JPH0499843A - Case hardening steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] "Objective of the Invention" (Industrial Application Field) The present invention relates to case hardening steel, and aims to provide a case hardening steel that is energy saving, process saving, and has less distortion during carburizing. be.

(Prior Art) Processing processes for parts using case hardened steel include hot forging, normalizing, cutting and carburizing. That is, when forming by hot forging, the heating temperature is about 1200°C, the crystal grain size is large, and the hardness after forging is high, resulting in poor machinability. Therefore, generally, after hot forging, normalization is performed to refine the structure and reduce hardness before cutting. Because it increases thermal energy and increases costs, the use of Ti, Zr, and 11 in Japanese Patent Publication No. 61-54844
The crystal grains are made finer by containing one or more types of f.

Furthermore, after case hardened steel is processed into its final shape, it is heated at about 920° C. for several hours and carburized, so the grain size tends to increase. A lot of research has been done on this, and Aj!
Precipitates such as , Nb, and Ti are used to prevent coarsening of crystal grains, and in Japanese Patent Publication No. 61-54844, Ti is used as a countermeasure.

(Problem to be Solved by the Invention) The technique using precipitates as described above to prevent crystal grain coarsening has a large difference in heating temperature between hot forging and carburizing, so it is difficult to prevent crystal grain coarsening using the same precipitates. It is difficult to achieve this goal.

Furthermore, the method disclosed in Japanese Patent Publication No. 61-54844 does not necessarily effectively achieve the original purpose of normalizing, which is to reduce hardness or improve machinability.

Therefore, it has not yet been possible to obtain a case-hardening steel that avoids complicated and high-cost manufacturing processes, has good machinability, and does not cause grain coarsening during carburization.

"Structure of the Invention" (Means for Solving the Problems) As a result of intensive research to solve the problems in the prior art as described above, the present invention has been developed by adding a small amount of Ti, adding S, and controlling the hardenability index. This method succeeded in improving machinability without normalizing and preventing grain coarsening during carburizing by controlling the balance between AA and N, and is as follows. .

C: 0.15-0.35wt%, Si: 0.5wt%
Below, Mn: 1.5wt% or less, Cr: 1.5w
t% or less, Mo: 0.25wt% or less, s: o, 03~o, oswt%, Ti: 0.005~0.015wt%, All/N"
: 1.5~2.8 T layer D+: 85~901m [Itanshi D+=7.95, Hago (100, 64Si)
(1+4.IMn) (1+2.33Cr) (1+3.
14Mo) ), with the remainder consisting of Fe and inevitable impurities.

(Function) The component composition range in the present invention as described above is changed by wt% (
(hereinafter simply referred to as "%") is as follows.

C: 0.15-0.35%.

C requires 0.15% or more to ensure strength, but
If it exceeds 0.35%, the toughness of the core will decrease;
15 to 0.35%.

Si: 0.5% or less.

It is an element necessary for deoxidation, but if it exceeds 0.5%, toughness will deteriorate, so it should be kept at 0.5% or less.

Mn: 1.5% or less.

Although it is necessary to ensure hardenability, if it exceeds 1.5%, the hardenability becomes too large, so it is set to 1.5% or less.

Cr: 1.5% or less.

Cr is also necessary to ensure hardenability, but if it exceeds 1.5%, the hardenability becomes too large, so Cr is added at 1.5%.
is the upper limit.

Mo: 0.25% or less.

Like Mn and Cr, M6 is also necessary to obtain hardenability, but if this MO exceeds 0.25%, the hardenability becomes too large, so it is necessary to keep it below 0.25%. It is.

S: 0.03-0.05%.

S is an element that improves machinability and requires 0.03% or more. However, if it exceeds 0.05%, it will have a negative effect on fatigue properties, toughness, etc., so this is the upper limit, and 0.03
-0.05%.

Ti: 0.005-0.015%.

Ti combines with N to form TiN, which has the effect of preventing coarsening of austenite crystal grains when heated to high temperatures such as in hot forging, and therefore requires a content of 0.005% or more, but on the other hand, TiN particles Since C is hard, an increase in its amount will have a negative effect on machinability, so it should be kept at 0.015% or less.

In addition to combining with Ti to form TiN, N is added in large amounts and also combines with AA, which has the effect of preventing coarsening of austenite crystal grains during carburizing (Ti/3.42
+0.01% or more is required, but on the other hand, if it exceeds 0.02%, the toughness deteriorates, so the content should be 0.02% or less.

8β/N": 1.5 to 2.8 It combines with N to form AAN and prevents coarsening of austenite crystal grains during carburizing, but the remaining N combined with Ti (
It is effective when the ratio to N'' (available nitrogen) is 1.5 to 2.8. Outside this range, the crystal grains become coarse, so it is set within the above range.

