JP6055343B2 - Nonmagnetic steel excellent in low-temperature bending workability and method for producing the same - Google Patents

Description

  The present invention relates to a structural member such as a linear motor car or a power generation / transmission facility exposed to a strong magnetic field, a structural member such as a medical facility in which a weak magnetic field is a problem, or a material constituting a nonmagnetic part of an electromagnetic control component. It is.

  In a structural member exposed to a magnetic field environment, a non-magnetic material that is not magnetized by an external magnetic field is usually used for a component that does not allow an adverse effect on the performance of the equipment body. As a typical nonmagnetic material, austenitic stainless steel such as SUS304 and SUS316 has been conventionally known. However, since austenitic stainless steel contains a lot of rare alloys such as Ni and Cr, there is a problem that the manufacturing cost of the member is increased. In addition, when processing strain is applied, the austenite structure transforms into a processing-induced martensite structure and exhibits ferromagnetism, so that the magnetic permeability increases with an increase in processing strain and the characteristics as a non-magnetic material decrease. There was also a problem.

  As a nonmagnetic material other than the austenitic stainless steel, a high Mn nonmagnetic steel with an increased amount of C and Mn that stabilizes the austenite structure has been developed. For example, the present applicant discloses in Patent Document 1 a nonmagnetic steel in which alloy elements are appropriately controlled and the microstructure is an austenitic structure. Among alloy elements, C is 0.5 to 0.8%, Mn is 8 to 14.6%, Ni is 0.1% or less (not including 0%), and Cr is 1.8 to 3.0%. Is controlling. However, this non-magnetic steel has not been able to sufficiently cope with the recent increase in strength (particularly high proof stress), and there is still room for improvement. Further, bending workability was not taken into consideration.

  In view of this, the present applicant has proposed a nonmagnetic steel in Patent Document 2 that achieves both high yield strength and low magnetic permeability with a low-alloy alloy and is excellent in bending workability. This non-magnetic steel has a P value determined from the contents of C, Si, Mn, Cr and N, in particular, with appropriate control of various component compositions, and a Mn content of 5.0 to 12%. The austenite structure is defined as 99% by area or more of the microstructure.

JP 2011-111666 A JP 2012-107325 A JP 2001-240942 A

  In the said patent document 2, the workability of a nonmagnetic steel is evaluated by performing the press bending (three-point bending) at room temperature, and confirming the presence or absence of a fracture | rupture at a bending angle of 180 degrees, and surface properties. As described above, the conventional high Mn nonmagnetic steel is assumed to be plastically processed at room temperature, and for example, it has not been considered to perform bending in a cold region in winter. However, when bending is performed at a low temperature of about −20 ° C., it has been newly found that a problem of cracking occurs, and there is room for improvement in terms of bending workability at low temperatures.

  For example, Patent Document 3 is known as a technique for stabilizing the austenite phase of high-Mn nonmagnetic steel to a low temperature region. This document describes that the magnetic permeability at a very low temperature of a high-Mn nonmagnetic steel sheet can be further lowered by further stabilizing the austenite phase by increasing the amount of Mn. %, Cr-containing high Mn non-magnetic steel seamless steel pipes containing 5.0 to 10.0% are disclosed. Expanding the stable region of the austenite phase by adding alloying elements such as Mn and Cr can be expected to work effectively to maintain non-magnetism in the use environment after component molding. Therefore, the ductility of the material is lowered, and it is not possible to exert a sufficient effect on the suppression of microcracks and the processing-induced martensite at the time of plastic processing, and there is a possibility that bending workability at low temperature may be decreased. That is, the high temperature Mn non-magnetic steel seamless steel pipe proposed in this document is exposed to electromagnetic stress in a cryogenic environment such as a liquid helium temperature (4.2 K) such as a superconducting magnet reel. The amount of strain when forming into a steel pipe shape is small, for example, plastic with a large amount of strain introduced when forming into a part shape by bending or cold forging, such as rebar It was not intended for deformation.

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to achieve high strength, high yield strength and low magnetic permeability, excellent bending workability at room temperature, and further at low temperature. The object is to provide a nonmagnetic steel excellent in bending workability. Moreover, the other object of this invention is to provide the manufacturing method of the said nonmagnetic steel.

  As a result of intensive studies in order to solve the above problems, even if an element that stabilizes the austenite phase is added in excess of the appropriate value, the occurrence of microcracks due to increased work hardening and the occurrence of grain boundary carbides It has been found that the austenite phase is lacking in stabilizing elements due to the formation of the austenite phase, the ductility of the austenite phase is lowered, and the bending workability at low temperatures is lowered. Therefore, the component composition is appropriately controlled to stabilize the austenite phase, the area ratio of the austenite structure is set to 99.0% or more, and the austenite grain size number is set to 8.0 to 10.5. The inventors have found that high yield strength and low magnetic permeability can be achieved, furthermore, bending workability at room temperature can be improved, and further, bending workability at low temperature can be improved, and the present invention has been completed.

