JP2002356745A - Ca CONTAINING FREE CUTTING STAINLESS STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide Ca containing free cutting stainless steel whose machinability can be improved without deteriorating the corrosion resistance thereof. SOLUTION: The Ca containing free cutting stainless steel has a composition containing, by weight, <=0.15% C, 0.10 to 1.00% Si, 0.10 to 2.00% Mn, <=0.045% P, 0.010 to 0.050% S, <=15.0% Ni, 11.0 to 20.0% Cr, <=3.0% Mo, <=0.15% N, 10 to 100 ppm Ca and 20 to 200 ppm O, and the balance Fe with inevitable impurities, and in which Ca2 SiO4 accounts for >=50% of the total piece of oxide inclusions. Preferably, the content of Al included as impurities is controlled to <=0.005%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Ca free-cutting stainless steel which has excellent machinability, can be widely used as structural members, mechanical parts and electronic equipment, and has excellent corrosion resistance.

[0002]

2. Description of the Related Art There are many types of various components such as structural members, mechanical components, and electronic devices which are manufactured by obtaining a predetermined component shape by cutting. Among these parts, those requiring excellent corrosion resistance are conventionally SUS303,
Free-cutting stainless steel such as SUS430F has been used. In these steels, in addition to components for maintaining the basic characteristics of stainless steel, such as Cr and Ni, S, which is a machinability improving element, is added to steel in which only the basic components are added to improve machinability. It is characterized by being improved in comparison.

[0003] However, these free-cutting stainless steels are naturally improved in machinability by adding a machinability improving element, but conversely deteriorate in corrosion resistance. It cannot be applied to parts that require high corrosion resistance, and steel with only basic components added is used, knowing that such parts have poor machinability Was normal.

[0004] In addition, among components of equipment which have conventionally used free-cutting stainless steel, some components require both high dimensional accuracy and excellent corrosion resistance in order to improve their performance (for example, Chemical plant, nuclear related parts)
Has recently increased. Even for such parts, considering the required level of corrosion resistance, conventional free-cutting stainless steel cannot be used, so it is difficult to improve the low tool life and without deteriorating basic performance such as corrosion resistance. It has been strongly desired to provide steel with improved machinability.

[0005] In order to solve such a problem, a study has been made on a method capable of improving machinability without deteriorating the corrosion resistance of conventional stainless steel containing only basic components (not including machinability improving elements). It has been actively pursued, and patent applications have been filed. In particular, Ca has attracted attention as an element capable of improving machinability without significantly lowering corrosion resistance as compared with S. For example, JP-A-4-41651, JP-A-6-41651
JP-A-145908, JP-A-7-150308, JP-A-7-331391, and the like.

JP-A-4-41651 discloses that Mn / S is 3
Hereinafter, by regulating the average particle size of sulfide to 1.2 μm or less, machinability can be improved without deterioration of corrosion resistance even when S, which is a machinability improving element, is added. is there. And as a machinability improving element, S
e, it is described that the addition of Ca is effective,
No particular study has been made on the composition of the Ca oxide-based inclusions described below. Among patents to which Ca is added as a machinability improving element, there are many other patents that have been filed without any consideration on the composition of Ca oxide-based inclusions described below.

Further, JP-A-6-145908, JP-A-7-150308, and JP-A-7-331391 disclose the use of Ca as a machinability improving element to adjust the content of steel in a specific range, thereby reducing the amount of steel contained in steel. Gehrenite 2CaO.Al
₂ O ₃ · SiO ₂ , anorthite CaO · Al ₂ O ₃ · 2
A feature is that oxide inclusions having a low melting point such as SiO ₂ are generated, and machinability is improved by the presence of the inclusions.

[0008] This measure for improving machinability has hitherto been mainly applied to structural steel. That is, by generating low-melting-point oxide-based inclusions in steel, when mainly cut with a cemented carbide, a thick deposit of a complex oxide containing Ca occurs on the tool surface, and the deposit layer is formed. It protects the tool surface and improves the machinability by preventing the tool from being directly rubbed by the chips and preventing the elements in the tool from migrating into the chips due to thermal diffusion and deteriorating the tool performance. is there.

