JP2007186764A - Free-cutting ferritic stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a free-cutting stainless steel containing no harmful free-cutting elements and having good corrosion resistance. <P>SOLUTION: The free-cutting ferritic stainless steel material has a composition comprising, by mass%, 0.01 to 0.5% C, 1.0% or less Si, 1.0% or less Mn, 0.005 to less than 0.05% S, 10 to 30% Cr, 0.6% or less Ni, 0.5 to 4.0% Cu, 0.02 to 1.0% Ti, and optionally, 1.0% or less Nb, 3.0% or less Mo, 1.0% or less Zr, 1.0% or less Al, 1.0% or less V, and at least one of rare earth metals (REM) in an amount of 0.05% or less, with the balance substantially consisting of Fe, and has such a metal structure that a Cu phase with a particle diameter of 0.01 μm or larger and a Ti compound phase containing S and C as constituent elements with a particle diameter of 0.1 to 10 μm are dispersed in a matrix. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a ferritic free-cutting stainless steel material suitable for a machine part material that requires excellent corrosion resistance and machinability.

  Due to the remarkable development of the precision machine industry and the increase in demand for home appliances and furniture furnishings that require corrosion resistance, stainless steel has come to be used even where stainless steel has not been used. In particular, with the automation and labor saving of machine tools, there is an increasing demand for stainless steel materials with excellent machinability in order to improve the productivity of equipment and parts that require cutting.

  Up to now, free-cutting stainless steel containing S, Pb, Se, Bi, Ti, Zr or the like as a machinability improving element has been developed and used.

JP 2002-38238 A JP-A-2005-139531 JP 2005-89850 A

  S, which is effective as a machinable element, causes a significant decrease in hot workability, ductility and corrosion resistance, and causes anisotropy in mechanical properties when added in a large amount. For this reason, the amount of S added is desired to be minimized. Free-cutting stainless steel with improved machinability by adding Pb may cause harmful Pb elution during use, and is difficult to recycle from the viewpoint of environmental protection. There is a need for machined stainless steel. In addition, Se has a poor yield in addition to steel, and is a harmful element, which causes problems for environmental measures. Bi is effective for improving the machinability like Pb, but the hot workability is remarkably lowered when the addition amount is increased.

  The present invention seeks to develop and provide a free-cutting stainless steel material that does not contain harmful machinable elements and has good corrosion resistance.

As a result of detailed studies, the inventors have found that in a ferritic stainless steel in which a Cu phase and a specific Ti-based compound phase coexist in a matrix, free machinability is significantly improved.
That is, in the present invention, by mass%, C: 0.01 to 0.5%, Si: 1.0% or less, Mn: 1.0% or less, S: 0.005 to less than 0.05%, Cr: 10-30%, Ni: 0.6% or less, Cu: 0.5-4.0%, Ti: 0.02-1.0%, Nb: 1.0% or less as required , Mo: not more than 3.0%, Zr: not more than 1.0%, Al: not more than 1.0%, V: not more than 1.0%, rare earth element (REM): not less than 0.05% A ferritic free-cutting stainless steel material having a metal structure in which the balance is substantially Fe and has a Cu phase and a Ti-based compound phase containing S and C as constituent elements dispersed in a matrix is provided.

  Here, “the balance is substantially Fe” means that an element that does not impair the effects of the present invention is allowed, and includes “the balance is made of Fe and inevitable impurities”. The Cu phase is a precipitated phase mainly composed of metal Cu, and includes so-called ε-Cu.

  The Cu phase is preferably dispersed in a distribution state in which the cross-sectional structure of the steel material has a particle size of 0.01 μm or more occupying 0.2% or more in area ratio. The Ti-based compound phase is preferably dispersed in a distribution state in which the grain size of 0.1 to 10 μm occupies 1% or more of the area ratio in the cross-sectional structure of the steel material.

  A steel material having such a metal structure can be produced by subjecting the steel sheet after hot rolling or cold rolling to final annealing in which the steel sheet is heated to a temperature range of 800 to 1100 ° C. However, when the heating temperature is over 800 to 900 ° C., a holding time exceeding at least 1 hr is secured as the time for holding in the temperature range.

