US20150292068A1 - Ferritic stainless steel having excellent heat resistance - Google Patents

Ferritic stainless steel having excellent heat resistance Download PDF

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US20150292068A1
US20150292068A1 US14/439,456 US201314439456A US2015292068A1 US 20150292068 A1 US20150292068 A1 US 20150292068A1 US 201314439456 A US201314439456 A US 201314439456A US 2015292068 A1 US2015292068 A1 US 2015292068A1
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stainless steel
ferritic stainless
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Shinichi Teraoka
Akihiro Fukuda
Masaaki Kobayashi
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Nippon Steel Stainless Steel Corp
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Nippon Steel and Sumikin Stainless Steel Corp
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
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    • 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
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Definitions

  • the present invention relates to a material for sheet structure which is used at a high temperature, in particular relates to ferritic stainless steel which exhibits corrosion resistance at ordinary temperature and which is resistant to embrittlement due to use at a high temperature, such as a material for an automobile exhaust system.
  • Ferritic stainless steel is inferior to austenitic stainless steel in workability, toughness, and high-temperature strength, but does not contain a large amount of Ni, so is inexpensive. Further, it has a small heat expansion, so in recent year has been used for roofing and other building materials or materials for parts of automobile exhaust systems becoming high in temperature and other applications where thermal strain becomes a problem. In particular, when used as material for parts of exhaust systems of automobiles, high-temperature strength, corrosion resistance at ordinary temperature, and high toughness associated with high-temperature use are important. In general, SUH409L, SUS429, SUS430LX, SUS436J1L, SUS432, SUS444, and other steels are used as ferritic stainless steel suitable for these applications.
  • PLT 1 discloses a material using 0.05 to 2% of Sn to raise the high-temperature strength.
  • PLT 2 discloses the technique of adding 0.005 to 0.10% of Sn to improve the surface quality of stainless steel sheet. Further, in recent years, scrap iron containing surface-treated steel sheet has been used as raw materials, and so large amounts of Sn exceeding 0.05% have come to be included in stainless steel as unavoidable impurities.
  • PLT 1 Japanese Patent Publication No. 2000-169943A
  • PLT 2 Japanese Patent Publication No. H11-92372A
  • An object of the present invention is to provide ferritic stainless steel which does not deteriorate in toughness at ordinary temperature even if exposed to a high temperature over a long period of time like in a material for an automobile exhaust system.
  • the inventors engaged in various studied on the drop in toughness at ordinary temperature of ferritic stainless steel containing Sn after long term exposure to high temperatures. First, they investigated temperature range at which a drop in toughness is caused when using the SUS430LX containing 0.3% of Sn, and they found that the temperature range was 500 to 800° C. In addition, particularly, the temperature at which a drop in toughness occurred in a short time was 700° C. and it was learned that a large drop in toughness occurred in just 1 hour. As shown in FIG. 1 , the mode of fracture surface which occurs due to brittle fracture differs from a general cleavage fracture surface and has the characteristic of a grain boundary fracture surface.
  • the inventors cooled a sample to a low temperature in an AES (Auger electron spectroscopy) apparatus, then broke it and analyzed the grain boundary fracture surface, and remarkable Sn segregation was observed at a thickness of about 1 nm. That is, it was believed that the drop in toughness due to long term use at a high temperature occurred due to Sn grain boundary segregation.
  • AES Alger electron spectroscopy
  • the inventors investigated the effects on toughness when adding the stabilizing elements Ti and Nb alone and when adding them together and were able to develop steel resistant to the drop in toughness due to high-temperature use.
  • the present invention was reached based on these discoveries.