  That is, the nonmagnetic steel excellent in the low temperature bending workability according to the present invention that has solved the above-mentioned problems is C: 0.8 to 1.2% (meaning mass%. Hereinafter, the same applies to chemical components. ), Si: 0.1 to 0.6%, Mn: more than 13%, 20% or less, Al: 0.001% or more, less than 0.02%, P: 0.040% or less (excluding 0%) ), S: 0.045% or less (excluding 0%) and N: 0.025 to 0.05%, the balance being iron and inevitable impurities, and 99.0 area% or more of the microstructure is It is an austenite structure and has a gist in that the austenite grain size number is 8.0 to 10.5.

The non-magnetic steel further includes
(A) Cr: 1.5% or less (excluding 0%),
(B) Cu: 0.1% or less (not including 0%) and / or Ni: 0.1% or less (not including 0%),
(C) B: 0.006% or less (excluding 0%),
(D) V: 0.2% or less (excluding 0%),
Etc. may be contained.

In the nonmagnetic steel according to the present invention, a steel material satisfying the above-described component composition is heated at a heating temperature T in the range of 1000 to 1250 ° C., and the Z value represented by the following formula (1) is 1800 to 2300. After being held as such, it can be manufactured by hot rolling and cooling at an average cooling rate in the temperature range of 750 to 500 ° C. after finish rolling at 100 ° C./min or more. In the following formula (1), t means a holding time (minute) at the heating temperature T.
Z value = T × log (t) (1)

  According to the present invention, after appropriately controlling C, Si, Mn, and N acting as an austenite stabilizing element, 99.0 area% or more of the microstructure is an austenite structure, and the austenite grain size number is in a predetermined range. Therefore, the austenite phase can be stabilized even at a low temperature without impairing the ductility of the austenite phase, which is a nonmagnetic phase. As a result, it is possible to provide a nonmagnetic steel that can achieve high strength, high proof stress, and low magnetic permeability, has good bending workability at room temperature, and is excellent in bending workability at low temperature.

FIG. 1 is a schematic diagram showing the shape of a test piece used in the greeble test. FIG. 2 is a schematic diagram showing a heat pattern in a greeble test.

  The nonmagnetic steel according to the present invention has C of 0.8 to 1.2%, Si of 0.1 to 0.6%, Mn of over 13%, 20% or less, Al of 0.001% or more, 0 0.02% or less (excluding 0%), S: 0.045% or less (excluding 0%), and N in the range of 0.025 to 0.05% ing. The reason for specifying such a range is as follows.

  C is an element effective for stabilizing the austenite phase, which is a nonmagnetic phase. If the content is less than 0.8%, the nonmagnetic properties are impaired. In addition, when bending is performed at room temperature or low temperature, the austenite phase becomes unstable, and processing-induced martensite is generated in the bent portion to generate cracks. Therefore, the C content is set to 0.8% or more. The amount of C is preferably 0.83% or more, and more preferably 0.85% or more. However, when the amount of C is excessive, coarse carbonitrides are produced, which causes a decrease in nonmagnetic properties and a deterioration in toughness. In addition, the work hardenability of the austenite phase is increased, and forgeability and machinability are greatly reduced. Therefore, the C amount is set to 1.2% or less. The amount of C is preferably 1.1% or less, and more preferably 1.0% or less.

  Si acts as a deoxidizer during melting and has the effect of lowering the temperature (Ms point) at which austenite transforms into martensite, and thus is effective as an austenite phase stabilizing element that substitutes for Cr. Moreover, it is an element which acts as a solid solution strengthening element of the austenite phase and contributes to improvement of 0.2% proof stress. Therefore, in the present invention, the Si amount is determined to be 0.1% or more. The amount of Si is preferably 0.15% or more, and more preferably 0.2% or more. However, when the amount of Si is excessive, hot workability is impaired, the productivity of the steel material is greatly reduced, and a decarburized layer is generated, thereby increasing the relative magnetic permeability. Moreover, when it adds excessively, the stacking fault energy of steel materials will reduce, and it will cause the deterioration of the bending workability at normal temperature and the bending workability at low temperature with ductility fall. Therefore, the Si amount is set to 0.6% or less. The amount of Si is preferably 0.50% or less, and more preferably 0.4% or less.

Mn is an important element as an austenite phase forming element, and is an element necessary for improving bending workability at normal temperature and bending workability at low temperature. Therefore, in the present invention, the amount of Mn is more than 13%, preferably 13.5% or more, more preferably 14.0% or more. However, when Mn is contained excessively, a Mn ₃ P compound or coarse MnS precipitates at the grain boundary, the hot ductility is remarkably lowered, and the productivity of the steel material is remarkably deteriorated. In addition, since ε martensite transformation is likely to occur, it leads to an increase in crack generation sources during bending, and cracks occur. Therefore, the amount of Mn is set to 20% or less. The amount of Mn is preferably 18.0% or less, more preferably 16.0% or less.

