Low-carbon sulfur-containing free-cutting steel with excellent cuttability

Abstract

A low-carbon resulfurized free-machining steel is excellent in machinability and contains 0.02% to 0.15% by mass of C; 0.004% by mass or less (exclusive of 0%) of Si; 0.6% to 3% by mass of Mn; 0.02% to 0.2% by mass of P; 0.35% to 1% by mass of S; 0.005% by mass or less (exclusive of 0% by mass) of Al; 0.008% to 0.03% by mass of 0; and 0.007% to 0.03% by mass of N, with the remainder being iron and inevitable impurities, in which the ratio [Mn] / [S] of the manganese content [Mn] to the sulfur content [S] is within the range of 3 to 4, and the carbon content [C], the manganese content [Mn] and the nitrogen content [N] satisfy the following Expression (1): 10[C]×[Mn]−0.94+1226[N]2≦1.2, wherein [C], [Mn] and [N] represent the contents on the percent by mass basis of carbon, manganese, and nitrogen, respectively.

Description

TECHNICAL FIELD[0001]The present invention relates to low-carbon resulfurized free-machining steels which are free of harmful lead (Pb) and exhibit good finished surface roughness.BACKGROUND ART[0002]Low-carbon resulfurized free-machining steels are widely used as steels for hydraulic parts of gear box units of automobiles, as well as for small parts, such as screws and printer shafts, which do not require strength so high. When better finished surface roughness and chip disposability are required, lead-sulfur free-machining steels comprising the low-carbon resulfurized free-machining steels combined with lead (Pb) are used.[0003]Lead (Pb) contained in free-machining steels is an element vary effective for improving machinability, but is harmful to the human body. In addition, lead-containing free-machining steels have some problems typically in fume of lead upon ingot making and chip disposability. Accordingly, free-machining steels exhibiting good machinability without adding lead...

AI Technical Summary

Benefits of technology

The present invention provides a low-carbon resulfurized free-machining steel that exhibits good machinability, even being free from lead, and can be produced with good productivity by a continuous casting process while suppressing blow holes. The steel contains specific amounts of carbon, silicon, manganese, phosphorus, sulfur, aluminum, oxygen, and nitrogen, with the ratio of manganese to sulfur falling within a specific range. The steel also exhibits good finished surface roughness and chip disposability. The invention solves the problem of improving the machinability of lead-free steels while maintaining good productivity.

Problems solved by technology

Lead (Pb) contained in free-machining steels is an element vary effective for improving machinability, but is harmful to the human body.

In addition, lead-containing free-machining steels have some problems typically in fume of lead upon ingot making and chip disposability.

These conventional techniques are useful from the viewpoint of improving the machinability of free-machining steels, but none of them can realize machinability in view of finished surface roughness upon forming as good as with lead-containing steels.

The continuous casting process is believed to be disadvantageous for improving the machinability of steels.

However, if a steel is high both in oxygen content and nitrogen content, blow holes caused by carbon monoxide gas (CO gas) and nitrogen gas (N2 gas) are often formed, which may deteriorate the finished surface roughness of the steel.

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