JP2008127590A - Austenitic stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide austenitic stainless steel having excellent strength, ductility and corrosion resistance. <P>SOLUTION: The stainless steel has a composition which consists of, by weight, 0.005 to 0.25% C, <2.0% Si, >0.2 to <10.0% Mn, ≤0.03% P, ≤0.05% S, 15.0 to 30.0% Cr, 0.05 to 8.0% Mo, 0 to 8.0% W, ≤0.030% Al, ≤0.020% O, >0.8 to 1.30% N and the balance essentially Fe with inevitable impurities and in which the values of Nieq, PRE and OCRE, each defined as follows, are made to ≥20, ≥5 and ≥25, respectively: Nieq=[Ni]+0.41[Cr]-0.01[Cr]<SP>2</SP>-2[Si]-0.4([Mo]+[W])+[Co]+0.6([Cu]+[Mn])+10([C]+[N]); PRE=ä[Cr]+3.3([Mo]+0.5[W])+16[N]}/[Mn]; OCRE=[Cr]+1.5([Mo]+0.5[W])+[Ni]+[Cu]-0.2[Mn]+2[N]. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel.

  Conventionally, maraging steels such as 18Ni series, which have a relatively good balance between strength and ductility, are used for structural members that require both high strength and high ductility. Further, precipitation hardened stainless steels such as 17-4PH and 17-7PH, metastable austenitic stainless steels such as SUS301, and the like are used at sites where further corrosion resistance is required.

  However, these steel types often have insufficient corrosion resistance in a highly corrosive environment containing chlorides, for example, in a marine environment.

  On the other hand, in applications where strength, ductility and corrosion resistance are required at the same time, the use of Ni-based alloys, titanium alloys, etc. may be considered. However, these alloys are expensive and prone to excessive quality. Therefore, development of inexpensive and highly reliable stainless steel is required, and the need for higher strength, higher ductility, and higher corrosion resistance is increasing.

  For example, in Patent Document 1, as stainless steel having excellent corrosion resistance in a marine environment, Cr: 16.52%, Ni: 14.23%, Mo: 2.25%, N: 0.00. 52%, C: 0.004%, Si: 0.01%, Mn: 0.01%, P: 0.001%, S: 0.001%, Al: 0.05%, O: 0.002 An austenitic stainless steel is disclosed in which the balance is Fe and the balance is Fe and inevitable impurities.

  Further, for example, in Patent Document 2, as stainless steel having high strength and high corrosion resistance, C: 0.15% or less, Si: 1.0% or less, Mn: 3.0 to 12.0% by weight% , P: 0.030% or less, Ni: 0.50% or less, Cr: 15.0-21.0%, N: 0.70-1.50%, Al: 0.020% or less, and O : An austenitic stainless steel containing 0.020% or less and the balance being substantially Fe is disclosed.

JP 2000-309857 A JP 2002-235153 A

  However, the conventionally known austenitic stainless steel has room for improvement in the following points.

  That is, in the austenitic stainless steel of Patent Document 1, Mn is hardly added and Ni is further added to improve corrosion resistance.

  However, in such a component range, in order to sufficiently dissolve Cr-based nitrides harmful to ductility and corrosion resistance, for example, heat treatment at a very high temperature exceeding 1250 ° C. is required. Such a solution heat treatment at a high temperature is disadvantageous because the crystal grains are coarsened and the manufacturing cost is increased. Also, depending on the temperature constraints on the existing heat treatment equipment, a large number of undissolved Cr-based nitrides are likely to remain, and sufficient corrosion resistance may not be obtained.

  In addition, the austenitic stainless steel of Patent Document 2 contains a large amount of N and suppresses the Mn content. Therefore, it can be said that the corrosion resistance is improved and the strength is moderately high.

  However, for use under more severe conditions, it is desirable to further improve the strength and corrosion resistance.

  Therefore, the problem to be solved by the present invention is to provide an austenitic stainless steel excellent in strength, ductility and corrosion resistance.

The austenitic stainless steel according to the present invention is, by weight, C: 0.005 to 0.25%, Si: less than 2.0%, Mn: more than 0.2 to less than 10.0%, P: 0.00. 03% or less, S: 0.05% or less, Cr: 15.0 to 30.0%, Mo: 0.05 to 8.0%, W: 0 to 8.0% or less, Al: 0.030% Nieq containing O: 0.020% or less and N: more than 0.8 to 1.30%, the balance being substantially made of Fe and inevitable impurities, and defined by the following formula (1) And the value of PRE defined by the following equation (2) is 5 or more, and the value of OCRE defined by the following equation (3) is 25 or more.
^{Nieq = [Ni] +0.41 [Cr} ] -0.01 [Cr] 2 -2 [Si] -0.4 ([Mo] + [W]) + [Co] +0.6 ([Cu] + [ Mn]) + 10 ([C] + [N]) (1)
PRE = {[Cr] +3.3 ([Mo] +0.5 [W]) + 16 [N]} / [Mn] (2)
OCRE = [Cr] +1.5 ([Mo] +0.5 [W]) + [Ni] + [Cu] −0.2 [Mn] +2 [N] (3)

  Here, the austenitic stainless steel is selected from Ni: more than 0.5 to less than 7.0%, Cu: 0.01 to 4.0%, and Co: 0.01 to 5.0%. You may further contain 1 type, or 2 or more types.

