Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to an austenitic stainless steel. More particularly, the present invention relates to a high strength austenitic stainless heat resistant steel which is to be used in constructing high temperature machines and equipment, such as power plant boilers, and is excellent in resistance to cracking at weld heat affected zone due to grain boundary embrittlement during use at high temperatures.
• the Patent Document 1 discloses a Cu-, Nb- and N-containing austenitic stainless steel excellent in high temperature strength and ductility in which the ratio of Nb (%) / Cu (%) is 0.05 to 0.2 and the content of undissolved Nb, after a solution heat treatment, is within the range of 0.04 ⁇ Cu (%) to 0.085 ⁇ Cu (%).
• Patent Document 2 discloses an austenitic stainless steel improved in hot workability by employing such contents of Ca, Mg, O and S that satisfy the relationship 3.0 ⁇ ⁇ (Ca + Mg) - 0.1 ⁇ O ⁇ / S ⁇ 15.0.
• the Patent Document 3 discloses an austenitic stainless steel excellent in high temperature strength and hot workability, which contains 2 to 6% of Cu and one or more elements selected from Y, La, Ce and Nd for a total content level of 0.01 to 0.2% and has a numerical value of a formula expressed in terms of the Mn, Mg, Ca, Y, La, Ce and Nd contents and the Al, Cu and S contents which is in a specified range.
• Patent Document 4 discloses an austenitic stainless steel, excellent in creep characteristics and hot workability, which satisfies the three relationships, namely the relationship between P and Cu, the relationship between sol. Al and N, and the relationship between O and Cu, respectively.
• HZ weld heat affected zone
• Non-Patent Documents 1 and 2 it is pointed out that the welded portion of 18Cr-8Ni type austenitic stainless heat resistant steels, undergo intergranular cracking in the HAZ after a long period of heating.
• the Non-Patent Document 3 describes investigations made in search of means for preventing intergranular cracking in the HAZ in the welded portion of 18Cr-8Ni-Nb type austenitic stainless heat resistant steels, if heated for a long period of time.
• Non-Patent Documents 1 and 2 suggest that such carbides as M 23 C 6 and NbC may be factors influencing the intergranular cracking in the HAZ; however, the mechanisms of action thereof have not been elucidated.
• the Non-Patent Document 3 proposes measures based on a finding, from the welding process viewpoint, that the reductions in welding residual stress by application of an appropriate post weld heat treatment are effective in preventing cracking. According to that document, differences in strength between grains strengthened by Nb(C, N) and grain boundaries are factors which cause of intergranular cracking in the HAZ; however, there is no mention of factors causing grain boundary (intergranular) embrittlement.
• Non-Patent Documents 1 to 3 suggest nothing about measures, from the material viewpoint, for preventing cracking in the HAZ on the occasion of using high strength austenitic stainless heat resistant steels, such as those recently proposed in the said Patent Documents 1 to 4, for instance, for a prolonged period of time.
• the present inventors made detailed investigations of cracks occurred in the welded portions used at high temperatures for a long period of time in the austenitic stainless steels used as materials for constructing machines and equipment to be used at high temperatures for a prolonged period of time, in order to prevent cracking in the HAZ and to provide steels which have excellent resistance to cracking due to grain boundary embrittlement.
• the present invention has been accomplished on the basis of the above-described findings.
• the main points of the present invention are the austenitic stainless steels shown in the following (1) to (4).
• An austenitic stainless steel which comprises by mass percent, C: 0.05 to 0.15%, Si: not more than 1.0%, Mn: not more than 2.0%, Ni: 6 to 13%, Cr: 15 to 25%, N: 0.03 to 0.15%, sol.
• P ⁇ 2 Nb + 2 ⁇ V + Ti
• each element symbol represents the content by mass percent of the element
• An austenitic stainless steel which comprises by mass percent, C: 0.04 to 0.18%, Si: not more than 1.5%, Mn: not more than 2.0%, Ni: more than 13% to not more than 30%, Cr: 15 to 30%, N: 0.10 to 0.35%, sol.
• each element symbol represents the content by mass percent of the element
• REM rare earth element
• the present invention (1) to (4) related to the austenitic stainless steels are referred to as “the present invention (1)” to “the present invention (4)", respectively. They are sometimes collectively referred to as “the present invention”.
