JP6623719B2 - Austenitic stainless steel - Google Patents

Description

  The present invention relates to an austenitic stainless steel.

  Materials used for high-temperature parts in power plants and petrochemical plants are required to have high-temperature creep strength and resistance to steam oxidation. As a material satisfying such characteristics, Nb-containing austenitic stainless steel TP347H specified by American Society of Mechanical Engineers (ASME) SA213 and SA213M is frequently used. TP347H has excellent creep strength at high temperatures. ASME SA213 / SA213M specifies that the austenite grain size number is 7 or less in order to obtain excellent creep strength at high temperatures.

  Japanese Patent Application Laid-Open No. 2012-255198 describes a method for producing an austenitic stainless steel pipe excellent in high-temperature creep strength and steam oxidation resistance.

By the way, as disclosed in Non-Patent Documents 1 to 3, in 347 series stainless steel containing Nb, when used as a welded structure at a high temperature, stress relaxation cracking or strain-induced cracking occurs in the weld during use. It is known that embrittlement cracks called precipitation hardening cracks are likely to occur. Non-patent Documents 1 to 3 are described as intragranular precipitation of carbides such as M ₂₃ C ₆ and NbC are affected contribute to cracking. Non-Patent Document 3 proposes that as a countermeasure from the viewpoint of the welding process, reduction of welding residual stress by applying appropriate post-heat treatment is effective in preventing cracking.

  As measures from the material side to prevent embrittlement cracking, WO 2009/044796 and WO 2009/044802 disclose Sb according to the amount of carbide-forming elements such as Nb, Ti and V. A high-N high-strength stainless steel and a low-C high-corrosion-resistant stainless steel capable of preventing embrittlement cracking during high-temperature use by adjusting the amount of embrittlement elements such as P and P are disclosed.

JP 2012-255198 A International Publication No. 2009/044796 International Publication No. 2009/044802

R. N. Younger et al .: Journal of The Iron and Steel Institute, October (1960), p. 188 R. N. Younger et al .: British Welding Journal, Decmber (1961), p. 579 Uchiki et al .: Ishikawajima Harima Technical Report, Vol. 15 (1975) No. 2, p. 209

  Japanese Patent Application Laid-Open No. 2012-255198 does not mention any crack that may occur in a welded portion, and does not suggest a method for preventing the crack.

  If the post-heat treatment described in Non-Patent Document 3 is applied to reduce the residual stress, it is possible to prevent embrittlement cracking occurring in the weld during use. However, it is difficult to perform a high-temperature and long-time heat treatment on a large structure such as an actual plant from the viewpoint of securing dimensional accuracy due to deformation. Also, even if it could be implemented, its application would cause a significant increase in cost.

  The stainless steels disclosed in WO 2009/044796 and WO 2009/044802 exhibit excellent resistance to embrittlement cracking without performing post-heat treatment. However, in the case of stainless steel containing a large amount of Nb from the viewpoint of preventing sensitization of the welded portion and containing a large amount of C from the viewpoint of ensuring high-temperature strength, even if the techniques described in these documents are applied, the brittle cracks may occur. It was found that there was a case where it was not possible to prevent.

  An object of the present invention is to provide an austenitic stainless steel capable of stably obtaining excellent resistance to embrittlement cracking of a weld heat-affected zone when used as a welded structure at a high temperature for a long time.

The austenitic stainless steel according to one embodiment of the present invention has a chemical composition of 0.04 to 0.12% C, 0.05 to 1.0% Si, and 1.0 to 2 Mn in mass%. 0.5%, P: 0.045% or less, S: 0.002% or less, Cu: 0.02 to 1%, Ni: 9 to 13%, Cr: 16 to 20%, Mo: 0.02 to 1 %, Nb: more than 0.5% and 1.2% or less, N: 0.01% or more and less than 0.10%, B: 0.0002 to 0.004%, Al: 0.001 to 0.02% , O: 0.001 to 0.02%, V: 0 to 0.3%, Ti: 0 to 0.3%, Co: 0 to 1%, Ca: 0 to 0.01%, Mg: 0 to 0% 0.01%, REM: 0 to 0.1%, balance: Fe and impurities, the crystal grain size number is 2 to 10, and satisfies the following formula (1).
0.0012- [grain size] / 10000 ≦ [% B] ≦ 0.0015 + 2.5 × [grain size] / 10000 ... (1)
In the formula (1), the crystal grain size number is substituted for [crystal grain size], and the content of B is substituted for [% B] in mass%.

