JPH09157791A - Free cutting steel excellent in hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a free cutting steel excellent in hot workability and machinability. SOLUTION: This low carbon sulfur-lead incorporated free cutting steel excellent in high temp. ductility is the one contg., as alloy elements, by weight, 0.02 to 0.15% C, 0.6 to 1.5% Mn, 0.10 to 0.40% S, 0.10 to 0.40% Pb, 0.010 to 0.020% O and 0.007 to 0.020% N and furthermore contg., at need, one or >=two kinds selected from 0.005 to 0.15% Te, 0.02 to 0.30% Se, 0.03 to 0.20% Bi and 0.0030 to 0.0200% Sn, and in which the content of P is limited to <=0.050%, Ti+Zr+Nb to <=0.020%, Si to <=0.005% and Al to <=0.0010%, and the balance substantial Fe, and in which the average area of sulfide inclusions in the surface layer part in the cross section vertical to the rolling direction of the steel is regulated to 5 to 20μm<2> .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a free-cutting steel containing S and Pb, which are additive elements for improving machinability, and more particularly to a low carbon sulfur lead having excellent hot workability and machinability. Regarding composite free-cutting steel.

[0002]

2. Description of the Related Art Low-carbon sulfur-lead composite free-cutting steel is inferior in hot workability to carbon steel for machine structures. In particular, in the latter stage of hot rolling using ingots and continuously cast slabs as raw materials, cracks are likely to occur due to the decrease in ductility of the surface where the temperature has decreased. Therefore, in the production of low-carbon sulfur-lead composite free-cutting steel, it is necessary to remove the surface defects of the billet, which is an intermediate material for hot rolling, with a grinder or the like.

[0003]

[Problems to be Solved by the Invention] It is effective to reduce the manufacturing cost to reduce the billet maintenance work in the intermediate material of the hot rolling of the low carbon sulfur lead free-cutting steel. For this purpose, it is effective to raise the heating temperature before hot rolling as much as possible and keep the temperature at the latter stage of hot rolling high. However, even if the heating temperature before rolling is too high, large-scale cracking occurs at the beginning of rolling. There is a limit because it may occur. The cause of the large-scale cracks at the initial stage of rolling is due to the local melting of grain boundaries, and it is effective to reduce the elements forming the inclusions such as S and Pb that improve the machinability. However, at the same time, machinability deteriorates. Therefore, it is difficult to raise the heating temperature before rolling without degrading the machinability, which is the most important characteristic of low carbon sulfur lead composite free cutting steel,

[0004]

[Means for Solving the Problems] As a result of investigating the relationship between the chemical composition of the low carbon sulfur lead composite free cutting steel and the ductility and machinability at high temperature, the present inventors have found the following.

The rapid deterioration of ductility at a temperature rise of 1250 ° C. or more becomes remarkable due to the increase of P and free-cutting inclusion forming elements such as S, Te, Pb and Bi. Ductility tends to improve as the content of these decreases, but the effect of P is particularly remarkable. Therefore, the temperature at which ductility sharply deteriorates (ductility lowering temperature) rises due to the decrease in the P content, and it becomes possible to raise the heating temperature in hot working. However, the effect is saturated when P is less than 0.050%.
Also, the cutting tool life, which is generally discussed as machinability, is significantly deteriorated by the reduction of free-cutting inclusion forming elements such as S, Pb, Te, and Bi, but the reduction of P has a relatively small effect and is low. It is optimal as a means for improving the hot workability of carbon-sulfur-lead composite free-cutting steel. However, the reduction of P facilitates the formation of the built-up cutting edge and increases the finished surface roughness in turning.

The increase in the finished surface roughness due to the reduction of P can be recovered by increasing the N (nitrogen) content, but due to the increase of N, carbonitrides having a high melting point crystallize during solidification, and The carbonitride of No. 2 acts as a crystallization nucleus of sulfide to promote the eutectic reaction, refine the sulfide, and make the composition easy to elongate in the rolling direction by hot rolling. In particular, in low carbon sulfur free-cutting steel that does not add elements such as Si and Al, which have a high deoxidizing ability, the formation temperature of oxides is lower than the crystallization temperature of sulfides, and it cannot form crystallization nuclei of sulfides. In the presence of carbonitrides with a high formation temperature, carbonitrides act as crystallization nuclei, making sulfides extremely fine and elongated. The size and shape of sulfides have a great influence on the life of the cutting tool, and the larger the sulfides and the closer they are spherical, the greater the effect of extending the tool life. Therefore, if the increase in the finished surface roughness due to the reduction of P is compensated for by the increase of N, the sulfides become fine and elongated and the cutting tool life is deteriorated. The present inventor has earnestly studied a method for effectively and economically preventing the above-mentioned harmful effects caused by the increase of N, and as a result, Ti, Zr, and Nb forming carbonitride having a high generation temperature are reduced as much as possible. By doing so, it was discovered that it is possible to reduce the amount of carbonitride crystallization, minimize the refinement of sulfides, and suppress the deterioration of the cutting tool life to a practically negligible level.