  The austenitic stainless steel is Ti: 0.01 to 0.5%, Nb: 0.01 to 0.5%, V: 0.01 to less than 1.0%, Ta: 0.01 to 0 It may further contain one or more selected from 0.5% and Zr: 0.01 to 0.5%.

  Further, the austenitic stainless steel is one type selected from B: 0.0005 to 0.01%, Ca: 0.001 to 0.01%, and Mg: 0.001 to 0.01%. Two or more kinds may be further contained.

  The austenitic stainless steel may further contain one or more selected from Te: 0.005 to 0.05% and Se: 0.01 to 0.20%. .

  In the austenitic stainless steel, the maximum diameter of the Cr-based nitride is preferably 2 μm or less in the solution heat treatment state, and the area ratio of the Cr-based nitride is preferably 1% or less.

  The austenitic stainless steel preferably has a tensile strength of 1000 MPa or more, a drawing of 65% or more, and a breaking elongation of 50% or more in a solution heat treatment state.

  The austenitic stainless steel has a tensile strength of 1600 MPa or more, a drawing of 60% or more, and a breaking elongation of 20% or more by applying cold working with a cross-section reduction rate of 20 to 40%. Good to be.

  The austenitic stainless steel has a tensile strength of 1800 MPa or more, a drawing of 50% or more, and a breaking elongation of 10% or more by applying cold working with a cross-section reduction rate of 40 to 60%. Good to be.

  The austenitic stainless steel has a tensile strength of 2000 MPa or more, a drawing of 45% or more, and a breaking elongation of 5% or more by applying cold working with a cross-section reduction rate of 60 to 80%. Good to be.

  In the austenitic stainless steel according to the present invention, the content of essential components is within a specific range, and the values of Nieq, PRE, and OCRE defined by the above formulas (1) to (3) are specified. Is greater than or equal to the value. Therefore, it has high strength, high ductility, and high corrosion resistance as compared with the prior art.

  Here, when a specific amount of one or more selected from Ni, Cu and Co is further contained, it is easy to improve the stability of the austenite phase. It is also effective in improving corrosion resistance, toughness, impact properties, and the like.

  In addition, when a specific amount of one or more selected from Ti, Nb, V, Ta and Zr is further contained, it is effective for improving the strength, refining crystal grains, and the like.

  Further, when a specific amount of one or more selected from B, Ca and Mg is further contained, it is effective for improving hot workability.

  Further, when a specific amount of one or more selected from Te and Se is further contained, it is effective for improving machinability.

  Further, when the solution heat treatment is performed at an optimum temperature according to the composition of the steel, the maximum diameter of the Cr-based nitride is 2 μm or less, and the area ratio of the Cr-based nitride is 1% or less, Alternatively, when cold working is performed at a specific cross-section reduction rate, the balance between strength and toughness is excellent while maintaining high corrosion resistance.

  Hereinafter, the austenitic stainless steel (hereinafter sometimes referred to as “the present stainless steel”) according to an embodiment of the present invention will be described in detail. This stainless steel contains the following elements, with the balance being substantially made of Fe and inevitable impurities.

  Further, the values of Nieq, PRE, and OCRE defined by a specific formula are greater than or equal to a specific value. The types, component ranges, reasons for limitation, technical significance of each formula, and the like are as follows. In addition, the unit of a component ratio is weight%.

C: 0.005-0.25%
C is an austenite-generating element, contributes to the stabilization of the austenite phase, and is also effective in suppressing nitrogen blow. Further, C is an interstitial element and thus contributes to improvement in strength. In order to obtain the effect, the lower limit of the C content is set to 0.005% or more. The lower limit of the C content is preferably 0.01% or more, more preferably 0.02% or more.

  However, excessive addition of C tends to reduce the solubility of N. In addition, due to the generation of Cr-based carbides, the solid solution Cr of the parent phase is lowered, and the corrosion resistance tends to be deteriorated. Therefore, the upper limit of the C content is 0.25% or less. The upper limit of the C content is preferably 0.20% or less, more preferably 0.15% or less.

Si: Less than 2.0% Si acts as a deoxidizer. As a deoxidizing element in general steel, Al described later is more effective than Si. However, when Al is added excessively in high nitrogen steel, AlN is generated, which causes a significant decrease in corrosion resistance and toughness. Therefore, it is preferable to use Si as a main deoxidizer together with Mn, which is an essential element. In order to obtain the effect, the lower limit of the Si content is preferably 0.01% or more.

  On the other hand, Si is a ferrite-forming element. For this reason, when Si is added excessively, the austenite phase becomes unstable, inducing nitrogen blow, and ensuring non-magnetism tends to be difficult. Moreover, not only becomes harmful at the time of forging, but also the toughness tends to decrease.