  ADVANTAGE OF THE INVENTION According to this invention, when used as a welded structure at high temperature for a long time, the austenitic stainless steel which can obtain the excellent brittle crack resistance of the weld heat affected zone stably is obtained.

FIG. 1 is a cross-sectional view illustrating the shape of the groove of the plate material manufactured in the example.

  The present inventors firstly presented representatives of the high-N high-strength stainless steel described in WO 2009/044796 and the low-C high-corrosion-resistant stainless steel described in WO 2009/044802. A material simulating the composition and a stainless steel containing both high Nb and high C were prepared, and the cracking properties of these stainless steels were investigated.

  As a result, in the aging process at a temperature of 600 to 650 ° C. simulating use in a plant, grain boundary cracking occurs only in stainless steel containing high Nb and high C at the same time, and high N high strength stainless steel and No cracking occurred in the low C high corrosion resistant stainless steel. In addition, the cracks generated in the stainless steel containing both high Nb and high C were determined to be typical embrittlement cracks, since the cracked fracture surface had almost no ductility.

As a result of detailed structure observation during the aging process, a large amount of MX phase (NbC) was precipitated in the grains of the stainless steel simultaneously containing high Nb and high C. In contrast, the intragranular precipitates observed in the high-N high-strength stainless steel and the low-C high-corrosion-resistant stainless steel mainly consisted of the Z phase (NbCrN) or Cr ₂ N, and the precipitation of the MX phase was small. Then, it was found that the MX phase in stainless steel containing both high Nb and high C was precipitated in a shorter time.

  Generally, embrittlement cracks that occur in the weld during use are difficult to deform within the grains due to precipitation of a large amount of carbides, nitrides, and carbonitrides in the grains, and are accompanied by relaxation of welding residual stress. It is believed that creep deformation concentrates at the grain boundaries and leads to openings.

This indicates that the MX phase mainly precipitates because the MX phase precipitates at an early stage when the residual stress is hardly relaxed compared to the Z phase or Cr ₂ N, and the strengthening ability differs depending on the type of the precipitate phase. It is considered that cracking occurred only in the stainless steel containing both high Nb and high C, which is a natural intragranular precipitation phase.

  Based on the above investigation results, further investigations were made on a method for preventing embrittlement cracking during use of stainless steel containing both high Nb and high C. Specifically, in mass%, in mass%, C: 0.04 to 0.12%, Si: 0.05 to 1.0%, Mn: 1.0 to 2.5%, P: 0. 045% or less, Cu: 0.02 to 1%, Ni: 9 to 13%, Cr: 16 to 20%, Mo: 0.02 to 1%, Nb: more than 0.5% to 1.2% or less, An austenitic stainless steel containing N: 0.01% or more and less than 0.10%, Al: 0.03% or less, O: 0.02% or less was examined. As a result, the following became clear.

  1) In order to prevent cracking, it is effective to make B essential. The required amount depends on the initial crystal grain size of the stainless steel. The coarser the grain, the more B is required. Specifically, according to the crystal grain size number specified by ASTM (American Society for Testing and Material: American Society for Testing Materials), B containing {0.0012- [crystal grain size] / 10000} (%) or more is contained. It is effective to do so.

  The reason is as follows. B is an element that segregates at the grain boundary to increase the bonding force, and segregates at the grain boundary during welding or during use at a high temperature to prevent grain boundary cracking during use. The coarser the grain size of the stainless steel, the smaller the grain boundary area per unit volume, and the greater the degree of creep deformation concentration at a specific grain boundary during use. Therefore, the coarser the initial crystal grains of the stainless steel, the higher the grain boundary fixing force needs to be.

  On the other hand, since B is also an element that lowers the melting point, if B is excessively contained, the grain boundary is partially melted during welding, and liquefaction cracking occurs. The coarser the grain size of the stainless steel, the more the thermal stress during welding tends to concentrate on a specific grain boundary. Therefore, the upper limit of the B content is limited as the initial crystal grain size of the stainless steel becomes coarse. Specifically, the B content needs to be not more than {0.0015 + 2.5 × [crystal grain size] / 10000} (%).

  2) In order to surely obtain the above-mentioned effect of B, it is necessary to control the content of elements that segregate at the grain boundaries and reduce the bonding strength of the grain boundaries. In particular, it is necessary to strictly control and reduce the content of S, which has a large adverse effect. Specifically, S needs to be 0.002% or less. When S is contained in excess of this amount, during welding or during use, S segregates before B, so that the segregation site of B disappears and the effect of B cannot be obtained.

