JPH09291341A - Austenitic free cutting stainless steel for cold working - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an austenitic stainless steel combining excellent machinability and cold workability such as cold heading without deteriorating its hot workability and corrosion resistance. SOLUTION: This stainless steel is the one having a compsn. contg., by weight, <=0.050% C, <=1.00% Si, <=2.00% Mn, <=0.010% S, 9.00 to 13.00% Ni, 17.00 to 20.00% Cr, 0.05 to 0.30% Pb, <=0.050% N, 0.001 to 0.020% B and <=0.20% Cu, in which the value α expressed by the formula, α=Ni+27×C +23×N+0.1×Mn+0.3×Cu-1.2×(Cr+Mo)-0.5×Si+10 lies in the range of -1 to 0, and furthermore, the value β expressed by the formula, β=551-462×(C+N)-9.2×Si-8.1×Mn-13.7×Cr-29×Ni-18.5×Mo is regulated to <=-30, and the balance Fe with inevitable impurities.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has excellent machinability without deteriorating hot workability and corrosion resistance, and is suitable as a material to be cut after cold working such as cold forging. The present invention relates to an austenitic free-cutting stainless steel for hot working.

[0002]

2. Description of the Related Art Conventionally, austenitic stainless steel has been widely used in various fields because of its excellent corrosion resistance, heat resistance and mechanical properties. Recently, it has been widely used for construction purposes, and is used for parts requiring strength and corrosion resistance such as screws, bolts and anchor bolts.
These parts are formed by cold working a material such as a wire rod and then finishing by cutting. However, stainless steel is generally poor in machinability, and one or more kinds of free-cutting elements such as S, Se, Te, Ca, Pb, and Bi are added to impart free-cutting property. Especially SUS30
Austenitic stainless steels such as No. 4 and the like have a high cold work hardening property, and therefore are not suitable for strong working such as cold heading.
Therefore, for cold working, in order to reduce the work hardenability, a method of adding Cu or further increasing the amount of Ni is used.

However, although free-cutting steel containing S (for example, SUS303) exhibits stable machinability because MnS combined with Mn is dispersed in the steel, on the other hand, the inclusions are expanded by hot forging. However, it causes anisotropy in mechanical properties, which deteriorates the cold forgeability. In addition, MnS also adversely affects the corrosion resistance. The addition of Se and Te is not as great as S,
Similarly, cold forgeability and corrosion resistance are poor. Also for Ca, the expanded oxide deteriorates the cold workability. Further, Pb is a free-cutting element that does not affect mechanical properties and corrosion resistance, but for example, simply adding Pb to an ordinary austenitic stainless steel such as SUS304 has a high cold work hardening property,
It cannot be said that it is suitable for cold heading. On the other hand, steel grades such as SUSXM7 whose work hardenability is moderated by adding Cu are excellent in cold forgeability but poor in machinability.

Various solutions have been taken to date for these problems. For example, JP-A-61-677
No. 60 gazette secures free machinability by adding P and S and Ca as required, and cold forgeability by adding Cu, but P alone is not sufficient to improve machinability. The addition of Ca and Ca deteriorates the cold forgeability as described above. Further, in JP-A-63-18039, Pb and Ca and 0.5
Although Cu is added by up to 5%, there is a problem that the cold forging property is deteriorated by the addition of Ca and the hot workability is deteriorated due to the simultaneous addition of Cu and Pb.

Further, the addition of Se and Ca and Cu disclosed in Japanese Patent Laid-Open No. 2-50937 may cause deterioration of cold forgeability due to Ca and deterioration of corrosion resistance due to Se. As described above, among the free-cutting elements of S, Se, Te, Ca, Pb, and Bi, except Pb and Bi, the cold forgeability is
On the other hand, there is a drawback that corrosion resistance or both deteriorate.
The Pb-containing and Bi-containing steels also have a problem that the hot workability is significantly deteriorated when Cu is added in order to improve the cold forgeability.

[0006]

As described above, it has been difficult in the prior art to satisfy each characteristic. Therefore, the present invention is to provide an austenitic stainless steel having both excellent machinability and cold workability such as cold forging without impairing hot workability and corrosion resistance.