  Al has the effect of reducing the diffusion rate of C in austenite and reducing the adverse effects of surface decarburization during hot rolling. In order to exert such an effect, in the present invention, the Al amount needs to be 0.001% or more, preferably 0.002% or more, more preferably 0.003% or more. However, when the amount of Al becomes excessive, solid solution N effective for stabilizing the austenite phase precipitates as AlN, so that the austenite phase becomes unstable and the relative magnetic permeability increases. Moreover, since Al has the effect | action which raises the temperature (Ms point) which a martensitic transformation starts, a process induction martensite produces | generates in a bending process part, and especially low-temperature bending workability cannot be improved. Therefore, the Al content is determined to be less than 0.02%. The amount of Al is preferably 0.015% or less, and more preferably 0.010% or less.

P is an inevitable impurity, and if contained excessively, Mn ₃ P precipitates at the austenite grain boundaries, and the hot workability is remarkably lowered, so that the productivity of the steel material is deteriorated. Moreover, the weldability and bending workability of the steel material are also impaired. Therefore, it is desirable to reduce the amount of P as much as possible, but it is preferable to make it 0.040% or less in consideration of economy. The amount of P is more preferably 0.035% or less, still more preferably 0.030% or less. The amount of P is desirably reduced as much as possible, and the lower limit is not particularly limited, but is usually about 0.003%.

  S is an unavoidable impurity. If it is excessively contained, the hot workability is remarkably deteriorated, so that the productivity of the steel material is deteriorated. Moreover, when it precipitates as MnS after hot rolling, since the solid solution Mn effective for stabilization of an austenite phase is reduced, the relative magnetic permeability is increased. Therefore, it is desirable to reduce the amount of S as much as possible, but it is preferable to make it 0.045% or less in consideration of economy. The amount of S is more preferably 0.03% or less, still more preferably 0.015% or less, and particularly preferably 0.010% or less. The amount of S is desirably reduced as much as possible, and the lower limit is not particularly limited, but is usually about 0.001%.

  N, like C, is an element that is effective for stabilizing the austenite phase, and effectively acts to increase the strength. If the N content is less than 0.025%, processing-induced martensite is likely to be generated during bending, and cracks are generated in the bent part both in bending at normal temperature and bending at low temperature. Therefore, the N amount is determined to be 0.025% or more. The amount of N is preferably 0.028% or more, more preferably 0.030% or more. However, when the amount of N is excessive, defects such as blow holes are easily generated in the steel, and the productivity of the steel material is significantly reduced. Moreover, bending workability and cold workability are reduced. Therefore, the N content is 0.05% or less, preferably 0.045% or less, more preferably 0.04% or less.

  The balance of the nonmagnetic steel is inevitable impurities other than iron and P and S.

  The nonmagnetic steel may further contain (a) Cr, (b) Cu and / or Ni, (c) B, (d) V.

[(A) Cr: 1.5% or less (excluding 0%)]
Cr is an element useful for stabilizing the austenite phase. In addition, precipitation of fine carbides and dispersion thereof is effective in improving 0.2% proof stress. In order to exhibit such an action effectively, Cr is preferably contained in an amount of 0.2% or more, more preferably 0.3% or more, and further preferably 0.4% or more. However, even if Cr exceeds 1.5%, the 0.2% yield strength improvement effect is saturated. Moreover, when it contains excessively, it will become easy to produce | generate a (delta) ferrite phase and a coarse carbide | carbonized_material, and 0.2% yield strength will fall on the contrary. In addition, non-magnetic properties and bending workability at normal and low temperatures are also reduced. Therefore, the Cr content is preferably 1.5% or less, more preferably 1.3% or less, and still more preferably 1.0% or less.

[(B) Cu: 0.1% or less (not including 0%) and / or Ni: 0.1% or less (not including 0%)]
Cu and Ni are elements that contribute to stabilization of the austenite phase, and also act to improve toughness. In order to effectively exhibit such an action, it is preferable to contain both Cu and Ni in an amount of 0.01% or more, and more preferably 0.03% or more. However, when Cu is contained excessively exceeding 0.1%, the hot workability is remarkably lowered, so that the productivity of the steel material is deteriorated. Therefore, the Cu content is preferably 0.1% or less, more preferably 0.09% or less, and still more preferably 0.08% or less. On the other hand, if Ni is contained in excess of 0.1%, the austenite phase is excessively stabilized and the yield strength is reduced by 0.2%. Therefore, the Ni content is preferably 0.1% or less, more preferably 0.09% or less, and still more preferably 0.08% or less.