  By the way, although it was confirmed that embrittlement cracking could be prevented by these measures, it was found that a new problem would arise if the study was continued.

  When stainless steel is used as a structure, it is often assembled by welding. When welding stainless steel, a filler metal is usually used. However, even in the case of small-sized thin-walled parts or thick-walled members, gas shield arc welding may be performed without using a filler material in initial layer welding or tack welding. At this time, if the penetration depth is insufficient, the unmelted butted surface remains as a defect, and the required strength in the welded joint cannot be obtained. S increases the susceptibility to embrittlement cracking as described above, but also has the effect of increasing the penetration depth. Therefore, it was found that when the S content was controlled to 0.002% or less from the viewpoint of brittle cracking resistance, the problem of insufficient penetration easily occurred.

  In order to solve the problem of insufficient penetration, welding heat input may be increased. However, an increase in welding heat input is not preferable because it promotes the coarsening of crystal grains in the heat affected zone and increases the susceptibility to embrittlement cracking.

  3) Therefore, as a result of studying to solve this problem, as a result of containing O, which has been conventionally recognized as an impurity, in the range of 0.001 to 0.02%, the resistance to embrittlement cracking is ensured. It has been found that the penetration depth can be increased. This is considered to be because when O dissolves in the molten pool, it acts as a surface active element similarly to S, thereby affecting the direction of convection in the molten pool and activating heat transfer in the vertical direction of the molten pool. . In addition, they have found that it is necessary to control the contents of Al and Si, which have a strong affinity for O, in order to reliably obtain the effect of O.

  The present invention has been completed based on the above findings. Hereinafter, the austenitic stainless steel according to one embodiment of the present invention will be described in detail.

[Chemical composition]
The austenitic stainless steel according to the present embodiment has the chemical composition described below. In the following description, “%” of the content of an element means mass%.

C: 0.04 to 0.12%
Carbon (C) stabilizes the austenite structure and combines with Cr and Nb to form fine carbides, thereby improving the creep strength during high-temperature use. In order to obtain this effect sufficiently, the content needs to be 0.04% or more. However, when the Nb carbide is excessively contained, a large amount of Nb carbides precipitate in the grains and the brittle crack resistance is reduced. In addition, a large amount of Cr carbide precipitates, leading to sensitization of the weld. Therefore, the upper limit is set to 0.12% or less. The lower limit of the C content is preferably 0.06%, and more preferably 0.07%. The upper limit of the C content is preferably 0.11%, and more preferably 0.10%.

Si: 0.05 to 1.0%
Silicon (Si) is an element that has a deoxidizing effect and is necessary for ensuring corrosion resistance and oxidation resistance at high temperatures. In order to obtain the effect, it is necessary to contain 0.05% or more. However, when the content is excessive, the stability of the structure is reduced, and the toughness and the creep strength are reduced. Furthermore, since Si has a strong affinity for O, it combines with O in the weld pool during welding to form slag, and the effect of O to increase the penetration depth is lost. Therefore, an upper limit is set to 1.0% or less. The lower limit of the Si content is preferably 0.1%, and more preferably 0.2%. The upper limit of the Si content is preferably 0.8%, and more preferably 0.7%.

Mn: 1.0-2.5%
Manganese (Mn) has a deoxidizing effect similarly to Si. Mn also contributes to the stability of the austenitic structure. Mn further increases the degree of concentration of the arc and contributes not only to an increase in the penetration depth. In order to obtain these effects, it is necessary to contain 1.0% or more. However, if it is contained excessively, the steel becomes brittle and the creep strength is reduced. Therefore, an upper limit is set to 2.5% or less. The lower limit of the Mn content is preferably 1.2%, and more preferably 1.3%. The upper limit of the Mn content is preferably 2.2%, and more preferably 2.0%.

P: 0.045% or less Phosphorus (P) is an element that is contained in steel as an impurity and segregates in the heat affected zone during welding to increase the liquefaction cracking sensitivity. P also reduces the creep ductility after prolonged use. Therefore, the P content is limited to 0.045% or less by setting an upper limit. The P content is preferably 0.04% or less, more preferably 0.038% or less. In addition, it is preferable to reduce the P content as much as possible, but the extreme reduction leads to an increase in steelmaking cost. Therefore, the lower limit of the P content is preferably 0.0005%, and more preferably 0.0008%.