• the austenitic stainless steels of the present invention have high strength and excellent resistance to cracking due to grain boundary embrittlement in welded portions during use at high temperatures. Consequently, they can be used as materials for constructing machines and equipment, such as power plant boilers, which are to be used at high temperatures for a prolonged period of time.
• C is an element having an austenite-stabilizing effect and at the same time it forms fine intragranular carbonitrides with N and thereby it contributes toward improvements in high temperature strength.
• the content of C it is necessary that the content of C be not less than 0.04%.
• C content is excessive, in particular when it exceeds 0.18%, C causes over precipitation of fine intragranular carbonitrides during use at high temperatures. Thereby this inhibits the intragranular deformation and causes stress concentration at grain boundary to increase the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ.
• the content of C is set to 0.04 to 0.18%.
• the lower limit of the C content is preferably 0.05% and the upper limit thereof is preferably 0.13%.
• the content of C is preferably 0.05 to 0.15%.
• the content range of C in this case is more preferably 0.07 to 0.13%.
• the C content range of 0.04 to 0.15% is preferable out of the above-mentioned range of 0.04 to 0.18%.
• Si is an element having a deoxidizing effect. It is also effective in increasing corrosion resistance and oxidation resistance at high temperatures. However, when the content thereof is excessive, in particular at a content level exceeding 1.5%, it deteriorates the stability of the austenite phase, thus creep strength and toughness are deteriorated. Therefore, the content of Si is set to not more than 1.5%. The content of Si is preferably not more than 1.0%.
• the more preferable Si content in the practice of the present invention is not more than 0.8%.
• the lower limit of the Si content is preferably 0.02%.
• Mn like Si, has a deoxidizing effect. Mn also contributes toward stabilization of the austenite phase. However, when the content thereof is excessive, in particular at a content level exceeding 2.0%, it causes embrittlement and thus deteriorates the creep ductility and toughness. Therefore, the content of Mn is set to not more than 2.0%. More preferably, the content of Mn is not more than 1.5%.
• the lower limit of the Mn content is preferably 0.02%.
• Ni is effective in obtaining the austenitic microstructure and also is an essential element for ensuring the structural stability during a long period of use and thus obtaining the desired level of creep strength.
• the Ni content In order to sufficiently produce the effects mentioned above within the Cr content range to be mentioned below, it is necessary that the Ni content be not less than 6%.
• the lower limit of the Ni content is preferably 7% and the upper limit thereof is preferably 28%.
• Cr is an essential element for ensuring the oxidation resistance and corrosion resistance at high temperatures and, in order to obtain the said effects, it is necessary that the Cr content be not less than 15%. However, when the content thereof is excessive, in particular at a content level exceeding 30%, it deteriorates the stability of the austenite phase at high temperatures and thus causes a decrease in creep strength. Therefore, the content of Cr is set to 15 to 30%.
• the preferable lower limit of the Cr content is 16% and the preferable upper limit thereof is 28%.
• the combination of the Ni content and Cr content is preferably as follows, as defined in the present invention (2): Ni: 6 to 13% and Cr: 15 to 25%, more preferably Ni: 7 to 12% and Cr: 16 to 20%.
• the combination of the Ni content and Cr content is preferably as follows, as defined in the present invention (3): Ni: more than 13% to not more than 30% and Cr: 15 to 30%, more preferably Ni: 15 to 28% and Cr: 18 to 28%.
• N is an austenite-forming element and is also an element soluble in the matrix and forms, like C, fine intragranular carbonitrides and thus it is an essential element for ensuring the creep strength at high temperatures.
• the content of N is required to be not less than 0.03%.
• N is also effective in enhancing corrosion resistance.
• the N content is set to 0.03 to 0.35%.
• the preferable lower limit of the N content is 0.05% and the preferable upper limit thereof is 0.30%.
• the content of N is preferably 0.03 to 0.15%.
• the content range of N in this case is more preferably 0.05 to 0.12%.
• the content of N is preferably 0.10 to 0.35%.
• the content range of N in this case is more preferably 0.15 to 0.30%.
• Al has a deoxidizing effect but, at high addition levels, it markedly impairs the cleanliness and deteriorates the workability and ductility; in particular, when the Al content exceeds 0.03% as sol. Al ("acid-soluble Al"), it causes marked decreases in workability and ductility. Therefore, the content of sol. Al is set to not more than 0.03%.
