JP3385145B2 - Low carbon free-cutting steel with excellent finished surface roughness and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material which is cut and used as various automobile parts and industrial machine parts, and more particularly to a low carbon free-cutting steel excellent in finished surface roughness.

[0002]

2. Description of the Related Art A method for improving the machinability of steel is roughly classified into a method of adding a free-cutting element and a heat treatment. Regarding free-cutting elements, free-cutting steel to which elements such as sulfur, lead, tellurium and calcium are added alone or in combination is used in large quantities in various automobile parts and industrial machine parts.

[0003] Especially in the case of low carbon free cutting steel, AI
SI12L14 steel, SUM22L, SUM23L, SU
M24L and the like are standardized in AISI and JIS.
Since manganese sulfide and lead particles are added together, they are widely used because of their excellent tool life and chip disposability. However, manganese sulfide is detrimental to mechanical properties and oil leakage resistance, and it also leaves a problem with the surface roughness during plunge cutting.Avoid these drawbacks as long as sulfur, which is a free-cutting element, is used. I can't.

Next, regarding the heat treatment method, there is a method of reducing the hardness by converting pearlite into a spherical cementite + ferrite structure by spheroidizing annealing. Carbon content is 0.5
In the case of a high carbon steel exceeding 10%, spheroidizing annealing is effective for improving machinability and is used for tool steel and the like. However, in the case of low carbon steel, for example, as described in “Heat Treatment of Steel” (edited by the Iron and Steel Institute of Japan, 1988, P46, published by Maruzen Co., Ltd.), spheroidizing treatment causes excessive softening, cementite, and the like. Is not utilized for impairing the finished surface roughness, because of the non-uniform distribution. It is expected to improve this point and develop a low-carbon free-cutting steel that does not use sulfur.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to develop a low carbon free-cutting steel excellent in finished surface roughness by a heat treatment method without using sulfur. Therefore, the problem to be solved is to develop knowledge and methods on the chemical composition of steel and a new heat treatment method for uniformly dispersing fine spherical cementite in a ferrite matrix.

[0006]

The present invention has been made to solve the problems, and its gist is as follows. (1)% by mass , C: 0.05 to 0.40%, Si:
0.15-1.5%, Mn: 0.3-0.8%, P:
0.03 to 0.08%, S ≦ 0.025 %, Al: 0.
01-0.04%, Pb: 0.01-0.05%, balance Fe and unavoidable impurities, and its microstructure is composed of a ferrite matrix in which fine spherical cementite is uniformly dispersed. Low carbon free cutting steel with excellent finished surface roughness. (2) C: 0.05 to 0.40% by mass %, Si:
0.15-1.5%, Mn: 0.3-0.8%, P:
0.03 to 0.08%, S ≦ 0.025 %, Al: 0.
A steel material containing 01 to 0.04% and Pb: 0.01 to 0.05% and the balance Fe and unavoidable impurities was cooled by a water cooling device installed on the rear surface of the hot rolling line. _Is A _r3 point or more, the cooling end temperature is M _s
Below the point, after cooling at an average cooling rate of 30 to 100 ° C./s, further natural cooling is performed, and then a heating temperature of 670 to 72 ° C.
By annealing at 0 ° C , the fine spherical cementite is made uniform.
One dispersed surface finish of excellent production method of low-carbon free cutting steel characterized by the this <br/> to microstructure comprising a ferrite matrix.

That is, the inventors of the present invention have made various investigations and have first clarified the cause of the poor finish surface roughness of the presently proposed spheroidized and annealed carbon steel. In order to improve the finished surface roughness, a one-phase structure or a structure equivalent thereto is the best, and in the case of a two-phase structure of ferrite + pearlite, fracture occurs at the interface due to the difference in the degree of deformation of each phase It was found that the finished surface roughness was poor.
If the carbide is finely and uniformly dispersed, the structure approaches a one-phase structure, and fracture is unlikely to occur between the ferrite and the fine cementite, so that the constituent cutting edge is unlikely to be formed.

