JP2000319753A - Low carbon sulfur base free-cutting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce low carbon sulfur base free-cutting steel having excellent machinability without being incorporated with Pb and particularly excellent in hot workability in the production. SOLUTION: This steel contains, by weight, 0.03 to 0.20% C, <=0.03% Si, 0.5 to 3.0% Mn, 0.02 to 0.40% P, >0.4 to 1.0% S, <0.01% Pb, <=0.005% Al, 0.005 to 0.040% O, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low carbon sulfur free-cutting steel having excellent machinability without containing Pb.

[0002]

2. Description of the Related Art Conventionally, as a material for small parts manufactured by cutting with an automatic lathe, such as soft screws and nipples that do not require much strength, sulfur free-cutting steel containing S added to low carbon steel is known. Was used. In the sulfur free-cutting steel, the machinability is better as the S content is higher. However, because of the deterioration of hot workability due to the addition of S, generally,
The upper limit of the S content was about 0.35% by mass, and at most 0.40% by mass.

[0003] As a free-cutting steel having further excellent machinability, a composite free-cutting steel containing Pb, Te, Ca and the like in addition to S has been developed. In particular, Pb is known as an element that improves the machinability without impairing the material strength of steel.
Low-carbon sulfur + Pb-based free-cutting steels such as 2L to 24L are widely and widely used due to their excellent turning workability. By the way, Pb is required as a harmful substance that inhibits health through its effect on blood production, to reduce the Pb content in air, food, and drinking water.

In the case of Pb-containing steel, when the steel is heated to a high temperature, such as when the steel is melted, heated to a high temperature for hot working, or cut, the Pb in the steel becomes fume. Reduce the Pb content in steel from the standpoint of safety and health, environmental protection, etc., as it may be scattered and require consideration in the treatment of industrial waste such as steel slag and chips. Was required.

[0005]

SUMMARY OF THE INVENTION The present invention provides a low-carbon sulfur-based free-cutting steel which has excellent machinability without containing Pb and which is particularly excellent in hot workability in production. The purpose is to:

[0006]

As a result of various studies to solve the above-mentioned problems, it has been found that by controlling the S content in steel, it is possible to reduce the carbon content of the conventionally used low carbon even without Pb. We have developed a low-carbon sulfur-based free-cutting steel that has a turning property equal to or higher than that of sulfur + Pb-based free-cutting steel. That is, in the low-carbon sulfur-based free-cutting steel of the present invention, C: 0.03 to 0.20%, Si: 0.03% or less, Mn: 0.5 to 3.0%, P: 0.02 to 0.40%, S: more than 0.4 and 1.0% or less, Pb: less than 0.01%, Al: 0.005% or less, O: 0.005 to 0.040% And the balance is made up of Fe and unavoidable impurities.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the content of chemical components in the low-carbon sulfur-based free-cutting steel of the present invention will be described below. C: 0.03 to 0.20% C is added to secure the strength of steel.
%, The strength is low, and the C content is less than 0.03%, the hot workability is poor. Therefore, the lower limit of the C content is 0.03%.
%. However, if it is contained excessively, the hardness of the steel becomes too high and the machinability decreases. Therefore, the upper limit of the C content is set to 0.20%.

[0008] Si: not more than 0.03% Si may be used as a deoxidizing agent. However, if it is contained more than 0.03%, a hard oxide is formed and the machinability is reduced. The upper limit of the content is 0.03%.

Mn: 0.5 to 3.0% Mn is added to combine with S in steel to form a sulfide and prevent red hot embrittlement of the steel due to S. If the Mn content is less than 0.5%, FeS is generated and the hot workability of the steel is deteriorated, and cracks occur in hot working such as hot rolling and hot forging. 0.5%.
If the content of Mn exceeds 3.0%, the hardness of the steel is increased and the machinability is impaired, so the upper limit of the Mn content is set to 3.0%. In addition, it is preferable that the ratio of the Mn content and the S content be 1.5 to 5.0.

P: 0.02 to 0.40% P is added for improving the machinability of steel, especially the surface finish. When the P content is less than 0.02%, the effect of improving machinability is not sufficiently exhibited, so the lower limit of the P content is set to 0.02%. However, if P is contained excessively, the steel becomes brittle, and 0.4%
%, The lower limit of the P content is 0.4%, since the toughness significantly decreases when P is contained in excess of%.

S: more than 0.4 and not more than 1.0% S is a well-known element for improving the machinability of steel. The higher the S content, the better the machinability. It has been known. However, even when added together with Mn, the hot workability decreases as the S content increases, so that the upper limit of the S content has been practically limited to 0.4% or less. In a steel having a chemical composition defined by the present invention, S can be contained up to a content of 1.0% without impairing hot workability. If the S content exceeds 1.0%, the hot workability is significantly reduced.
%.