[0009]

However, when this method is applied to stainless steel as it is, there are the following problems. That is, in order to form anorthite or gehlenite in steel, Al must be added inevitably, so that hard Al ₂ O ₃ inclusions are inevitably generated. As a result, the tool is damaged and the machinability is reduced. Further, since stainless steel has a lower thermal conductivity than structural steel, the temperature of the tool edge during cutting becomes higher than that of structural steel. For this reason, most of the low melting point anorsite or gehlenite flows down with the chips without sufficiently forming a thick deposit of the complex oxide containing Ca on the tool surface.

Therefore, the conventionally proposed method of improving machinability by adding Ca cannot sufficiently improve the machinability of stainless steel, and greatly improves the machinability of stainless steel as compared with the above method. There has been a strong demand for the development of new technologies that can be used.

The present invention is directed to a Ca free-cutting stainless steel capable of solving the conventional problems described above and obtaining excellent machinability without deteriorating other required characteristics such as corrosion resistance and hot workability. The purpose is to enable provision.

[0012]

According to the present invention, C: 0.15% or less and Si: 0.10 to 1.00 by weight%.
%, Mn: 0.10 to 2.00%, P: 0.045% or less, S: 0.010 to 0.050%, Ni: 15.0%
Hereinafter, Cr: 11.0 to 20.0%, Mo: 3.0% or less, N: 0.15% or less, Ca: 10 to 100 ppm,
O: contains 20 to 200 ppm, the balance being Fe and unavoidable impurities, and 50% of the total number of oxide-based inclusions
% Or more is Ca ₂ SiO ₄ in the Ca free-cutting stainless steel.

What should be noted in the present invention is that stainless steel having the above-mentioned specific composition is used, and Ca ₂ SiO ₄ which is an oxide-based inclusion is formed in a large amount in the steel so that excellent machinability is obtained. The point is to secure.

FIG. 1 shows an equilibrium diagram of the CaO--Al ₂ O ₃ --SiO ₂ system. The formation of hard Al ₂ O ₃ inclusions damages the tool and degrades machinability, so if Al is not added to suppress the generation of Al ₂ O ₃ , oxide inclusions to be generated The composition of CaO (point A) to SiO ₂ (point B)
The composition is between Among the compositions during this period, as a result of examining oxide-based inclusions having a higher melting point than anorthite and gehlenite and having a large machinability improving effect, Ca ₂ SiO ₄ was adopted as the inclusions. However, they have found that the machinability can be improved as intended by setting the number of the inclusions to 50% or more of the total number of the oxide-based inclusions.

Incidentally, the term “Ca ₂ SiO ₄ ” used herein refers to FIG.
In the state diagram of FIG. The range of X is not a specially set range, but is exactly the same as the range of Ca ₂ SiO ₄ described in the above-described state diagram. The composition and number of the oxide-based inclusions can be easily measured by using SEM, EPMA, or the like. That is, when the composition and the content ratio of the oxide-based inclusions are analyzed using SEM and EPMA, CaO, Al ₂ O ₃ ,
The content ratio of each of SiO ₂ can be determined.
By plotting this value on the phase diagram shown in FIG. 1 and examining whether the position is within the range of X defined by the present invention, whether this inclusion is an inclusion having the composition specified by the present invention is determined. Can be easily determined.

However, when the composition of oxide-based inclusions is actually detected using SEM or EPMA, a small amount of inclusions other than the above three types (eg, MnO) may be detected. Therefore, in the present invention, in order to clarify the scope of right, these inclusions are ignored and plotted on the state diagram of FIG. 1 to determine whether each inclusion has a composition in the range of X. Shall be.

However, it can be measured according to the specifications of the measuring device.
Since the size of the smallest inclusion is different,
Ri Ca_TwoSiO_FourPrevents the total number of measurement results from changing
Therefore, in the present invention, the target inclusion is 2 μm or more.
Size (length of the longest part of the inclusion)
I do. Among oxide-based inclusions having a size of 2 μm or more, Ca
_TwoSiO_FourThe present invention relates to a case where the ratio of the number of
Range.

Next, the reasons for limiting the numerical values such as the component composition of the Ca free-cutting stainless steel of the present invention will be described below. C: 0.15% or less C forms a carbide by combining with Cr, which is an element maintaining corrosion resistance, which is a basic characteristic of stainless steel, and reduces Cr that contributes to ensuring corrosion resistance. Is the element that exerts the effect. It also has the effect of forming a solid solution in the substrate and increasing the hardness. If it exceeds 0.15%, the hardness increases sharply and adversely affects the machinability, and the corrosion resistance is greatly deteriorated. Therefore, the upper limit is set to 0.15%.