  Therefore, the upper limit of the Si content is less than 2.0%. The upper limit of the Si content is preferably less than 1.0%, more preferably 0.50% or less.

Mn: more than 0.2 to less than 10.0% Mn is an austenite generating element and contributes to stabilization of the austenite phase. In addition, Mn is effective in suppressing nitrogen blow because it significantly increases the solubility of N. Furthermore, Mn is effective for lowering the solid solution temperature of Cr-based nitride described later. In addition, Mn is effective as a deoxidation and desulfurization element.

  In order to obtain the effect, the lower limit of the Mn content is set to more than 0.2%. The lower limit of the Mn content is preferably more than 2.0%.

  On the other hand, excessive addition of Mn tends to deteriorate the corrosion resistance. Therefore, the upper limit of the Mn content is less than 10.0%. The upper limit of the Mn content is preferably 8.0% or less.

P: 0.03% or less P is an impurity element of steel. However, an excessive reduction of P causes an increase in cost. On the other hand, excessive addition of P tends to reduce hot workability, grain boundary strength, and toughness. Therefore, the upper limit of the P content is 0.03% or less.

S: 0.05% or less S, like P, is an impurity element of steel. However, reduction of S more than necessary causes an increase in cost. On the other hand, if the addition of S becomes excessive, the corrosion resistance deteriorates due to the formation of MnS. Therefore, the upper limit of the S content is 0.05% or less. The upper limit of the S content is preferably 0.01% or less.

Cr: 15.0-30.0%
Since Cr significantly increases the solubility of N, it is not only effective for suppressing nitrogen blow, but is also an important element that greatly contributes to improvement of corrosion resistance and strength. In order to obtain the effect, the lower limit of the Cr content is set to 15.0% or more. The lower limit of the Cr content is preferably more than 21.0%.

  On the other hand, since Cr is a ferrite-forming element, if added excessively, the austenite phase tends to be destabilized and non-magnetic can not be maintained. Further, during the solution heat treatment, the amount of undissolved Cr-based nitride is increased, and the toughness is remarkably lowered. In addition, it promotes precipitation of the σ phase that causes deterioration of toughness. Therefore, the upper limit of the Cr content is 30.0% or less. The upper limit of the Cr content is preferably 27.0% or less.

Mo: 0.05-8.0%
Mo increases the solubility of N and significantly improves the corrosion resistance. Mo contributes to improving the strength as a solid solution strengthening element. In order to obtain the effect, the lower limit of the Mo content is set to 0.05% or more. The lower limit of the Mo content is preferably 0.10% or more.

  On the other hand, excessive addition of Mo destabilizes the austenite phase. Therefore, it induces nitrogen blow and tends to make it difficult to ensure non-magnetism. In addition, due to the formation of the embrittlement phase, the toughness of the ductile material is lowered and tends to be harmful during forging. Further, during the solution heat treatment, the amount of undissolved Cr-based nitride tends to increase, and the corrosion resistance tends to be remarkably reduced.

  Therefore, the upper limit of the Mo content is set to 8.0% or less. The upper limit of the Mo content is preferably less than 5.0%, more preferably less than 2.5%.

W: 0 to 8.0%
W, like Mo, contributes to an improvement in corrosion resistance and also contributes to an improvement in strength as a solid solution strengthening element. In order to obtain the effect, the lower limit of the Mo content is preferably 0.05% or more.

  On the other hand, when W is added excessively, the tough ductility is lowered due to the formation of an embrittled phase, which tends to be harmful during forging. In addition, during the solution heat treatment, the amount of undissolved Cr-based nitride tends to increase, and the corrosion resistance tends to be significantly reduced.

  Therefore, the upper limit of the W content is 8.0% or less. The upper limit of the W content is preferably 1.5% or less.

Al: 0.030% or less Al, like Si and Mn, is very effective as a deoxidizing element. On the other hand, when Al is added excessively, the generation of AlN is likely to proceed, and the corrosion resistance and toughness are significantly reduced. Therefore, the upper limit of the Al content is 0.030% or less. The upper limit of the Al content is preferably 0.025% or less, more preferably 0.020% or less.

O: 0.020% or less O is regulated to reduce the cleanliness of the steel and to deteriorate the corrosion resistance. In the present stainless steel, the upper limit of the O content is 0.020% or less. The upper limit of the O content is preferably 0.015% or less, more preferably 0.010% or less.

N: Over 0.8% to 1.30%
N is one of the heaviest elements in this stainless steel. N is an interstitial element and is very effective in improving strength, stabilizing the austenite phase, and improving corrosion resistance.

  In order to obtain the effect, the lower limit of the N content is set to more than 0.8%. On the other hand, excessive addition of N tends to induce nitrogen blow. In addition, a large amount of undissolved Cr-based nitride and a large amount of Ti, Nb, and V-based nitride remain in the steel when Ti, Nb, and V are contained, as will be described later, during the solution heat treatment. Let Therefore, corrosion resistance and toughness tend to be remarkably reduced. Also, the impact characteristics at room temperature tend to be lowered.