  • the solution to the problem of the present invention that is, the ferritic stainless steel of the present invention, is as follows:
  • Ferritic stainless steel containing, by mass %, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.50%, and Nb: 0.05 to 0.60%, restricted to C: 0.015% or less, Si: 1.5% or less, Mn: 1.5% or less, N: 0.020% or less, P: 0.035% or less, and S: 0.015% or less, containing a balance of Fe and unavoidable impurities, satisfying formula 1 and formula 2, and having a grain boundary Sn concentration of 2 at % or less when berformdnq heat treatment at a temperature of 600 to 750° C. so that an L-value shown by formula 3 becomes 1.91 ⁇ 10 4 or more:
  • T temperature (° C.)
  • t time (h)
  • the ferritic stainless steel according to (1) or (2) further containing, by mass %, one or more of Ti: 0.32% or less, Ni: 1.5% or less, Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% or less, Al: 0.3% or less, and B: 0.0020% or less:
  • a method of production of ferritic stainless steel according to any one of (1) to (5) comprising annealing stainless steel of a composition of (1) (3), or (4) at a cold-rolled strip annealing temperature of 850° C. to 1100° C. and then cooling from the cold-rolled strip annealing temperature by a cooling rate of 5° C./s or more in temperature range of 800 to 500° C.
  • the stabilizing elements Nb and Ti are optimized, and so stainless steel sheet which has little deterioration of the toughness even when used at a high temperature and further is excellent in corrosion resistance is obtained.
  • FIG. 1 shows photos of ferritic stainless steels of the present embodiment and comparative steels which are hot rolled annealed sheets of thickness 4.0 mm as is, and shows photos of fractured surfaces of test pieces showing brittle fracture in a Charpy impact test for ferritic stainless steels of the present embodiment and comparative after heat treating at 700° C. for 1 hour.
  • FIG. 2 is a graph which shows ductile brittle transition temperatures measured by conducting V-notch Charpy impact tests on subsize test pieces of thickness 4.0 mm for ferritic stainless steels of the present embodiment and comparative steels which are hot rolled annealed sheets of thickness 4.0 mm as is, and which shows ductile brittle transition temperatures measured by conducting the V-notch Charpy impact tests on the test pieces for ferritic stainless steels of the present embodiment and comparative steels after heat-treating at 700° C. for 1 hour.
  • FIG. 3 is a graph which shows the relationship between a ductile brittle transition temperature (DBTT) measured by conducting V-notch Charpy impact tests on subsize test pieces of thickness 4.0 mm and an indicator (GBSV) showing the tendency of the grain boundary segregation of Sn when using ferritic stainless steels of the present embodiment and comparative steels that are hot rolled annealed sheets of thickness 4.0 mm and further heat-treating the ferritic stainless steels at 700° C. for 1 hour.
  • DBTT ductile brittle transition temperature
  • GBSV indicator
  • FIG. 4 is a graph which shows the relationship between Sn concentration at the grain boundary and ductile brittle transition temperature (DBTT) when measuring the Sn concentration at the grain boundary fracture surface by AES and measuring the DBTT by a Charpy impact test and using ferritic stainless steels of the present embodiment and comparative steels which are hot rolled annealed sheets of thickness 4.0 mm and further heat treating the ferritic stainless steels at 700° C. for 1 hour.
  • DBTT ductile brittle transition temperature
  • the content is preferably as small as possible. Therefore, the upper limit is made 0.015%. However, excessive reduction causes an increase in the refining cost, so the lower limit may also be 0.001%. Further, if considered from the viewpoint of the corrosion resistance, the lower limit is preferably made 0.002% and the upper limit is preferably made 0.009%.
  • the upper limit is preferably made 0.018%. More preferably, the upper limit may be made 0.015%.
  • Si is an element which is useful as a deoxidizing agent and is an element which improves the high-temperature strength and oxidation resistance.
  • the deoxidizing effect is improved along with the increase in the amount of Si.
  • the effect is manifested at 0.01% or more and stabilizes at 0.05% or more, so the lower limit may be made 0.01%. Note that, if considering the oxidation resistance in adding Si, the lower limit is more preferably made 0.1% and the upper limit is more preferably made 0.7%.
