JP4120354B2 - Nonmagnetic stainless steel and method of manufacturing the same - Google Patents

Description

【００３１】
表２ 試験結果

【００３２】
表３ 試験結果

【００３３】
【発明の効果】
本発明によって、さきに開示した非磁性ステンレス鋼において得られた高い耐食性と製造性を維持したまま、その既知の鋼では達成できなかった、０．２％耐力が９６０ＭＰａを超える高い強度が実現した。これにより本発明は、ドリルカラーを代表とする非磁性ステンレス鋼の諸用途において、きびしい要求に応えることができる。[0001]
[Industrial application fields]
The present invention relates to a non-magnetic stainless steel and a method for manufacturing a part using the steel as a material. The steel of the present invention is non-magnetic indispensable characteristics, and for the manufacture of parts made of steel wires including forged products, steel bars, and ultrafine wires (wire diameters of 100 μm or less) that require corrosion resistance and strength. Is preferred. This material is particularly useful when applied to non-magnetic drill collars for drilling oil wells and large diameter products with a diameter of 60 mm or more.
[0002]
[Prior art]
The applicant has already disclosed that stainless steel having the following alloy composition is suitable as a steel that is nonmagnetic and has both high strength and corrosion resistance (Japanese Patent Laid-Open No. 2000-87187). The steel, by weight, C: ≦ 0.06%, Si: ≦ 0.50%, Mn: 13-16%, P: ≦ 0.040%, S: ≦ 0.030%, Cu: 0 .35 to 1.00%, Ni: 2.50 to 5.50%, Cr: 17.0 to 21.0%, Mo + W: 0.5 to 1.0%, N: 0.38 to 0.60 %, O: ≦ 0.0100% and sol-Al: 0.05% or less, and the balance is substantially Fe, provided that 86 ([% Ni] + [% Cu]) ≧ 13 [% It is a steel having an alloy composition that satisfies the condition of [Cr] +19 [% Mo] +9 [% W] +2 [% Mn].
[0003]
When the characteristics achieved by this nonmagnetic stainless steel are seen in the examples, the 0.2% proof stress is at a level of 776 to 955 MPa. At this strength level, the steel of the previous invention has good corrosion resistance (result of salt spray test A), is non-magnetic (permeability is 1.003 or less), and has good manufacturability.
[0004]
Recently, however, the drill collar is required to have a very high strength exceeding a level of 960 MPa (equivalent to 140,000 psi) with a 0.2% yield strength. In particular, high strength is strongly demanded for a material of a member for embedding a measuring instrument called “MWD color”. On the other hand, the nonmagnetic stainless steel described above could not be answered.
[0005]
[Problems to be solved by the invention]
In view of the current state of the art that such non-magnetic stainless steel is required to have higher strength, the basic object of the present invention is that the 0.2% proof stress is at a level exceeding 960 MPa. It is an object of the present invention to provide a nonmagnetic stainless steel that maintains the good corrosion resistance and manufacturability of the steel disclosed in (1).
[0006]
An additional object of the present invention is to provide a method of manufacturing a part having high strength made of this nonmagnetic stainless steel. The main target is drill collar parts for oil well drilling as described above.
[0007]
[Means for Solving the Problems]
The non-magnetic stainless steel of the present invention that achieves the above-mentioned basic object is, in mass %, C: 0.06% or less, Si: 0.40% or less, Mn: 15.5 to 17%, P: 0 0.040% or less, S: 0.010% or less, Cu: 0.35 to 2.00%, Ni: 2.50 to 4.00%, Cr: 17.0 to 21.0%, Mo + W: 0.0. 5~1.5%, N: 0.48 ~0.65% , O: 0.01% or less, sol-Al: 0.05% or less and B: containing 0.001 to 0.010 percent, The balance is Fe and inevitable impurities , but an alloy that satisfies the condition of 86 ([% Ni] + [% Cu]) ≧ 13 [% Cr] +19 [% Mo] +9 [% W] +2 [% Mn] It has a composition, and 0.2% proof stress has a high strength of 960 MPa or more.
[0008]
The method for producing a non-magnetic stainless steel part according to the present invention that achieves the above-mentioned incidental object is to produce steel having an alloy composition as defined in claim 1 under the condition that the surface temperature of the workpiece is 850 ° C. or less. It is characterized by performing hot working with a reduction in area of at least%.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The main difference between the known steel disclosed in the aforementioned JP 2000-87187 and the steel of the present invention is that in the present invention, 1) the N content higher than that of the known steel is adopted, and 2) The addition of a predetermined amount of B is essential. In the following, the significance of this difference will be explained together with the matters already described for known steels.
[0010]
The technical idea when developing a known steel was to improve the strength of conventional high manganese non-magnetic stainless steel by increasing the amount of N. In order to ensure the corrosion resistance of steel, it is necessary to add a certain amount of Cr and Mo (or Mo + W). Therefore, even in known steels, in addition to a large amount of Mn, a predetermined amount of Cr and Mo + W The nitrogen blow in the solidification process, which tends to occur in high-N steel added with a large amount of Mn, Cr, and Mo, is suppressed by adding appropriate amounts of Ni and Cu.
[0011]
In the present invention, in order to realize a high strength that has not been achieved in the past, a higher N content, which has not been performed with known steels, is realized, and processing at a high processing rate at a relatively low temperature is achieved. Take advantage of the work hardening that results. In this case, the technical idea of preventing the deterioration of the corrosion bending characteristics that emerges as a weak point by adding a large amount of B, which was not adopted in the known steel, is the technical idea underlying the new steel. It is.
[0012]
The reason why the alloy composition of the nonmagnetic stainless steel of the present invention is limited as described above will be described below in relation to that of known steel.
[0013]
C: 0.06% or less If C is present in the steel, carbides containing Cr are precipitated, and various properties such as corrosion resistance are deteriorated. Therefore, the C content is preferably low. In particular, in order to obtain high strength, it is inevitable that this steel is processed in a temperature range where carbides are precipitated, so the C content should be reduced as much as possible. In the previous invention, the upper limit was 0.08%, but in the present invention, the upper limit was 0.06%. Preferably, it is 0.04% or less.
[0014]
Si: 0.40% or less Useful as a deoxidizing agent, but adding more than necessary lowers the solubility of N in molten steel and the solid solubility of solidified steel, and also promotes precipitation of intermetallic compounds. It is not preferable to make it exist in a large amount. The upper limit of 0.50% of the previous invention was reduced to 0.40% in the present invention.
[0015]
Mn: 15.5-17%