  Therefore, the upper limit of the N content is set to 1.30% or less. The upper limit of the N content is preferably 1.20% or less.

  The stainless steel contains at least the essential components. Here, this stainless steel has a value of Nieq defined by the following formula (1) of 20 or more.

^{Nieq = [Ni] +0.41 [Cr} ] -0.01 [Cr] 2 -2 [Si] -0.4 ([Mo] + [W]) + [Co] +0.6 ([Cu] + [ Mn]) + 10 ([C] + [N]) (1)
However, [X] represents the weight% of the element X.

  The above “Nieq” is an index of the stability of the austenite phase. (1) Formula is prescribed | regulated using content of the main austenite formation element and a ferrite formation element.

  When the Nieq is 20 or more, the austenite phase can be stably maintained even after a strong cold working.

  On the other hand, when the Nieq is less than 20, a work-induced martensite phase is likely to be generated after cold working. The work-induced martensite phase not only increases the magnetic permeability and makes it difficult to maintain non-magnetic properties, but also increases the ductile-brittle transition temperature and impairs toughness, particularly impact properties, and therefore needs to be suppressed. The work-induced martensite phase also harms the stress corrosion cracking resistance after cold work. The lower limit of Nieq is preferably 22 or more.

  Further, this stainless steel has a PRE value defined by the following formula (2) of 5 or more.

PRE = {[Cr] +3.3 ([Mo] +0.5 [W]) + 16 [N]} / [Mn] (2)
However, [X] represents the weight% of the element X.

  The “PRE” is an index of corrosion resistance. The formula (2) is defined using N, Cr, Mo and W as elements for improving the corrosion resistance, and Mn as an element for deteriorating the corrosion resistance.

  When the PRE is 5 or more, it is possible to obtain good corrosion resistance (for example, approximately equivalent to or higher than SUS836L, which is super stainless steel). The lower limit value of PRE is preferably 7 or more.

  Further, this stainless steel has an OCRE value of 25 or more defined by the following formula (3).

OCRE = [Cr] +1.5 ([Mo] +0.5 [W]) + [Ni] + [Cu] −0.2 [Mn] +2 [N] (3)
However, [X] represents the weight% of the element X.

  The “OCRE” is an index of stress corrosion cracking resistance. Formula (3) is defined using N, Cr, Mo, W, Ni and Cu as elements for improving the stress corrosion cracking resistance, and Mn as an element for degrading the stress corrosion cracking resistance.

  When the OCRE is 25 or more, good stress corrosion cracking resistance can be obtained even in a high strength state where the tensile strength after cold working exceeds 2000 MPa. The lower limit of OCRE is preferably 27 or more.

  In any of the above equations, when there is no component constituting the equation, 0 is substituted for calculation.

  In addition to the essential elements described above, the present stainless steel may further contain one or more elements selected from Ni, Cu and Co as necessary. The component ranges of Ni, Cu and Co and the reasons for their limitation are as follows.

Ni: more than 0.5 to less than 7.0% Ni is an austenite forming element and contributes to stabilization of the austenite phase. Furthermore, it is effective in improving corrosion resistance, toughness, and impact characteristics. In order to obtain the effect, the lower limit of the Ni content is set to more than 0.5%.

  On the other hand, excessive addition of Ni tends to significantly reduce the solubility of N. Further, during the solution heat treatment, there is a tendency to increase the amount of undissolved Cr-based nitride and deteriorate the corrosion resistance and toughness.

  Therefore, the upper limit of the Ni content is less than 7.0%. The upper limit of the Ni content is preferably less than 5.0%, more preferably less than 3.0%.

Cu: 0.01 to 4.0%
Cu is an austenite generating element and contributes to stabilization of the austenite phase. Furthermore, it is effective in improving corrosion resistance, toughness, and impact characteristics.

  In order to obtain the effect, the lower limit of the Cu content is set to 0.01% or more. The lower limit of the Cu content is preferably 0.02% or more, more preferably 0.05% or more.

  On the other hand, excessive addition of Cu tends to increase the amount of undissolved Cr-based nitride during the solution heat treatment and deteriorate the corrosion resistance. Also, hot workability tends to decrease.

  Therefore, the upper limit of the Cu content is 4.0% or less. The upper limit of the Cu content is preferably 2.0% or less, more preferably 1.5% or less.

Co: 0.01-5.0%
Co is an austenite generating element and contributes to stabilization of the austenite phase. In order to obtain the effect, the lower limit of the Co content is set to 0.01% or more. The lower limit of the Co content is preferably 0.1% or more.

  On the other hand, excessive addition of Co causes an increase in cost. In addition, during the solution heat treatment, the amount of undissolved Cr-based nitride tends to increase, and the corrosion resistance and toughness ductility tend to be remarkably reduced.

  Therefore, the upper limit of the Co content is 5.0% or less. The upper limit of the Co content is preferably 4.0% or less, more preferably 3.0% or less.

  In addition to the elements described above, the present stainless steel may further contain one or more elements selected from Ti, Nb, V, Ta, and Zr as necessary. The component ranges of Ti, Nb, V, Ta, and Zr and the reasons for their limitation are as follows.