• the lower limit of the sol.Al content is not particularly restricted. However the lower limit of the sol.Al content is preferably 0.0005%.
• Nb not more than 1.0%
• V not more than 0.5%
• Ti not more than 0.5%
• Nb, V and Ti constitute an important group of elements forming the basis of the present invention. That is to say, these elements precipitate intragranularly in the form of fine carbonitrides and thus act as essential elements for ensuring the creep strength at high temperatures.
• the content of these elements is excessive, in particular when the contents of Nb and V exceed 1.0% and 0.5% respectively, the carbonitrides rapidly become coarsened during use at high temperatures, causing extreme decreases in creep strength and toughness.
• Ti if the content thereof exceeds 0.5% it causes a marked increase in the susceptibility to liquid cracking on the occasion of welding. Therefore, the content of each of Nb, V and Ti is set to as follows: Nb: not more than 1.0%, V: not more than 0.5% and Ti: not more than 0.5%.
• the upper limit of each of the contents of the above-mentioned elements is preferably as follows: 0.8% for Nb, 0.4% for V, and 0.04% for Ti.
• the steels of the present invention can contain only one or a combination of two or more of the above-mentioned elements selected from Nb, V and Ti.
• the value of the said parameter P2 mentioned hereinabove should be set to not less than 0.2 and, in order to reduce susceptibility to cracking in the coarse-grained HAZ, it is necessary that the upper limit to the value of the said parameter P2 should be set to [1.7 - 10 ⁇ P1], as described later herein.
• the value of P1 defined by the said formula (1), that is to say, by [S + ⁇ (P + Sn) / 2 ⁇ + ⁇ (As + Zn + Pb + Sb) / 5 ⁇ ], exceeds 0.06, it becomes impossible to prevent the decrease in the grain boundary binding force and, therefore the occurrence of cracking due to grain boundary embrittlement in the coarse-grained HAZ during use at high temperatures of the austenitic stainless steel, according to the present invention, which contains 15 to 30% of Cr and 6 to 30% of Ni cannot be avoided. Therefore, it is necessary that the value of the parameter P1 should be set to not more than 0.06.
• the value of the parameter P1 is preferably not more than 0.04. It is also preferable that the value of the parameter P1 be reduced as low as possible.
• the value of P2 defined by the said formula (2) that is to say, by [Nb + 2(V + Ti)]
• the value of P2 is set to not less than 0.2 to not more than [1.7 - 10 ⁇ P1].
• the lower limit of the value of the parameter P2 is preferably 0.3, and more preferably 0.4.
• the upper limit of the value of the parameter P2 is preferably set to [1.5 - 10 ⁇ P1], and more preferably [1.3 - 10 ⁇ P1].
• the value of the parameter P2 is preferably set to not less than 0.3 to not more than [1.7 - 10 ⁇ P1].
• the more preferable lower limit of the value of the parameter P2 is 0.4.
• the more preferable upper limit of the value of the parameter P2 is [1.5 - 10 ⁇ P1].
• the value of the parameter P2 is preferably set to not less than 0.2 to not more than [1.3 - 10 ⁇ P1].
• the more preferable lower limit of the value of the parameter P2 is 0.3.
• the more preferable upper limit of the value of the parameter P2 is [1.2 - 10 ⁇ P1].
• the austenitic stainless steels according to the present inventions (1) to (3) can further selectively contain, according to need, one or more elements of each of the following groups of elements in lieu of a part of Fe:
• one or more of the first to third groups of elements may be added, as optional element(s), to the above-mentioned steels and thereby contained therein.
• First group Cu: not more than 4%, Mo: not more than 2%, W: not more than 2%, Co: not more than 1%, Ta: not more than 0.1%, Zr: not more than 1% and Hf: not more than 0.1%
• Each of Cu, Mo, W, Co, Ta, Zr and Hf being elements of the first group, if added, has the effect of enhancing the high temperature strength.
• the said elements may be added to the steels and thereby contained therein. The elements, which are in the first group, are now described in detail.
• Cu precipitates finely at high temperatures. Therefore, Cu is an effective element which enhances high temperature strength. Cu is also effective in stabilizing the austenite phase.
• the content of Cu is set to not more than 4%.
• the content of Cu is preferably set to not more than 3.8%, and more preferably not more than 3.5%.
• the content of Cu is still more preferably set to not more than 3%.