A large amount of Mn is required to ensure non-magnetism and N solubility in molten steel. In the previous invention, 13 to 16% was selected, but in the present invention, a larger amount of 15.5 to 17% was adopted. Addition of a large amount exceeding the upper limit deteriorates hot workability and corrosion resistance, and causes nitrogen blowing during solidification.
[0016]
P: 0.040% or less P is an impurity that segregates at the grain boundaries and deteriorates the properties of the steel. Therefore, the lower the content, the better. In consideration of the manufacturing cost, 0.040% was set as the allowable limit.
[0017]
S: 0.010% or less S, like P, is an impurity that adversely affects hot workability and corrosion resistance, so its amount is desirably low. In the steel of the previous invention, 0.030% was set as the allowable limit, but in the present invention, a more severe limit of 0.010% was set.
[0018]
Cu: 0.35-2.00%, Ni: 2.50-4.00%
Cu and Ni are added in order to stably add N, which is useful for improving the strength, as described in the previous invention. That is, nitrogen blow is suppressed by increasing the ratio of the austenite phase in which the solid solubility of N is large during solidification. Cu and Ni itself contribute to the improvement of corrosion resistance. The lower limit of each addition amount is a limit for reliably obtaining such an effect. On the other hand, excessive addition lowers the solubility of N in the molten metal, raises the cost, and lowers the SCC characteristics, and must be avoided. The upper limit was determined from this viewpoint. In the previous invention, the addition range of Cu: 0.35-1.00% and Ni: 2.50-5.50% was defined. In the present invention, the addition range was changed to increase the amount of Cu. Ni was reduced.
[0019]
Cr: 17.0-21.0%, Mo + W: 0.5-1.5%
As described above, these components are essential for ensuring the corrosion resistance of steel. However, adding too much is not preferable because it tends to help nitrogen blow during solidification and may reduce phase stability. In the previous invention, Cr: 17.0 to 19.0% and Mo + W: 0.5 to 1.0% were specified. However, in the present invention, a higher addition is added to both Cr and Mo + W with an increase in cost. A quantity range was selected.
[0020]
N: 0.48 to 0.65%
As described repeatedly, N is actively used in the present invention as an extremely effective element for enhancing the strength and corrosion resistance of steel and ensuring non-magnetism. There is a danger of causing nitrogen blow, and from the viewpoint of ensuring the acquisition of a sound product, a large amount of addition has not been conventionally performed. In the previous invention, the addition amount range was defined as 0.38 to 0.60%, but in the examples, the upper limit was 0.46%. In the present invention, this practical limit was overcome, and a large amount of 0.48 to 0.65% was added to achieve the desired high strength.
[0021]
O: 0.01% or less O is a component that lowers the cleanliness of the steel, and is a component harmful to hot workability, corrosion resistance, toughness, and the like, so a smaller amount is preferable. 0.01% is an acceptable limit.
[0022]
sol-Al: 0.05% or less, preferably 0.01% or less
sol-Al also lowers the cleanliness of steel and lowers hot workability, corrosion resistance, toughness and the like. The upper limit of 0.05% is the same as the previous invention, but the steels of the examples have a lower sol-Al content of 0.001 to 0.002%.
[0023]
B: 0.001 to 0.010%, preferably 0.0015 to 0.0040%
In order to obtain the desired high strength, it is recommended that the steel of the present invention is subjected to strong processing at a relatively low temperature to produce a part. According to the inventors' experience, it has been found that if the steel disclosed above is processed at a low temperature, the corrosion bending properties are degraded. The novel point of the present invention is that, by adding B, a high strength can be obtained while maintaining the corrosion bending characteristics even when the low temperature strong processing is performed.
[0024]
The addition of B also improves the hot workability itself. Since this effect has been known, even in known steels, it is supposed to be selectively added as a hot workability improving element together with Ca, Mg, REM and the like. However, in the previous example in which B: 0.0011% was added in the previous invention (No. 12), the N content was low (0.41%), so the 0.2% proof stress was only 893 MPa. Excessive addition of B not only lowers the local melting temperature but also reduces the cleanliness, so an upper limit of 0.010% was set. A preferable range is 0.0015 to 0.0040%.
[0025]
86 ([% Ni] + [% Cu]) ≧ 13 [% Cr] +19 [% Mo] +9 [% W] +2 [% Mn]
As already disclosed, this component condition is necessary to suppress the occurrence of nitrogen blow when the steel solidifies.
[0026]
As described above for hot working with a surface temperature of 850 ° C. or lower and a processing rate of 30% or higher, processing at a processing start temperature of 850 ° C. or lower and a processing rate of 30% or higher is as follows. It is preferable for obtaining intended high strength using work hardening. Since it is difficult to ensure strength in processing at a high temperature and a low processing rate, this processing condition may actually be essential.
[0027]
【Example】
A non-magnetic stainless steel having the alloy composition shown in Table 1 was melted in an AOD furnace to produce a 3.6-ton ingot. The ingot was divided and hot forged at 1100 ° C. to form a 300 mm square billet. Each billet was processed into round bars having various diameters at the processing start temperature and processing rate shown in Table 2, and then blast-cooled. Comparative Example 5 is Example No. 12 of the previous invention.
[0028]
In Table 2, the display in each column has the following meaning.
Cu + Ni: 86 ([% Ni] + [% Cu])
Cr + Mo + W + Mn: 13 [% Cr] +19 [% Mo] +9 [% W] +2 [% Mn]
Ingredient conditions: “Cu + Ni” ≧ “Cr + Mo + W + Mn” is satisfied when the condition is satisfied;
Processing size: Diameter of the round bar obtained by processing (mm)
Processing conditions: ○ when the conditions of the surface temperature of 850 ° C. or less and the processing rate of 30% or more are satisfied, and × otherwise
[0029]
The manufacturability at the time of production of the bar was recorded, a test piece was collected from the obtained forged product, and the corrosion resistance, tensile properties, corrosion bending resistance and magnetic permeability were measured by the following measurement methods. The results are shown in Table 3.
Manufacturability: X for when a crack that is too large to be relieved occurs during forging, ○ otherwise
Salt spray test (35 ° C, 5% NaCl, 96 hours)
A: No corrosion B: Some corrosion
Bending angle 180 degrees 20mm x 70mm x 5mmt
Permeability: VSM method External magnetic field 2000Oe
[0030]