Si: 0.10 to 1.00% Si is added as a deoxidizing agent during steelmaking (Si is not used as Al in the present invention, so Si is mainly used as a deoxidizing agent. ), And are also necessary for forming oxide-based inclusions that are a feature of the present invention. Si content is 0.10
%, The number of Ca ₂ SiO ₄ to be generated cannot be sufficiently secured, and the machinability is insufficiently improved.
10%. Further, if Si is too large, the toughness is reduced. Therefore, the upper limit is set to 1.00%.

Mn: 0.10 to 2.00% In the present invention, excellent machinability is ensured by the effects of both generation of Ca ₂ SiO ₄ and generation of MnS. Ca ₂ SiO ₄ , an oxide-based inclusion, improves machinability by adhering to the tool surface and protecting the tool surface, whereas MnS, a sulfide-based inclusion, improves chip crushability. This has the effect of improving machinability. And, while this effect has a great effect at the time of high-speed cutting with cemented carbide, the above-mentioned oxide-based inclusions have an effect at the time of cutting with both cemented carbide and high-speed tool steel. Demonstrate. Therefore, in order to ensure excellent machinability, C
a ₂ SiO ₄ as well, MnS also must be sufficiently produced, and the lower limit was made 0.10%. However, if the content is too large, the hot workability decreases, so the upper limit was made 2.00%.

P: 0.045% or less P is an element inevitably contained as an impurity in a small amount. However, P is an element that easily causes segregation and lowers hot workability. did.

S: 0.010 to 0.050% S is an element effective for improving the machinability of steel.
It is an element that has the effect of producing nS and improving the chip crushing property. To sufficiently obtain this effect, 0.010%
The above content is necessary. However, if contained in a large amount, the corrosion resistance is significantly reduced, so the upper limit is 0.050.
%.

Ni: 15.0% or less The effect of improving machinability by adding Ca to the free-cutting stainless steel of the present invention is exerted regardless of the structure of the stainless steel. That is, Ni is an austenite-forming element, and other components have some influence, but
If the content is increased or decreased within the range of 0% or less, the obtained stainless steel becomes ferritic, martensitic, or austenitic. However, the above-described machinability improving effect can be similarly obtained in any structure state, so that the above-mentioned wide range is set. However, even if it is contained in a large amount, the effect of improving the corrosion resistance of Ni is saturated, and an economically viable effect cannot be obtained. Therefore, the upper limit is set to 15.0%.

Cr: 11.0 to 20.0% Cr is a basic element for maintaining excellent corrosion resistance, which is a basic feature of stainless steel, and must be contained at least 11.0%. However, if it is contained in a large amount, the hot workability deteriorates and the production becomes difficult, so the upper limit was made 20.0%.

Mo: 3.0% or less Mo is an element effective for improving corrosion resistance like Cr. However, if contained in a large amount, the hot workability is reduced, so the upper limit was made 3.0%.

N: 0.15% or less N is a strong austenite-forming element. When stably obtaining an austenite structure, the desired effect can be obtained by adding a small amount. However, N has a large effect of improving the hardness by solid solution strengthening,
Since the addition of a large amount adversely affects the machinability, the upper limit is set to 0.
15%.

Ca: 10 to 100 ppm Ca is the most important element in the present invention for the purpose of improving machinability. In the present invention, Ca ₂ SiO ₄ which is an oxide-based inclusion is generated by adding Ca to improve machinability. Therefore, in order to obtain the effect, a content of at least 10 ppm is required. However, if it is contained in a large amount, nozzle clogging occurs during melting and production becomes difficult, so the upper limit was made 100 ppm.

O: 20 to 200 ppm In the present invention, in order to improve machinability, it is necessary to produce Ca ₂ SiO _4. Therefore, it is necessary to contain O in an amount necessary for the production, and the lower limit is set to 20 ppm. . However, if O is contained in a large amount, oxide inclusions other than Ca ₂ SiO ₄ are likely to be generated, and it is difficult to achieve 50% or more of Ca ₂ SiO ₄ inclusions. Therefore, the upper limit was set to 200 ppm.