S: 0.002% or less Sulfur (S) is an element contained in steel as an impurity like P, and is segregated in the weld heat affected zone during welding to increase liquefaction cracking sensitivity. S further segregates at grain boundaries during long-term use, greatly increasing the susceptibility to embrittlement cracking. In order to prevent these in the range of chemical composition other than S in the present embodiment, the S content needs to be 0.002% or less. The S content is preferably at most 0.0012%, more preferably at most 0.001%. On the other hand, S is an element effective for increasing the penetration depth during welding. Therefore, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.

Cu: 0.02 to 1%
Copper (Cu) enhances the stability of the austenitic structure and contributes to the improvement in creep strength. Cu further suppresses grain boundary segregation of S, and reduces the susceptibility to embrittlement cracking to a considerable extent. In order to obtain the effect, it is necessary to contain 0.02% or more. However, an excessive content lowers the hot workability of steel. Therefore, the upper limit is set to 1% or less. The lower limit of the Cu content is preferably 0.04%, and more preferably 0.05%. The upper limit of the Cu content is preferably 0.8%, and more preferably 0.6%.

Ni: 9 to 13%
Nickel (Ni) is an essential element for ensuring the stability of the austenite structure during long-term use. In order to obtain a sufficient effect in the range of the chemical composition other than Ni in the present embodiment, it is necessary to contain 9% or more of Ni. However, Ni is an expensive element, and a large content causes an increase in cost. Therefore, the upper limit is set to 13% or less. The lower limit of the Ni content is preferably 9.5%, more preferably 10%. The upper limit of the Ni content is preferably 12.5%, and more preferably 12%.

Cr: 16-20%
Chromium (Cr) is an essential element for securing oxidation resistance and corrosion resistance at high temperatures. Cr also forms fine carbides and contributes to securing creep strength. In order to obtain the effect, it is necessary to contain 16% or more. However, if the Cr content exceeds 20%, the stability of the austenitic structure at high temperatures is degraded, leading to a decrease in creep strength. The lower limit of the Cr content is preferably 16.5%, and more preferably 17%. The upper limit of the Cr content is preferably 19.5%, and more preferably 19%.

Mo: 0.02 to 1%
Molybdenum (Mo) is an element that forms a solid solution in the matrix and contributes to improvement in creep strength and tensile strength at high temperatures. In addition, Mo also contributes to improvement of corrosion resistance. In order to obtain the effect, it is necessary to contain 0.02% or more. However, when the content is excessive, the deformation resistance in the grains increases, and the brittle crack resistance may decrease to a considerable extent. Furthermore, since Mo is an expensive element, its excessive content causes an increase in cost. Therefore, the upper limit is set to 1% or less. The lower limit of the Mo content is preferably 0.04%, and more preferably 0.05%. The upper limit of the Mo content is preferably 0.8%, and more preferably 0.6%.

Nb: more than 0.5% and not more than 1.2% Niobium (Nb) precipitates as fine carbonitrides in the grains in the present embodiment and contributes to improvement in creep strength and tensile strength at high temperatures. In addition, Nb has an effect of fixing C, suppresses precipitation of Cr carbide, and suppresses sensitization of the heat affected zone. In order to sufficiently obtain the effect, it is necessary to contain more than 0.5%. However, if it is contained excessively, it precipitates in large quantities as carbides in the grains, increases the susceptibility to embrittlement cracking, and also causes a decrease in creep ductility and toughness. Furthermore, the susceptibility to liquefaction cracking during welding increases. Therefore, the upper limit is set to 1.2% or less. The lower limit of the Nb content is preferably 0.55%, more preferably 0.6%. The upper limit of the Nb content is preferably 1.1%, and more preferably 1.0%.

N: 0.01% or more and less than 0.10% Nitrogen (N) stabilizes the austenite structure and contributes to improvement of high-temperature strength by solid solution or by precipitation of nitride. In order to obtain the effect, it is necessary to contain 0.01% or more. However, if it is contained excessively, it binds with Nb and reduces the amount of Nb that binds with C, thereby deteriorating the sensitization resistance of the welded portion. Therefore, the upper limit is set to less than 0.10%. The lower limit of the N content is preferably 0.02%, and more preferably 0.04%. The upper limit of the N content is preferably 0.08%, and more preferably 0.07%.