Ti: 0.01 to 0.5%
Ti combines with C and N and contributes to improvement of strength and refinement of crystal grains. In order to obtain the effect, the lower limit of the Ti content is set to 0.01% or more. The lower limit of the Ti content is preferably 0.02% or more.

  On the other hand, excessive addition of Ti causes a large amount of oxides and nitrides to remain in the steel and significantly reduces toughness.

  Therefore, the upper limit of Ti content is 0.5% or less. The upper limit of the Ti content is preferably 0.4% or less, more preferably 0.2% or less.

Nb: 0.01 to 0.5%
Nb combines with C and N like Ti, and contributes to improvement of strength and refinement of crystal grains. In order to obtain the effect, the lower limit of the Nb content is set to 0.01% or more. The lower limit of the Nb content is preferably 0.02% or more.

  On the other hand, excessive addition of Nb causes a large amount of oxides and nitrides to remain in the steel and significantly reduces the toughness.

  Therefore, the upper limit of Nb content is 0.5% or less. The upper limit of the Nb content is preferably 0.4% or less, more preferably 0.2% or less.

V: 0.01 to less than 1.0% V, like Ti and Nb, combines with C and N to contribute to improvement in strength and refinement of crystal grains. In order to obtain the effect, the lower limit of the V content is set to 0.01% or more. The lower limit of the V content is preferably 0.02% or more, more preferably 0.03% or more.

  On the other hand, excessive addition of V causes a large amount of oxides and nitrides to remain in the steel and significantly reduces toughness.

  Therefore, the upper limit of V content is less than 1.0%. The upper limit of the V content is preferably 0.9% or less, more preferably 0.5% or less.

Ta: 0.01 to 0.5%
Ta, like Ti, Nb, and V, combines with C and N and contributes to improvement in strength and refinement of crystal grains. In order to obtain the effect, the lower limit of the Ta content is set to 0.01% or more. The lower limit of the Ta content is preferably 0.02% or more.

  On the other hand, excessive addition of Ta causes a large amount of oxides and nitrides to remain in the steel and significantly reduces toughness.

  Therefore, the upper limit of the Ta content is 0.5% or less. The upper limit of the Ta content is preferably 0.4% or less, more preferably 0.2% or less.

Zr: 0.01 to 0.5%
Zr combines with C and N like Ti, Nb, V, and Ta, and contributes to improvement of strength and refinement of crystal grains. In order to obtain the effect, the lower limit of the Zr content is set to 0.01% or more. The lower limit of the Zr content is preferably 0.02% or more.

  On the other hand, excessive addition of Zr causes a large amount of oxides and nitrides to remain in the steel and significantly reduces toughness.

  Therefore, the upper limit of the Zr content is 0.5% or less. The upper limit of the Zr content is preferably 0.4% or less, more preferably 0.2% or less.

  In addition to the elements described above, the present stainless steel may further contain one or more elements selected from B, Ca and Mg as necessary. The component ranges of B, Ca and Mg and the reasons for their limitation are as follows.

B: 0.0005 to 0.01%
B is effective for improving the strength and hot workability. In order to obtain the effect, the lower limit of the B content is set to 0.0005% or more.

  On the other hand, excessive addition of B tends to degrade the hot workability and deteriorate the corrosion resistance.

  Therefore, the upper limit of the B content is set to 0.01% or less. The upper limit of the B content is preferably 0.005% or less, more preferably 0.003% or less.

Ca: 0.001 to 0.01%
Ca is effective for improving hot workability and machinability. In order to obtain the effect, the lower limit of the Ca content is set to 0.001% or more.

  On the other hand, excessive addition of Ca tends to impair hot workability.

  Therefore, the upper limit of the Ca content is set to 0.01% or less. The upper limit of the Ca content is preferably 0.008% or less.

Mg: 0.001 to 0.01%
Mg is effective in improving hot workability. In order to obtain the effect, the lower limit of the Mg content is set to 0.001% or more.

  On the other hand, excessive addition of Mg tends to impair hot workability.

  Therefore, the upper limit of Mg content is 0.01% or less. The upper limit of the Mg content is preferably 0.008% or less.

  In addition to the elements described above, the present stainless steel may further contain one or more elements selected from Te and Se as necessary. The component ranges of Te and Se and the reasons for their limitation are as follows.

Te: 0.005 to 0.05%
Te contributes to improvement of machinability. In order to obtain the effect, the lower limit of the Te content is set to 0.005% or more. The lower limit of the Te content is preferably 0.01% or more.

  On the other hand, excessive addition of Te deteriorates corrosion resistance, toughness, and hot workability. Therefore, the upper limit of Te content is set to 0.05% or less. The upper limit of the Te content is preferably 0.04% or less.

Se: 0.01-0.20%
Se contributes to improvement of machinability. In order to obtain the effect, the lower limit of the Se content is set to 0.01% or more.

