatment, lime-based dephosphorizing material and iron oxide are added to the hot metal,
After dephosphorizing the hot metal by blowing oxygen gas, soda ash, lime, carbide, etc. are blown into the hot metal to desulfurize the hot metal. In this way, dephosphorization and desulfurization are performed, and as shown in FIGS.
The content is reduced to 0.010 wt% or less and the S content is reduced to 0.007 wt% or less to enable slagminium blowing in a converter.

Next, this low-phosphorus, low-sulfur hot metal is charged into a converter, and calcined lime, light-burnt dolomite, and silica are added to the converter as auxiliary materials, and a Cr source (for example, ferrochrome) is added to the converter. Add and blow with a vertical blowing converter.

At this time, since the titanium as the Cr source is removed by oxidation, the titanium content becomes 10 ppm or less when the steel is taken out from the converter (see FIG. 2 (a)).

As described above, when the Ti content is extremely low, the performance as a bearing by the thrust type rolling fatigue test shown in FIG. 2 (b), that is, the life (B ₁₀ ) becomes very excellent. Incidentally, in FIG. 2 (b), the Ti content is 10 pp.
It can be seen that B ₁₀ becomes extremely short when m is exceeded. In FIG. 2 (b), the contact pressure is 500 kg / mm ² , the rotation speed is 1000 rpm,
The lubricating oil is spindle oil.

The molten steel discharged from the converter in this way is subjected to molten steel treatment. In the method for producing case-hardening steel having excellent rolling fatigue properties according to the present invention, it is sent to a molten steel processing facility having an ASEA or LF type arc heating function and a slag refining function. In this case, lime, by adding CaF ₂ , by increasing the slag basicity, it is possible to desulfurize and deoxidize, the sulfur content is 0.005 wt% or less, the oxygen content may be 7 ppm or less. it can. Further, C in the steel needs to be 0.05 wt% or less for maintaining strength, and if too much, the toughness deteriorates, so it is necessary to suppress C to 0.35 wt% or less.

Thus, finally, as shown in FIGS. 1 to 3, clean steel having P 0.005 wt% or less, S 0.005 wt% or less, Ti 10 ppm or less, and O ₂ 7 ppm or less is obtained.

In electromagnetic stirring in the mold and in the final stage of solidification during continuous casting, continuous casting can prevent reoxidation at the time of pouring molten metal as compared to the ingot casting process, and reduce inclusions by using TD and electromagnetic waves of molten steel in the mold. The inclusions are floated and separated by stirring, and the center segregation of the slab by electromagnetic stirring at the final stage of solidification in the mold is reduced to a level without problems. The center segregation is significantly reduced while being removed.

Fig. 4 (a) shows a comparison of the degree of segregation in the electromagnetic stirring in the mold, and in Fig. 4 (b) the segregation degree in the slab of the electromagnetic stirring twice in the mold + the final stage of the molten metal solidification. The segregation degree ΔC is 1.0 when the electromagnetic stirring is performed twice in the mold and the final stage of melting and solidification as shown in b), and the segregation degree is 1/2 or less as compared with the case of only in the mold. You can see that

By combining these processes, it is possible to stably produce a large amount of ultra-clean, case-hardening steel with a long fatigue life.

[Example] An example of a method for producing case-hardening steel having excellent rolling fatigue properties according to the present invention will be described.

Example Hot Metal Pretreatment In a hot metal pretreatment furnace, 23 kg / ton of CaO-based dephosphorizing agent was added, and at the same time, 10 kg / ton of soda ash-based desulfurizing agent was added to treat hot metal.

[P] = 0.006 wt% and [S] = 0.004 wt%,
It became possible to blow slag minimum in the converter. The content of P and S is the P content of JIS SCR20H standard ≦ 0.030wt
%, S content is much lower than 0.030 wt%.

Converter treatment (upper and lower blowing converter) Using 50kg / ton of slag and adding Fe-Cr alloy, the Cr yield was 90% and the [Cr] content was 1.00wt%.

Further, the Ti content was 10 ppm or less after the steel was taken out from the converter.

The out-of-furnace refining O ₂ content was reduced to 6 ppm, and then continuous casting in the next step was performed.

Continuous casting Magnetic stirring was performed twice in the mold and at the end of solidification.

 Table 1 shows the components of the case-hardening steel produced.

As a comparative example, steel manufactured by the conventional vacuum melting method is shown.

The rolling fatigue life of the two case hardening steels manufactured in this way is shown in FIG.

Test conditions Load 500kg / mm ² Speed 1000rpm Lubricating oil # 60spindle oil ○ Case-hardening steel obtained by the method for manufacturing case-hardening steel with excellent rolling fatigue according to the present invention ● Case-hardening steel obtained by comparative example It can be seen from FIG. 5 that the fatigue life of the case-hardening steel 1 obtained by the method for manufacturing a case-hardening steel having excellent rolling fatigue life according to the present invention is superior to that of the case-hardening steel 2 obtained by the comparative example. .

[Effects of the Invention] As described above, since the method for producing a case-hardening steel having excellent rolling contact fatigue resistance according to the present invention has the above-described configuration, it is possible to produce a super-clean case-hardening steel. The case-hardened steel produced has an excellent effect of extending rolling fatigue life.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) (b) is a diagram showing reduction of P and S by hot metal pretreatment, and FIG. 2 (a) (b) is reduction of Ti and Ti content in a converter. Fig. 4 shows the relationship between B and B ₁₀ , Fig. 3 shows the reduction of O ₂ content by refining outside the furnace, Fig. 4 (a)
(B) is a diagram showing the degree of segregation between the electromagnetic stirring in the mold and the two electromagnetic stirring in the mold + the molten metal at the end of solidification, and FIG. 5 is a diagram showing the relationship between the fatigue life and the cumulative damage probability.

Claims (1)

(57) [Claims] 1. In the hot metal pretreatment, P 0.010 wt% or less, S
After reduction to 0.007 wt% or less, the total Cr content of the product was added to the furnace under slag minimum blowing in a converter and Ti was reduced to 10 ppm or less, and then O ₂ 7
The molten metal reduced to ppm or less is subjected to electromagnetic stirring in the mold during the continuous casting and at the end of solidification to reduce the center segregation of the slab, and finally in the steel of C 0.05 to 0.35 wt%, P 0.005 wt% or less, S 0.005 A method for producing a case-hardening steel having excellent rolling fatigue resistance, characterized by comprising a clean steel containing wt% or less, Ti 10 ppm or less, and O ₂ 7 ppm or less.