D, 15~9Qmm The hardening index D1 is defined as below 8511, the hardenability is too low and incomplete quenching tends to occur, while above 90 Tsuru, the hardness after hot forging is high (too much). It has an adverse effect on machinability.Therefore, it should be 85n or more and 9011 or less.

(Example) The present inventors melted steels with different D1 values within the composition range of the present invention, rolled them to 60 mφ, and hot-forged them to 30 mmφ by heating at 1200°C. Figure 1 summarizes the relationship between hardness and tool life.

That is, as shown in FIG. 1, when the D1 value exceeds 90 m, the hardness increases and the machinability deteriorates. Figure 2 shows the microstructure depending on the Ti content. In other words, Figure 2 shows the microstructures of two steel types with almost the same D1 value in the presence and absence of Ti, after hot forging into 30 mm diameter. It is known that the addition of Ti to ferrite and bainite makes the grains finer, lowers the hardenability, and makes it difficult for bainite to form.

Furthermore, steels with various addition amounts of i and N were produced, and 1200
'c, forge-stretched to 3Qmi+φ, then 92
Carburizing was performed at 0° C. for 5 hours, and prior austenite grains were observed. Figure 3 shows the presence or absence of coarse particles (particle size of 5 or less).
1% or more is required, and A6/N”=1.5 to 2.8
It is known that it is necessary to

A specific manufacturing example of the product according to the present invention will be described below.

The present inventors melted steel having the chemical composition shown in Table 1 below, heated it to 1200°C, and forged it to a diameter of 30 mm.

The specimens obtained as described above were then tested for hardness, machinability, carburized grain size, and Jominy hardenability. That is, hardness is measured using a Brinell hardness tester, and machinability is measured using a Brinell hardness tester.
SKH51) Using tipper, cutting speed 120 m/mi
The tool life was measured in turning with a depth of cut of 21'11 and a feed of 0.1 vs/r6. Also, the carburized particle size is 850~
After holding the temperature between 1100°C at 25°C intervals for 5 hours, the prior austenite grains were measured by quenching, and the temperature at which coarse grains with a grain size of 5 or less were generated was defined as the coarsening temperature. Furthermore, the Jominy hardenability test was conducted based on JIS GO561.

The results of these test measurements are shown in Table 2 below.

According to Table 2, that is, the results of Table 2, inventive steels 1 to 3 according to the present invention have good machinability as hot-forged, and the grain size during carburization is The coarsening temperature is also high.

On the other hand, Comparative Steel 1 does not contain Ti and has a high Ti value, so it has high hardness after forging and poor machinability. Comparison steel 2
Although the machinability after forging is good, the DI value is low and the strength after carburizing cannot be obtained. In addition, Comparative Steel 3 has poor machinability and low grain coarsening temperature after carburizing because the amount of Ti added is too large and AA/N''' is 2.8 or more. Since 8β/N” is 1.6 or less, the crystal grain coarsening temperature after carburizing is low. Furthermore, Comparative Steel 5 has a low S content, so its machinability is poor. Comparative steel 6 was normalized with ordinary SCM420 and has good machinability, but if as-forged, the hardness becomes too high and the machinability is poor.

"Effects of the Invention" According to the present invention as explained above, machinability does not deteriorate even if normalizing is not performed after hot forging, and crystal grain coarsening does not easily occur even after carburizing. This invention is industrially very effective because it sufficiently saves the process and energy mentioned above, and also reduces distortion during carburizing, and provides a case-hardening steel that is preferred at low cost.

[Brief explanation of drawings]

The drawings show the technical content of the present invention;
The figure shows photographs comparing microscopic metal structures with and without Ti. (A) shows the case without the addition of Ti, (B) shows the case with the addition. Figure 3 shows the difference between available nitrogen, soβ, Aβ, and coarse grains. This is a chart summarizing the state of occurrence. No. oka (mm) No. 5 o LAL (%) No. 5 (A) (B)

Claims (1)

[Claims] C: 0.15 to 0.35 wt%, Si: 0.5 wt% or less, Mn: 1.5 wt% or less, Cr: 1.5 wt% or less, Mo: 0.25 wt% or less, S :0.03~0.05wt%, Ti:0.005~0.015wt%, N:(Ti/3.42)+0.01~0.02wt%,
Al/N^*: 1.5 to 2.8 (However, N^* = N-(Ti/3.42)) D_I: 8
5~90mm [However, D_I=7.95√C(1+0.64Si)(1
+4.1Mn) (1+2.33Cr) (1+3.14M
o)] with the remainder consisting of Fe and inevitable impurities.