JPH06104857B2 - Method for manufacturing free-cutting planks with excellent fatigue characteristics - Google Patents

Description

TECHNICAL FIELD The present invention is a material for machine tools such as hydraulic manifolds, bolsters, and platens, and has excellent chip disposability and excellent fatigue properties when drilling holes. The present invention relates to a method for manufacturing a free-cutting thick plate having a strength of 40 to 80 kg f / mm ² .

(Prior Art) Steel materials for machine tools such as hydraulic manifolds, bolsters, and platens are thick materials and are often drilled. JIS G 5502 FCD40, FC with good machinability
Spheroidal graphite cast iron such as D45 is used. However, cast iron has a high product price, has small latent defects, has poor fatigue characteristics, and is likely to cause oil leakage through the latent defects. For this reason, spheroidal graphite cast iron is used only at low stress levels.

In addition to this, there is JIS G 4804 "Sulfur and sulfur composite free-cutting steel" as free-cutting steel, but in the S single system, MnS expands to more than 30 μm by rolling or forging, and it becomes the fracture starting point of fatigue fracture and fatigue characteristics Markedly reduced. For this reason, it is difficult to manufacture a free-cutting thick plate having an S single system and excellent fatigue characteristics.

Further, in the S + Pb system, Pb is harmful, and fine chips generated during processing are not preferable for the human body.

JIS G 32, a forged steel product, is used as a steel material for machine tools, which is usually used as a steel material with no small latent defects and good fatigue properties.
There are 01 SF40A and SF45A, but the product price is high because it goes through the forging process. Further, since the chips are wrapped around the drill, it takes manpower to process the chips, and the processing efficiency is reduced. It also shortens the tool life.

This poor chip disposability poses a problem for unmanned processing. Therefore, it is desired to establish a method for manufacturing a free-cutting thick plate which has a low product price and excellent fatigue characteristics.

(Problems to be Solved by the Invention) The present invention is based on the conventional JIS standard FCD40, FCD45 and SF40A, SF45A.
Replaces JIS G 4804 with excellent fatigue strength and tensile strength.
It is an object to provide a method for manufacturing a free-cutting thick plate of 40 to 80 kg f / mm ² .

(Means for Solving the Problem) The gist of the present invention is that the weight ratio is C: 0.10 to 1.00.
%, Si: 0.05-1.00%, Mn: 0.10-2.00%, sol.Al:0.001
~ 0.50%, N: 0.0020 ~ 0.20%, S: 0.025 ~ 0.50% as the basic component and the balance Fe and unavoidable impurities, or a steel containing Cr: 0.10 ~ 1.00% in this steel It is characterized by cooling at a cooling rate in the temperature range of 1100 ° C to 800 ° C during post-cooling or after AlN solution heat treatment of 5 ° C / hr or more and 10 ° C / min or less, and then reheating and rolling, and if necessary. Accordingly, it is a method for producing a free-cutting thick plate having excellent fatigue properties, in which normalization is performed after rolling.

(Action) In order to improve the free-cutting property, in the work body during cutting, 1) embrittlement action due to inclusions forming a constituent cutting edge that acts as a stress concentration source in the work body, and 2) deformation resistance in the work body It is necessary to increase the lubrication action that small inclusions act as a lubricant, and 3) the tool protection action that protects the cutting edge during cutting by the inclusions with small deformation resistance in the work body.

However, MnS acts as a built-up edge during cutting, and peels off at the interface of MnS with low deformation resistance.
Lubrication and tool protection work together to improve machinability.
In particular, when MnS is finely and uniformly dispersed and precipitated, it is known that the effect of dividing chips is strong.

On the other hand, MnS has been conventionally stretched by rolling or forging to 30μ
It is known that when the value exceeds m, it becomes a starting point of fatigue fracture and the fracture characteristics are significantly deteriorated.

The inventors of the present invention have further finely dispersed AlN with 30 or more particles / mm.
As a precipitation nucleus of MnS, by uniformly dispersing and precipitating fine MnS having a diameter of 3 μm or less, MnS does not become a starting point of fatigue fracture, and does not become a propagation path of fatigue cracks. It was discovered that it is possible to develop a free-cutting thick plate with excellent chip disposability.

As a method for uniformly dispersing and precipitating fine MnS with fine AlN as a nucleus, 1
Cooling rate in the temperature range from 100 ℃ to 800 ℃ is 5 ℃ / hr or more, 10
It was newly found that cooling was performed at ℃ / min or less.

In the case of this cooling rate, fine AlN precipitates of less than 1 μm are generated and become MnS precipitation nuclei that precipitate at 950 ° C to 800 ° C.

In addition, it was confirmed that fine AlN of less than 1 μm precipitates if the cooling rate is 5 ° C./hr to 10 ° C./min even when the thickness reduction process is performed in the high temperature range (1100 to 1000 ° C.) of the above temperature range. Was done.

MnS with this fine AlN as a core is unlikely to cause coarsening due to false coarsening even after reheating during rolling.
The distribution form of can be maintained. When the cooling rate exceeds 10 ° C / min, the precipitation of AlN is remarkably suppressed, so that MnS whose core is AlN does not precipitate. On the other hand, in the case of less than 5 ° C./hr, huge massive AlN having a size of 10 μm or more is deposited. This huge precipitate does not match well with MnS, so M
Even if nS is precipitated, it is reheated to cause coarsening of particles and deteriorate fatigue characteristics.