The number of Ca ₂ SiO ₄ inclusions is 50% or more. Ca ₂ SiO ₄ is an important inclusion for improving the machinability in the present invention. To protect the tool surface and improve tool life. In order to sufficiently obtain the above-mentioned effect, the number ratio of Ca ₂ SiO _{4 in} the generated oxide-based inclusion must be at least 50% or more. As described above, in order to achieve this ratio, the components are adjusted to the above ranges, and an element other than Al is used as a deoxidizing agent to suppress the generation of Al-based oxide-based inclusions. There is a need.

Also, as in the invention of claim 2, claim 1
Pb: 0.30% as a machinability improving element
Hereinafter, Bi: 0.30% or less, Se: 0.30% or less,
By adding one or more of Te: 0.30% or less, machinability can be further improved as compared with the invention steel of claim 1. The reason for limiting the range of the content of these elements will be described below.

Pb: 0.30% or less, Bi: 0.30%
Hereinafter, Se: 0.30% or less, Te: 0.30% or less Pb, Bi, Se, and Te are elements that can further improve the machinability of the present invention, and Ca is added as in the present invention. To form Ca ₂ SiO ₄ oxide,
Addition of a small amount of b, Bi, Se, and Te elements can improve machinability without deteriorating corrosion resistance. However, if added in a large amount, the hot workability decreases and the production becomes difficult, so the upper limit was set to 0.30% for all four elements.

In the Ca free-cutting stainless steel according to the first and second aspects, it is preferable that the content of Al contained as an impurity is regulated to 0.005% or less. Thereby, the generation ratio of Ca ₂ SiO ₄ can be reliably improved. The reason for the limitation will be described below.

Al: 0.005% or less In order to sufficiently obtain the effect of improving the machinability of the present invention, as the oxide-based inclusions, Ca ₂ SiO ₄ should be formed at 50% or more of the total number of the oxide-based inclusions. It is essential that Al be used as a deoxidizing element. Conventionally, even though Al is used as a deoxidizing agent, the Al content in that case is about 0.015 to 0.030%, which is almost equal to the level of impurities. There are a number of patents that do not mention anything as substantially impurities. In the present invention, Al is not used as a deoxidizing agent in order to reliably obtain a machinability improving effect. Further, in order to prevent the formation of anorthite and gehlenite, it is desirable that the upper limit is also strictly regulated, and the upper limit is set to 0.005%.

[0034]

EXAMPLES Next, the effects obtained by the Ca free-cutting stainless steel of the present invention will be clarified by examples. Tables 1 and 2 show the chemical components of the test steel.

[0035]

[Table 1]

[0036]

[Table 2]

Table 1 shows test steels of ferritic stainless steel, and Table 2 shows test steels of austenitic stainless steel. As shown in Tables 1 and 2, the test steels are C steel corresponding to the conventional steel, a comparative steel B obtained by adding Ca, which is a machinability improving element, to C steel, and further have substantially the same components as the B steel. Prepared three types of steel A, which is a steel of the present invention, manufactured without using Al as a deoxidizing agent. As conventional steel, SUS430, SUS410L, SUS434,
SUS304, SUS316, and SUS301 were selected, and Ca and Al were added to each of them.
Steel manufactured without using it during deoxidation (ie, the steel of the present invention)
Was prepared, and the influence of the Ca addition and deoxidation methods was investigated. About the steel of the present invention, Pb which is a machinability improving element,
Steels to which Bi, Se, and Te were added were also prepared and evaluated at the same time. In addition, to further confirm the effects of other components,
X, Y, and Z steels having some components outside the scope of the present invention were also evaluated. Thus, the reason why A, B, and C steels having almost the same components other than Ca and Al were prepared is that when the amounts of Ni and Cr change, the levels of corrosion resistance and machinability greatly change. This is because the difference in the effect due to the presence or absence of Ca, Al, or not can not be clearly evaluated.

For the test steel, a steel ingot was manufactured in a 2 ton VIM melting furnace, which was hot-rolled, the ferritic stainless steel was annealed, and the austenitic stainless steel was subjected to solution heat treatment. It was prepared by processing into a test piece shape. First, a composition analysis of oxide inclusions in the steel was performed for all of the test steels. For analysis, oxide-based inclusions were extracted by the bromine-methanol method, and 50 inclusions having a size of 2 μm or more were selected from the extracted inclusions, and component analysis was performed one by one using EPMA. . Among the inclusions analyzed, CaO, Al ₂ O ₃ , S
Although a small amount of a composition other than iO ₂ was also detected, as described above, only the content of the inclusions of the three compositions described above (however, correction was made so that the total of the inclusions of the three compositions was 100%,
The results in Table 3 also show the corrected values. ) Was plotted on the phase diagram shown in FIG. 1 to determine whether each of the inclusions was within the specified range. Table 3 shows the results.

