EP2036994B1 - Cr-haltiger stahl mit hervorragenden wärmeermüdungseigenschaften - Google Patents

Cr-haltiger stahl mit hervorragenden wärmeermüdungseigenschaften Download PDF

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EP2036994B1
EP2036994B1 EP07767965A EP07767965A EP2036994B1 EP 2036994 B1 EP2036994 B1 EP 2036994B1 EP 07767965 A EP07767965 A EP 07767965A EP 07767965 A EP07767965 A EP 07767965A EP 2036994 B1 EP2036994 B1 EP 2036994B1
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high temperature
heat fatigue
strength
fatigue characteristics
aging
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EP2036994A1 (de
EP2036994A4 (de
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Junichi Hamada
Haruhiko Kajimura
Nobuhiko Hiraide
Toshio Kojima
Daiharu Doi
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Nippon Steel Stainless Steel Corp
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Nippon Steel and Sumikin Stainless Steel Corp
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    • 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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/16—Selection of particular materials
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes

Definitions

  • the present invention relates to a Cr-containing steel superior in heat fatigue characteristics optimal for use for exhaust system parts etc. particularly requiring high temperature strength and oxidation resistance.
  • Exhaust manifolds, front pipes, center pipes, and other exhaust system parts of automobiles carry high temperature exhaust gas discharged from engines, so the materials forming the exhaust parts are required to have oxidation resistance, high temperature strength, heat fatigue characteristics, and various other characteristics.
  • austenitic stainless steel is superior in heat resistance and workability, but is large in coefficient of heat expansion, so when applied to a part repeatedly subjected to heating and cooling like an exhaust manifold, heat fatigue breakage easily occurs.
  • ferritic stainless steel has a smaller coefficient of heat expansion compared with austenitic stainless steel, so is superior in heat fatigue characteristics.
  • steel adjusted in amounts of addition of alloy elements such as Cr, Mo, and Nb is used.
  • alloy elements such as Cr, Mo, and Nb
  • the amounts of these alloy elements added are increasing. The most important characteristic, that is, the heat fatigue life, does not necessarily become longer. Further, excessive increase of the amount of addition of alloy elements leads to an increase in costs, so sometimes has the defect of not being economical.
  • Japanese Patent Publication (A) No. 7-145453 discloses ferritic stainless steel superior in oxidation resistance, high temperature strength, and heat fatigue characteristics for automobile exhaust manifolds. It is art for the case where the Cr amount is a relatively low Cr content of 11 to 14% and adding Si to improve the oxidation resistance high temperature strength, and heat fatigue characteristics at 900°C or more. Among these, the heat fatigue characteristics are the characteristics under the conditions of 50% restraint at 200 to 900°C. The invention steels became longer in heat fatigue life, but when the restraint ratio became longer, the temperature becomes about 800°C, and the cycle imparted becomes longer, sufficient properties could not be obtained.
  • Japanese Patent Publication (A) No. 9-279316 discloses an invention controlling the Si/Mn to make the yield strength at 900°C 15 MPa or more and improve the high temperature characteristics. In this case as well, just defining the tensile yield strength of the final sheet at 900°C was insufficient in a long usage environment. Further, Mn is added in an amount of 0.7 to 1.3%, so the ductility is low, and the formability when working a part and the high temperature ductility drop and thereby the heat fatigue life drops.
  • Japanese Patent Publication (A) No. 2002-105605 discloses the technical idea of adjusting the ingredients to make the 0.2% yield strength after holding at 900°C for one hour 18 MPa or more.
  • holding at a high temperature for one hour or more improves the strength in the usage environment, but when being subjected to a thermal cycle, sometimes just the improvement of the high temperature strength is not enough for improvement of the heat fatigue life.
  • JP 2000 336 462 discloses a Ferritic stainless steel for exhaust manifolds with good high temperature strength which contains titanium and niobium.
  • Japanese Patent Publication (A) No. 9-279312 Japanese Patent Publication (A) No. 2000-169943 , and Japanese Patent Publication (A) No. 10-204590 disclose steels containing B as ferritic stainless steels superior in high temperature characteristics, but B is added from the viewpoint of improvement of the workability.