• the lower limit of the Cu content is preferably set to 0.02%.
• the lower limit of the Cu content is more preferably 0.05%.
• Mo dissolves in the matrix and is an element which makes a contribution to the enhancement of high temperature strength, in particular to the enhancement of creep strength at high temperatures.
• the content of Mo is increased, the stability of the austenite phase is deteriorated; hence the creep strength is rather low, and moreover, the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ increases.
• the content of Mo exceeds 2%, the creep strength markedly deteriorates. Therefore, if Mo is added, the content of Mo is set to not more than 2%.
• the content of Mo is preferably set to not more than 1.8%.
• the lower limit of the Mo content is preferably set to 0.05%.
• the lower limit thereof is more preferably set to 0.08%.
• W also dissolves in the matrix and is an element which makes a contribution to the enhancement of high temperature strength, in particular to the enhancement of creep strength at high temperatures.
• the content of W is increased, the stability of the austenite phase is deteriorated; hence the creep strength is rather low, and moreover, the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ increases.
• the content of W exceeds 2%, the creep strength markedly deteriorates. Therefore, if W is added, the content of W is set to not more than 2%.
• the content of W is preferably set to not more than 1.8%.
• the lower limit of the W content is preferably set to 0.05%.
• the lower limit thereof is more preferably set to 0.08%.
• Co is an austenite-forming element; it increases the stability of the austenite phase and makes a contribution to the enhancement of high temperature strength, in particular to the enhancement of creep strength.
• Co is a very expensive element and, therefore, an increased content thereof results in an increase in cost. In particular when the content of Co exceeds 1%, the cost markedly increases. Therefore, if Co is added, the content of Co is set to not more than 1%.
• the content of Co is preferably set to not more than 0.9%.
• the lower limit of the Co content is preferably set to 0.03%.
• the lower limit of the Co content is more preferably 0.05%.
• Ta dissolves in the matrix or precipitates as a carbonitride; it is an element which makes a contribution to the enhancement of high temperature strength, although the effect is not so much compared with Mo, W, V, Nb or Ti.
• the amount of precipitates increases and thereby toughness deteriorates and moreover the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ becomes higher.
• the content of Ta exceeds 0.1%, toughness markedly deteriorates and the incidence of cracking due to grain boundary embrittlement in the coarse-grained HAZ also markedly increases. Therefore, if Ta is added, the content of Ta is set to not more than 0.1%.
• the content of Ta is preferably set to not more than 0.09%.
• the lower limit of the Ta content is preferably set to 0.002%.
• the lower limit of the Ta content is more preferably 0.005%.
• Zr also dissolves in the matrix or precipitates as a carbonitride; it is an element which makes a contribution to the enhancement of high temperature strength, although the effect is not so much compared with Mo, W, V, Nb or Ti.
• the Zr content is increased, the amount of precipitates increases and thereby toughness deteriorates and moreover the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ becomes higher.
• the content of Zr exceeds 0.1%, toughness markedly deteriorates and the incidence of cracking due to grain boundary embrittlement in the coarse-grained HAZ also markedly increases. Therefore, if Zr is added, the content of Zr is set to not more than 0.1%.
• the content of Zr is preferably set to not more than 0.09%.
• the lower limit of the Zr content is preferably set to 0.002%.
• the lower limit of the Zr content is more preferably 0.005%.
• Hf also dissolves in the matrix or precipitates as a carbonitride; it is an element which makes a contribution to the enhancement of high temperature strength, although the effect is not so much compared with Mo, W, V, Nb or Ti.
• Hf content is increased, the amount of precipitates increases and thereby toughness deteriorates and moreover the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ becomes higher.
• toughness markedly deteriorates and the incidence of cracking due to grain boundary embrittlement in the coarse-grained HAZ also markedly increases. Therefore, if Hf is added, the content of Hf is set to not more than 0.1%.
• the content of Hf is preferably set to not more than 0.09%.
• the lower limit of the Hf content is preferably set to 0.002%.
• the lower limit of the Hf content is more preferably 0.005%.
• the steels of the present invention can contain only one or a combination of two or more of the above-mentioned Cu, Mo, W, Co, Ta, Zr and Hf.
• Second group B: not more than 0.012%
• B which is the element of the second group, if added, has the effect of strengthening the grain boundaries. In order to obtain this effect, B may be added to the steels and thereby contained therein. B, which is in the second group, is now explained in detail.