Pb: less than 0.01% Pb is a well-known element for improving machinability.
However, due to the harmful effects of Pb fume generated when steel is heated to high temperatures, such as melting, hot working, and cutting of Pb-containing steel, and due to consideration for the treatment of industrial waste such as steel slag and chips. Preferably, the content is as low as possible. In the steel of the present invention, the Pb content is less than 0.01% as the lower limit of detection in the general analysis method.

Al: 0.005% or less Al is an element having an extremely high affinity for oxygen, and is often used as a deoxidizing agent during steel melting, but is a hard oxide formed as a deoxidizing product. The steel content impairs the machinability of steel, so that its content is preferably low. The upper limit of the Al content is set to 0.005% as a limit that does not significantly affect the machinability.

O: 0.005 to 0.040% O is an element that greatly affects the form of sulfide formed in steel. That is, if the O content in steel is low,
The sulfides formed in the molten steel are reduced in size, and are further elongated in the axial direction during hot working such as hot rolling and forging to have an elongated shape, thereby reducing the contribution to the machinability of the steel.
The effect on the sulfide morphology is that the O content is 0.1%.
Since it becomes remarkable at less than 005%, the lower limit of the O content is set to 0.1%.
005%. In addition, if O is contained excessively, erosion of the refractory increases during smelting of steel, making smelting difficult.In addition, a large amount of hard oxide eluted from the refractory reduces the machinability of the steel. Since it is damaged, the upper limit of the O content is set to 0.040%.

[0015]

EXAMPLES The steels shown in Table 1 were melted by a high-frequency induction furnace and cast into 150 kg ingots.

[0016]

[Table 1]

D / 4 of the ingot (1/4 of the diameter of the ingot)
A high-temperature, high-speed tensile test specimen having a diameter of 6 mm and a length of 110 mm centered on a position parallel to the height direction of the steel ingot with the position as the center was used. The remaining ingot was forged by hot forging at a forging ratio of 8 to form a round bar having a diameter of 55 mm.
A normalizing treatment of air cooling at 0 ° C. × 60 minutes was performed to obtain a test material for a cutting test.

In order to evaluate the hot workability, a high-temperature and high-speed tensile test was performed on the high-temperature and high-speed tensile test piece. With the distance between the grippers being 80 mm, both ends of the test piece are gripped, the temperature is increased to 1300 ° C. at a heating rate of 100 ° C./sec by direct energization, maintained at 1300 ° C. for 60 seconds, and then 1000 ° C. at a cooling rate of 10 ° C./sec. After cooling at 1000 ° C. for 60 seconds, the test piece was broken by pulling in the axial direction at a pulling speed of 50 mm / sec. The temperature was measured and controlled by a thermocouple spot-welded to the central surface of the test piece. After cooling, the squeezed portion of the broken portion of the test piece was measured and used as an index for evaluating hot workability. Table 1 shows the values of the drawing obtained by the high-temperature high-speed tensile test.

The test material for the cutting test was subjected to a cutting test using an NC lathe under the following cutting conditions. The turning time until the average flank wear width of the lateral flank of the tool became 100 μm was defined as the tool life, and was used as an index for evaluating the machinability of the test material. Table 1 shows the measurement results of the tool life. Tool: Carbide K10 Cutting speed: 150 m / min Feed: 0.1 mm / rev. Depth of cut: 1mm Cutting oil: Oily

From Table 1, it can be seen that Examples of the present invention are Comparative Examples 1
(Drawing equivalent to or more than JIS SUM24L steel) indicates that the hot workability is good. In addition, the examples show that the tool life is equal to or longer than that of Comparative Example 1 including Pb even though the Pb content is below the analysis limit, and that the machinability is excellent.

[0021]

As described above, according to the low-carbon sulfur-based free-cutting steel of the present invention, Pb is not contained, the productivity is not deteriorated, and the same or higher than that of the conventional steel is obtained. A free cutting steel having a tool life can be provided, and the industrial advantage is extremely large.

Claims (1)

[Claims] C: 0.03 to 0.20%, Si: 0.03% or less, Mn: 0.5 to 3.0%, P: 0.02 to 0.40% by mass% S: more than 0.4 to 1.0% or less, Pb: less than 0.01%, Al: 0.005% or less, O: 0.005 to 0.040%, the balance being Fe and unavoidable impurities Low carbon sulfur free-cutting steel characterized by the following.