JP2682945B2 - Graphite free-cutting steel with excellent tool life and finished surface roughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to free-cutting steel containing graphite, sulfur and lead as main free-cutting elements, and more particularly to graphite free-cutting steel excellent in tool life and finished surface roughness.

[0002]

2. Description of the Related Art Graphite is known to exhibit excellent lubricity due to weak interlayer bonding force, and is widely used as a lubricant. For example, graphite in tool steel and cast iron is used as a free-cutting precipitate, and a dispersion obtained by dispersing graphite in a solvent such as water, oil or a solvent is used as a lubricant release agent during hot plastic working.

Free-cutting elements and inclusions for improving the machinability of steel include tellurium, selenium, lead, bismuth and Mn.
S, Ca-based oxides and the like are well known, but graphite is described in No. 3, vol. 30
(1966), 279 and No. 12, vol. 52
(1988), 1285, it has been found that the workability of steel is improved by dispersing graphite in ferrite.
The research was announced for the first time in 6 years. After that, many research reports on the effect of graphite on machinability were made, and regarding practical use, features 53-15450, 53-1
No. 5451, No. 53-46774, No. 54-5367,
It is disclosed in Japanese Laid-Open Patent Publication Nos. 54-11773 and 54-30366.

However, the graphite free-cutting steel based on these findings is the existing free-cutting steel (lead free-cutting steel, sulfur free-cutting steel) in terms of tool life, which is the most important point in judging the quality of the work system. , Calcium free-cutting steel) and the surface roughness after cutting is extremely poor, so even though it is excellent in mechanical properties such as cold workability and toughness, it is still industrial. It has not been manufactured and sold on a commercial scale.

There is a strong demand from industry for the development of graphite free-cutting steel that surpasses the existing free-cutting steels in terms of tool life and finished surface roughness.

[0006]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention provides a graphite free-cutting steel by newly finding a method for improving the tool life and the surface roughness of the cut finish surface of the graphite free-cutting steel. To do.

[0007]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is that the chemical component value is expressed by weight%.
As an indication, (1) C: 0.20 to 1.0%, Si: 0.1 to 0.5
%, Mn: 0.3 to 1.5%, P: 0.02% or less,
B: 0.001-0.004%, S: 0.04-0.3
5%, N: 0.002-0.01% , balance iron
And it consists unavoidable impurities, and, in the ferrite matrix, the average cross-sectional area: 5 to 30 [mu] m 0.2 to ² graphite
Excellent graphite cutting steel tool life and finished surface roughness, characterized in that it comprises from 0 to 1.0% free.

(2) C: 0.20 to 1.0%, Si:
0.1-0.5%, Mn: 0.3-1.5%, P: 0.
02% or less, B: 0.001 to 0.004%, S: 0.
04-0.35%, containing N: 0.002-0.01%
In addition, 0.05 to 1 or 2 kinds of Pb and Bi are added.
0.35% content, balance is iron and unavoidable impurities
And the average cross-sectional area in the ferrite matrix:
Excellent graphite cutting steel tool life and finished surface roughness, characterized in that it comprises including a graphite 5 to 30 [mu] m ² 0.20 to 1.0%.

(3) C: 0.20 to 1.0%, Si:
0.1-0.5%, Mn: 0.3-1.5%, P: 0.
02% or less, B: 0.001 to 0.004%, S: 0.
04-0.35%, containing N: 0.002-0.01%
In addition, 0.002 of 1 or 2 of Te and Se is added.
~ 0.02%, balance is iron and inevitable impurities
And the average cross-sectional area in the ferrite matrix:
Is an excellent graphite cutting steel tool life and finished surface, characterized in that it comprises including a graphite 5 to 30 [mu] m ² 0.20 to 1.0%.

That is, as a result of various studies by the present inventors, the cause of the tool life of the currently proposed graphite free-cutting steel cannot exceed that of the existing free-cutting steel was first clarified. Graphite free-cutting steel usually has a high Si content of 0.5% or more in order to shorten the annealing time for graphitization.
As a result, the amount of Si dissolved in ferrite increases, the roughness increases, and the tool life does not improve as expected,
As a countermeasure, it was found that reducing the Si content is effective. The annealing time can be shortened by adding a trace amount of B.