That is, the free-cutting steel according to the present invention has a weight ratio of C: 0.02 to 0.15% and Mn: 0.6 to 1.
5%, S: 0.10 to 0.40%, Pb: 0.10
0.40%, O: 0.010 to 0.020%, N: 0.
007 to 0.020% as a basic component, and further P:
0.050% or less, Ti + Zr + Nb: 0.020% or less, Si: 0.005% or less, Al: 0.0010% or less, the balance consisting essentially of Fe, and perpendicular to the rolling direction of the steel material. A low-carbon sulfur-lead composite free-cutting steel excellent in high-temperature ductility, characterized by having an average area of sulfide-based inclusions in the surface portion of the cross section of 5 to 20 μm ² , and further, if necessary, Te: 0.005-0.15
%, Se: 0.02 to 0.30%, Bi: 0.03 to
It is also possible to use low-carbon free-cutting steel containing one or more selected from 0.20% and Sn: 0.0030 to 0.0200%.

The reasons for limiting the claims of the free-cutting steel of the present invention will be described below.

C: 0.02 to 0.15% C is an element that improves the strength of the steel, but at the same time reduces the ductility. In the region where the content is extremely low, the ductility of the steel decreases to an appropriate degree. It has the effect of improving machinability.
For this purpose, the content must be 0.02% or more by weight, but if the content exceeds 0.15%, the hardness of the work material becomes high and the machinability deteriorates. Therefore, the content of C is set to 0.02 to 0.15%.

Mn: 0.6 to 1.5% Mn is an element that forms a sulfide and its content is 0.6%.
If it is less than 1.5%, the hot workability deteriorates, and if it exceeds 1.5%, the work hardening of the work material becomes remarkable and the machinability deteriorates. Therefore, the Mn content is set to 0.6 to 1.5%.

S: 0.10 to 0.40% S is an element that forms (Mn, Cr) S which is effective in improving machinability, but if the content is less than 0.10%, the effect is small. On the other hand, if it exceeds 0.40%, the hot workability is significantly deteriorated. Therefore, the S content is 0.10 to 0.40%.

Pb: 0.10 to 0.40% Pb is an element effective for improving the machinability, and the content thereof is 0.
If it is less than 10%, the effect is small, and if it exceeds 0.40%, the hot workability and ductility are remarkably deteriorated. Therefore, P
The content of b is 0.10 to 0.40%.

O: 0.010 to 0.020% O (oxygen) is an element that influences the crystallization form of sulfide,
If it is less than 0.010%, the sulfide becomes fine and the life of the carbide cutting tool deteriorates. On the other hand, if it exceeds 0.020%, the amount of oxides increases and the life of the HSS tool deteriorates. Therefore, the content of O is set to 0.010 to 0.020%.

P: 0.050% or less P is an element that reduces the finished surface roughness during cutting, but if it is contained in excess of 0.050%, it may cause hot working of ingots and continuously cast slabs. The hot ductility in the initial stage decreases, and the maximum temperature that can be hot-worked decreases. Therefore, the P content is 0.050% or less. From the viewpoint of hot workability and finished surface roughness, the preferable range is 0.040-
It is 0.050%.

N: 0.007 to 0.020% N is an element having the effect of preventing an increase in the finished surface roughness in cutting that occurs when the P content is reduced, and if less than 0.007%. The effect is small, 0.02
If it exceeds 0%, casting defects such as blowholes are likely to occur. Therefore, the N content is 0.007 to 0.
020%.

Ti + Zr + Nb: 0.020% or less Ti, Zr and Nb combine with N to form carbonitrides during solidification, and these carbonitrides act as crystallization nuclei of sulfides, and eutectic sulfides It promotes the crystallization of and refines sulfides.
The effect is that the total content of Ti, Zr and Nb is 0.
When it exceeds 020%, it becomes remarkable and the life of the cutting tool is deteriorated. Therefore, the total content of Ti, Zr and Nb is set to 0.020% or less.