However, in the current spheroidized annealing structure of low carbon steel, cementite in the pearlite part is melted and spheroidized,
It is made of ferrite + finely divided cementite. In the cutting stress field, the community of fine cementite behaves similarly to the pearlite structure, and fractures at the interface between the ferrite and the community to form the constituent cutting edge. Therefore, the finished surface roughness deteriorates.

[0009] Cementite was finely dispersed and uniformly dispersed.
In order to achieve this, new knowledge about manufacturing methods is needed. Starting
The authors decided that the steel material immediately after hot rolling was
The cooling start temperature is set to A by the water cooling device installed on the rear surface.
_r3Above the point, the cooling end temperature is M _SBelow the point, the average cooling rate
After cooling at 30 to 100 ° C./s, further natural cooling,
Then, spheroidizing annealing is performed at a heating temperature of 650 ° C to 720 ° C.
Found that the cementite is finely and uniformly dispersed by
It was In addition to the martensitic transformation strain,
The quenching adds rolling strain remaining in the martensite.
Therefore, the total amount of strain contained in martensite increases,
As a result, it is considered that the number of locations where cementite has occurred has increased.
To be

During the spheroidizing annealing, if the cementite is graphitized, the graphite causes formation of the constituent cutting edge. It was newly found that Pb is effective as a method for preventing this. The present invention is made by applying this knowledge,
We succeeded in developing a low carbon free-cutting steel with excellent finished surface roughness.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the present invention will be described below. Regarding the item (1), C has a lower limit of 0.05% in order to secure strength. If the hardness becomes too large, the tool life will decrease, so the upper limit was made 0.40%.

Si is an essential element because it has a small bonding force with carbon atoms in steel and is one of the powerful elements that accelerates the decomposition of pearlite plate-like cementite. The lower limit must be 0.15% to promote the decomposition of pearlite plate-like cementite. If it exceeds 1.5%, the hardness increases due to an increase in the content of Si dissolved in the ferrite phase, and the tool life deteriorates. Therefore, the upper limit value is limited to 1.5%.

The amount of Mn is required to be the amount required to fix and disperse sulfur in the steel as MnS and the amount required to form a solid solution in the matrix to secure strength, and the lower limit value is It is 0.3%. If the amount of Mn increases, the decomposition of pearlite plate-like cementite is significantly impaired, so the upper limit was made 0.8%. P exists as a phosphorus compound precipitated at grain boundaries in steel, and as a solid solution in ferrite. Since it has the effect of reducing the shear strength of steel and improving the finished surface roughness, its lower limit is 0.03%, and if the hardness of the base material is too large, the tool life will decrease,
The upper limit is 0.08%.

S is present as MnS inclusions in combination with Mn. When the amount of MnS inclusions in steel increases, the tool and Mn
The chances of contact with S inclusions increase, and the MnS inclusions plastically deform on the tool rake face to form a film. As a result, the chance of contact between the ferrite and the tool is reduced, so that the adhesion is suppressed and the quality of the finished surface is improved. From the viewpoint of machinability, it is better to add a large amount, but mechanical properties,
Since the oil leakage resistance is impaired, the upper limit was made 0.025 %.

Al removes oxygen in the steel as oxide inclusions. Further, in order to adjust the grain size, 0.01
% Or more must be added. Deoxidizing effect is 0.040%
Therefore, the upper limit is set to 0.040%. Pb is
Although the mechanism is not clear, addition of a small amount has an effect of suppressing graphitization. 0.01 to prevent the precipitation of graphite
% Or more is required. If it exceeds 0.05%, the decomposition of pearlite plate-like cementite will be delayed, so the upper limit is set to 0.
It was set to 05%.

The reasons for limiting the chemical components and manufacturing conditions of the invention of item (2) will be described. C, Si, Mn, P, S, A
l and Pb are exactly the same as in (1). Next, manufacturing conditions will be described. The steel material immediately after the hot finish rolling is forcibly cooled by the water cooling device installed on the extension line of the hot rolling line in order to superimpose the rolling strain generated by the hot rolling on the martensitic transformation strain. As a result, the number of sites for producing cementite is increased, and the cementite is refined. In order to obtain the martensitic structure, the cooling start temperature measured on the steel surface must be A _r3 point or higher and the cooling end temperature must be M _s point or lower. The lower limit of the average cooling rate is set to 30 ° C./s in order to obtain the martensitic transformation structure and to make the processing strain remain to facilitate cementite formation, and the upper limit is set to 100 ° C./s. This is because the amount of martensitic transformation does not increase even if it is rapidly cooled. Lower limit of annealing temperature is 670 ℃
The reason is that the cementitization time is shortened. The upper limit is limited to 720 ° C. to prevent the growth of cementite.