The cause of the poor finished surface roughness was clarified next. By graphitizing, the microstructure of steel becomes
The two-phase structure of ferrite and pearlite becomes one ferrite phase in which graphite is dispersed. Therefore, it was found that the cutting tool and ferrite adhere to each other, and further grow to form the constituent cutting edge, so that the finished surface roughness becomes particularly poor. Then, it was found that in order to improve the finished surface roughness of the graphite free-cutting steel, it is sufficient to suppress the adhesion and the formation of the constituent cutting edge.

One way to prevent adhesion is to insert a lubricant at the interface between the ferrite phase and the tool. As a concrete method, Pb,
It has been found that forming a film of MnS or the like is effective. It is also an effective method to suppress the growth after adhesion. It has been found that reducing the size of graphite is effective for that purpose.

As a result, the present invention was accomplished by succeeding in developing a graphite free-cutting steel excellent in tool life and finished surface roughness. The reasons why the claims of the present invention steel are defined as above are shown below. Regarding the item (1), C is an indeterminate element for producing graphite, and its lower limit value is limited to 0.2% from the viewpoint of securing a certain amount of graphite which is effective for the tool life. The upper limit is set to 1% or less in order to prevent a decrease in workability during hot rolling or hot forging and generation of surface scratches.

Although Si has a small bonding force with carbon atoms in steel and is one of the effective elements for promoting graphitization, it forms a solid solution in the ferrite matrix to increase the catalysis and shorten the tool life. Its upper limit should be 0.5%. The lower limit is 0.1% to deoxidize steel
is necessary. The amount of Mn needs to be the amount required to fix and disperse sulfur in steel as MnS and the amount required to form a solid solution in the matrix to secure strength, and the lower limit value is 0.3%. Is. If the amount of Mn increases, the machinability is impaired, so the upper limit was made 2.0%.

Since P exists as a phosphorus compound precipitated at grain boundaries in the steel and P as a solid solution in ferrite, it improves the machinability and at the same time significantly impairs the hot workability, so its upper limit is 0.02. %. B produces BN which shortens the time of graphitization annealing. To obtain a sufficient shortening effect, 0.001% or more must be added. 0.0
If it exceeds 04%, the shortening effect is saturated, so the upper limit was made 0.004%.

S is combined with Mn and exists as MnS inclusions. When the amount of MnS inclusions in steel increases, tool and MnS
The chance of contact with inclusions increases, and the MnS inclusions plastically deform on the tool rake face to form a coating. as a result,
Adhesion is suppressed because the chance of contact between the ferrite and the tool is reduced. In order to suppress the adhesion, the lower limit value of S needs to be 0.04%. The upper limit was 0.35% from the viewpoint of hot workability.

N was added to produce BN.
The amount of N necessary for converting 0.001 to 0.004% B into BN is 0.002 to 0.01%. The amount of graphite and the average cross-sectional area are the most important factors governing tool life and finished surface roughness. Since almost all of C in steel is graphitized,
The amount of graphite is approximately equal to the C content. Therefore, the reason for limiting the amount of graphite is exactly the same as the reason described in the section of C%. When the average cross-sectional area of graphite increases, both the adhesion and the growth of the coagulate increase because the dispersion density of graphite decreases.
The upper limit is limited to 30 μm ² . If the average cross-sectional area of graphite becomes extremely small, the action as a free-cutting element disappears, so its lower limit must be 5 μm ² .

Next, the reason for limiting the components of the item (2) of the present invention will be described. C, Si, Mn, P, B, S, N, and graphite are exactly the same as the item (1). Pb and Bi have the effect of suppressing the adhesion at the interface between the tool and the work material, and are elements that significantly improve the finished surface roughness.
If it is less than 05%, the effect is small, and if it exceeds 0.35%, the hot workability deteriorates, so the upper limit was made 0.35% and the lower limit was made 0.05%.