  • B is added from the viewpoint of improvement of the workability.
  • the role of B in the improvement of the workability is the improvement of the grain boundary strength through grain boundary precipitation and improvement of the secondary workability, but in the present invention, the addition of B makes the precipitate finer and improves the high temperature strength.
  • the above three patents describe the addition of V, but V was not added from the viewpoint of improvement of the corrosion resistance of the weld zone and the improvement of the workability due to the C and N being fixed.
  • the present invention provides, as a material able to handle exhaust temperatures near 800°C, a relatively inexpensive Cr-containing steel having superior oxidation resistance, high temperature strength, and heat fatigue characteristics in an environment where it is used at a high temperature for a long time or is repeatedly subjected to heating and cooling.
  • the inventors investigated the relationship between the ingredients and high temperature deformation characteristics in relation to the oxidation resistance, high temperature strength, and heat fatigue characteristics of Cr-containing steel. Among these, considering in particular environments subjected to a thermal cycle, they carefully investigated how the deformation characteristics in the high temperature region and also the deformation characteristics in the low temperature region act on the heat fatigue life. Further, the inventors engaged in repeated studies to achieve the above object and as a result obtained the following discoveries.
  • the invention features adding Cr and Si from the viewpoint of mainly the oxidation resistance, adding Nb and Ti in combination from the viewpoint of improving the high temperature characteristics, and further adjusting the ingredients in the novel ingredients in addition of V and B so as to secure strength and ductility at the time of long term use and greatly improve the heat fatigue characteristics.
  • the gist of the present invention is as follows:
  • N like C, degrades the formability and corrosion resistance and causes a drop in the high temperature strength, so the smaller the content the better, so the content was made 0.015% or less.
  • excessive addition leads to an increase in the refining costs, so further 0.001 to 0.010% is preferable.
  • Si is an element important for improving the oxidation resistance and high temperature characteristics in the present invention.
  • the oxidation resistance and the high temperature strength are improved along with the increase of the Si amount.
  • Si promotes the precipitation of intermetallic compounds mainly comprised of Fe and Nb called a "Laves phase" at a high temperature. If the Laves phase excessively precipitates, the amount of soluble Nb required for securing high temperature strength ends up being reduced. Further, if excessively adding Si, the ordinary temperature workability deteriorates and also the ductility during long term use is reduced and a drop in the heat fatigue life is caused. From these viewpoints, the upper limit was made 1.0%, further preferably 0.8 to 0.9%.
  • Mn is an element added as a deoxidizing agent and improving the high temperature strength. Its effects are manifested from 0.2% or more. Further, in steel in which it is added combined with Ti, it is learned that the addition of Mn suppresses the oxidation of Ti at the time of continuous oxidation and improves the oxidation resistance. On the other hand, addition over 1.5% lowers the ductility and also forms MnS to reduce the corrosion resistance. Further, excessive addition causes deterioration of the oxidation resistance. Therefore, the content was made 0.2 to 1.5%. Furthermore, considering the high temperature ductility and the scale adhesion, 0.3 to 1.0% is preferable.
  • P is a solution strengthening element like Mn and Si, so in terms of material quality, the lower the content the better, so an upper limit of 0.03% is preferable. However, excessive reduction leads to an increase in the refining costs, so a lower limit of 0.01% is preferable. Furthermore, if considering the refining costs and corrosion resistance, 0.012 to 0.025% is preferable.
  • S is an element degrading the corrosion resistance and oxidation resistance, but the effect of bonding with Ti or C to improve the workability is expressed from 0.0001%, the lower limit was made 0.0001%. On the other hand, excessive addition causes the S to bond with the Ti and C to reduce the amount of soluble Ti and increase the coarseness of the precipitates, whereby the high temperature strength falls, so the upper limit was made 0.01%. Furthermore, if considering the refining costs and the high temperature oxidation characteristics, 0.0010 to 0.0090% is preferable.
  • Cr is an element essential for securing the oxidation resistance in the present invention. If less than 13%, this effect is not manifested, while if over 15%, it reduces the workability and degrades the toughness, so the content was made 13 to 15%. Furthermore, if considering the high temperature ductility and manufacturing costs, 13.2 to 14.5% is preferable.