• B segregates on the grain boundaries and also disperses carbides precipitating on the grain boundaries finely, and is an element which makes a contribution to strengthening the grain boundaries.
• an excessive addition of B lowers the melting point; in particular, when the content of B exceeds 0.012%, the decrease of the melting point becomes remarkable, and therefore, in the step of welding, the liquation cracking on the grain boundaries in the HAZ vicinity to the fusion line occurs. Therefore, if B is added, the content of B is set to not more than 0.012%.
• the content of B is preferably not more than 0.010%.
• the lower limit of the B content is preferably set to 0.0001%.
• the lower limit of the B content is more preferably 0.0005%.
• Third group Ca: not more than 0.02%, Mg: not more than 0.02% and REM: not more than 0.1%.
• Each of Ca, Mg and REM being elements of the third group, if added, has the effect of increasing the hot workability. In order to obtain this effect, the said elements may be added to the steels and thereby contained therein.
• the elements, which are in the third group, are now described in detail.
• Ca has a high affinity for S and so, it has an effect of improving the hot workability. Ca is also effective, although to a slight extent, in reducing the possibility of cracking due to grain boundary embrittlement in the coarse-grained HAZ which is caused by the segregation of S on the grain boundaries.
• an excessive addition of Ca causes deterioration of the cleanliness, in other words, an increase of the index of cleanliness, due to the binding thereof to oxygen; in particular, when the content of Ca exceeds 0.02%, the deterioration of the cleanliness markedly increases and the hot workability rather deteriorates. Therefore, if Ca is added, the content of Ca is set to not more than 0.02%.
• the content of Ca is preferably not more than 0.015%.
• the lower limit of the Ca content is preferably set to 0.0001%.
• the lower limit of the Ca content is more preferably 0.001%.
• Mg also has a high affinity for S and so, it has an effect of improving the hot workability. Mg is also effective, although to a slight extent, in reducing the possibility of cracking due to grain boundary embrittlement in the coarse-grained HAZ which is caused by the segregation of S on the grain boundaries.
• an excessive addition of Mg causes deterioration of the cleanliness due to the binding thereof to oxygen; in particular, when the content of Mg exceeds 0.02%, the deterioration of the cleanliness markedly increases and the hot workability rather deteriorates. Therefore, if Mg is added, the content of Mg is set to not more than 0.02%.
• the content of Mg is preferably not more than 0.015%.
• the lower limit of the Mg content is preferably set to 0.0001%.
• the lower limit of the Mg content is more preferably 0.001%.
• REM has a high affinity for S and so, it has an effect of improving the hot workability. REM is also effective in reducing the possibility of cracking due to grain boundary embrittlement in the coarse-grained HAZ which is caused by the segregation of S on the grain boundaries.
• an excessive addition of REM causes deterioration of the cleanliness due to the binding thereof to oxygen; in particular, when the content of REM exceeds 0.1%, the deterioration of the cleanliness markedly increases and the hot workability rather deteriorates. Therefore, if REM is added, the content of REM is set to not more than 0.1%.
• the content of REM is preferably not more than 0.08%.
• the lower limit of the REM content is preferably set to 0.001%.
• the lower limit of the REM content is more preferably 0.005%.
• the term "REM” refers to a total of 17 elements including Sc, Y and lanthanoid collectively, and the REM content means the content of one or the total content of two or more of the REM.
• the steel of the present invention may contain one alone or a combination of two or more of the above-mentioned Ca, Mg and REM.
• the austenitic stainless steel according to the present invention (4) is defined as the one which contains one or more elements of one or more groups selected from the first to third groups listed below in lieu of a part of Fe in the austenitic stainless steel according to any one of the present inventions (1) to (3).
• the austenitic stainless steels according to the present inventions (1) to (4) can be produced by selecting the materials to be used in the melting step based on the results of careful and detailed analyses so that, in particular, the contents of Sn, As, Zn, Pb and Sb among the impurities may fall within the above-mentioned respective ranges, namely Sn: not more than 0.1%, As: not more than 0.01%, Zn: not more than 0.01%, Pb: not more than 0.01% and Sb: not more than 0.01% and the values of P1 and P2 respectively defined by the said formula (1) and formula (2) satisfy the conditions P1 ⁇ 0.06 and 0.3 ⁇ P2 ⁇ 1.7 - 10 ⁇ P1, respectively and then melting the materials using an electric furnace, an AOD furnace or a VOD furnace.