Al: 0.001 to 0.02%
Aluminum (Al) is contained as a deoxidizing agent. Since Al has a strong affinity with O, it combines with O in the weld pool during welding to form slag, and eliminates the effect of O to increase the penetration depth. Therefore, the upper limit is set to 0.02% or less. The Al content is preferably as low as possible in order to improve the penetration depth, but since the extreme reduction leads to an increase in steel making costs, the Al content is set to 0.001% or more. The lower limit of the Al content is preferably 0.002%. The upper limit of the Al content is preferably 0.015%, and more preferably 0.01%.

O: 0.001 to 0.02%
Oxygen (O) is generally contained in steel as an impurity, but has an effect of increasing the penetration depth during welding. Therefore, in this embodiment, 0.001% or more is contained in order to utilize the effect. . However, if it is contained excessively, hot workability is reduced and toughness and ductility are deteriorated. Therefore, the upper limit is set to 0.02% or less. The lower limit of the O content is preferably 0.0015%, and more preferably 0.002%. The upper limit of the O content is preferably 0.015%, and more preferably 0.01%.

B: 0.0002-0.004%
Boron (B) is an element that is easily segregated at the grain boundaries, and is an essential element for increasing the grain boundary fixing force and reducing the susceptibility to embrittlement cracking. In order to obtain the effect, it is necessary to contain 0.0002% or more. However, if the content is excessive, the susceptibility to liquefaction cracking during welding increases. Therefore, the upper limit is set to 0.004% or less. It is necessary that the B content further satisfies the following expression (1) according to the crystal grain size of the steel. A preferable range of the B content is 0.001 to 0.002% when the grain size number is 2 to 7, and 0.0005 to 0.003% when the grain size number is more than 7 and 10 or less. is there.

  The balance of the chemical composition of the austenitic stainless steel according to the present embodiment is Fe and impurities. The impurities referred to here are elements mixed from ores and scraps used as a raw material of steel, or elements mixed from the environment in the manufacturing process.

  The chemical composition of the austenitic stainless steel according to the present embodiment may contain one or more elements selected from the following first to third groups, instead of a part of the above Fe. The following elements are all optional elements. That is, none of the following elements may be contained in the austenitic stainless steel according to the present embodiment. Moreover, only a part may be contained.

  More specifically, for example, only one group may be selected from the first to third groups, and one or more elements may be selected from the group. In this case, it is not necessary to select all elements belonging to the selected group. Alternatively, a plurality of groups may be selected from the first to third groups, and one or more elements may be selected from each group. Also in this case, it is not necessary to select all the elements belonging to the selected group.

First group V: 0 to 0.3%, Ti: 0 to 0.3%
Elements belonging to the first group are V and Ti. These elements form fine carbides or carbonitrides and contribute to the improvement of creep strength, and therefore may be contained as necessary.

V: 0 to 0.3%
Vanadium (V), like Nb, combines with carbon or nitrogen to form fine carbides or carbonitrides, and contributes to improvement in creep strength. This effect can be obtained if V is contained at all. However, if contained excessively, a large amount of carbide or carbonitride precipitates, resulting in a decrease in brittle crack resistance and creep ductility. Therefore, the upper limit is set to 0.3% or less. The lower limit of the V content is preferably 0.01%, and more preferably 0.03%. The upper limit of the V content is preferably 0.25%, and more preferably 0.2%.

Ti: 0 to 0.3%
Titanium (Ti), like V, combines with carbon or nitrogen to form fine carbides or carbonitrides, and contributes to improvement in creep strength. This effect can be obtained if any amount of Ti is contained. However, if contained excessively, a large amount of carbide or carbonitride precipitates, resulting in a decrease in brittle crack resistance and creep ductility. Therefore, the upper limit is set to 0.3% or less. The lower limit of the Ti content is preferably 0.01%, and more preferably 0.03%. The upper limit of the Ti content is preferably 0.25%, and more preferably 0.2%.

Second group Co: 0-1%
The element belonging to the second group is Co. Co may be included as necessary, since it enhances the stability of the austenitic structure and contributes to the improvement in creep strength.

Co: 0-1%
Cobalt (Co), like Ni, is an austenite-forming element and enhances the stability of the austenitic structure and contributes to the improvement in creep strength. This effect can be obtained if Co is contained even in a small amount. However, since Co is an extremely expensive element, excessive content causes a large increase in cost. Therefore, the upper limit is set to 1% or less. The lower limit of the Co content is preferably 0.01%, and more preferably 0.03%. The upper limit of the Co content is preferably 0.9%, and more preferably 0.8%.

Third group Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0 to 0.1%
Elements belonging to the third group are Ca, Mg, and REM. These elements have an effect of improving hot workability during the production of steel, and therefore may be contained as necessary.