  • Ni is effective for improvement of toughness and improvement of the high temperature salt corrosion resistance.
  • austenite forming element has a detrimental effect on the oxidation resistance, and is expensive, so the content was made 0.2% or less.
  • Cu is effective for improvement of the high temperature strength, but lowers the ductility and has a detrimental effect on the oxidation resistance, so the content was made 0.2% or less.
  • Mo improves the corrosion resistance, suppresses high temperature oxidation, and is effective for improvement of the high temperature strength through solution strengthening. However, it causes a drop in high temperature ductility and also is expensive, so was made 0.2% or less, more preferably 0.1% or less.
  • Nb is an element necessary for improving the high temperature strength through solution strengthening and strengthening by increased fineness of the precipitate. Further, it has the role of fixing C and N as carbonitrides and contributing to the formation of a recrystallized texture having an effect on the corrosion resistance and r value of the product sheet. These effects appear from 0.3% or more.
  • the solution strengthening ability is lost, so even if excessively added, the effect ends up becoming saturated. Further, with excessive addition, the ductility falls and the heat fatigue life ends up becoming shorter in the low temperature region.
  • the soluble Nb amount is secured. In this case, this action is saturated at 0.55%, so the content was made 0.3 to 0.55%. Further, if considering the formability, grain boundary corrosion, and manufacturing costs, 0.32 to 0.45% is preferable.
  • Ti is an element bonding with C, N, and S and improving the corrosion resistance, grain boundary corrosion resistance, and deep drawability. Further, in combined addition with Nb, addition of a suitable amount improves the high temperature strength after long term exposure to a high temperature, improves the high temperature ductility, and improves the heat fatigue characteristics. These effects are expressed from 0.05% or more, but addition over 0.2% causes the soluble Ti amount to increase and degrades the formability and also causes formation of surface defects, a drop in toughness, and a deterioration of the oxidation resistance, so the content was made 0.05 to 0.2%. Furthermore, if considering the manufacturability, 0.05 to 0.15% is preferable.
  • V forms fine carbonitrides upon addition at 0.01% or more and gives rise to a precipitation strengthening action to contribute to an improvement of the high temperature strength.
  • the low temperature ductility falls and conversely the heat fatigue life ends up falling, so the upper limit was made 0.2%.
  • 0.08 to 0.15% is preferable.
  • A1 is an element added as a deoxidizing element and also improving the oxidation resistance and the high temperature strength by solution strengthening and is an essential element in the present invention. This action appears from 0.015%, but addition over 1.0% causes hardening, the formation of surface defects, and the deterioration of the weldability, so the content was made 0.015 to 1.0%. Furthermore, if considering the refining costs, 0.03 to 0.7% is preferable.
  • B is an element improving the secondary workability at the time of press forming of a product.
  • Ti-containing steel is susceptible to secondary work cracks, so B is an essential element in the present invention.
  • B is considered to form (Fe, Cr)23(C,B)6 or Cr2B in the high temperature region and precipitate at the grain boundaries, but in a system of ingredients where Si is added, it was learned that these precipitates do not form and there is the effect of making the above-mentioned Laves phase finely precipitate.
  • the Laves phase causes a reduction in the soluble Nb amount and usually ends up making the structure coarser, so there is almost no high temperature strengthening ability in particular after long aging, but the addition of B causes fine precipitation, so B has precipitation strengthening ability and contributes to the improvement of the high temperature strength.
  • B causes fine precipitation of the Laves phase is believed to be the drop in the interfacial energy due to grain boundary precipitation and the increased difficulty of grain boundary precipitation.
  • the product sheet was made into a pipe by seam welding (outside diameter: 38.1 mm) and used for a heat fatigue test.
  • the heat fatigue test was conducted by a computer controlled electrical hydraulic controlled fatigue tester.
  • the temperature cycle given was made a pattern of raising the temperature from 200 to 800°C in 120 sec, holding it at 800°C for 30 sec, then cooling down to 300°C for 120 sec and further cooling down to 200°C for 90 sec.
  • the heating was performed by a high frequency induction heating coil, while the cooling was performed by feeding air into the test tube.