• a slab, a bloom or a billet is produced by casting the molten metal, which is prepared by a melting process, into an ingot by the so-called "ingot making method” and subjecting the ingot to hot working, or by continuous casting.
• the said material is subjected to hot rolling into a plate or a coil shaped sheet.
• any of such materials is subjected to hot working into a tubular product by the hot extrusion pipe manufacturing process or Mannesmann pipe manufacturing process.
• the hot working may use any hot working process.
• the hot working may include the hot extrusion pipe manufacturing process represented by the Ugine-Sejournet process and/or the rolling pipe manufacturing process (Mannesmann pipe manufacturing process) represented by the Mannesmann-Plug Mill rolling process or the Mannesmann-Mandrel Mill rolling process or the like.
• the hot working may include the typical process of manufacturing a steel plate or a hot rolled steel sheet in coil.
• the end temperature of the hot working is not particularly defined, but may be preferably set to not less than 1030°C. This is because if the end temperature of the hot working is less than 1030°C, the dissolution of the carbonitrides of Nb, Ti and V becomes insufficient, and therefore the creep strength and ductility may be impaired.
• the cold working may be carried out after the hot working.
• the cold working may include the cold drawing pipe manufacturing process in which the pipe produced by the above-mentioned hot working is subjected to drawing and/or the cold rolling pipe manufacturing process using a cold pilger mill.
• the cold working may include the typical process of manufacturing a cold rolled steel sheet in coil.
• the final heat treatment after the above-mentioned hot working or the final heat treatment after a further cold working following the hot working may be carried out at a heating temperature of not less than 1030°C.
• the upper limit of the said heating temperature is not particularly defined, but a temperature exceeding 1350°C may cause not only high temperature intergranular cracking or a deterioration of ductility but also very coarse grains. Moreover, the said temperature may cause a marked deterioration of workability. Therefore, the upper limit of the heating temperature is preferably set to 1350°C.
• Austenitic stainless steels A1, A2, B1 and B2 having the chemical compositions shown in Tables 1 and 2 were melted using an electric furnace and cast to form ingots. Each ingot was hot worked by a hot forging and a hot rolling, and then, was subjected to a heat treatment comprising heating at 1200°C, followed by water cooling and, thereafter subjected to machining to produce steel plates having a thickness of 20 mm, a width of 50 mm and a length of 100 mm.
• the steels A1 and A2 shown in Tables 1 and 2 are steels having chemical compositions which fall within the range regulated by the present invention.
• the steels B1 and B2 are steels of comparative examples in which the values of the parameters P1 and P2 are out of the ranges regulated by the present invention.
• Table 1 Steel Chemical composition (% by mass) The balance: Fe and impurities C Si Mn Cu Ni Cr Mo Co sol.Al B N Nb V Ti S P A1 0.07 0.24 0.80 2.79 8.93 18.95 0.31 0.15 0.002 0.0047 0.12 0.500 0.072 0.003 0.0020 0.031 A2 0.06 0.38 1.15 0.08 19.70 24.56 0.11 0.39 0.005 0.0018 0.24 0.444 0.030 0.003 0.0003 0.017 B1 0.08 0.22 0.75 2.85 8.92 18.95 0.35 0.15 0.007 0.0030 0.10 0.595 0.192 0.040 0.0090 0.038 B2 0.06 0.36 1.18 0.08 20.10 25.12 0.10 0.42 0.004 0.0014 0.26 0.480 0.210 0.082 0.0120 0.028
• the steel plates made of the steels A1, A2, B1 and B2 were machined for providing each of them with a shape of V-groove with an angle of 30° in the longitudinal direction and a root thickness of 1 mm. Then each of them was subjected to four side-restrained welding onto a commercial SM400C steel plate, 25 mm in thickness, 200 mm in width and 200 mm in length, as standardized in JIS G 3106 (2004) using "DNiCrFe-3" defined in JIS Z 3224 (1999) as a covered electrode.
• each steel plate was subjected to multilayer welding in the groove using the "YNiCr-3" defined in JIS Z 3334 (1999) as a welding wire by the TIG welding method under a heat input condition of 9 to 15 kJ/cm.