Lastly, the reason for limiting the components in the item (3) will be described. C, Si, Mn, P, B, S, N and graphite are exactly the same as those in the item (1). Te and Se have the effect of suppressing the adhesion at the interface between the tool and the work material, and are elements that significantly improve the finished surface roughness.
If it is less than 02%, the effect is small, and if it exceeds 0.02%, the hot workability deteriorates, so the upper limit was made 0.02% and the lower limit was made 0.002%.

Reference will now be made to the means for producing the steel of the present invention. The steel of the present invention can be easily produced by ordinary steel making and rolling processes. The graphitization heat treatment is a method of rapidly cooling immediately after rolling and then annealing in a heating furnace. Next, the effects of the present invention will be described more specifically with reference to examples.

[0021]

EXAMPLES Tables 1 and 2 show the chemical composition of the sample steel, the average cross-sectional area of graphite, the cutting performance and the surface condition after hot working.
The average cross-sectional area of graphite was determined by dividing the sum of the cross-sectional areas of graphite contained in the steel material rolling direction cross section of 1 square mm by the total number of graphite. The cross-sectional areas of individual graphites were measured using an optical microscope with a magnification of 200.

[0022]

[Table 1]

[0023]

[Table 2]

The cutting test method is as follows. Cutting method: Turning by lathe, Cutting condition: V = 200m /
min, d = 2.0 mm, f = 0.2 mm / rev, tool shape:
(-6, -6,6,6,15,15,0.8), tool grade: Al ₂ O ₃ -TiCN-TiC cemented carbide tools were CVD coated, lubricating oil: a 71 / min. The tool life was defined as the cutting time until the crater wear depth of the tool rake face became K _T = 50 μm.

The tool life indexes shown in Tables 1 and 2 are expressed as the ratio of the tool life of each test sample steel to the tool life of carbon steel S50C for machine structural use (comparative steel H). The tool life of the graphite free-cutting steel of the present invention is 9 to 1 as compared with that of the comparative steel (H).
4 is extremely excellent. The finished surface roughness is significantly superior to that of the comparative steels containing graphite (L, M),
It is about the same as lead free-cutting steel (K) containing no graphite. The graphite free-cutting steels for comparison (L, M, N) are remarkably inferior to the graphite free-cutting steels of the present invention in terms of finished surface roughness, tool life and surface condition after hot working.

[0026]

As is clear from the above examples, according to the present invention, it is possible to provide a graphite free-cutting steel which is extremely excellent in tool life and surface roughness after cutting. There are some very prominent.

Claims (3)

(57) [Claims] 1. A C: 0.20 to 1.0 in terms of weight%.
%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.5
%, P: 0.02% or less, B: 0.001 to 0.004
%, S: 0.04 to 0.35%, N: 0.002 to 0.
01%, with the balance being iron and unavoidable impurities ,
And, in the ferrite matrix, the average cross-sectional area: 5
Excellent graphite cutting steel tool life and finished surface roughness, characterized in that it comprises including a graphite 30 [mu] m ² 0.20 to 1.0%. 2. C: 0.20 to 1.0 in % by weight.
%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.5
%, P: 0.02% or less, B: 0.001 to 0.004
%, S: 0.04 to 0.35%, N: 0.002 to 0.
Containing 01%, even, Pb, one or two of Bi containing from 0.05 to 0.35%, the balance being iron and inevitable not
Made of pure object, and, in the ferrite matrix, the average cross-sectional area: 5 to 30 [mu] m to ² graphite 0.20 to 1.0%
Excellent graphite cutting steel tool life and finished surface roughness and having free. 3. C: 0.20 to 1.0 in % by weight.
%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.5
%, P: 0.02% or less, B: 0.001 to 0.004
%, S: 0.04 to 0.35%, N: 0.002 to 0.
Containing 01%, even, Te, one or two of Se containing from 0.002 to 0.02%, the balance being iron and unavoidable
Consists of impurities and in the ferrite matrix,
Average cross-sectional area: 0.20 to 1.0 for graphite with 5 to 30 μm ²
Excellent graphite cutting steel of% tool life and finished surface, characterized in that it comprises free.