  According to the present invention, the machinability of the ferritic stainless steel is improved by the technique of dispersing the Cu phase and the Ti-based compound phase in the matrix. Therefore, the amount of S that has been conventionally added in large amounts as a machinable element can be greatly reduced, and the corrosion resistance is remarkably improved. In addition, since no machinable elements such as Pb and Se harmful to the human body are added, no special elements such as Bi are required, so that environmental problems, recyclability, and material costs have been improved. It is advantageous compared to machined stainless steel. The free-cutting stainless steel material of the present invention is suitable as a material for machining parts that require corrosion resistance, and is expected to be used in a wide range of fields.

  Stainless steel is generally poor in machinability and is counted as one of difficult-to-cut materials among widely used metal materials. Causes of poor machinability include low thermal conductivity, high degree of work hardening, and easy adhesion.

  As a means for improving machinability, it is effective to disperse relatively brittle inclusions or precipitates in the matrix. However, the machinability improving means using S-based inclusions or precipitates that have been conventionally used has the disadvantage of reducing the corrosion resistance of stainless steel, and the high corrosion resistance inherent in stainless steel cannot be fully utilized. It was accompanied.

Therefore, the inventors examined the machinability improvement effect for various precipitated phases. As a result, it was found that a Cu phase such as ε-Cu can be used for improving machinability.
Cu phase has been conventionally used as a precipitation strengthening means for stainless steel (Patent Document 3). However, the Cu phase effective for improving the strength is a very finely dispersed precipitate, and it is difficult to greatly improve the machinability. However, if a Cu phase grown to a slightly inferior precipitation strengthening capacity or a Cu phase coarsened to a degree that does not sufficiently contribute to precipitation strengthening is dispersed, the machinability can be remarkably improved. I understood. Such Cu phase is considered to contribute to the improvement of machinability by reducing the cutting resistance and extending the tool life by the lubrication on the rake face of the tool at the time of cutting and the friction based on the heat conduction action. It is done.

However, the machinability cannot be sufficiently improved by simply dispersing the Cu phase as described above. Since the Cu phase is soft, hard particles that can be the starting point of cracks during cutting are further required. As a result of research, it has been found that it is extremely effective to disperse a Ti-based compound phase containing S and C as constituent elements as such hard particles. This Ti-based compound phase is considered to be mainly composed of a compound represented by the composition formula Ti ₄ S ₂ C ₂ . The Ti-based compound phase becomes a starting point of cracks during cutting, and contributes to reduction of cutting resistance and improvement of tool life.

  Thus, the free-cutting stainless steel material of the present invention has a structure in which a soft Cu phase and a hard Ti-based compound phase are combined and precipitated. During cutting, a hard Ti-based compound phase containing S and C as constituent elements serves as a crack starting point, the crack propagates to the soft Cu phase, and the lubricating action of the Cu phase is exhibited, cutting resistance is reduced and tool life is reduced. It is thought that this results in a significant improvement in machinability as a result.

  In order to improve machinability, a relatively coarse Cu phase needs to be dispersed in the matrix. As a result of various studies, it is desirable that the Cu phase is dispersed in a distribution state in which the grain size of 0.01 μm or more occupies 0.2% or more of the area ratio in the cross-sectional structure of the steel. However, if there is a large amount of a very coarse Cu phase, the distance between the Cu phases becomes too large and smooth propagation of cracks is difficult to occur, and the steel material becomes brittle. It is preferable that

  Regarding the Ti-based compound phase, it is desirable that the cross-sectional structure of the steel material is dispersed so that the particle size of 0.1 to 10 μm occupies 1% or more in terms of the area ratio.

  The particle size of these precipitated phases is represented by the diameter of the longest part of the individual particles observed in the cross-sectional structure of the steel material. The particle size and area ratio of the precipitated phase can be determined by observing a sample obtained by polishing a cross section of a steel material, but can also be measured by observing at an acceleration voltage of 200 kV using a transmission electron microscope, for example. Note that the upper limit of the abundance (area ratio) of the Cu phase and the Ti-based compound phase is inevitably restricted by the chemical composition described later, and thus need not be specified.

As the heat treatment for generating the Cu phase and the Ti-based compound phase, it is necessary to set the conditions such that a relatively coarse Cu phase is sufficiently generated. That is, a heat treatment can be employed in which the steel material is heated and held in a temperature range of more than 800 ° C. and not more than 1100 ° C. When the heating temperature is over 800 ° C. to 900 ° C., the steel material is held in the temperature range for more than 1 hr. Is desirable. Cooling after heating sufficiently generates a brittle Cu phase that is effective for improving machinability, for example, by slowly cooling so that the average cooling rate between 800 and 300 ° C. is 5 ° C./min or less. This is advantageous. Precipitation of the Cu phase is also promoted by adding elements such as Nb, Ti, Mo, Zr, etc., which easily form carbonitrides and precipitates.
The Ti-based compound phase can be properly precipitated by adopting the above heat treatment conditions in consideration of proper precipitation of the Cu phase.