[0039]

[Table 3]

As is clear from Table 3, the test steels B and C, which are Al deoxidized test steels, have a higher Al ₂ content than the steel A of the present invention.
The inclusion of O ₃ was high, so that very few inclusions fulfilled the requirements of the present invention. On the other hand, the steel A of the present invention does not use Al as a deoxidizing agent and suppresses the Al content to a low content, so that the content of Al ₂ O _{3 in the} inclusions is all 20% or less. It was confirmed that 60 to 98% of all the inclusions were within the composition range defined in the present invention.

Next, as a performance evaluation of the test steel, results of a turning tool life test for evaluating machinability and an immersion test in a corrosive liquid for evaluating corrosion resistance will be described.

In the turning tool life test, a φ60 round bar was prepared, evaluated under the conditions shown in Table 4, and the time until the life reached was measured. And the result is based on the time of the conventional steel C-1 steel for ferritic stainless steel, and based on the time of the conventional steel C-4 steel for austenitic stainless steel. The results are shown as an integer ratio when the time until the tool life of steel is 100.

[0043]

[Table 4]

The immersion test, which is an evaluation of the corrosion resistance, was performed in a 5% NaCl + 2% H ₂ O ₂ aqueous solution at 40 ° C. by φ20 × 20 mm.
Was immersed for 24 hours, and the weight change (that is, corrosion loss) before and after the test was measured to evaluate the test piece. Table 3 shows the results.

As is apparent from Table 3, the present invention A
It can be seen that the steel maintains substantially the same corrosion resistance as the corresponding comparative steel and conventional steel, but the machinability is greatly improved. In particular, the steel of the present invention to which Pb, Bi, Se, and Te were added in addition to Ca as a machinability improving element was particularly excellent in machinability. In addition, although the comparative steel B has slightly better machinability than the conventional steel C due to the addition of Ca, the difference is not so large that a sufficient improvement effect cannot be obtained. I understand. From this result, when Ca is used to improve the machinability of stainless steel, it is not sufficient to simply add Ca, and it is necessary to make the composition of the generated inclusions as specified in the present invention. Proven to be.

Further, among the components of the present invention, X, Y, and Z steels whose S and N contents do not satisfy the conditions of the present invention are X
Steel is excellent in machinability but has poor corrosion resistance. Steel Y is conversely excellent in corrosion resistance but poor in machinability.
It was confirmed that the machinability of the steel was superior to that of the conventional steel, but was slightly inferior to that of the steel of the present invention. From these results, it was confirmed that the steel A having both components and types of inclusions within the ranges specified in the present invention can obtain excellent performance in both corrosion resistance and machinability.

[0047]

As described above, according to the present invention, the composition range is limited to the above specific range, and the number of Ca ₂ SiO ₄ generated by the addition of Ca as a machinability improving element is reduced by oxidation. By setting the content to 50% or more of the total number of physical inclusions, a free-cutting stainless steel with improved machinability without reducing corrosion resistance can be provided.

[Brief description of the drawings]

FIG. 1 is a CaO—Al ₂ O ₃ —SiO ₂ phase diagram illustrating the range of the composition of Ca ₂ SiO ₄ specified in the present invention.

Claims (3)

[Claims] C. 0.15% or less by weight, Si:
0.10-1.00%, Mn: 0.10-2.00%,
P: 0.045% or less, S: 0.010 to 0.050
%, Ni: 15.0% or less, Cr: 11.0 to 20.0
%, Mo: 3.0% or less, N: 0.15% or less, Ca:
Ca free-cutting stainless steel containing 10 to 100 ppm, O: 20 to 200 ppm, the balance being Fe and unavoidable impurities, wherein 50% or more of the total number of oxide-based inclusions is Ca ₂ SiO _4. steel. 2. The steel according to claim 1, further comprising P
b: 0.30% or less, Bi: 0.30% or less, Se:
A Ca free-cutting stainless steel containing one or more of 0.30% or less and Te: 0.30% or less. 3. The Ca free-cutting stainless steel according to claim 1, wherein Al contained as an impurity is set to 0.005% or less.