Ca: 0 to 0.01%
Calcium (Ca) improves hot workability during steel production. This effect can be obtained as long as Ca is contained. However, if it is contained excessively, it bonds with O to significantly reduce cleanliness, and rather reduces hot workability. Therefore, the upper limit is set to 0.01% or less. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.008%, and more preferably 0.006%.

Mg: 0 to 0.01%
Magnesium (Mg), like Ca, improves hot workability during steel production. This effect can be obtained if any amount of Mg is contained. However, if it is contained excessively, it bonds with O to significantly reduce cleanliness, and rather reduces hot workability. Therefore, the upper limit is set to 0.01% or less. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.008%, and more preferably 0.006%.

REM: 0-0.1%
Rare earth elements (REMs), like Ca and Mg, improve hot workability during steel production. This effect can be obtained if REM is contained at all. However, if it is contained excessively, it bonds with O to significantly reduce cleanliness, and rather reduces hot workability. Therefore, the upper limit is set to 0.01% or less. The lower limit of the REM content is preferably 0.0005%, more preferably 0.001%. The upper limit of the REM content is preferably 0.08%, and more preferably 0.06%.

  Note that “REM” is a generic term for a total of 17 elements of Sc, Y and lanthanoid, and the REM content indicates the total content of one or more elements of REM. REM is generally contained in misch metal. Therefore, for example, it may be contained in the form of a misch metal so that the REM content falls within the above range.

[Grain size number]
The austenitic stainless steel according to the present embodiment has a grain size number of 2 to 10 specified in ASTM E112. In the welded structure using the austenitic stainless steel according to the present embodiment, in order to impart sufficient embrittlement cracking resistance and liquefaction cracking resistance to the heat affected zone of the weld, the welded structure is welded even when subjected to a heat cycle by welding In order to prevent crystal grains in the heat-affected zone from becoming excessively coarse, it is necessary to make the initial crystal grains into fine grains having a crystal grain size number of 2 or more. However, when the crystal grains become fine grains having a crystal grain size number exceeding 10, it becomes difficult to secure necessary creep strength. Therefore, the crystal grain size number needs to be 10 or less.

  In order to set the crystal grain size number within the above range, it is necessary to appropriately set production conditions. The austenitic stainless steel according to the present embodiment is formed by, for example, forming a raw material having the above-described chemical composition by hot working or cold working, then holding the material at a temperature of 1000 to 1300 ° C. for 3 to 60 minutes, and then performing a solid solution treatment. It is manufactured by performing heat treatment. The solution heat treatment is preferably carried out at a temperature of 1,080 to 1,280 ° C. for 3 to 45 minutes, followed by water cooling, and more preferably at a temperature of 1,150 to 1,250 ° C., for 3 to 45 minutes, followed by water cooling. The grain size number decreases as the temperature of the solution heat treatment increases, and decreases as the holding time of the solution heat treatment increases.

[About Equation (1)]
The austenitic stainless steel according to the present embodiment satisfies the following equation (1).
0.0012- [grain size] / 10000 ≦ [% B] ≦ 0.0015 + 2.5 × [grain size] / 10000 ... (1)
In the formula (1), the crystal grain size number is substituted for [crystal grain size], and the B content is substituted for [% B] in mass%.

  B is an element that segregates at the grain boundary to increase the bonding force, and segregates at the grain boundary during welding or during use at a high temperature to prevent grain boundary cracking during use. As the crystal grains of the austenitic stainless steel are coarser, the grain boundary area per unit volume becomes smaller, and the degree of concentration of creep deformation at a specific grain boundary during use becomes larger. Therefore, as the initial crystal grains of the austenitic stainless steel are coarser, the grain boundary fixing force needs to be further increased, and a large amount of B needs to be contained. Specifically, the B content needs to be not less than {0.0012- [crystal grain size] / 10000}%.

  On the other hand, since B is also an element that lowers the melting point, if B is excessively contained, the grain boundary is partially melted during welding, and liquefaction cracking occurs. As the crystal grains of the austenitic stainless steel are coarser, the thermal stress during welding tends to concentrate on specific grain boundaries. Therefore, the upper limit of the B content is limited as the initial crystal grains of the austenitic stainless steel become coarser. Specifically, the B content needs to be not more than {0.0015 + 2.5 × [crystal grain size] / 10000} (%).