  • Compressive strain was imparted so that the restraint ratio became a constant ratio with respect to the amount of free expansion. That is, for example, in the case of 50% restraint, compressive strain is mechanically imparted so as to give an amount of expansion of half the free expansion.
  • the chemical composition of the tested materials are shown in Table 1.
  • the Steel A is steel complying with the present invention, while the Steel B is a comparative steel.
  • the Steel B is a generally used heat resistant stainless steel sheet.
  • Table 1 No. C N Si Mn P S Ni Cu Cr Mo Nb Ti V Al B A 0.005 0.012 0.82 0.31 0.03 0.0012 0.11 0.03 13.8 0.01 0.35 0.14 0.08 0.05 0.0003 B 0.005 0.015 0.93 0.30 0.02 0.0009 0.15 0.02 14.1 0.01 0.45 0.005 0.08 0.03 0.0001
  • the steels of the present invention have higher lifetimes than the comparative examples at all of the restraint ratios. This is because even if imparting aging of the high temperature strength, that is, a long term thermal cycle, there is almost no drop in strength and, further, a high ductility is maintained in the low temperature region of the thermal cycle. While receiving the thermal cycle, at the high temperature, the material is given a compressive load, while when held at 800°C, creep deformation or stress-relief phenomena occur, so the increase of the 0.2% yield strength at 800°C is considered effective for extension of the heat fatigue life. On the other hand, in the cooling process from 800°C to 200°C, the material is given tensile stress.
  • This tensile stress is a stress remarkably larger than the compressive stress in the high temperature region.
  • damage damage (defects) occur in the thermal cycle, there is remarkable plastic deformation at that location. Therefore, the increase in the ductility of the material at 200°C (drawability) is believed to be effective in suppressing the progression of damage in the cooling process.
  • FIG. 3 and FIG. 4 show the tensile strength and drawability at a high temperature after aging at 800°C.
  • the high temperature strength is a high strength of 20 MPa or more and the ductility is a high one of a drawability value at 200°C of 35%.
  • the strength at the highest temperature is high and the ductility at the lowest temperature is high. Due to this, as shown in FIG. 2 , no matter what the restraint ratio, this is believed to lead to an improvement in the heat fatigue life.
  • prior inventions only improvement of the strength at the highest temperature when subjected to a thermal cycle was the technical idea, but in the present invention it was discovered that improvement of the ductility at the lowest temperature results in remarkable improvement of the heat fatigue life.
  • the improvement in the drawability value at 200°C is believed to be due to the securing of the elongation at break at ordinary temperature mentioned above and the suppression of aging deterioration. That is, the fact that the balance of addition of Ti and Nb is important became clear from the present invention.
  • the soluble Nb amount and soluble Ti amount have an effect on the improvement of the high temperature strength at 800°C.
  • FIG. 5 shows the relationship between the soluble Nb amount+soluble Ti amount after aging at 800°C and the high temperature strength at 800°C.
  • the high temperature strength at 800°C becomes 20 MPa or more. Due to this, to obtain a high temperature strength of 20 MPa or more and improve the heat fatigue life, the soluble Nb amount+soluble Ti amount was made 0.08% or more.
  • a suitable amount of Ti is added in combination with Nb to improve the high temperature strength and high temperature ductility after long aging and improve the heat fatigue characteristics, but conversely acts to degrade the oxidation resistance.
  • Nb oxidation resistance
  • spinal type oxides mainly containing TiO 2 , Cr, and Mn are formed at the outside layer and Cr 2 O 3 is formed at the inside layer.
  • the inside layer Cr 2 O 3 film becomes thicker and the oxidation resistance deteriorates.
  • FIG. 6 shows the Ti/Mn and the film thickness of the Cr 2 O 3 inside layer after continuous oxidation at 900°C for 200 hours.
  • the thickness of the Cr 2 O 3 inside layer scale is over 5 ⁇ m, scale peeling etc. occurs and the oxidation resistance deteriorates, but when Mn/Ti ⁇ 3, the Cr 2 O 3 inside layer scale becomes thinner and the oxidation resistance becomes superior.