  • Mn is an element which is added as a deoxidizing agent and an element which contributes to the rise in high-temperature strength in the medium temperature region. Further, it is an element whereby during long term use, Mn-based oxides form at the surface and contribute to the-effect of suppressing adhesion of scale (oxides) and abnormal oxidation. To cause this effect to be manifested, Mn may be added so that the content of Mn in the stainless steel of the present invention becomes 0.01% or more. Note that, if considering the high-temperature ductility or adhesion property of the scale and suppression of abnormal oxidation, the lower limit is more preferably made 0.1 and the upper limit is more preferably made 1.0%.
  • P is an element with a large solution strengthening ability, but is a ferrite stabilizing element and further is an element harmful to corrosion resistance and toughness, and so the content is preferably as small as possible.
  • P is contained as an impurity in the ferrochrome material of stainless steel. Removal of P from the melt of stainless steel is extremely difficult, so 0.010% or more is acceptable. Further, the content of P is substantially determined by the purity and amount of the ferrochrome material used.
  • the content of P in the ferrochrome material is preferably low, but ferrochrome containing low P is expensive, and so the content is set to a range not causing the quality or corrosion resistance to greatly deteriorate, that is, 0.035% or less. Note that, the content is preferably 0.030% or less.
  • the content of S is preferably as small as possible. Considering a range not affecting the corrosion resistance, the upper limit is made 0.015%. Further, the smaller the content of S, the better the corrosion resistance, but to lower the S, the desulfurization load increases and the manufacturing cost increases, so the lower limit may be 0.001%. Note that, preferably, the lower limit is made 0.001% and the upper limit is made 0.008%.
  • Cr is an essential element for securing oxidation resistance and corrosion resistance in the present invention if less than 13.0%, these effects are not manifested, while if over 21.0%, a drop in workability or deterioration of toughness is caused, so the lower limit is made 13.0 and the upper limit is made 21.0%. Furthermore, if considering the manufacturability and high temperature ductility, the upper limit is preferably made 18.0%.
  • Sn is an element which is effective for improvement of the corrosion resistance or high-temperature strength. Further, it also has an effect of not causing a great deterioration of the mechanical properties at ordinary temperature.
  • the effect on the corrosion resistance is manifested at 0.01% or more, so the lower limit is made 0.01%.
  • the contribution to high-temperature strength stably manifests with addition of 0.05% or more, and so the preferable lower limit is made 0.05%.
  • the upper limit is made 0.50%. Note that, if considering the oxidation resistance etc., the lower limit is preferably made 0.1%.
  • the upper limit is preferably made 0.3%, The manifestation of the embrittlement phenomenon at high-temperature use becomes remarkable by inclusion of Sn: 0.05% or more, but by jointly adding Nb as explained below, the embrittlement phenomenon due to inclusion of Sn can be suppressed. Further, to make the DBTT (ductile brittle transition temperature) less than 50° C., the upper limit of content of Sn is more preferably made 0.21%.
  • Nb is an element which forms carbonitrides and thereby has the effect of suppressing sensitization due to precipitation of chrome carbonitrides at the stainless steel and the drop in corrosion resistance.
  • the effect is manifested at 0.05% or more.
  • the inventors found the fact that this also has the effect of suppressing grain boundary embrittlement in the steel containing Sn.
  • the two effects of improvement of corrosion resistance and suppression of grain boundary embrittlement are manifested at 0.05% or more, so the lower limit is made 0.05%.
  • the content is preferably made 0.09% or more, If 0.2% or more, the effects can be substantially reliably obtained.
  • excessive addition causes the problem of a drop in the manufacturability due to the formation of Laves phases.