  The embodiment of the present invention has been described above, but the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  Laboratory ingots of the materials of the acronyms A to L having the chemical compositions shown in Table 1 were cast to produce ingots.

  The prepared ingot was subjected to hot forging and hot rolling in a temperature range of 1000 to 1150 ° C. to obtain a sheet material having a sheet thickness of 20 mm. This was further cold-rolled to a plate thickness of 16 mm, and then subjected to a solution heat treatment at a holding temperature of 1230 ° C. Thereafter, it was formed into a thickness of 14 mm, a width of 50 mm, and a length of 100 mm by machining. In addition, the holding time in the solution heat treatment was changed variously in the range of 3 to 30 minutes to produce materials having different crystal grain sizes. Separately from this plate material, a sample for structure observation was collected from the solution heat-treated plate material, and the grain size number of the structure was measured in accordance with ASTM E112.

[Welding workability]
A groove processing shown in FIG. 1 was performed on an end face parallel to the longitudinal direction of this plate material. Numerical values in FIG. 1 represent dimensions. The end faces of the grooved two plate materials were abutted to each other, and one-time welding was performed by gas tungsten arc welding without using a welding material, thereby producing two welded joints for each symbol.

  Out of the obtained welded joints, those in which the back bead was formed over the entire length of the welding line in both joints were judged as having good welding workability and passed. Among the acceptable joints, those in which the width of the back bead was 2 mm or more over the entire length were regarded as “good”, and those in which even some of the joints had a width of less than 2 mm were regarded as “good”. In addition, a part where the back bead was not formed even in a part of the two joints was determined as “impossible”.

[Weld crack resistance]
The above-mentioned welded joint obtained by welding only the first layer is stipulated in JIS Z 3224 (2010) on a commercially available steel plate (thickness 30 mm, width 200 mm, length 200 mm) equivalent to SM400B specified in JIS G 3106 (2008). Was subjected to constraint welding on four sides using a coated arc welding rod ENi6625 of No.3. Then, using a TIG wire corresponding to SNi6625 specified in JIS Z 3334 (2011), lamination welding is performed in the groove by TIG welding with a heat input of 10 to 15 kJ / cm, and two joints are formed for each algebra. Produced.

  One of the produced welded joints was subjected to aging heat treatment at 650 ° C. × 500 hours. Samples were taken from each of the five spots of the as-welded welded joint and the welded joint subjected to the aging heat treatment so that the observation surface was the cross section of the joint (a cross section perpendicular to the weld bead). After the sample was mirror-polished and corroded, it was inspected with an optical microscope to check for cracks in the heat affected zone. Weld joints in which cracks were not observed in all five samples were evaluated as "good", and weld joints in which cracks were observed in one sample were evaluated as "acceptable", and were judged as acceptable. Weld joints in which cracks were observed in two or more samples were rated as "impossible".

[Creep rupture test]
From the as-welded welded joint that passed the weld crack resistance test, a round bar creep rupture test piece was collected so that the weld metal was at the center of the parallel portion. A creep rupture test was performed under the conditions of 650 ° C. and 216 MPa at which the target rupture time of the base material was about 1000 hours. A material that was fractured in the base material and whose rupture time was 90% or more of the rupture time of the base material (that is, 900 hours or more) was regarded as “pass”.

[Performance evaluation results]
Table 2 shows the crystal grain size number and the performance evaluation results. In addition, "-" in the column of "creep rupture test" indicates that the creep rupture test was not performed.

  Symbols A1 to A4, B1 to B4, C-2, C-3, D-3, D-4, E-3, E-4, F-3, and L-2 to L-4 are welded joints. In addition to having good workability at the time of the preparation of the material, it had excellent resistance to embrittlement cracking and liquefaction cracking, and also had the required creep strength. In particular, the evaluation results of the welding crack test are all for the symbols A1 to A4, B1 to B4, C-3, E-4, and L2 to L4 in which the relationship between the crystal grain size number and the B content satisfies the preferable conditions. It was "good".

  In the abbreviations C-1, F-1 and F-2, embrittlement cracking occurred in the heat affected zone after aging heat treatment. This is probably because the B content was too small in relation to the crystal grain size.

  The required creep strength was not obtained with the acronyms C-4 and F-4. This is probably because the crystal grains were too fine.

  In the abbreviations D-1, D-2, and E-2, liquefaction cracks occurred in the heat affected zone during welding. This is probably because the B content was too large in relation to the crystal grain size.