  On the other hand, excessive addition of Se deteriorates corrosion resistance, toughness, and hot workability. Therefore, the upper limit of the Se content is 0.20% or less. The upper limit of the Se content is preferably 0.10% or less.

  Here, in the stainless steel, it is preferable that the maximum diameter of the Cr-based nitride is 2 μm or less and the area ratio of the Cr-based nitride is 1% or less in a solution heat treatment state.

  Note that the maximum diameter and area ratio of the Cr-based nitride were observed as 10 arbitrary fields of view in a backscattered electron image 400 times that of a scanning electron microscope. Is obtained by image analysis. The area ratio of the Cr-based nitride is the total area ratio of the Cr-based nitride including the Cr-based nitrides of 2 μm or less and 2 μm or more.

  This stainless steel is subjected to a solid solution heat treatment at a heat treatment temperature corresponding to the composition of the steel in order to solidify the Cr-based nitride generated during hot working such as forging or rolling into the austenite matrix. The details of the solution heat treatment will be described later.

  The stainless steel may be cold worked. When cold working is performed, the strength can be particularly improved. It can be selected in consideration of the balance between required strength and ductility.

  The stainless steel preferably has a tensile strength of 1000 MPa or more, a drawing of 65% or more, and an elongation at break of 50% or more in the solution heat treatment state.

  Further, when cold working with a cross-section reduction rate of 20 to 40% is applied, it is preferable that the tensile strength is 1600 MPa or more, the drawing is 60% or more, and the elongation at break is 20% or more.

  In addition, when cold working with a cross-section reduction rate of 40 to 60% is applied, it is preferable that the tensile strength is 1800 MPa or more, the drawing is 50% or more, and the elongation at break is 10% or more.

  In addition, when cold working with a cross-section reduction rate of 60 to 80% is applied, it is preferable that the tensile strength is 2000 MPa or more, the drawing is 45% or more, and the elongation at break is 5% or more.

  The tensile strength, drawing, and elongation at break are values measured according to JIS Z 2241.

  Next, an example of the manufacturing method of this stainless steel is demonstrated. In order to obtain the above-described stainless steel, for example, the following may be performed.

  For example, the alloy having the above-described chemical composition is melted in a melting furnace such as a pressurizable electric furnace or a pressurizable high-frequency induction furnace, and cast into a steel ingot. Next, the obtained steel ingot is hot-worked to obtain a steel material having a desired shape. Next, after the hot working, a solution heat treatment is performed. As a result, Cr-based nitrides can be sufficiently dissolved to make the structure uniform.

  Examples of the solution heat treatment include a method of heating to an optimum temperature corresponding to the steel composition, holding the temperature for a certain period of time, and quenching.

  The lower limit of the heat treatment temperature during the solution heat treatment is preferably 1050 ° C. or higher. On the other hand, the upper limit of the heat treatment temperature during the solution heat treatment is preferably 1250 ° C. or less, more preferably 1200 ° C. or less, and most preferably 1150 ° C. or less.

  This is because, if the heat treatment temperature is within the above range, it is easy to obtain a steel type excellent in balance of strength, ductility, and corrosion resistance while ensuring the quality of the steel and suppressing an increase in equipment cost.

  In the steel composition described above, when the heat treatment temperature is less than 1050 ° C., it becomes difficult to eliminate Cr-based nitrides having a diameter of more than 2 μm, and undissolved Cr-based nitrides having a diameter of more than 2 μm remain after the solution heat treatment. There is a tendency to become easier. In this case, due to the presence of coarse Cr-based nitrides, ductility and corrosion resistance are likely to decrease.

  On the other hand, when the heat treatment temperature exceeds 1250 ° C., the crystal grains tend to become coarse, the surface properties of the steel deteriorate, and the equipment costs increase.

  Therefore, these should be taken into consideration when selecting the heat treatment temperature.

  The heat treatment time is not particularly limited. Depending on the dimensions of the material, it is preferable to select from the range of 0.1 to 2 hours.

  In addition, when the strength is particularly important, cold working may be performed in addition to the solution heat treatment. Moreover, you may perform an aging treatment etc. for the further hardness improvement.

  Specifically, the aging treatment may be performed at a temperature of 400 to 600 ° C., preferably 500 to 600 ° C. for about 0.1 to 2 hours.

  In the above, this stainless steel and its manufacturing method were demonstrated. The use of this stainless steel is not particularly limited, and can be applied to various uses that require strength, ductility and corrosion resistance.

  Specifically, this stainless steel is, for example, a high strength, high corrosion resistance material such as a bolt, nut, cylinder liner, shaft, hub, connector, bearing, race, rail, gear, pin, screw, roll, turbine blade, Molds, dies, drills, valves, valve seats, blades, nozzles, gaskets, rings, springs, golf iron faces, industrial furnace members, chemical plant members, chemical manufacturing members, food manufacturing plant members, food manufacturing equipment members It is suitable for oil drilling members, oil refining plant members, waste incinerator members, steam turbine members, gas turbine members, nuclear reactor members, aircraft members, biomass plant members and the like.