[0007]

The present invention has been made in order to solve the above problems, and the gist of the present invention is, in weight%, C: 0.050% or less, Si:
1.00% or less, Mn: 2.00% or less, S: 0.01
0% or less, Ni: 9.00-13.00%, Cr: 1
7.00 to 20.00%, Pb: 0.05 to 0.30
%, N: 0.050% or less, B: 0.001 to 0.02
0%, Cu: 0.20% or less, and the following formula α
Is in the range of -1 to 0, the value represented by the formula β is -30 or less, and the balance is Fe and inevitable impurities. Free-cutting stainless steel. α = Ni + 27 × C + 23 × N + 0.1 × Mn + 0.3
XCu-1.2x (Cr + Mo) -0.5xSi + 10 [beta] = 551-462x (C + N) -9.2xSi-8.
1 x Mn-13.7 x Cr-29 x Ni-18.5 x M
o

Hereinafter, the present invention will be described in detail. As a result of various investigations, the present inventors have found that the following measures are effective in order to have excellent machinability and cold forging without deterioration of hot workability and corrosion resistance. . That is, the free-cutting property was imparted by adding only Pb, which is a free-cutting element that does not affect the corrosion resistance and mechanical properties. The improvement of cold forging property suppresses the generation of MnS and the like and reduces the occurrence of cracks due to cold working. Therefore, S is limited to 0.010% or less, and the formation of work-induced martensite by cold working. In order to suppress the above and suppress the increase of the deformation resistance, the component balance was adjusted so that the value represented by the formula β would be −30 or less. The deterioration of hot workability due to the addition of Pb is controlled by adding an appropriate amount of B and limiting the value represented by the formula α to the range of -1 to 0 to control the amount of δ-ferrite in the steel remaining during solidification. It was found that embrittlement can be completely suppressed by limiting the Cu content to 0.20% or less to prevent it, and to coexist with Pb and inhibit hot workability.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components of the steel of the present invention are shown below. C: 0.050% or less C is an austenite stabilizing element together with Ni and Mn, but it also forms a solid solution with other elements to increase the strength and deteriorate the corrosion resistance. Especially when strength is required, it is preferable that the amount be small, and in the present invention, if it exceeds 0.050%, cold forging property deteriorates, so the upper limit is made 0.050%. Si: 1.00% or less Si is an element effective as a deoxidizer during steel making, but is also a strong ferrite stabilizing element, and if it exceeds 1.00%, the austenite phase becomes unstable, so the upper limit is set. 1.0
0%.

Mn: 2.00% or less Mn is a very effective element as both a deoxidizing element and an austenitizing element. However, addition of 2.00% or more deteriorates cold forgeability and corrosion resistance. Also deteriorates, so the upper limit was made 2.00%. S: 0.010% or less S is usually added as an element that improves machinability,
In the present invention, the free-cutting property is not required because it is imparted by Pb, and conversely 0.010% or more deteriorates cold forging property and corrosion resistance, so the upper limit was made 0.010%.

Ni: 9.00-13.00% Ni is a main element in austenitic stainless steel and stabilizes the austenitic phase and provides corrosion resistance. SUS304 which is a basic steel of 18-8 stainless steel
Although the amount of Ni was 8.00 to 12.00%, it was set to 9.00 to 13.00% in order to suppress work hardening. Cr: 17.0 to 20.00% Cr is also a basic element in austenitic stainless steel together with Ni. If it is 17.0% or less, the corrosion resistance is poor, and if it is more than 20.00%, the austenite phase becomes unstable in the above Ni range, so that 17.0 to 20.
00%.

Pb: 0.05 to 0.30% Pb exists in the steel in a dispersed state and is melted by the heat generated during cutting without causing anisotropy in mechanical properties, so that it is between the tool and the steel. It is a free-cutting element that acts as a lubricant and also cuts chips to improve machinability. Further, unlike S free-cutting steel, it does not deteriorate corrosion resistance. Although the hot workability is remarkably deteriorated, it is sufficiently improved by adding an appropriate amount of B, limiting Cu and adjusting the component balance as described above. 0.05
% Or less, the effect of improving the machinability is small, and if 0.30% or more, the effect commensurate with the increase of the added amount cannot be obtained, so the content was made 0.05 to 0.30%.

N: 0.050% or less N is an austenite stabilizing element, but like C, it forms a solid solution to increase the strength. If it is 0.050% or more, the cold forging property deteriorates, so the upper limit was made 0.050%. B: 0.001 to 0.020% B is added to improve hot workability in an austenitic high alloy. In particular, it is an effective element for a steel type that contains Pb and is apt to cause grain boundary embrittlement during heat.
If the content is 0.001% or less, the effect is poor, and if the content is 0.020% or more, the compound B is precipitated at the grain boundaries and becomes harmful to the hot workability, so the content was made 0.001 to 0.020%.

Cu: 0.20% or less Cu has an effect of reducing work hardenability in austenitic stainless steel, and is an austenite-stabilizing element, which makes it difficult for martensite to be generated due to cold working and increases deformation resistance. It is a useful element for cold heading, because it is contained. However, when it coexists with Pb, there is a drawback that the hot workability is significantly deteriorated. However, when the content is 0.20% or less, the deterioration of the hot workability due to Cu does not occur even with Pb-added steel, and this does not pose a problem, so the content was limited to 0.20% or less.