[(C) B: 0.006% or less (excluding 0%)]
B is an element useful for increasing the grain boundary strength of the austenite structure and improving the productivity of the steel material. In order to exhibit such an action effectively, the B content is preferably 0.0010% or more, more preferably 0.0011% or more, and further preferably 0.0012% or more. But if excessively contained, Fe ₂ B is precipitated along the austenite grain boundaries, the productivity of the steel decreases the grain boundary strength is deteriorated. In addition, the 0.2% proof stress decreases, and the bending workability at normal and low temperatures deteriorates. Accordingly, the B content is preferably 0.006% or less, more preferably 0.005% or less, and still more preferably 0.004% or less.

[(D) V: 0.2% or less (excluding 0%)]
V is a carbide-forming element and is an element that effectively acts to improve 0.2% yield strength. Moreover, since V carbide precipitates in both the crystal grains and the grain boundaries, it is possible to reduce the influence of the decrease in grain boundary strength caused by the remaining carbides at the crystal grain boundaries. In order to effectively exert such effects, V is preferably contained in an amount of 0.02% or more, more preferably 0.03% or more, and further preferably 0.04% or more. However, when it contains excessively, hot workability will fall remarkably and the productivity of steel materials will worsen. Therefore, the V amount is preferably 0.2% or less, more preferably 0.1% or less, and still more preferably 0.08% or less.

  In the nonmagnetic steel according to the present invention, after satisfying the above component composition, 99.0% by area or more of the microstructure is an austenite structure. By setting the area ratio of the austenite structure to 99.0% or more, nonmagnetic characteristics can be secured. Further, when the area ratio of the austenite structure is less than 99.0%, for example, when grain boundary carbides are excessively produced, bending workability at normal temperature and bending workability at low temperature deteriorate. Therefore, in the present invention, the area ratio of the austenite structure is 99.0% or more, preferably 99.5% or more, more preferably 99.9% or more, and most preferably 100.0%.

  The austenite grain size number is 8.0 to 10.5. When the austenite grain size is less than 8.0, the tensile strength and 0.2% proof stress cannot be secured, the grain boundary strength is lowered, and the bending workability at normal temperature and low temperature is deteriorated. Therefore, in the present invention, the crystal grain size number is 8.0 or more, preferably 8.3 or more, more preferably 8.5 or more. However, if the grain size number becomes too large, the austenite phase becomes too fine, so the area of the grain boundaries that form carbide nuclei increases, making it easier for grain boundary carbides to form during cooling after hot rolling, and low-temperature bending. Workability is reduced. Therefore, the crystal grain size number is 10.5 or less, preferably 10.0 or less, more preferably 9.5 or less.

  The method for calculating the area ratio of the austenite structure in the microstructure and the crystal grain size number will be described in the Examples section described later.

The nonmagnetic steel according to the present invention is expressed by the following formula (1) after melting and casting a steel material satisfying the above-described component composition according to a conventional method, and heating at a heating temperature T in the range of 1000 to 1250 ° C. It can hold | maintain so that Z value may be set to 1800-2300, Then, it hot-rolls, It can manufacture by cooling by making the average cooling rate in the temperature range of 750-500 degreeC after finish rolling into 100 degreeC / min or more. In the following formula (1), t means a holding time (minute) at the heating temperature T.
Z value = T × log (t) (1)

  This will be described in detail below.

[Heating temperature T: 1000 to 1250 ° C.]
By setting the heating temperature T to 1000 ° C. or higher, the alloy components can be completely dissolved in the matrix phase, and the grain boundary carbides generated in the cooling process after casting can be re-dissolved, so austenite An organization can be secured. The heating temperature T is preferably 1050 ° C. or higher, more preferably 1060 ° C. or higher. However, when the heating temperature T exceeds 1250 ° C., in addition to the economic cost reduction of heating costs, it causes grain boundary precipitation of Mn-based compounds and coarsening of austenite crystal grains, resulting in a decrease in hot ductility and a significant decrease in low-temperature bending workability. Invite. Accordingly, the heating temperature T is set to 1250 ° C. or lower, preferably 1200 ° C. or lower, more preferably 1150 ° C. or lower.

[Z value: 1800-2300]
In the present invention, after the heating temperature T is controlled within the above range, the Z value calculated based on the heating temperature T (° C.) and the holding time t (minutes) at the heating temperature T is in the range of 1800 to 2300. It is necessary to control the heating temperature T and the holding time t so that The Z value is a parameter set by the inventors by repeating various experiments.

  By controlling the Z value in the range of 1800 to 2300, grain boundary embrittlement and decarburization after hot rolling can be suppressed, and the grain boundary carbides generated during casting can be re-dissolved. That is, when the Z value is less than 1800, the re-solution of the grain boundary carbides becomes insufficient, the austenite structure cannot be ensured in a predetermined amount or more, and the relative magnetic permeability is lowered. Further, a large amount of carbide is generated at the grain boundary, and the 0.2% yield strength is lowered. In addition, since a large amount of grain boundary carbides is generated, the amount of solute C that contributes to stabilizing the austenite structure is decreased. Therefore, bending is performed both when bent at room temperature and when bent at low temperature. Work-induced martensite is generated in the machined part, causing cracks and causing cracks. Therefore, the Z value needs to be 1800 or more, preferably 1900 or more, more preferably 2000 or more. However, if the Z value exceeds 2300, the amount of austenite structure formed cannot be ensured, and the nonmagnetic characteristics deteriorate. Further, decarburization occurs after hot rolling, and the tensile strength and 0.2% yield strength are not achieved. Moreover, since grain boundary embrittlement occurs and toughness decreases, cracking occurs particularly when bending is performed at a low temperature. Therefore, the Z value is 2300 or less. The Z value is preferably 2250 or less, more preferably 2200 or less.

