JPH0711059B2 - Low-carbon free-cutting steel with excellent machinability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a low carbon sulfur free-cutting steel and a sulfur composite free-cutting steel bar which have improved machinability by controlling the pearlite structure in the steel.・ It relates to wire rods.

<Prior art> As a steel material to be used for cutting, free-cutting sulfur steel containing a large amount of low-carbon S and sulfur-free cutting steel containing S and Pb are specified in JIS, etc. Used in large quantities for cutting.

It is well experienced that these steels may have significantly different machinability even though they have the same chemical composition. As a cause for this, many studies have been conducted relating to the amount and morphology of sulfide and Pb particles added as free-cutting inclusions,
Little consideration has been given to the microstructure of these steels.

This type of steel manufactured by a conventional method has a ferrite-pearlite structure, but depending on manufacturing conditions, pearlite exhibits a so-called striped structure in which it is continuous in the rolling direction.

This striped structure is generated along with the P and Mn microsegregation. The conventional free-cutting steel produced by the soul-making method and produced through the high temperature and long temperature soaking process has the P and Mn microsegregation. Is dispersed and disappears, so the structure is such that pearlite is relatively evenly distributed.

Further, the continuous cast material which has recently begun to be applied to this type of steel is more likely to form a pearlite stripe structure because the heating before rolling is performed at a low temperature for a short time as compared with the ingot material. The interval of the striped pearlite at this time varies depending on the size of the steel material, but in the normally rolled one it is 0.001 × (steel material diameter) to 0.002 × (steel material diameter)
mm is the center. In any case, pearlite structure control has not been carried out until now, and various microstructures were formed depending on the manufacturing conditions.

<Problems to be solved by the invention> Low-carbon free-cutting steel is machined by an automatic lathe and becomes a part of equipment, etc., but the ratio of the machining cost to the part manufacturing cost is large, so that the machinability is improved. There is a strong demand. Until now, the amount and morphology of free-cutting inclusions such as sulfides and Pb grains have been investigated, but problems such as surface defects and the formation of huge Pb grains associated with free-cutting inclusions have been identified. Yes, there was a limit.

<Means for Solving Problems> The present invention intends to provide a free-cutting steel of good quality with improved machinability without significantly changing the free-cutting inclusions in the conventional free-cutting steel. Is.

The present inventors have considered that the pearlite structure, which has not been considered so much in the past, has a great influence on the machinability of the low carbon free-cutting steel, and a fringe that is continuous in the rolling direction rather than a structure in which pearlite is uniformly distributed. It has been found that the machinability is improved when the striped pearlite has a large texture and the striped pearlite spacing is large.

The machinability referred to here indicates the quality of the tool wear and the cut surface roughness in a relatively low speed cutting process using an automatic lathe generally used as a cutting process for low carbon free cutting steel.

In such cutting, the component cutting edge is attached to the tool cutting edge, which is repeatedly generated and dropped to progress groove-shaped wear, and this groove-shaped wear is transferred to the work material, so that the cutting surface roughness is reduced. Although deteriorated, when the pearlite is continuously distributed in the rolling direction and the ratio of the stripe interval (R) to the steel material diameter (D) is 0.003 or more, the constituent cutting edge is small and stable,
Therefore, the groove-like wear is small and the cutting surface roughness is good.

Fig. 1 shows a low carbon free-cutting steel SUM23L with a diameter of 25.4 to 32 mm, using an automatic lathe, a tool SKH57, a cutting speed of 78 to 83 m / rev, and a tool feed of 0.
Shows the relationship between the cutting surface roughness and the striped pearlite spacing of steel when 2,000 co-cuts under the conditions of 03mm / rev, total tool feed amount 4.7mm / part, cutting width 10mm, cutting oil, water-insoluble oil 120l / min. The cutting surface roughness was small and good when (striped pearlite spacing (R)) / (steel bar diameter (D)) was 0.003 or more.

Such a striped structure having a large strip interval can be obtained by heating before rolling at a low temperature for a short time to prevent the microsegregation of P and Mn from diffusing and disappearing and by gradually cooling after rolling. .

The striped pearlite continuous in the rolling direction mentioned here means that a straight line is drawn in the widthwise center of each band of the striped pearlite stretched in the rolling direction in the micrograph as shown in the embodiment of FIG. The cumulative length ratio of pearlite on the straight line to the total length of the straight line is defined as the continuity rate of the striped pearlite, and this value indicates 50% or more, and the interval between the striped pearlites continuous in the rolling direction (R) Is the average value of the intervals between the center lines.

Next, the reasons for limiting the constituent features of the present invention will be described.

In the first invention, when C is less than 0.05%, the striped pearlite is insufficient and a stable cutting edge is not generated. If it exceeds 0.15%, the strength is increased and the machinability is reduced.
It was set to 0.05 to 0.15%.