  The ferritic free-cutting stainless steel material of the present invention can be manufactured in accordance with a general stainless steel sheet manufacturing process. For example, a steel having the composition described later is melted and manufactured by the process of “hot rolling → final annealing” or “hot rolling → (annealing) → cold rolling → final annealing” according to the target plate thickness. it can. At that time, the above heat treatment is performed in the final annealing. In addition, after hot rolling or after annealing, pickling is performed as necessary. If the target plate thickness is set in the range of 1.5 to 5 mm, it can be widely applied to various cutting parts using a steel plate as a raw material.

Hereinafter, the component elements will be described.
C is an important element constituting a Ti-based compound phase that improves machinability. In order to exhibit the machinability improving effect, 0.01% by mass of C is necessary. It is effective to contain more than mass%, and it is particularly preferable to contain 0.02 mass% or more. It is more preferable to ensure a C content exceeding 0.03 mass%. However, since excessive C content causes a decrease in manufacturability and corrosion resistance, the upper limit of the C content is limited to 0.5% by mass, and it is more preferable to add in the range of 0.15% by mass or less.

  Si is an element effective for improving corrosion resistance, and can be added as a deoxidizer for steel. However, if the Si content is excessive, the steel becomes hard due to solid solution strengthening, which leads to a decrease in manufacturability and workability. Moreover, it becomes a factor which reduces hot workability. Accordingly, the Si content is limited to 1.0% by mass or less.

  Mn improves the manufacturability and has the effect of fixing S in steel as MnS. Although MnS is effective in improving machinability, it is a factor that degrades corrosion resistance. Therefore, in the present invention, it is preferable to suppress the formation of MnS. For this reason, Mn content is restrict | limited to 1.0 mass% or less.

  S is an important element that forms a Ti-based compound phase together with C, and 0.005% by mass or more of S is necessary to obtain the effect of improving machinability. However, since excessive S content deteriorates the inherent corrosion resistance of stainless steel, the S content is limited to less than 0.05% by mass in the present invention.

  Cr is an essential element for maintaining corrosion resistance, and a content of at least 10% by mass is necessary. However, excessive Cr content causes a decrease in hot workability and a decrease in toughness, so the content is 30% by mass or less.

  Ni is effective for improving the corrosion resistance, but is excessively undesirable because it is an austenite-forming element. In the present invention, the Ni content is limited to 0.6% by mass or less.

  Cu is an important element for improving corrosion resistance and machinability in the present invention. As described above, in order to obtain the effect of improving the machinability by the Cu phase, it is necessary to contain 0.5% by mass or more of Cu. It is more preferable to ensure a Cu content of 0.7% by mass or more, and it is more preferable to set the Cu content to 1.0% by mass or more. However, excessive Cu content deteriorates hot workability and adversely affects manufacturability, workability, and the like, so the upper limit of Cu content is limited to 4.0 mass%. When particularly good workability is required, it is preferable to contain Cu in the range of 3.0% by mass or less.

  Ti is a deoxidizing and denitrifying agent and, in the present invention, is an important element for generating a Ti-based compound phase that becomes a crack base point during cutting as described above. In order to obtain the machinability improving effect by the Ti-based compound phase, at least 0.02% by mass of Ti is necessary, but it is more effective to secure a Ti content of 0.05% by mass or more, More preferably, the Ti content is 0.1% by mass or more. However, excessive Ti content deteriorates manufacturability and workability, and easily causes wrinkles on the product surface, so the Ti content is limited to a range of 1.0% by mass or less. Usually, a good machinability improving effect is obtained in a Ti content range of 0.5 mass% or less.

  Nb contributes to uniform dispersion of the Cu phase. That is, since the Cu phase has a strong tendency to precipitate around the Nb-based precipitate, to uniformly disperse the Cu phase, a structure in which Nb is added and the Nb-based precipitate is finely precipitated at a high temperature is obtained. It is advantageous. At that time, it is more effective to secure Nb content of 0.01 mass% or more. However, excessive addition of Nb adversely affects manufacturability and workability. Therefore, when Nb is added, the addition is performed in the range of 1.0% by mass or less.