  The Z value may be calculated by substituting the average temperature during the holding period into the equation (1) as the heating temperature T (unit: ° C.).

[Hot rolling]
After holding the Z value so as to satisfy 1800-2300, hot rolling is performed according to a conventional method. Since the austenite structure is maintained up to room temperature after the end of hot rolling, the finish rolling temperature at the time of hot rolling is not particularly limited from the viewpoint of the structure transformation. In order to reduce the load on the steel sheet, the finish rolling temperature is preferably 800 ° C. or higher. The finish rolling temperature is more preferably 850 ° C. or higher. Although an upper limit is not specifically limited, For example, it is 900 degreeC.

  When cooling after finish rolling, in the present invention, it is necessary to cool at an average cooling rate in a temperature range of 700 to 500 ° C. at 100 ° C./min or more. Since the austenite phase in high-Mn nonmagnetic steel is a metastable phase, if it is kept for a certain time or more in the temperature range of 700 to 500 ° C., grain boundary carbides precipitate and the amount of austenite structure formed cannot be secured, and nonmagnetic Characteristics are degraded. Further, precipitation of grain boundary carbides cannot improve bending workability (particularly bending workability at a low temperature). Therefore, the average cooling rate in the 700-500 degreeC temperature range after finish rolling shall be 100 degrees C / min or more. The average cooling rate is preferably 110 ° C./min or more, more preferably 120 ° C./min or more. Although an upper limit is not specifically limited, Usually, it is about 180 degreeC / min.

  After cooling to 500 ° C., it may be cooled according to a conventional method, for example, it may be cooled to room temperature.

  The non-magnetic steel according to the present invention constitutes a structural member such as a linear motor car or power generation / transmission facility exposed to a strong magnetic field, a structural member such as a medical facility in which a weak magnetic field is a problem, or a non-magnetic portion of an electromagnetic control component. It can be used as a material to be used.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel of the composition shown in the following Table 1 (the balance is inevitable impurities other than iron and P, S) is melted in a 150 kg vacuum furnace, and the resulting steel ingot is forged and processed into a wire rod of φ20 mm × 1 m, A 155 mm × 155 mm square was manufactured.

  First, a greeble test was performed using the wire (φ20 mm × 1 m) to evaluate manufacturability. Specifically, the test piece shown in FIG. 1 was sampled from the above-mentioned φ20 mm × 1 mm wire, and a hot tensile test was performed with a processing for master tester manufactured by Fuji Electric Koki.

  The test conditions were carried out with the heat pattern shown in FIG. 2 simulating continuous casting. That is, it is heated to 1300 ° C. at an average temperature rising rate of 10 ° C./second, held at this temperature for 5 minutes, then cooled to a predetermined test temperature T1 at an average cooling rate of 5 ° C./second, and this test temperature T1 for 2 minutes. Retained. After the holding, the drawing value was measured at a pulling speed of 0.01 mm / second, and the gas was quenched after breaking. The test temperature T1 was set at five levels of 700 ° C, 800 ° C, 900 ° C, 1000 ° C, and 1100 ° C.

  When the drawing value is 20% or more at all the five levels of the test temperature, the manufacturability is evaluated as good (passed), and a circle mark is shown in the column of the grease drawing in Tables 2 and 3 below. Was judged to have mass productivity. On the other hand, when the drawing value is less than 20% at at least one of the above five levels, it is evaluated that the manufacturability is bad (failed), and an x mark is placed in the column of the grease drawing in Tables 2 and 3 below. It was judged that mass production was not possible with actual equipment. When the aperture value in the greeble test was less than 20%, the subsequent test was not performed.

  About what was evaluated as favorable (passed) manufacturability, the following test was done using the square material of 155mmx155mm manufactured separately from the said φ20mmx1m wire. That is, after welding a square material of 155 mm × 155 mm to a dummy billet, it is heated to a heating temperature T (° C.) shown in Tables 2 and 3 below, and holding times t (minutes) shown in Tables 2 and 3 below at this temperature. After being held, it was hot-rolled to obtain a φ16 mm wire, cooled on a cooling conveyor to 500 ° C. or lower, and then wound into a coil and cooled to room temperature.

  Tables 2 and 3 below show the Z value (= T × log (t)) calculated based on the heating temperature T and the holding time t and the average cooling rate (° C./min) in the temperature range of 750 to 500 ° C. Show. In addition, the heating temperature T shown in following Table 2 and Table 3 means the average temperature in a holding | maintenance period.