  • the steels of the compositions of ingredients shown in Table 2 were produced and formed into slabs.
  • the slabs were hot rolled to obtain 5 mm thick hot rolled coils.
  • the hot rolled coils were annealed and pickled, were cold rolled to 2 mm thicknesses, then were annealed and pickled to obtain the product sheets.
  • the annealing temperatures of the cold rolled sheets were made 980 to 1050°C to make the crystal grain sizes the #6 to 8 or so. From the thus obtained product sheets, high temperature tensile test pieces were taken and subjected to tensile tests at 200°C and 800°C. Further, the sheets were aged at 800°C for 100 hours, then were subjected to high temperature tensile tests similar to the above.
  • the product sheets were made into pipes by seam welding (outside diameter: 38.1 mm) and used for heat fatigue tests.
  • the temperature cycle given was made a pattern of raising the temperature from 200 to 800°C in 120 sec, holding it at 800°C for 30 sec, then cooling down to 300°C for 120 sec and further cooling down to 200°C for 90 sec.
  • the restraint ratio was made 50%.
  • a width 20 mm, length 25 mm test piece was cut out from the product sheet, polished by emery paper to #600, then subjected to a continuous oxidation test at 900°C for 200 hours in the atmosphere.
  • the thickness of the Cr 2 O 3 inside layer scale was found by examination of the cross-section by an SEM (scan type electron microscope).
  • the inside layer scale thicknesses of the present invention steels were good ones of 5 ⁇ m or less.
  • the Si is outside the scope of the present invention.
  • the small Mn/Ti Steel Nos. 14, 17, 23, 24, and 26 have inside layer scale thicknesses of over 5 ⁇ m and are inferior in oxidation resistance.
  • the method of production of steel sheet is not particularly defined.
  • the hot rolling conditions and hot rolled sheet thickness, the hot rolled sheet and cold rolled sheet annealing temperature, the atmosphere, etc. may be suitably selected. Further, it is also possible to impart patent rolling or a tension leveler after cold rolling and annealing. Furthermore, the product sheet thickness may be selected in accordance with the required part thickness.

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Claims (4)

  1. Cr-haltiger Stahl mit hervorragenden Wärmeermüdungseigenschaften, dadurch gekennzeichnet, dass er in Masse-% enthält:
    C: höchstens 0,01 %,
    N: höchstens 0,015 %,
    Si: 0,8 bis 1,0 %,
    Mn: 0,2 bis 1,5 %,
    P: höchstens 0,03 %,
    S: höchstens 0,01 %,
    Ni: höchstens 0,2 %,
    Cu: höchstens 0,2 %,
    Cr: 13 bis 15 %,
    Mo: höchstens 0,1 %,
    Nb: 0,3 bis 0,55 %,
    Ti: 0,05 bis 0,2 %,
    V: 0,01 bis 0,2 %,
    Al: 0,015 bis 1,0 % und
    B: 0,0002 bis 0,0010 %,
    (Nb + 1,9 x Ti)/(C + N) ≤ 50 erfüllt und als Rest Fe und unvermeidliche Verunreinigungen hat.
  2. Cr-haltiger Stahl mit hervorragenden Wärmeermüdungseigenschaften nach Anspruch 1, dadurch gekennzeichnet, dass eine 0,2 %-Dehngrenze bei 800 °C nach mindestens 100-stündiger Alterung bei 800 °C mindestens 20 MPa beträgt und ein Ziehfähigkeitswert bei 200 °C mindestens 35 % beträgt.
  3. Cr-haltiger Stahl mit hervorragenden Wärmeermüdungseigenschaften nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass eine lösliche Nb-Menge + lösliche Ti-Menge nach mindestens 100-stündiger Alterung bei 800 °C mindestens 0,08 % beträgt.
  4. Cr-haltiger Stahl mit hervorragenden Wärmeermüdungseigenschaften nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass er Mn/Ti ≥ 3 erfüllt.
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DE102013004905A1 (de) 2012-03-23 2013-09-26 Salzgitter Flachstahl Gmbh Zunderarmer Vergütungsstahl und Verfahren zur Herstellung eines zunderarmen Bauteils aus diesem Stahl
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