JPS59205453A - Free cutting steel and preparation thereof - Google Patents

Abstract

PURPOSE:A free cutting steel secured in good cut property by making sulfide impurities macro-spherical, obtained by compositely adding Te, Pb and Bi to S while reducing Al in oxide impurities by lowering Al-content. CONSTITUTION:A steel ingot containing 0.2% or less C, 0.2% or less Si, 2.0% or less Mn, 10% or less P, 0.02% or less N, 0.002% or less Al, 0.4-0.50% S, 0.02-0.50% Te, 0.01-0.40% Pb and 0.01-0.40% Bi so as to adjust Te+Pb+Bi to 0.20% or more and further containing 0.0040-0.030% O and comprising the remainder of substantially Fe is prepared. In the next step, this steel is melted and continuously cast to obtain free cutting steel with high quality wherein MnS impurities with a long diameter of 5mum or more, a short diameter of 2mum or more and a long diameter/short diameter ratio of 5 or less occupies 50% or more of total MnS impurities and the average Al2O3-content in oxide impurities is 15% or less.

Description

[Detailed description of the invention] The present invention relates to an improved free-cutting steel and a method for manufacturing the same.

In the production of low-carbon, sulfur free-cutting steel known as "ultra-super free-cutting steel," the conventional practice has been to increase the oxygen content so that the sulfide shape becomes as spherical as possible.
However, a high amount of oxygen means the presence of a large amount of oxide-based inclusions, which increases defects such as scratches in the steel material, leading to problems such as reduced strength and poor appearance.

When the amount of oxygen is reduced using a deoxidizing material, sulfide-based inclusions are expanded and the rigidity is reduced.

The present inventors have conducted extensive research on low carbon sulfur free-cutting steel J5 with the aim of making sulfide inclusions into giant spheres to ensure good rigidity and to provide a product free of defects. As a result, if Te1Pb and B1 are added in combination to S, and the AI content is lowered to reduce the amount of A1 in the oxide inclusions, machinability is good even within a certain range of oxygen content, and there are no defects. The present invention was achieved by discovering that it is possible to obtain steel with a small amount of carbon.

The free-cutting steel of the present invention has C: 0.2% or less, si: 0.2
% or less and Mn: 2.0% or less, P: 0
．． 10% or less, N: 0.0296 or less, A1: 0
．． 002% or less r at T, further Ics: 0.04~0.
50%, Te: 0.002-0.50%, PI):
0.01-0.40%tJJ: Bi: 0.01
~0.40%, Te +Pb +Bi: 0.2
Contained in an amount of 0% or more, o:o, 0040-0.
030%, the remainder is essentially Fe, the major axis is 5μ or more, the minor axis is 2μ or more, and the ratio of major axis / minor axis is 5 or less. 50% or more, and the content of AI 203 in the oxide inclusions is 15% or less on average.

The reasons for limiting the composition of each alloying element are as follows.

C: 0.2% or less C not only gives appropriate hardness to the material, but also acts as a deoxidizing element. In this type of free-cutting steel, the upper limit is 0.2% in order to obtain high stiffness. The preferred range is 0.05 to 0.1%.

Si: 0.2% or less Si is a deoxidizing agent, but if its amount exceeds 0.2%, the effect of adding 5k-Te + Pb + Bi in combination is diminished.

Mn: 2.0% or less The lower the Mn content, the better for machinability, but if it is desired to improve hot workability, it may be added up to 2.0%.

P: 0.10% or less P has a positive effect on machinability, but if present in a large amount, it hardens the matrix and reduces hot workability, so it should be within the above limit.

N:<), 02% or less Since the matrix is hardened in the same way as P, the above limit should not be exceeded.

Al: 0.002% or less As mentioned above, one of the characteristics of the steel of the present invention is to suppress the amount of AI to a very small amount. The amount of AI 203 in the physical inclusions is reduced as much as possible within the limits described below.

S: 0.04 to 0.50% This element is central to improving machinability, and 0.04% or more is required. The upper limit was set at 0.5% due to the influence on hot workability.

Te: 0.002~0.50%, Pb: 0.01~
0.40%, Bi: 0.01-0.40%, Te +P
b + Bi: 0.20% or more These elements form low-melting inclusions and override the function of spheroidizing sulfide-based inclusions, but their effect becomes more pronounced when combined.

For this purpose, the content must be at least the above-mentioned lower limit for each and at least 0.20% as a whole. As with S, the upper limit was determined based on hot workability.

Regarding the shape and size of the sulfide-based inclusions, the closer to a spherical shape and the larger the size, the better. Among all sulfide-based inclusions, the proportion (volume fraction) of giant spherical sulfides with a major axis of 5 μ or more, a minor axis of 2 μ or more, and a ratio of major axis / minor axis of 5 or less is 50
% or more, the stiffness improvement effect reaches a satisfactory level.

Oxide-based inclusions are generally Mn01Si02 and F
When eO is the main ingredient and the amount of AI 203 increases,
Its high hardness causes significant wear on cutting tools. The AI content should be kept as low as possible within the above-mentioned limits, and its proportion in oxide inclusions should be 15% or less.

Although the above-mentioned method for producing the free-cutting steel of the present invention is arbitrary, high-quality products can also be obtained by the continuous casting method, which has become widely used due to its high productivity. This is one advantage of the steel of the invention.

In general, the continuous casting method has a faster cooling rate than the conventional ingot casting method, so the sulfides in the steel are more likely to become fine.
Although it was difficult to expect an improvement in rigidity, since the sulfide becomes gigantic spheres according to the present invention, a continuous casting method can be adopted.

