JPH0425344B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a free-cutting austenitic stainless steel with excellent drawing workability. Austenitic stainless steel has excellent corrosion resistance and heat resistance, so it is used in a wide range of fields as structural members and mechanical parts. Machine parts such as bolts, nuts, screws, shafts, pins, etc. are drawn into the specified shape and then finished into products by cutting. Free-cutting austenitic stainless steel containing free-cutting elements such as S, Se, and Pb is used to improve its properties. However, SUS303, a typical free-cutting austenitic stainless steel, has an S content of about 0.20%.
In steel containing Ni, sulfides spread in the rolling direction tend to become starting points for cracks during cold working, resulting in a problem in that cold workability is significantly reduced. Therefore, in order to prevent the occurrence of cold working cracks, free-cutting austenitic stainless steels have been used in which the machinability has been sacrificed and the S content has been reduced to 0.05 to 0.20%. On the other hand, austenitic stainless steel is a steel that produces martensite during cold working and is easily work hardened. This tendency also applies to austenitic stainless steel.
The smaller the amount of Ni and Cr and the lower the austenite stability, the larger the cracks are, and cold work cracks occur due to work hardening. In particular, when the amount of deformation of the surface layer is large, such as during drawing, only the surface layer undergoes significant work hardening, resulting in a large difference in hardness between it and the inside, and large residual stress exists in the surface layer. According to
Pulling process cracks are likely to occur. In this way, conventional S free-cutting austenitic stainless steels reduce the amount of S, sacrificing machinability, and also suffer from problems such as drawing cracks due to the work hardening properties of austenitic stainless steels themselves. Both the drawing properties and the drawing properties were unsatisfactory. The object of the present invention is to obtain an inexpensive free-cutting austenitic stainless steel having excellent drawing workability and eliminating the drawbacks of conventional steels. The present inventors have investigated the effects of C+N and Ni+2Cu on drawing cracking of S-containing austenitic stainless steel.
, the interaction between C+N and Ni+2Cu, and
As a result of intensive research into the balance of Cr, Ni, Mn, C, N, Cu, etc., we succeeded in developing the steel of the present invention. That is, in 0.30Si-1.6Mn-18Cr-0.25S steel, C+N is 0 to 0.15% and Ni+2Cu is 8.0 to 12.0%.
%, and found the optimum amount of C+N and Ni+2Cu for drawing cracking. As is clear from Figure 1, C+N
As the Ni+2Cu content decreases and the Ni+2Cu content increases, the incidence of drawing cracks decreases. In other words, the amount of C+N is 0.07% or less and the amount of Ni+2Cu is
At 9.50% or higher, no drawing cracks occur. This is due to the decrease in work hardenability due to the decrease in the amount of C+N and the stabilization of the austenite phase due to the increase in the amount of Ni+2Cu, which controls the formation of martensite during drawing and prevents the occurrence of drawing cracks. This was prevented. By finding the minimum content of Ni+Cu necessary to prevent drawing cracks from FIG. 1, it was possible to obtain a free-cutting austenitic stainless steel at a low cost. Figure 2 shows 0.02C−0.30Si−1.6Mn−8.5Ni−18Cr
This study investigated the effect of Cu on drawing cracks in −0.30S−0.10N steel, and also
Figure 3 shows 0.02C−0.30Si−1.6Mn−8.5Ni−18Cr−
This study investigated the influence of S on drawing cracking in 0.5Cu-0.01N steel. As is clear from Figures 2 and 3, by using the above composition, by setting the Cu content to 0.25 to 1.0%, and even when the S content was contained up to 0.40%, pultrusion cracking was prevented. It was discovered that this does not occur. In this way, the steel of the present invention has a C of 0.05% or less and a Si of 1.00.
% or less, Mn2.0% or less, Ni8.0~11.0%, Cr17.0~
19.0%, S0.20~0.40%, N0.04% or less, Cu0.25~
Contains 1.00%, and C+N0.07% or less, Ni+
2Cu9.50% or more, and if necessary Mo0.1
By containing ~0.6%, we succeeded in obtaining an inexpensive austenitic stainless steel with excellent drawing workability and free machinability. The reasons for limiting the composition of the steel of the present invention will be explained below. Like N, C is an element that improves work hardening properties, thereby deteriorating drawing workability, and also forms chromium carbide, reducing corrosion resistance. In the present invention, it is desirable to reduce the upper limit as much as possible. It was set to 0.50% or less. Si is an element necessary for deoxidation during steel manufacturing, but the inclusion of more than necessary impairs drawing workability due to solid solution strengthening and also reduces hot workability, so the upper limit should be 1.00% or less. And so. Mn is used as a deoxidizing agent like Si, and
It is an element that stabilizes the austenite phase. It is also an element that combines with S to form MnS and improves machinability. However, if it is contained in a large amount, hot workability is reduced, so the upper limit was set at 2.00%. Ni is an important element that improves corrosion resistance, lowers the hardness of the matrix, stabilizes the austenite phase, suppresses the austenite → martensitic transformation, reduces work hardenability, and prevents cracking during drawing. , it is necessary to contain 8.0% or more. However, since Ni is an expensive element,
Its content should be kept to the minimum necessary, and the upper limit was set at 11.0%. Cr is the most important element for improving corrosion resistance and must be contained at least 17.0%. However, when its content increases,
The upper limit was set at 19.0% because the ferrite-austenite balance in the high temperature range is disrupted, resulting in a significant decline in hot workability and also in drawing workability. S is extremely effective as an element for improving machinability in austenitic stainless steel, and the machinability improves as the content increases, so it must be contained in an amount of 0.20% or more. However, since S is an element that deteriorates corrosion resistance and also reduces hot workability and cold workability, the upper limit was set at 0.40%. Like C, N is an element that reduces drawing workability by increasing work hardenability, and in the present invention, it is desirable to reduce it as much as possible, and the upper limit is set to 0.04%. Cu reduces work hardenability due to its synergistic effect with Ni, and as is clear from Figure 4, it reduces the difference in hardness between the surface layer and the center during drawing.
It is the most important element in the present invention, which prevents drawing cracks, improves machinability, and also improves corrosion resistance. It must be contained at least 0.25%, and the lower limit was set at 0.25%. However, if it is contained in a large amount, hot workability will be significantly impaired due to red heat brittleness, so the upper limit was set at 1.00%. Mo is an element that increases corrosion resistance and must be contained at 0.10% or more. However, since Mo is an expensive element, the upper limit was set at 0.60%. Regarding C+N, as mentioned above, both C and N are elements that deteriorate drawing workability by increasing work hardening properties, and if the sum of C+N exceeds 0.07%, drawing cracks are likely to occur, so it should be reduced as much as possible. It is desirable that the upper limit be set at 0.07%. Regarding Ni + 2Cu, it is an important element in the present invention, which reduces work hardenability due to the synergistic effect of Ni and Cu and prevents drawing cracks.
If the sum of the two is less than 9.5%, drawing cracks are likely to occur, so the lower limit was set at 9.5%. Next, the characteristics of the steel of the present invention will be clarified in Examples by comparing it with conventional steel and comparative steel. Table 1 shows the chemical composition of these test steels.