If Mn is less than 0.50%, surface defects are likely to occur during hot rolling, and it is difficult to manufacture. If it exceeds 2.00%, strength increases and machinability deteriorates, so the content was made 0.50 to 2.00%.

If P is less than 0.030%, the ferrite hardness is too low to reduce the cutting surface roughness, but if it exceeds 0.100%, the strength becomes high and the machinability deteriorates, so the content was made 0.030 to 0.100%.

If S is less than 0.08%, the machinability improvement effect is small, and 0.45%
%, The hot brittleness becomes remarkable and manufacturing becomes difficult.
It was set to ~ 0.45%.

The ratio between the striped pearlite spacing (R) and the steel material diameter (D) is
If it is less than 0.003, attachment of the constituent cutting edge becomes remarkable, and tool wear and cutting surface roughness are deteriorated, so the ratio of (R) to (D) is set to 0.003 or more.

In the second invention, the reason for limiting the contents of C, Mn, P, and S and the reason for limiting the ratio of R) to (D) are the same as in the first invention.

Pb is added when it is necessary to further improve the machinability, but its effect is small at less than 0.03%, and if it exceeds 0.35%, large Pb grains tend to be generated and the quality deteriorates.
It was set to ~ 0.35%.

<Example> Steels having the components shown in Table 1 were melted in a converter, and then formed and continuously cast. These steels were heated / rolled and drawn under the conditions shown in the second form. Table 2 shows the continuity of pearlite at this time, the interval (R) between striped pearlites, and the ratio of the interval (R) between the striped pearlites and the steel material diameter (D).

In Comparative Steel Nos. A and B, pearlite has a relatively uniformly distributed structure because it is heated at a high temperature for a long time before billet rolling and before bar steel and wire rod rolling. Also No.C, D
Is rolled after heating at a relatively low temperature for a short period of time, but after rolling No.C is an example in which the wire is cooled in hot water, and No.B is an example in which the steel bar is allowed to cool.In this case, pearlite is striped. It is distributed continuously, but the fringe spacing is relatively small.

On the other hand, in Nos. E, F, and G of the examples of the present invention, the heating before rolling is performed at a low temperature for a short time, and the cooling after rolling is gradually cooled. That is, No. E is an example of cooling in the cover to gradually cool the coil including the end portion, and No. F and G are examples of cooling in the steel bar gradually cooling cover. No. H is an example in which the material is cooled and then put in an annealing furnace.

The structures of these steel materials exhibit a striped pearlite structure with a large stripe interval. After these steel materials were drawn into the dimensions shown in Table 2, 2,000 parts were cut using a 6-axis automatic lathe. Machining conditions are tools: SKH57, cutting speed 78-83m / re
v, tool feed 0.03 mm / rev, cutting oil: water-insoluble oil 120 l / min.
Is. The cutting results are shown in Table 3, and all of the steels of the present invention have smaller tool wear and smaller cutting surface roughness than the comparative steels of the same steel type, and are good.

FIG. 2 shows a micrograph of a continuous striped pearlite structure of Example No. F as a typical example of the steel of the present invention, and shows the interval (R) of the striped pearlite and the continuity rate.

<Effects of the Invention> As described above, according to the steel according to the present invention, tool wear and cutting surface roughness in automatic lathe cutting can be improved, and tool life can be improved. As a result, the tool cost can be reduced, the tool setting time can be shortened, and the surface quality of the parts can be improved. Therefore, the present invention has a great industrial advantage.

[Brief description of drawings]

Fig. 1 shows the tool SKH5 made of low carbon free cutting steel SUM23L with an automatic lathe.
7, cutting speed 83m / rev, tool feed 0.03mm / rev, total tool feed 4.7mm / part, cutting width 10mm, cutting oil, water-insoluble oil 120l / m
Fig. 2 is a diagram showing the relationship between the cutting surface roughness and the striped pearlite spacing of steel when 2,000 co-cuts are made under the in. condition, and Fig. 2 is an embodiment of the present invention.
It is a microscope photograph which shows the striped pearlite metallographic structure of No.F.

Claims (2)

[Claims] 1. C0.05-0.15%, Mn0.50-2.00%, P0.03-
In low carbon free cutting steel containing 0.100% and S0.08 to 0.45%,
A low-carbon free-cutting steel with excellent machinability, characterized in that the ratio of the interval (R) between striped pearlites continuous in the rolling direction and the steel material diameter (D) is 0.003 or more. 2. C0.05-0.15%, Mn0.50-2.00%, P0.03-
In low carbon free-cutting steel containing 0.100%, S0.08 to 0.45% and Pb0.03 to 0.35%, the ratio of the interval (R) between striped pearlites continuous in the rolling direction and the steel material diameter (D) is 0.003 or more. A low carbon free-cutting steel with excellent machinability.