  With respect to the coiled φ16 mm wire obtained by cooling, the microstructure identification, austenite fraction, and austenite grain size were measured by the following methods.

[Microstructure identification and austenite area ratio]
After embedding it in a supporting substrate with the cross section of the above-mentioned wire (φ16 mm wire) exposed, polishing, immersing it in a nital solution, and corroding it, the D / 4 position (D is the diameter) with an optical microscope Photographs were taken at observation magnifications of 100 and 400, and the presence or absence of precipitates such as carbides was examined, and the microstructure was identified. By corroding with a nital liquid, austenite grain boundaries, twins, and martensite structures can be determined from their shapes and colors in addition to the presence or absence of precipitates such as carbides.

  The fraction of the austenite structure was a value obtained by subtracting the total area ratio of the area ratio of precipitates such as carbides and the area ratio of the martensite structure from 100% using the photograph taken at the observation magnification of 100 times. The above-mentioned austenite grain boundaries and twins do not have an adverse effect on low-temperature bending workability, and precipitates such as carbides and martensite can be visually distinguished, so the austenite grain boundaries and twin areas The rate was previously removed based on the color tone change by image analysis. When it was difficult to distinguish between austenite grain boundaries and grain boundary carbides, photographs taken at an observation magnification of 400 times were used as a reference for judgment.

  In addition, fine precipitates having a particle size of 0.2 μm or less that cannot be identified by a photograph taken at an observation magnification of 100 times do not adversely affect bending workability, and thus were ignored when calculating the area ratio of the austenite structure.

  The procedure for calculating the area ratio of the austenite structure will be specifically described as follows. First, a photograph taken at an observation magnification of 100 times is opened with Photoshop (version 5.1), and austenite grain boundaries, twins, polishing wrinkles, and corrosion stains are identified and removed. It was a part that should be done. When it was difficult to distinguish between the portion to be removed and the grain boundary carbide, a photograph taken at an observation magnification of 400 times was referred to. Next, a part of the part to be removed was selected by the automatic selection tool of Photoshop, and a continuous part was automatically selected to remove it. While confirming that the remainder after the removal was not a precipitate or the like, this process was repeated to create a photograph in which austenite grain boundaries, twins, polishing flaws, corrosion stains, and the like were removed. The obtained photograph was binarized by image analysis to obtain a black-and-white photograph, and the area ratio of the portion shown in black was determined as the total area ratio of precipitates such as carbides and martensite structure. The value obtained by subtracting the total area ratio from 100% was defined as the area ratio of the austenite structure. The calculated area ratio (%) of austenite is shown in Tables 2 and 3 below.

[Austenite grain size number]
As for the crystal grain size number of austenite, the D / 4 position after being corroded by immersing in the above-mentioned nital liquid according to JIS G0551 was measured for four visual fields with an optical microscope, and the average value of the four visual fields measured was determined. The obtained grain size numbers of austenite are shown in Tables 2 and 3 below.

  Next, a JIS standard test piece was taken from a wire straightened with a roll straightening machine (φ16 mm) (hereinafter referred to as a straight straightening material) wound in a coil shape by hot rolling, and a tensile test was performed. . In addition, the magnetic properties and bending workability of the straightening material were evaluated. The scale and the decarburized layer formed on the surface of the straightening material are removed at the time of processing into a test piece shape used for a tensile test, evaluation of magnetic characteristics, and evaluation of bending workability.

[Tensile test]
A JIS No. 14A test piece was sampled from the straightening material so that the axis was in the longitudinal direction of the test piece, and the tensile strength and 0.2% proof stress were measured at room temperature according to JIS Z2241. The measurement results of tensile strength and 0.2% yield strength are shown in Tables 2 and 3 below. In the present invention, based on SD345 defined in JIS G3112, a case where the tensile strength is 490 MPa or more and the 0.2% proof stress is 345 to 440 MPa is regarded as acceptable, and at least the tensile strength or the 0.2% proof stress is at least. A case in which one of them deviated from the standard was regarded as a failure.

[Magnetic properties]
The magnetic properties of the straightening material were evaluated based on the relative permeability. The relative permeability was measured using a vibration sample type magnetization automatic measuring device (BHV-3.5 manufactured by Riken Denshi Co., Ltd.) by collecting a cube of 5 mm square from the straightening material. The measurement results of the relative magnetic permeability are shown in Tables 2 and 3 below. In the present invention, the case where the relative permeability was less than 1.1 was evaluated as “nonmagnetic steel”, and the case where the relative permeability was 1.1 or more was evaluated as “magnetic steel”.