  In addition, the stainless steel specifically includes, for example, marine-related equipment that requires corrosion resistance, beach environment members, marine structure structural members, seawater desalination plant members, seawater heat exchanger members, submarine cables, and submarine architecture. Suitable for structural members of objects, mooring ropes, fish nets for aquaculture, bridge wires in beach area, pumps for seawater, shafts, fastening members such as bolts, nuts and screws.

  Further, among the stainless steels, the steel types that sufficiently ensure the stability of the austenite phase are specifically, for example, non-magnetic, high-strength, high-corrosion resistant members such as springs, shafts, bearings for precision electronic components, Wires for printed circuit board manufacturing parts such as races, pins, dies, rails, meshes, bio-implant electrodes, MRI parts, bio-implants that support MRI, chemical manufacturing parts, hanger members, linear motor car members, semiconductor manufacturing equipment parts Effective for tweezers, bearings, scissors, blades, etc.

  Further, among the present stainless steels, the steel types with a reduced Ni content are specifically applicable to, for example, biomaterials and ornaments, and ornaments such as necklaces, earrings, and rings that directly touch the human body. , Wristwatches, wristbands, eyeglass frames, interdental brushes, dental materials such as artificial roots and orthodontic wires used in vivo, plates, bolts, nuts, springs, screws, wires, electrodes, It is suitable for implant materials such as artificial bones and artificial joints, medical instruments such as injection needles, knives, scalpels, scissors, forceps, and drills.

Hereinafter, the present invention will be described more specifically with reference to examples.
Alloys having chemical components shown in Tables 1 and 2 were melted by a pressurizable high frequency induction furnace and cast into a 50 kg steel ingot.

  Next, a test piece (diameter: about 150 mm × thickness: 10 mm) was cut out from the bottom of the steel ingot, and the presence or absence of nitrogen blowing was investigated. At this time, the presence / absence of nitrogen blow was investigated by visually confirming whether or not there was a blowhole-like defect on the surface of the test piece.

  Next, the steel ingot was homogeneously heated and then hot forged to produce round bars each having a diameter of 25 mm.

  Next, each of the obtained round bars was subjected to a solution heat treatment by cooling with water after being held at a temperature of 1050 to 1250 ° C. for 1 hour. Then, arbitrary 10 visual fields were observed with a 400 times backscattered electron image of a scanning electron microscope, and the maximum value of the equivalent circle diameter of the Cr-based nitride and the area ratio of the Cr-based nitride were measured by image analysis. .

  Next, test pieces suitable for each test were collected from the solution heat treated material, and the corrosion resistance, tensile properties, and Charpy impact properties were investigated by the following test methods.

<Corrosion resistance>
Corrosion resistance was evaluated by measuring the degree of corrosion after immersion in ferric chloride at 50 ° C. for 24 hours in accordance with JIS G 0578 (A).

<Tensile properties>
The tensile properties were evaluated by measuring tensile strength, drawing, and elongation at break according to JIS Z 2241.

<Charpy impact characteristics>
Charpy impact characteristics were evaluated by measuring Charpy impact values in accordance with JIS Z 2242 using a 2 mmV notch test piece.

  Moreover, about Example 2, 4, 6 and the comparative example 2, after solution heat treatment, it cold-processes by processing rate 30%, 50%, and 70% by swaging, extract | collects a test piece from these raw materials, The tensile properties were investigated.

  For Examples 2, 4, 6, and Comparative Examples 1, 6, and 10, after solution heat treatment, cold working was performed with a working rate of 70% by swaging, and test pieces were collected from these materials, The magnetic susceptibility and stress corrosion cracking properties were investigated.

  At this time, the magnetic permeability was measured with an external magnetic force 2000e using a VSM (Vibrating Sample Magnetometer).

  On the other hand, the stress corrosion cracking characteristics were obtained by taking a test piece having an annular notch having a diameter of 6 mm, a length of 40 mm, a notch bottom radius of 0.1 mm, and a notch diameter of 4 mm at the center, and 4.9% NaCl and 0. With a mixed solution of 1N HCl added dropwise to the notch at 45 ml per hour, 70% of the static bending strength was applied, and the fracture life up to 200 hours was measured.

  Tables 1 and 2 show the chemical components of the stainless steel produced, and Tables 3 to 5 show the test results. In the table, Comparative Example 7 is SUS836L (super stainless steel), Comparative Example 8 is SUS301 (metastable austenitic stainless steel), Comparative Example 9 is SUS630 (precipitation hardening type stainless steel), and Comparative Example 10 is , 18Ni-based maraging steel, each of which was heat-treated under commonly used temperature conditions.

  When the test results in Tables 3 to 5 are relatively evaluated, the following can be understood.

  That is, Comparative Example 1 (Cr excess, N excess, Nieq <20) is particularly inferior in Charpy impact characteristics. In addition, it is difficult to ensure non-magnetism and stress corrosion cracking is likely to occur.

  Comparative Example 2 (Mn deficiency, Ni excess, Co excess) is particularly inferior in corrosion resistance and tensile properties (drawing, elongation at break). Further, when cold working is performed after the solution heat treatment, drawing and elongation at break are low, and the balance between strength and toughness is lacking.