  • the upper limit of Nb was made 0.60%. Furthermore, from the viewpoint of the weldability and workability as a sheet, the lower limit is sometimes made 0.3% and the upper limit is sometimes made 0.5%. Further, the effect of suppression of grain boundary embrittlement in the steel containing Sn can be obtained even by joint addition of Ti and Nb. In this case as well, the effects are obtained with an amount of addition of Nb of 0.05% or more. However, in both sole addition of Nb and joint addition of Ti and Nb, the later explained CI value has to be adjusted to fall in a predetermined range.
  • Ti and Nb form carbonitrides and suppress the drop in corrosion resistance due to formation of chromium carbonitrides and sensitization. That is, an amounts of addition corresponding to the amounts of C and N in the steel are necessary.
  • the CI value is an indicator for causing the C and N in the steel to precipitate as carbonitrides of Ti and Nb and suppressing sensitization.
  • CI has to be 8 or more. However, if excessively adding Ti and Nb, they form large inclusions and lower the workability, so CI is made 26 or less. To stably secure corrosion resistance and workability, CI is preferably made 10 to 20.
  • GBSV is an indicator which shows the tendency of grain boundary segregation of Sn. The larger the value, the more remarkable the grain boundary segregation.
  • the coefficients of the elements which form the GBSV are for evaluating the effects on grain boundary segregation.
  • Sn is an element which is effective for high-temperature strength and corrosion resistance, but grain boundary segregation causes the toughness of the material to fall at 400° C. or less.
  • Nb and ho have not only actions of suppressing grain boundary segregation of Sn, but also effects of raising the grain boundary strength and have actions of suppressing embrittlement due to grain boundary segregation of Sn.
  • FIG. 3 it can be found that along with a drop in the GBSV, the ductile brittle transition temperature becomes lower and that if the GBSV becomes 0 or less, the ductile brittle transition temperature of a hot rolled annealed sheet of thickness 4.0 mm becomes 150° C. or less and that the toughness is greatly improved. Therefore, GBSV is set to 0 or less.
  • the inventors used the concentration of Sn at the grain boundary fracture surface (at %) as an indicator of Sn grain boundary segregation to investigate the relationship with the ductile brittle transition temperature. As shown in FIG. 4 , is was found that if the concentration of Sn at the grain boundaries exceeds 2.0 at %, the ductile brittle transition temperature rapidly increases and grain boundary embrittlement easily occurs. In a high temperature service environment as well, making the concentration of Sn at the grain boundaries 2.0 at % or less is important fbr suppressing grain boundary embrittlement due to Sn.
  • the L-value which is usually used as an indicator for evaluation of heat treatment and is shown by the formula 3
  • the inventors found the fact that segregation of Sn at the grain boundaries has a detrimental effect on the properties (transition temperature). Further, the inventors confirmed that in the case of the composition of components in the present invention, the grain boundary Sn concentration when performing heat treatment which gives an L-value of 1.91 ⁇ 10 4 or more becomes 2 at % or less. Note that, as a condition further simplifying the provision on the heat treatment conditions by the L-value, the grain boundary Sn concentration after performing heat treatment at 700° C. for 1 hour is preferably 2.0 at % or less.
  • the concentration of Sn at the grain boundaries is fractured and measured in an AES apparatus in an ultrahigh vacuum. Auger electrons are emitted not only from atoms at the surface, but also at several run inside from the surface, and so the value does not show just the concentration of Sn at the grain boundaries. Further, the precision of analysis differs with each apparatus. However, in principle, the concentration of Sn at the cleavage fracture surface is the same as the average concentration of Sn of the base material. Therefore, the concentration of Sn at the grain boundaries has been determined by calibrating the measurement values of Sn concentration at she cleavage fracture surface so that the concentration of Sn measured at the cleavage fracture surface becomes the average concentration of Sn of the base material.
  • the concentration of Sn at the grain boundaries 1.7 at % or less. Further, making the concentration lower than the concentration of Sn at the base material is difficult, so it is preferable to make 0.02 at % the lower limit.