[Bending workability]
The bending workability of the straightening material is determined by the presence or absence of breakage (cracking) when a bending test (3-point bending) is performed using a No. 2 test piece of φ14 mm collected according to JIS Z2248 and the bending angle is 180 °. The surface properties were confirmed and evaluated. The push bending test was performed based on two standards of 20 ° C. and −20 ° C. When there was no rupture and there was no abnormality in the surface properties, it was accepted (indicated by a circle in Tables 2 and 3), when it was broken or when defects such as wrinkles were observed on the surface were rejected (Table 2). In Table 3, it is indicated by an x mark). In this invention, the case where it passed when tested at -20 degreeC was evaluated as "it is excellent in low-temperature bending workability."

  The following Table 2 and Table 3 can be considered as follows. No. 1 to 6 and 13 to 20 are examples that satisfy the requirements defined in the present invention, have a desired tensile strength and 0.2% proof stress, and have a relative permeability less than a predetermined value. It can be seen that the bending workability at normal temperature and the bending workability at low temperature can be improved.

  On the other hand, no. Nos. 7 to 12, 21 to 29, 34, 36, and 37 are examples that do not satisfy the requirements defined in the present invention.

Of these, No. Nos. 7 to 12 are examples that do not satisfy the production conditions defined in the present invention. No. 7 was an example in which the heating temperature was too high. The crystal grain size number of the austenite structure was 7.5 and the crystal grains were coarsened, so the tensile strength and 0.2% yield strength were not achieved. In addition, since Mn-based compounds (for example, Mn ₃ P and MnS) are generated at the grain boundaries and the grain boundary strength is reduced, cracks occur both when bent at 20 ° C. and when bent at −20 ° C. There has occurred.

  No. No. 8 is an example in which the holding time t was long and the balance between the heating temperature T and the holding time t was poor, and the Z value exceeded 2300. For this reason, the amount of austenite structure formed cannot be secured. Moreover, since the Mn-PS system compound was generated to cause grain boundary embrittlement or decarburization occurred after hot rolling, the tensile strength and 0.2% yield strength were not achieved. Moreover, since the grain boundary embrittlement occurred, the toughness decreased, and cracking occurred when bending at -20 ° C.

  No. 9 and 10 are examples in which the average cooling rate in the temperature range of 750 to 500 ° C. was below the range defined in the present invention and was too small after hot rolling. As a result, a large amount of carbides were generated at the grain boundaries, the amount of austenite structure formed could not be ensured, and the relative permeability increased. Further, since a large amount of carbide was generated at the grain boundaries, the 0.2% yield strength was not achieved. In addition, since a large amount of grain boundary carbides is generated, the amount of solute C that contributes to stabilizing the austenite structure is reduced. Therefore, when bending is performed at −20 ° C., work-induced martensite is generated in the bent portion. Cause cracking and cracking. No. For No. 9, cracking also occurred when bending at 20 ° C.

  No. 11 is an example in which the balance between the heating temperature T and the holding time t is poor and the Z value is less than 1800. For this reason, carbide remained even after the hot rolling was completed, and the amount of austenite structure formed could not be ensured, so that the relative magnetic permeability increased. Further, since a large amount of carbide was generated at the grain boundaries, the 0.2% yield strength was not achieved. Further, since a large amount of grain boundary carbides is generated, the amount of solute C that contributes to stabilizing the austenite structure is decreased. Therefore, both when bent at 20 ° C. and when bent at −20 ° C. Then, work-induced martensite was generated in the bent part, causing cracks and causing cracks.

  No. No. 12 is an example in which the heating temperature T is low, the balance between the heating temperature T and the holding time t is poor, and the Z value is less than 1800. Even after the hot rolling was completed, carbide remained, and the amount of austenite structure formed could not be secured, so that the relative magnetic permeability increased. In addition, since a large amount of carbide was generated at the grain boundaries, the tensile strength and 0.2% yield strength were not achieved. Further, since a large amount of grain boundary carbides is generated, the amount of solute C that contributes to stabilizing the austenite structure is decreased. Therefore, both when bent at 20 ° C. and when bent at −20 ° C. Then, work-induced martensite was generated in the bent part, causing cracks and causing cracks.

  No. 21 to 29, 34, 36, and 37 are all examples that do not satisfy the component composition defined in the present invention.

  No. No. 21 is an example in which the amount of C was large, and since the amount of austenite structure formed could not be ensured, the relative permeability increased. Moreover, many grain boundary carbides were generated, and cracks occurred both when bent at 20 ° C. and when bent at −20 ° C. No. No. 22 is an example in which the amount of C was small, and the relative magnetic permeability was high. Further, the austenite structure became unstable, and when it was bent at 20 ° C. and when bent at −20 ° C., work-induced martensite was generated in the bent portion, and cracking occurred.

  No. No. 23 was an example in which the amount of Si was small. The strength of the austenite structure was insufficient, so the 0.2% yield strength was not achieved. No. No. 24 is an example in which the amount of Si was large, which resulted in the formation of a decarburized layer during hot rolling, and the relative permeability increased. Moreover, when it bent at 20 degreeC and when bent at -20 degreeC, the process induction martensite produced | generated in the bending process part, and the crack generate | occur | produced.