[Table] In Table 1, steels A to E are conventional steels such as SUS303,
Steel F is a comparison steel, and steels G to Q are steels of the present invention. Table 2 shows that after heating the test steel in Table 1 at 1050°C for 30 minutes and applying solid solution treatment called W and Q,
This figure shows the incidence of drawing cracks, machinability, and corrosion resistance. Regarding the incidence of cracking during drawing, a 10mmφ sample material was pickled with 5% sulfuric acid and then pickled with nitrofluoric acid.
The figure shows the crack occurrence rate when drawn into an 8.5 mm hexagonal shape (area reduction rate 20.3%), and drawing lubricant was used as the lubricant. Regarding machinability, we prepared five 40 mmφ x 10 mm materials, used a 5φSKH9 straight drill as the cutting tool, and measured the drilling time with a rotation speed of 1140 rpm and a thrust of 30 kg (weight free fall method). It is expressed as an index with steel A as 100. For corrosion resistance, test in boiling 5% sulfuric acid solution.
The figure shows the corrosion weight loss when immersed for 6 hours.

[Table] As is known from Table 2, conventional steel A,
Steel B has a high drawing crack occurrence rate of 56.17% due to its high C+N content of 0.12% and 0.09%, and steel A has a low machinability due to its low S content of 0.15%. It is also low. Furthermore, due to the low Cu content of 0.05% for C steel, the occurrence rate of drawing cracks is as high as 35%;
For steels E and F, the occurrence of drawing cracking was high at 12-50% due to the low Ni+2Cu content. In contrast, the steels G to Q, which are the steels of the present invention, have C
+N is 0.07% or less and Ni+2Cu is 9.5% or more to reduce work hardenability, so that no cracking occurs during drawing. Although the concentration was increased to 0.40%, no drawing cracks caused by S were generated. In addition, regarding machinability, by increasing the S content to 0.20 to 0.40%, compared to conventional steel A steel,
It has 1.3 to 1.6 times the machinability, and also contains a large amount of S at 0.20 to 0.40% for corrosion resistance, but the corrosion loss is 320 to 587 g/
m ² ·h, which was almost the same as that of steel A, which is a conventional steel. As described above, the steel of the present invention is an austenitic stainless steel that has corrosion resistance equivalent to that of conventional steel and also has excellent drawing workability and machinability. As mentioned above, in order to prevent the occurrence of drawing cracks, the steel of the present invention has the content of elements that increase work hardening such as C and N as low as possible, and also contains Ni and Cu.
By containing the necessary minimum amount of alloys that reduce work hardenability, such as alloys, to improve drawing workability at low cost and preventing cracking caused by S, it is possible to add the necessary amount of S, and improve machinability. It is also excellent in terms of features and has high practicality.

[Brief explanation of drawings]

Figure 1 shows the effects of C+N and Ni+ on drawing cracking.
Figure 2 is a diagram showing the relationship between pultrusion cracking occurrence rate and Cu, and Figure 3 is a diagram showing the relationship between pultrusion cracking and S.
Figure 4 is a diagram showing the influence of Ni and Cu on hardness after drawing. In addition, in Figures 1 to 4, the 10 mmφ test material is 8.5 mm hexagonal (area reduction rate).
20.3%).

Claims (1)

[Claims] 1. C0.05% or less, Si 1.00% or less in terms of weight ratio,
Mn2.00% or less, Ni8.0~11.0%, Cr17.0~19.0%,
Contains S0.20~0.40%, N0.04% or less, Cu0.25~1.00%, and C+N0.07% or less, Ni+2Cu9.50%
A free-cutting austenitic stainless steel with excellent drawing workability characterized by the balance being Fe and impurity elements. 2 C0.05% or less, Si1.00% or less in terms of weight ratio,
Mn2.00% or less, Ni8.0~11.0%, Cr17.0~19.0%,
Contains S0.20-0.40%, N0.04% or less, Cu0.25-1.00%, further contains Mo0.1-0.6%, and C+N0.07% or less, Ni+2Cu9.50% or more,
A free-cutting austenitic stainless steel with excellent drawing workability characterized by the balance being Fe and impurity elements.