Mo improves corrosion resistance and precipitates as an intermetallic compound such as Fe ₂ Mo effective as a nucleation site for the Cu phase. In order to fully exhibit these effects, it is preferable to contain 0.5 mass% or more of Mo. However, excessive Mo content adversely affects manufacturability and workability, so when adding Mo, it is performed in the range of 3.0% by mass or less.

  Zr precipitates as carbonitrides effective as Cu phase nucleation sites. In order to sufficiently obtain the effect, it is desirable to ensure a Zr content of 0.1% by mass or more. However, excessive Zr content adversely affects manufacturability and workability, so when Zr is added, it is performed in the range of 1.0% by mass or less.

  Al, like Mo, improves corrosion resistance and precipitates as an effective compound as a nucleation site for the Cu phase. In order to sufficiently obtain the effect, it is desirable to secure an Al content of 0.01% by mass or more. However, excessive Al content adversely affects manufacturability and workability. Therefore, when Al is added, it is performed in the range of 1.0% by mass or less.

  V precipitates as a carbonitride that is effective as a nucleation site for the Cu phase in the same manner as Zr. In order to sufficiently obtain the effect, it is desirable to ensure a V content of 0.1% by mass or more. However, excessive V content adversely affects manufacturability and workability, so when V is added, it is performed in the range of 1.0% by mass or less.

  Rare earth elements (REM) have the effect of improving hot workability, and it is effective to contain 0.001% by mass or more. Moreover, it becomes a precipitate effective for precipitation of the Cu phase and is dispersed in the matrix. However, excessive addition of REM, on the other hand, causes a decrease in hot workability. Therefore, when REM is added, the addition is performed in the range of 0.05% by mass or less.

  In addition, B is an element that fixes N and improves the corrosion resistance and workability, and its action is manifested by containing 0.0005% by mass or more of B. However, excessive B content adversely affects hot workability. Therefore, the addition of B is allowed in the range of 0.01% by mass or less. It is better that the P content is small, and it is desirable to regulate it to 0.1 mass% or less. Ca, Mg, Co and the like may also be mixed from the raw materials, but unless they are excessively contained, they do not particularly adversely affect the corrosion resistance and machinability. These elements are allowed to be contained within a range not inhibiting the effects of the present invention (for example, 0.1% by mass or less).

  Ferritic steel having the composition shown in Table 1 was melted in a 30 kg vacuum melting furnace, and the obtained ingot was hot rolled to a plate thickness of 3 mm in a temperature range of 1200 to 900 ° C. Thereafter, the hot-rolled sheet was heated to a temperature range exceeding 800 to 1100 ° C., and then subjected to final annealing under the condition of furnace cooling. In that case, in the sample which employ | adopted the heating temperature exceeding 800 degreeC-900 degreeC, time exceeding 1 hr was ensured as the heating holding time in the said temperature range. Further, the cooling rate in the final annealing was controlled so that the average cooling rate of 800 to 300 ° C. was about 0.5 ° C./min. Both surfaces of the steel plate after the final annealing were ground to obtain a test steel plate having a thickness of 2.5 mm.

[Tissue observation]
About the test piece cut out from each test steel plate, the cross section perpendicular to the rolling direction (C cross section) was polished and etched with hydrofluoric acid, and then this surface was observed with a scanning electron microscope (SEM), and the Cu phase and Ti The size and area ratio of the system compound phase were examined. The identification of the Cu phase and the Ti-based compound phase was performed by an EDS apparatus attached to the SEM.
As a result, all the steels of the examples of the present invention have a distribution state in which the Cu phase has a particle size of 0.01 μm or more and occupies 0.2% or more in area ratio, and the Ti-based compound phase has a particle size of 0 It was confirmed that the distribution of 0.1 to 10 μm occupied 1% or more in area ratio. In addition, the presence of S and C is confirmed in the Ti-based compound phase, and it is considered from the quantitative analysis results that the Ti-based compound phase is mainly composed of a compound represented by the composition formula Ti ₄ S ₂ C _2. It was.