Α = Ni + 27 × C + 23 × N + 0.1 ×
Mn + 0.3 × Cu-1.2 × (Cr + Mo) -0.5
XSi + 10: -1 to 0 The value calculated by the formula α is a factor that determines the amount of δ-ferrite in austenite. When the value of the expression α is lower than −1, the amount of δ-ferrite becomes too large, and cracks are likely to occur at the interface with the austenite phase during hot working. On the other hand, when the value of the expression α exceeds 0, the austenite phase is almost formed, so that impurities such as P, which are easily segregated in the δ-ferrite phase, segregate at the austenite grain boundaries, deteriorating the hot workability. Therefore, the value of the expression α is limited to −1 to 0.

Β = 551-462 × (C + N) -9.2
XSi-8.1xMn-13.7xCr-29xNi-
18.5 × Mo: −30 or less The value calculated by the formula β is an index showing the stability of austenite, and the larger this value, the more easily the work-induced martensite is generated due to the cold workability. The value of expression β is -3
When it is 0 or more, the martensite generated in cold working causes an increase in deformation resistance, which deteriorates cold workability. Therefore, the value of the expression β is limited to −30 or less. Also, originally β
The lower limit of the value should be set, but in the case of the present invention, it is better that the amount of processing-induced martensite is smaller, and there is no adverse effect due to the smaller value of the expression β, so the lower limit is not specified.

[0017]

EXAMPLES Table 1 shows the chemical composition of the example steels of the present invention and the comparative steels together with the values calculated by the formulas α and β. Ingots of these steels were melted in a high frequency induction melting furnace of 100 kg and subjected to the following tests.

[0018]

[Table 1]

(1) Machinability Table 2 shows the results of the drill piercing test using a high speed steel drill. In the test, forging the ingot to φ20 mm and then subjecting it to solution heat treatment to give a test material, and the time required to perforate a depth of 10 mm at a constant load and a constant peripheral speed was measured. Further, the machinability index in the table indicates that the drilling time of SUS304 is 100
Was evaluated. The time required for drilling is approximately Pb
And S content, and the perforation time becomes shorter as the content increases. The steel of the present invention has an extremely low S because it does not adversely affect the cold workability, but due to the effect of Pb, SUS
Perforation can be done in about 50% of 304.

[0020]

[Table 2]

(2) Cold forgeability Table 2 shows the rolling test results. Test the ingot by φ20m
m was subjected to solution heat treatment after forging, to prepare a test piece as a test material, and a constrained compression test was performed with a compression tester at a constant speed. Cold forgeability was evaluated by deformation resistance and critical upsetting rate. The deformation resistance was evaluated by the value when 50% compression was applied. Also, the critical upset rate is given by the following formula. (1−H / Ho) × 100 (%) where Ho: height of the test piece before compression H: height of the test piece when cracking occurs during constrained compression Formula β is the stability of the austenite phase. This value is an index indicating that the larger this value, the more unstable the austenite phase becomes, and the more easily the work-induced martensite is generated by cold working. Therefore, when the value represented by the expression β increases, the deformation resistance increases accordingly. The critical upset ratio is apt to be cracked at the interface with the ground when inclusions such as MnS that are stretched hot are present, and deteriorated when S exceeds 0.010%.

(3) Hot workability Table 2 shows the results of the hot workability test. In the test, a high-temperature high-speed tensile test was conducted, and the hot workability was evaluated by the reduction value at 1150 ° C. The hot workability is improved by adding B in an appropriate amount. However, when Cu exceeds 0.20%, the aperture value is lowered, and even if the value represented by the expression α exceeds 0, it deteriorates.

[0023]

As described above, the austenitic free-cutting stainless steel for cold working according to the present invention is a stainless steel exhibiting good machinability and cold forgeability without impairing hot workability and corrosion resistance. Steel is most suitable for materials that are formed by cutting after cold working such as cold heading.

Claims (1)

[Claims] 1. By weight%, C: 0.050% or less, Si: 1.00% or less, Mn: 2.00% or less, S: 0.010% or less, Ni: 9.00-13.00. %, Cr: 17.00 to 20.00%, Pb: 0.05 to 0.30%, N: 0.050% or less, B: 0.001 to 0.020%, Cu: 0.20% or less And the value represented by the following formula α is in the range of -1 to 0, the value represented by the formula β is -30 or less, and the balance is Fe and inevitable impurities. Austenitic free-cutting stainless steel for cold working. α = Ni + 27 × C + 23 × N + 0.1 × Mn + 0.3
XCu-1.2x (Cr + Mo) -0.5xSi + 10 [beta] = 551-462x (C + N) -9.2xSi-8.
1 x Mn-13.7 x Cr-29 x Ni-18.5 x M
o