By controlling the cooling rate after solidification cooling or after AlN solution heat treatment, control the precipitation morphology of MnS using fine AlN as precipitation nuclei as described above, and develop a free-cutting thick plate with excellent fatigue properties. Is a successful one.

Next, the reasons for limiting the components in the present invention are as follows.

C has the effect of forming a solid solution in martensite or precipitating as a carbide and increasing the strength of steel. In order to exert this effect, the content of 0.10% or more is required, but if it exceeds 1.00%, the weldability during machine tool production is impaired, so the content was limited to 0.10 to 1.00%.

Si is an element that inexpensively increases the hardness of steel. In order to exert this effect, it is necessary to contain 0.05% or more,
If the content exceeds 1.00%, the toughness decreases, so the content is 0.05 to 1.
Limited to 00%.

Mn has the effect of improving the strength and toughness of steel at low cost,
In addition, it has the effect of forming cutting edge as MnS and improving the cutting control. For this purpose, the content of 0.10% or more is required. On the other hand, if the content exceeds 2.00%, the fatigue properties are deteriorated due to the formation of huge MnS. Therefore, the content is 0.10
Limited to ~ 2.00%.

sol.Al is necessary as a deoxidizing element during the production of the mother steel sheet,
To obtain this effect, 0.001% or more is required. While 0.5
If the content exceeds%, the cleanliness of the steel sheet is significantly reduced. Therefore, the content of sol.Al is limited to 0.001 to 0.5%.

N precipitates as AlN during the production of the mother steel sheet and prevents the γ grains from coarsening to improve the toughness, and at the same time acts as MnS precipitation nuclei to impart free-cutting property. As the amount of N required for this,
0.0020% or more is required. On the other hand, if the content exceeds 0.20%, huge AlN precipitates and the toughness decreases. N, content
It was limited to 0.0020 to 0.20%.

S forms MnS and imparts free-cutting property. 0.0 for this
It is necessary to contain at least 25%. On the other hand, if the content exceeds 0.5%, the cleanliness of the steel sheet is significantly reduced. Therefore, the content of S is limited to 0.025 to 0.50%.

Further, the Cr added in the second invention will be described.

Although it is added to increase the strength of steel, Cr: 0.20% is required as the lower limit of the content in order to secure the desired effect on the above-mentioned action. However, even if the content of Cr exceeds 1.00%, the effect is saturated and it is not economical, so the content is defined as above.

The melting may be carried out by an electric furnace or a converter, and the casting may be carried out by any of the ordinary ingot making method, the continuous casting method and the unidirectional solidification method. Also, an AlN solution heat treatment may be performed before reheating and rolling. Furthermore, when normalizing after rolling, it is preferable to heat to 850 ° C to 980 ° C. On the other hand, for reheating-rolling, a conventionally known method is sufficient.

(Example) FIG. 1 shows the plate thickness, chemical composition, and manufacturing method of the example of the present invention and the comparative example.

The mechanical properties at this time are shown in Table 2.

The steel was melted in a converter.

After casting, each slab with and without slabs is 1000 ℃ ～ 1250
Plate rolling was performed by heating to ℃.

FIG. 1 shows Example Nos. 2, 3, 7, 13, 18 of the present invention and Comparative Examples No. 20, 21.
3 shows the chip length distribution for. Fig. 2 shows the shape of the fatigue test piece (ITCT test piece), and the numbers in the figure are the dimensions (m
m) is shown.

Therefore, it is apparent from Table 2 and the drawings that the products according to the present invention have good chip disposability and good fatigue properties. On the other hand, the comparative example does not satisfy these characteristics at the same time.

(Effect of the invention) As described in detail above, the cooling rate in the temperature range of 1100 ° C to 800 ° C during cooling after solidification or after solution heat treatment is limited,
Since finely precipitated AlN is used as MnS precipitation nuclei to finely and uniformly disperse MnS, a thick plate excellent in both machinability and fatigue properties can be obtained compared to conventional steel.

[Brief description of drawings]

FIG. 1 shows Example Nos. 2, 3, 7, 13, 18 of the present invention, Comparative Example No. 20,
Graph showing the chip length distribution for 21, Fig. 2 a, b,
It is explanatory drawing which shows the test piece shape used for the fatigue test.

Claims (3)

[Claims] 1. By weight ratio, C: 0.10 to 1.00%, Si: 0.05 to 1.00%, Mn: 0.10 to 2.00%, sol.Al:0.001 to 0.50%, N: 0.0020 to 0.20%, S: 0.025 to 0.50 %, The balance Fe and unavoidable impurities are cooled at a cooling rate in the temperature range of 1100 ° C to 800 ° C during solidification cooling or after AlN solution heat treatment at 5 ° C / hr or more and 10 ° C / min or less. A method for producing a free-cutting thick plate having excellent fatigue characteristics, which comprises reheating and rolling. 2. A free-cutting slab having excellent fatigue properties according to claim 1, which is a steel further containing Cr: 0.10 to 1.00% by weight, and the balance being Fe and unavoidable impurities. Production method. 3. The method for producing a free-cutting thick plate having excellent fatigue characteristics according to claim 1, wherein normalizing is performed after rolling.