Si: 0.005% or less Si is a deoxidizing element and affects the crystallization morphology of sulfide by crystallizing an oxide that becomes a crystallization nucleus of sulfide even if an extremely small amount is mixed, This is refined and the cutting tool life is deteriorated. The effect becomes remarkable when it exceeds 0.005%.
Therefore, the Si content is set to 0.005% or less.

Al: 0.0010% or less Al has the same effect as Si, and Si
The effect is greater than in the case of, and if it exceeds 0.0010%, the cutting tool life is deteriorated. Therefore, the Al content is 0.0010% or less.

Te: 0.005 to 0.15% Te is an element that improves machinability and is added as necessary in the second invention, but if it is less than 0.005%, the effect is small, and If it exceeds 15%, the hot workability is impaired. Therefore, the content of Te is set to 0.005 to 0.15%.

Se: 0.02 to 0.30% Se is an element that generally combines with Mn and S to form Mn (S, Se), which has a smaller adverse effect on hot workability than MnS. In the invention of 1), it is added as necessary for the purpose of improving machinability. If it is less than 0.02%, the effect of improving the machinability is small, and if it exceeds 0.30%, the hot workability is significantly impaired and the addition cost becomes high. Therefore, the content of Se is set to 0.02 to 0.30%.

Bi: 0.03 to 0.20% Bi is an element that forms metal inclusions like Pb and improves machinability. Especially, when Pb and Pb are added in combination, the adhesion of the constituent cutting edge in low speed cutting. And has the effect of suppressing the above, and is added as necessary in the second invention. If it is less than 0.03%, its effect is small, and if it exceeds 0.20%, the hot workability is significantly impaired. Therefore, the Bi content is 0.03 to
0.20%.

Sn: 0.0030 to 0.0200% Sn has the effect of suppressing the adhesion of the constituent cutting edges in low speed cutting when added in combination with Pb, and is added as necessary in the second invention. If it is less than 0.0030%, its effect is small, and if it exceeds 0.0200%, the hot workability is significantly impaired. Therefore, the Sn content is 0.0030 to 0.
It is set to 0200%.

Average area of sulfide inclusions: 5 to 20 μm
The size of sulfide inclusions in the cross section of ^two steels changes depending on the crystallization morphology of sulfide and hot working conditions, and its increase is effective in reducing the surface roughness. Especially in the outer circumference turning, the average area of sulfides on the surface layer has a significant effect on the finished surface roughness, and if it is less than 5 μm ² , a good finished surface cannot be obtained, and if it exceeds 20 μm ² , the glossiness of the finished surface deteriorates. Do. Therefore, the average area of sulfide-based inclusions is 5 to 20μ.
m ² .

[0024]

EXAMPLES The present invention will be described below with reference to examples. After manufacturing rolled steel with the chemical composition shown in Table 1, a round bar with a diameter of 10 mm was formed by cold drawing. D1 and C1 are 7 ton ingot cast materials, and the others are all continuous cast materials.

[0025]

[Table 1]

In Table 1, D1 and D2 are free-cutting steels corresponding to claim 1 of the present invention, and D3 to D7 are free-cutting steels of claim 2 of the present invention. Also, C
1 to C4 are comparative steels for the invention steels D1 and D2, and C5 and C6 are comparative steels for the invention steels D4 and D6, respectively. Comparative steels C1 and C2 are generally widely used low carbon sulfur lead composite free-cutting steel JIS-SU.
It is M24L. The comparative steels of C3 to C6 are steel grades prototyped for clarifying the effect of the present invention, and although they contain almost the same major alloying elements and free-cutting inclusion forming element amounts as the corresponding invention steels, N or P, Ti + Zr + N
One or more of the b amount and the average sulfide area is a steel type that deviates from the claims of the present invention. In Table 1, D * is the average area of the sulfide-based inclusions at a position 0.5 mm from the surface in the cross section. In addition, T * is the temperature at which the drawing value drops to one half of the maximum value on the higher temperature side than the temperature at which the highest drawing value is obtained in the high-speed hot tensile test. Furthermore, R * is the ten-point average roughness (Rz) of the finished surface of the part that has been turned under the conditions shown in Table 2, and L * is Table 2
The tool life is the number of machined parts for which the maximum flank wear of the cemented carbide tool is 0.3 mm in the cutting under the conditions shown in.