  In the abbreviation E-1, during welding, liquefaction cracks occurred in the weld heat affected zone, and after aging heat treatment, embrittlement cracks occurred in the weld heat affected zone. This is probably because the B content was too large in relation to the crystal grain size and the crystal grains were too coarse.

  In the abbreviations G-1 to G-4, embrittlement cracking occurred in the heat affected zone after aging heat treatment. This is probably because the B content was too small.

  In the abbreviations H-1 to H4, embrittlement cracking occurred in the heat affected zone after aging heat treatment. It is probable that although the abbreviations H-1 to H-4 had an appropriate B content, the S content was too large, so that the effect of B was not sufficiently obtained.

  With the abbreviations I-1 to I-4, the back bead was not sufficiently formed during welding. This is probably because the O content was too small.

  With the abbreviations J-1 to J-4, the back bead was not sufficiently formed during welding. It is considered that although the O content was appropriate for the abbreviations J-1 to J-4, the Al content was too large, so that the effect of O was not sufficiently obtained.

  With the abbreviations K-1 to K-4, the back bead was not sufficiently formed during welding. It is considered that although the O content was appropriate for the abbreviations K-1 to K-4, the effect of O was not sufficiently obtained because the Si content was too large.

  In the abbreviation L-1, during welding, liquefaction cracks occurred in the weld heat affected zone, and after aging heat treatment, embrittlement cracks occurred in the weld heat affected zone. This is probably because the crystal grains were too coarse.

  INDUSTRIAL APPLICATION This invention can be used suitably as a high temperature member, such as a power plant and a petrochemical plant.

Claims (3)

Chemical composition in mass%
C: 0.04 to 0.12%,
Si: 0.05 to 1.0%,
Mn: 1.0 to 2.5%,
P: 0.045% or less,
S: 0.002% or less,
Cu: 0.02 to 1%,
Ni: 9 to 13%,
Cr: 16-20%,
Mo: 0.02 to 1%,
Nb: more than 0.5% and 1.2% or less,
N: 0.01% or more and less than 0.10%,
B: 0.0002 to 0.004%,
Al: 0.001 to 0.02%,
O: 0.001 to 0.02%,
V: 0 to 0.3%,
Ti: 0 to 0.3%,
Co: 0 to 1%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0-0.1%,
The balance: Fe and impurities,
The crystal grain size number is 2 to 10,
Meet the following formula (1),
An austenitic stainless steel, wherein the chemical composition contains, in mass%, one or more elements selected from the following first to third groups .
First group V: 0.01 to 0.3%, Ti: 0.01 to 0.3%
Second group Co: 0.01-1%
Third group Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, REM: 0.0005 to 0.1%
0.0012- [grain size] / 10000 ≦ [% B] ≦ 0.0015 + 2.5 × [grain size] / 10000 ... (1)
In the formula (1), the crystal grain size number is substituted for [crystal grain size], and the content of B is substituted for [% B] in mass%. An austenitic stainless steel according to claim 1,
The chemical composition is, in mass%,
C: 0.07 to 0.10%,
Si: 0.2 to 0.7%,
Mn: 1.3 to 2.0%,
P: 0.038% or less,
S: 0.001% or less,
Cu: 0.05-0.6%,
Ni: 10 to 12%,
Cr: 17-19%,
Mo: 0.05 to 0.6%,
Nb: 0.6-1.0%,
N: 0.04 to 0.07%,
B: 0.001 to 0.002%,
Al: 0.002 to 0.01%,
O: 0.001 to 0.02%,
V: 0 to 0.3%,
Ti: 0 to 0.3%,
Co: 0 to 1%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0-0.1%,
The balance: Fe and impurities,
An austenitic stainless steel having a grain size number of 2 to 7. An austenitic stainless steel according to claim 1,
The chemical composition is, in mass%,
C: 0.07 to 0.10%,
Si: 0.2 to 0.7%,
Mn: 1.3 to 2.0%,
P: 0.038% or less,
S: 0.001% or less,
Cu: 0.05-0.6%,
Ni: 10 to 12%,
Cr: 17-19%,
Mo: 0.05 to 0.6%,
Nb: 0.6-1.0%,
N: 0.04 to 0.07%,
B: 0.0005 to 0.003%,
Al: 0.002 to 0.01%,
O: 0.001 to 0.02%,
V: 0 to 0.3%,
Ti: 0 to 0.3%,
Co: 0 to 1%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0-0.1%,
The balance: Fe and impurities,
An austenitic stainless steel having a grain size number of more than 7 and 10 or less.

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