When manufacturing by the continuous casting method, it is preferable to add the machinability-improving elements S, Te, Pb, and 13i to the molten steel in the tundish in order to obtain a high yield. This has the effect of promoting levitation.

Examples are shown below to demonstrate the effects of the present invention.

Steel having the composition shown in Table 1 was melted in a 70 ton electric arc furnace. The machinability-improving elements were added by the methods shown in the table below.

Δ: Added B in the furnace or ladle into the exposed hot water surface by Nigazol treatment, that is, bubbling with inert gas C: Added in the injection pipe D: Added molten steel in the tundish was cast by the following method, Example 1.2.3 and Comparative Examples A, 81G... Ingot (6,5 tons) Example 4.5 and Comparative Example D... Continuous casting method, blooming rolling, wire rod rolling, drawing and straightening. , a round bar with a diameter of 11 mm.

About this product, sulfide form, AI2 in oxide
Table 2 shows the results of analyzing 03. The morphology of sulfides was determined by analyzing micro samples using an image analyzer, and the morphology of oxides was determined by analyzing them with EPMA. Giant spherical sulfides (as mentioned above, refers to those with a major axis of 5 μ or more, a minor axis of 2 μ or more, and a ratio of major axis / minor axis of 5 or less. As mentioned above, the ratio is also a volume fraction. However, it is known that this corresponds to the area ratio value obtained directly through analysis, so we will directly record the area ratio data.

Table 1 □□□□□□A1□ (present invention) 1 0.06 0.012 1.00 0.0G
5 0.311 0.0010 0.009(A) 2 0.11 0.005 1.25 0.04
4 0.250 0.0007 0.011(A) 3 0.15 0.008 1.14. 0.0
55 0.273 0.0015 0.008(A) 4 0.08 0.152 1.30 0.07
5 0.350 0.0005 0.006(A) 5 0.09 0.010 1.21 0.06
8 0.314 0.008 0.007 (D) (Comparative example) A O,080,0351,100,0G5 0.
305 0.0041 0.008(A) B O,090,0121,080,0720,3
020,02050,008<A) C' 0.10 0.052 1.15 0.
052 0.29G 0.0015 0.005<
A) DO,080,0261,050,0680,3
330,00920,009(A) 〇 -Tamahada -2 Takumi Once i Te +Pb +
[3i0.0152 0.042 0.252 0.
092 0.386(C) (B) (
C) 0.0211 0.015 0.200 0.124
0.339(B) (B) (B) 0,0060 0.053 0.340 0.050
0.44.3(C) (B) (C) 0,009: Multi 0.040 0.280 0.12
0 0.440(D) <D) <8) 0.010! i 0.045 0.295 0.0
86 0.42G (D) (D) (D
) 0, 0350 - 0.250 - 0.250 (B
) 0.0030 - 0.150 0.100
0.250(B) (B) 0.0420 0.030 - 0.050
0.080(C) (C) 0.0380 − -- −
-1-"2"L No Giant spherical sulfide Major axis/minor axis Sulfide in oxide Average particle size Average ratio AI 203 engineering 1 ±LL- (%) (Invention) 1 72 5 3.5 3
．． 02 81 6 3.2
2.13 78 6 3.8
0.94 84 4 2.9
1.55 77 5 3
, 0 '1. 8 (Comparative example) A 24 1.5 6.0 15B
32 1.2 5.9 58G 3
5 1.6 5.3 30 5
0.813 The machinability of each of the 25 samples was evaluated. The test evaluated the machining efficiency in automatic lathe machining, that is, the amount that could be machined within a certain tool life, and the index was determined based on a comparative example steel, which has the lowest machining efficiency. The data are shown in Table 3. Table 3 reveals the excellent machinability of the steels of the invention.

(Present invention) 1 200 2 200 3 220 4 195 5 197 (Comparative example) A 130 B 140 C135 D 100

Claims (3)

[Claims] (1) Contains C: 0.2% or less, Si: 0.2% or less, Mn: 2.0% or less, P: 0° or less, N: 0.02% or less, Al: Q. 002% or less, and further S: 0.04 to 0.50
%, Te: 0.002-0.50%, Pb: Q, 0
1~0.40% and Bi: 0°01~0.40%
, containing Te + Pb + Bi: 0°20% or more, and also contains 0:0.0040 to 0.030%, the remainder substantially consists of Fe, and the major axis is 5μ
As mentioned above, M'nS inclusions with a short axis of 2μ or more and a ratio of major axis/breadth axis of 5 or less account for more than 50% of all MnS inclusions, and the content of AI 203 in the oxide inclusions. is an average of 1
Free-cutting steel characterized by a 5% or less content. (2) Contains C: 0.2% or less, Si: 0.2% or less, and Mn: 2.0% or less, p:o. 10% or less, N: 0.02% or less, AI: 0°002%
or less, and further includes S: 0.04 to 0.50%, Te
: 0.002-0.50%, Pb: 0.01-0.
40% and Bi: 0°01~0.40%, Te +
pb + Bi: Contained at 0°20% or more, o: o, o. By casting molten steel containing 40 to 0.030% Fe with the remainder substantially consisting of Fe using a continuous casting method, the major axis is 5 μ or more, the short axis is 2 μ or more, and the ratio of major axis / minor axis is 5 or less. A free-cutting steel characterized by obtaining a steel in which MnS-based inclusions account for 50% or more of the total MnS-based inclusions, and the content of AI 203 in the oxide-based inclusions is 15% or less on average. Production method. (3) The method for producing free-cutting steel according to claim 2, wherein some or all of S, Te, Pb, and Bi are added to molten steel in a tundish used for continuous casting.