  Comparative Example 3 (N deficiency, Nieq <20, PRE <5, OCRE <25) is particularly inferior in corrosion resistance and tensile properties (tensile strength).

  Comparative Example 4 (Cu excess) cracked during hot forging, resulting in poor manufacturability and subsequent tests could not be performed.

  In Comparative Example 5 (Si excess, N excess, Nieq <20), nitrogen blow occurred.

  Comparative Example 6 (Mn excess, PRE <5, OCRE <25) is particularly inferior in corrosion resistance. In addition, stress corrosion cracking is likely to occur.

  Comparative Example 7 (SUS836L) is particularly inferior in tensile properties (tensile strength).

  Comparative Example 8 (SUS301) is particularly inferior in corrosion resistance and tensile properties.

  Comparative Example 9 (SUS630) is particularly inferior in corrosion resistance.

  Comparative Example 10 (18Ni maraging steel) is particularly inferior in corrosion resistance. In addition, stress corrosion cracking is likely to occur.

  Thus, it turns out that at least 1 or more among the comparative examples 1-10 which do not satisfy | fill the conditions prescribed | regulated to this application are inferior among intensity | strength, ductility, and corrosion resistance.

  On the other hand, it turns out that Examples 1-10 which satisfy the conditions prescribed | regulated by this application are excellent in all of intensity | strength, ductility, and corrosion resistance. Moreover, even when it cold-processes after solution heat treatment, it turns out that it is excellent in the balance of strength-toughness ductility, maintaining high corrosion resistance. It can also be seen that non-magnetism is easily secured and the stress corrosion cracking resistance is excellent.

  As described above, the austenitic stainless steel according to the present invention has been described. However, the present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the scope of the present invention. is there.

Claims (10)

% By weight
C: 0.005-0.25%,
Si: less than 2.0%,
Mn: more than 0.2 to less than 10.0%,
P: 0.03% or less,
S: 0.05% or less,
Cr: 15.0-30.0%,
Mo: 0.05-8.0%,
W: 0 to 8.0% or less,
Al: 0.030% or less,
O: 0.020% or less, and
N: more than 0.8 to 1.30%,
And the balance substantially consists of Fe and inevitable impurities,
The value of Nieq defined by the following formula (1) is 20 or more,
The value of PRE specified by the following formula (2) is 5 or more,
An austenitic stainless steel having an OCRE value defined by the following formula (3) of 25 or more.
^{Nieq = [Ni] +0.41 [Cr} ] -0.01 [Cr] 2 -2 [Si] -0.4 ([Mo] + [W]) + [Co] +0.6 ([Cu] + [ Mn]) + 10 ([C] + [N]) (1)
PRE = {[Cr] +3.3 ([Mo] +0.5 [W]) + 16 [N]} / [Mn] (2)
OCRE = [Cr] +1.5 ([Mo] +0.5 [W]) + [Ni] + [Cu] −0.2 [Mn] +2 [N] (3) Ni: more than 0.5 to less than 7.0%,
Cu: 0.01-4.0%, and
Co: 0.01-5.0%
The austenitic stainless steel according to claim 1, further comprising one or more selected from the group consisting of: Ti: 0.01 to 0.5%,
Nb: 0.01-0.5%
V: 0.01 to less than 1.0%,
Ta: 0.01 to 0.5%, and
Zr: 0.01 to 0.5%,
The austenitic stainless steel according to claim 1 or 2, further comprising one or more selected from the group consisting of: B: 0.0005 to 0.01%,
Ca: 0.001 to 0.01%, and
Mg: 0.001 to 0.01%,
The austenitic stainless steel according to any one of claims 1 to 3, further comprising one or more selected from the group consisting of: Te: 0.005 to 0.05%, and
Se: 0.01-0.20%,
The austenitic stainless steel according to any one of claims 1 to 4, further comprising one or more selected from the group consisting of:   6. The austenite according to claim 1, wherein the maximum diameter of the Cr-based nitride is 2 μm or less in the solution heat treatment state, and the area ratio of the Cr-based nitride is 1% or less. Stainless steel.   The austenitic stainless steel according to any one of claims 1 to 6, which has a tensile strength of 1000 MPa or more, a drawing of 65% or more, and a breaking elongation of 50% or more in a solution heat treatment state. By adding cold working with a cross-section reduction rate of 20-40%,
The austenitic stainless steel according to any one of claims 1 to 6, having a tensile strength of 1600 MPa or more, a drawing of 60% or more, and a breaking elongation of 20% or more. By adding cold working with a cross-section reduction rate of 40-60%,
The austenitic stainless steel according to any one of claims 1 to 6, having a tensile strength of 1800 MPa or more, a drawing of 50% or more, and a breaking elongation of 10% or more. By adding cold working with a cross-section reduction rate of 60-80%,
The austenitic stainless steel according to any one of claims 1 to 6, having a tensile strength of 2000 MPa or more, a drawing of 45% or more, and a breaking elongation of 5% or more.