[0031]
Table 2 Test results

[0032]
Table 3 Test results

[0033]
【The invention's effect】
According to the present invention, while maintaining the high corrosion resistance and manufacturability obtained in the non-magnetic stainless steel disclosed above, a high strength with a 0.2% proof stress exceeding 960 MPa, which could not be achieved with the known steel, was realized. . Thereby, this invention can respond to a severe request | requirement in various uses of nonmagnetic stainless steel represented by a drill collar.

Claims (3)

In mass %, C: 0.06% or less, Si: 0.40% or less, Mn: 15.5 to 17%, P: 0.040% or less, S: 0.010% or less, Cu: 0.35 ~2.00%, Ni: 2.50~4.00%, Cr: 17.0~21.0%, Mo + W: 0.5~1.5%, N: 0.48 ~0.65%, O: 0.01% or less, sol-Al: 0.05% or less and B: 0.001 to 0.010%, with the balance being Fe and inevitable impurities , provided that 86 ([% Ni] + [% Cu]) ≧ 13 [% Cr] +19 [% Mo] +9 [% W] +2 [% Mn] The alloy composition satisfies that, 0.2% proof stress has a high strength of 960 MPa or more. Characteristic non-magnetic stainless steel. A nonmagnetic stainless steel characterized by subjecting steel having an alloy composition as defined in claim 1 to hot working with a reduction in area of 30% or more under the condition that the surface temperature of the workpiece is 850 ° C or lower. Manufacturing method of parts. A nonmagnetic drill collar produced by the method of claim 2.