[0027]

[Table 2]

The invention steels have higher ductility lowering temperatures (T *) than the comparative steels C1 and C2. Further, the finished surface roughness (R *) is equal to or smaller than that of the comparative steels C1 and C2, and the tool life (L *) is equal or long. C3 to C
6 The comparative steels are inferior to the invention steels in ductility lowering temperature, finished surface roughness and / or cutting tool life.

When examined in more detail, the invention steel D1 produced by the ingot and the invention steel D2 produced by continuous casting have higher ductility lowering temperatures than the respective comparison steels C1 and C2, and the finished surface roughness and the tool life. Are equivalent. On the other hand, Comparative Steel C3 has a high ductility lowering temperature because the P content falls within the scope of the claims of the present invention, but has a large finished surface roughness because the N content is low. Also,
Comparative steel C4 has a high ductility lowering temperature because the content of P is included in the scope of the claims of the present invention, but Ti, Zr and N
Since the total content of b (Ti + Zr ÷ Nb) is higher than the claimed range of the present invention, the sulfide is small (that is, D * is small), resulting in a short tool life, and because N does not act effectively, the finished surface roughness is small. It is also big. Invention Steels D3 to D6
Is a steel grade corresponding to claim 2 of the present invention. One of Te, Se, Bi and Sn is added to the steel grade D2 corresponding to the first claim.
Steels or steels with more than two types of machinability added, but both have higher ductility lowering temperature, smaller finished surface roughness, and longer tool life than Comparative Steel C2. On the other hand, the comparative steel C5 corresponding to the invention steel D4 has P and T
Since i + Zr + Nb is higher than the claimed range of the present invention, the ductility lowering temperature is low and the tool life is short. Further, the comparative steel C6 corresponding to the invention steel D6 has a N content lower than the claimed range of the present invention and a P content higher than the claimed range of the present invention.
Compared to invention steel D6, the ductility lowering temperature is low and the finished surface roughness is large.

In other words, the invention steel is SUM24 in terms of ductility lowering temperature, finish surface roughness and cutting tool life.
It is superior to L, and it is clear that all of the claims of N, P, Ti + Zr + Nb, and the average area of sulfide of the present invention must be satisfied in order to satisfy such characteristics.

[0031]

As described above, according to the present invention, P in the low carbon sulfur lead composite free-cutting steel is reduced, and the deterioration of the cut surface roughness due to this is compensated for by increasing N, and a higher N By increasing Ti, Zr, and Nb to prevent the refinement of sulfides due to aging, the heating temperature for hot working is increased compared to conventional steel types while maintaining the finished surface roughness and cutting tool life. Is possible. As a result, in the production of sulfur-lead composite free-cutting steel, it becomes possible to omit reheating in the intermediate stage of hot working, simplify the billet maintenance process, and improve the surface quality in the hot working state. The advantages of are extremely large.

Claims (2)

[Claims] 1. C: 0.02 to 0.15% by weight, M
n: 0.6 to 1.5%, S: 0.10 to 0.40%, P
b: 0.10 to 0.40%, O: 0.010 to 0.02
0%, N: 0.007 to 0.020% as a basic component,
Furthermore, P: 0.050% or less, Ti + Zr + Nb:
0.020% or less, Si: 0.005% or less, Al:
The average area of sulfide-based inclusions in the surface layer portion of the cross section perpendicular to the rolling direction of the steel material is 5 to 20 μm ² by limiting the content to 0.0010% or less. Low carbon sulfur lead composite free-cutting steel with excellent high temperature ductility. 2. C: 0.02 to 0.15% by weight, M
n: 0.6 to 1.5%, S: 0.10 to 0.40%, P
b: 0.10 to 0.40%, O: 0.010 to 0.02
0%, N: 0.007 to 0.020% as a basic component,
Furthermore, Te: 0.005 to 0.15%, Se: 0.0
2 to 0.30%, Bi: 0.03 to 0.20%, Sn:
Contains one or more selected from 0.0030 to 0.0200%, P: 0.050% or less, Ti +
Zr + Nb: 0.020% or less, Si: 0.005% or less, Al: 0.0010% or less, the balance consisting essentially of Fe, and sulfurization in the surface layer portion of the cross section perpendicular to the rolling direction of the steel material. The average area of material inclusions is 5 to 20 μm
Low carbon sulfur lead composite free cutting steels with excellent high temperature ductility, which is a ^2.