[Machinability test]
Each test steel sheet was examined for machinability as follows.
A cutting specimen was cut from the test steel sheet and subjected to a cutting test using a horizontal milling machine. Cutting conditions are as follows.
Dry type, cutting tool: 3.5 mmφ high speed end mill, peripheral speed: 10-30 m / min, feed rate: 10-50 mm / min, cutting depth: 0.5 mm

The test piece was fixed to a load cell, and the main component force generated in the test piece when cutting was performed under the above conditions was measured. The measured value was defined as cutting resistance. A cutting resistance of 10N or less was evaluated as ◎ (very good), a value exceeding 10N to 15N was evaluated as ◯ (good), a value exceeding 15N was evaluated as × (defective), and a value exceeding ◯ evaluation was determined to be acceptable.
Further, the finished surface roughness of the surface (cut surface) of the test piece after cutting was measured. The finished surface roughness was determined by measuring the arithmetic average roughness Ra defined in JIS B0601 in a direction perpendicular to the cutting direction using a stylus type surface roughness meter. When Ra is 0.5 μm or less, ◎ (very good), 0.5 to 1.0 μm is evaluated as ◯ (good), and more than 1.0 μm is evaluated as × (defect). The thing was determined to be acceptable.
Furthermore, visually observe the shape of the chips produced in the cutting test, evaluate that the shape is round and has good crushability, and that the fracture shape is poor and inferior in crushability is evaluated as x It was determined.
The results are shown in Table 2.

[Corrosion resistance test]
A salt spray test based on JIS Z2371 was performed.
The surface of the corrosion test piece (50 × 100 mm) cut out from the test steel plate was polished to # 600 with emery paper, degreased and washed, and then exposed to a 5 mass% sodium chloride aqueous solution spraying environment at 35 ° C. for 72 hours. The surface of the test piece after the test was visually observed. The case where no occurrence was observed was evaluated as ◯ (good), the case where the generation was observed was evaluated as x (defect), and the case where the evaluation was evaluated was determined as pass.
The results are shown in Table 2.

As can be seen from Table 2, all of the ferritic stainless steel materials of the examples of the present invention have excellent machinability and good corrosion resistance.
On the other hand, No. 21, which is a comparative example, was inferior in corrosion resistance because the S content was too high, and the cutting resistance was higher than that of the present invention example because the Cu content was insufficient. No. 22 was inferior in corrosion resistance because the S content was too high, and the cutting resistance was higher than that of the present invention example because the Ti content was insufficient. No. 23 was inferior in corrosion resistance because the S content was too high, and since the Cu and Ti contents were insufficient, the machinability improving effect by the Cu phase and the Ti-based compound phase could not be obtained, and the machinability was poor. In No. 24, since the S content and the Ti content were too low, the effect of improving the machinability by the Ti compound phase was not exhibited and the machinability was inferior. In No. 25, since the S content was too low, the effect of improving the machinability by the Ti-based compound phase was not exhibited, the machinability was inferior, and the Cr content was insufficient, and the corrosion resistance was also poor.

Claims (5)

  In mass%, C: 0.01 to 0.5%, Si: 1.0% or less, Mn: 1.0% or less, S: 0.005 to less than 0.05%, Cr: 10 to 30%, Ni: 0.6% or less, Cu: 0.5 to 4.0%, Ti: 0.02 to 1.0%, the balance being substantially Fe, comprising Cu phase, and S and C A ferritic free-cutting stainless steel material having a metal structure in which a Ti-based compound phase contained in an element is dispersed in a matrix.   Further, Nb: 1.0% or less, Mo: 3.0% or less, Zr: 1.0% or less, Al: 1.0% or less, V: 1.0% or less, rare earth element (REM): 0.05 2. The ferritic free-cutting stainless steel material according to claim 1, having a composition containing at least 1% or less.   3. The ferritic free-cutting stainless steel material according to claim 1, wherein the Cu phase is in a distribution state in which a grain size of 0.01 μm or more occupies 0.2% or more in area ratio in a cross-sectional structure of the steel material.   The ferritic free-cutting according to any one of claims 1 to 3, wherein the Ti-based compound phase is in a distribution state in which a grain size of 0.1 to 10 µm occupies 1% or more in terms of area ratio in a cross-sectional structure of a steel material. Stainless steel material.   The said metal structure is the final annealing which heats to the temperature range of 800 to 1100 degreeC with respect to the steel plate after hot rolling or after cold rolling, However, when the heating temperature is over 800 to 900 degreeC, it is in the said temperature range. The ferritic free-cutting stainless steel material according to any one of claims 1 to 5, wherein the ferritic free-cutting stainless steel material is obtained by performing final annealing for securing a holding time exceeding at least 1 hr as a holding time.