By adopting the method of cooling with water immediately after rolling, the heat energy of the red hot steel material after hot rolling can be utilized for quenching and reheating is not required, and as a result, there is an advantage that the heat treatment cost can be reduced. Next, the effects of the present invention will be more specifically shown by examples.

[0018]

EXAMPLES As an example of the steel bar of the present invention, Table 1 shows chemical components and production conditions. The diameter of the steel bar used in this test is 2
It is 5 mm. The steel bar was cooled by uniformly spraying 0.5 ton / m ² of cooling water per unit area on the entire surface of the steel bar with a cooling device installed on the extension of the hot rolling line. The cooling device is a pipe having a length of 20 m and having holes for supplying a large number of cooling water on the circumference, and the steel bar is cooled when passing through the center line of the pipe. The temperature was measured on the surface of the steel material with an optical thermometer. The average cooling rate was obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. Then, it was naturally cooled, and spherical cementite was finely precipitated in an off-line annealing furnace.

[0019]

[Table 1]

The cutting test method is as follows. Cutting method: Plunge cutting, Cutting conditions: V = 80 m / min, f =
0.03mm / rev, Tool shape: Cut-off type, Tool material type:
Cemented carbide tool coated with TiCN, lubricating oil: 71 / min
Is. The finished surface roughness was measured by a stylus type roughness meter.
The tool life was defined as the cutting time until the wear width of the front flank reaches 200 μm. The tool life index was expressed as a ratio to the tool life of Comparative Steel J.

In Table 2, presence or absence of community-like cementite,
The finish surface roughness and the tool life measurement result are shown. It has been proved that the finished surface roughness of the present invention having no lumpy cementite is remarkably excellent.

[0022]

[Table 2]

[0023]

As is clear from the above examples, according to the present invention, it is possible to provide a low carbon free-cutting steel having an excellent finished surface roughness, and the industrial effect is extremely remarkable. There is something.

Continuation of the front page (56) Reference JP-A-3-264648 (JP, A) JP-A-4-116125 (JP, A) JP-A-6-279849 (JP, A) JP-A-6-299240 (JP , A) JP-A-6-212351 (JP, A) JP-A-5-279793 (JP, A) JP-A-61-253343 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) C22C 38/00-38/60 C21D 6/00 C21D 8/00-8/10

Claims (2)

(57) [Claims] 1. A mass%, C: 0.05~0.40%, Si : 0.15~1.5%, Mn: 0.3~0.8%, P: 0.03~0. 08%, S ≦ 0.025 %, Al: 0.01 to 0.04%, Pb: 0.01 to 0.05%, and the balance Fe and inevitable impurities. A low-carbon free-cutting steel with excellent finished surface roughness, which consists of a ferrite body in which spherical cementite is uniformly dispersed. 2. In mass %, C: 0.05 to 0.40%, Si: 0.15 to 1.5%, Mn: 0.3 to 0.8%, P: 0.03 to 0. A steel material containing 08%, S ≦ 0.025 %, Al: 0.01 to 0.04%, Pb: 0.01 to 0.05% and the balance Fe and unavoidable impurities is hot-rolled. With the water cooling device installed on the rear side of the line, the cooling start temperature is _{Ar 3} points or higher, and the cooling end temperature is M _s.
Below the point, after cooling at an average cooling rate of 30 to 100 ° C./s, further natural cooling is performed, and then a heating temperature of 670 to 72 ° C.
By annealing at 0 ° C , the fine spherical cementite is made uniform.
One dispersed surface finish of excellent production method of low-carbon free cutting steel characterized by the this <br/> to microstructure comprising a ferrite matrix.