• each test specimen was subjected to an aging heat treatment at 650°C for 3000 hours, and a section thereof was mirror-like polished and etched, and then observed using an optical microscope. As a result, cracking due to grain boundary embrittlement was found to have occurred in the coarse-grained HAZ in the steels B1 and B2.
• test specimens 12 mm ⁇ 12 mm ⁇ 100 mm in size, were prepared from the middle part in the direction of thickness of each of the above-mentioned steel plates with a thickness of 20 mm, a width of 50 mm and a length of 100 mm.
• the above-mentioned test specimens were subjected to simulated HAZ thermal cycles of 1350°C for 5 seconds which simulates the weld thermal cycle at the coarse-grained HAZ.
• brimmed round bar creep test specimens with a soaked potion exposed to the simulated HAZ thermal cycles forming the parallel portion, 6 mm in diameter and 10 mm in length, were cut off from the above test specimens.
• the creep test specimens were subjected to a creep rupture test under the conditions of 650°C and 196 MPa corresponding to a desired base metal strength level of 3000 hours; regarding the steels A2 and B2, which had higher Cr and Ni contents and also had a higher level of creep strength; the creep test specimens were subjected to a creep rupture test under the conditions of 650°C and 216 MPa corresponding to a desired base metal strength level of 3000 hours.
• Austenitic stainless steels A3 to A13, B3 and B4 having the chemical compositions shown in Tables 4 and 5 were melted using an electric furnace and cast to form ingots. Each ingot was hot worked by a hot forging and a hot rolling, and then, was subjected to a heat treatment comprising heating at 1200°C, followed by water cooling and, thereafter subjected to machining to produce steel plates having a thickness of 20 mm, a width of 50 mm and a length of 100 mm.
• the steels A3 to A13 shown in Tables 4 and 5 are steels having chemical compositions which fall within the range regulated by the present invention.
• the steels B3 and B4 are steels of comparative examples in which the value of the parameter P2 is out of the ranges regulated by the present invention.
• Table 4 Steel Chemical composition (% by mass) The balance: Fe and impurities C Si Mn Cu Ni Cr Mo Co sol.Al B N Nb V Ti S P A3 0.08 0.23 0.80 2.90 9.33 18.54 - - 0.003 - 0.10 0.500 0.075 0.003 0.0010 0.026 A4 0.08 0.23 0.74 2.88 8.87 18.07 0.21 0.19 0.010 0.0041 0.11 0.520 0.067 0.004 0.0010 0.027 A5 0.08 0.24 0.78 2.87 8.73 18.38 0.22 0.16 0.011 0.0042 0.11 0.502 0.050 0.002 0.0005 0.033 A6 0.08 0.15 0.80 2.77 8.64 18.33 0.45 0.25 0.004 0.0043 0.10 0.489 0.050 - 0.0008 0.036 A7 0.08 0.21 0.82 2.96 8.72 18.39 0.37 0.17 0.007 0.0040 0.10 0.487 0.050 0.004 0.0005 0.032 A8 0.06 0.42 1.20 0.10 19.82 24.
• Test specimens 12 mm ⁇ 12 mm ⁇ 100 mm in size, were prepared from the middle part in the direction of thickness of each of the thus-obtained steel plates with a thickness of 20 mm, a width of 50 mm and a length of 100 mm.
• the above-mentioned test specimens were subjected to simulated HAZ thermal cycles of 1350°C for 5 seconds which simulates the weld thermal cycle at the coarse-grained HAZ. Then, brimmed round bar creep test specimens with a soaked portion exposed to the simulated HAZ thermal cycles forming the parallel portion, 6 mm in diameter and 10 mm in length, were cut off from the above test specimens.
• the creep test specimens were subjected to a creep rupture test under the conditions of 650°C and 196 MPa corresponding to a desired base metal strength level of 3000 hours; regarding the steels A8 to A13 and B4, which had higher Cr and Ni contents and also had a higher level of creep strength; the creep test specimens were subjected to a creep rupture test under the conditions of 650°C and 216 MPa corresponding to a desired base metal strength level of 3000 hours.
• the austenitic stainless steels of the present invention have high strength and excellent resistance to cracking due to grain boundary embrittlement in the welded portion during use at high temperatures. Consequently, they can be used as materials for constructing machines and equipment, such as power plant boilers, which can be used at high temperatures for a long period of time.

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