No. No. 25 was an example in which the amount of Mn was large. Mn ₃ P compounds and coarse MnS were precipitated at the grain boundaries, so that the hot workability was remarkably deteriorated and the productivity of the steel material was deteriorated. No. No. 26 is an example in which the amount of Mn is small, the austenite structure becomes unstable, and work-induced martensite is generated in the bent portion both when bent at 20 ° C. and when bent at −20 ° C. Cracking occurred. No. 27 is an example in which the amount of Mn was small, and the relative magnetic permeability was high. Moreover, the austenite structure became unstable, and when bending was performed at −20 ° C., work-induced martensite was generated in the bent portion and cracking occurred.

No. No. 28 was an example in which the amount of P was large, and the Mn ₃ P compound was precipitated at the grain boundary, so that the hot workability was remarkably deteriorated, and the productivity of the steel material was deteriorated. No. No. 29 was an example in which the amount of S was large. MnS was precipitated at the grain boundaries to cause grain boundary embrittlement, the hot workability was remarkably deteriorated, and the productivity of the steel material was deteriorated.

  No. No. 34 is an example in which the amount of Al was large, and since the solid solution N contributing to the stabilization of the austenite structure was precipitated as AlN, the relative permeability increased. In addition, Al raises the martensite transformation start temperature (Ms point) and generates work-induced martensite in the bending part. Therefore, cracking occurs particularly when bending is performed at −20 ° C., and low-temperature bending is performed. The sex could not be improved.

  No. No. 36 is an example in which the amount of N is small, and the austenite structure becomes unstable, and when the bending process is performed at 20 ° C. and when the bending process is performed at −20 ° C., induced martens are formed in the bending process part. The site was generated and cracked. No. No. 37 is an example in which the amount of N was large. Defects such as blow holes were generated in the steel material, and the productivity of the steel material was deteriorated.

  No. Reference numerals 30 to 33 and 35 are reference examples outside the range recommended in the present invention.

  No. No. 30 is an example in which Cu is excessively contained exceeding the range recommended in the present invention, and the hot workability is remarkably deteriorated, and the productivity of the steel material is deteriorated.

  No. No. 31 is an example in which Ni was excessively contained exceeding the range recommended in the present invention, austenite was excessively stabilized, and 0.2% proof stress was not achieved.

  No. No. 32 is an example in which V is excessively contained exceeding the range recommended in the present invention, and carbide precipitates in the particle size, causing intergranular embrittlement, and hot workability is remarkably deteriorated. Worsened.

  No. No. 33 is an example containing excessive Cr exceeding the range recommended in the present invention, because coarse carbides were generated at the grain boundaries, and the amount of austenite structure formed could not be secured, so the relative permeability increased. . In addition, because the coarse carbides formed at the grain boundaries, the target 0.2% yield strength was not achieved, and cracking occurred both when bent at 20 ° C. and when bent at −20 ° C. There has occurred.

No. No. 35 is an example of excessively containing B beyond the range recommended in the present invention. Fe ₂ B precipitates along the austenite grain boundary and the grain boundary strength decreases, so that the 0.2% proof stress is not targeted. Achieved. Also. Cracks occurred both when bending at 20 ° C. and when bending at −20 ° C.

Claims (6)

C: 0.8 to 1.2% (meaning mass%, hereinafter the same for chemical components),
Si: 0.1 to 0.6%,
Mn: more than 13%, 20% or less,
Al: 0.001% or more, less than 0.02%,
P: 0.040% or less (excluding 0%),
S: 0.045% or less (excluding 0%) and N: 0.025-0.05%,
The balance is iron and inevitable impurities,
99.0 area% or more of the microstructure is an austenite structure,
A nonmagnetic steel excellent in low temperature bending workability, characterized in that the austenite grain size number is 8.0 to 10.5.   The nonmagnetic steel according to claim 1, further comprising Cr: 1.5% or less (not including 0%). The nonmagnetic steel according to claim 1 or 2, further comprising Cu: 0.1% or less (not including 0%) and / or Ni: 0.1% or less (not including 0%).   The nonmagnetic steel according to any one of claims 1 to 3, further comprising B: 0.006% or less (not including 0%).   The nonmagnetic steel according to any one of claims 1 to 4, further comprising V: 0.2% or less (not including 0%). A method for producing the nonmagnetic steel according to any one of claims 1 to 5 ,
After heating the steel material satisfying the component composition of the nonmagnetic steel at a heating temperature T in the range of 1000 to 1250 ° C. and holding the Z value represented by the following formula (1) to be 1800 to 2300, A method for producing nonmagnetic steel excellent in low-temperature bending workability, characterized by performing hot rolling and cooling at an average cooling rate in a temperature range of 750 to 500 ° C after finish rolling at 100 ° C / min or more.
Z value = T × log (t) (1)
[In the above formula (1), t means the holding time (minutes) at the heating temperature T. ]

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