  • Ti 0.32% or less
  • Ni 1.5% or less
  • Cu 1.5% or less
  • Mo 2.0% or less
  • V 0.3% or less
  • Al 0.3% or less
  • B 0.0020% or less.
  • Ti like Nb, is an element which forms carbonitrides and thereby has the effect of suppressing sensitization due to precipitation of chrome carbonitrides in the stainless steel and the drop in corrosion resistance.
  • this has a larger effect in exacerbating grain boundary embrittlement in the steel containing Sn, so in the steel containing Sn, this is an element which should be decreased.
  • the effect on grain boundary segregation of Sn is manifested when the content of Ti exceeds 0.03%.
  • Nb it is possible to reduce the detrimental effect due to Ti.
  • the preferable upper limit which including Nb is 0.15%. Note that, this enters from the starting materials as an unavoidable impurity, so excessive reduction is difficult, so the content of Ti is preferably made 0.001% or more. From the viewpoint of the improvement of the workability by the reduction of inclusions, the lower limit is more preferably made 0.001 and the upper Limit is more preferably made 0.03%.
  • the upper limit is made 1.5%.
  • the upper limit is preferably 1.0%. More preferably, the upper limit is 0.5%. Due to this, Ni is suitably 0.1 to 0.5%.
  • Ni is an element which improves the corrosion resistance due to the synergistic effect with Sn. Joint addition with Sn is useful. Furthermore, Ni has the action of reducing the drop in workability (elongation and r-value) which accompanies the addition of Sn.
  • the lower limit of Ni is preferably made 0.2 and the upper limit is preferably made 0.4%.
  • Cu is effective for improving the corrosion resistance. In particular, it is effective for reducing the rate of progression of crevice corrosion after occurrence the crevice corrosion. To improve the corrosion resistance, inclusion of 0.1% or more is preferable. However, excessive addition causes deterioration of the workability. Therefore, Cu is preferably included with a lower limit of 0.1 and an upper limit of 1.5%. Cu is an element which improves the corrosion resistance by a synergistic effect with Sn. Joint addition with Sn is useful. Furthermore, Cu has the action of reducing the drop in workability (elongation and r-value) which accompanies the addition of Sn. When jointly adding this with Sn, the Cu is preferably included with a lower limit of 0.1 and an upper limit of 0.5%.
  • Cu is an element which is required for raising the high-temperature strength which is used for use as a member for a high temperature environment such as a high temperature exhaust system of an automobile.
  • Cu mainly exhibits a precipitation strengthening ability at 500 to 750° C. and acts to suppress plastic deformation of the material and raise the thermal fatigue-resistance by solution strengthening at temperatures above that Such a Cu precipitation hardening action or solution strengthening is manifested by addition of 0.2% or more.
  • the upper limit is made 1.5%.
  • the lower limit is preferably made0.5 and the upper limit is preferably made 1.0%.
  • Mo should be added as needed for improving the high-temperature strength and thermal fatigue-resistance. To exhibit these effects, the lower limit is preferably made 0.01%.
  • the upper limit of Mo is made 2.0%. Furthermore, from the viewpoint of the productivity and manufacturability, the lower limit is preferably made 0.05% and the upper limit is preferably made 1.5%.
  • V enters the alloy material of the ferritic stainless steel as an unavoidable impurity and is difficult to remove in the refining process, so generally is contained in 0.01 to 0.1% in range. Further, it forms fine carbonitrides and has the effect of giving rise to a precipitation strengthening action and contributing improvement of the high-temperature strength, and so it is an element which is deliberately added as needed. This effect is stably manifested by addition of 0.03% or more, so the lower limit is preferably made 0.03%.
  • the upper limit is made 0.3%. Note that, if considering the manufacturing cost and the manufacturability, the lower limit is preferably made 0.03% and the upper limit is preferably made 0.1%.
  • Al is an element, which is added as a deoxidizing element and also improves the oxidation resistance. Further, it is useful as a solution strengthening element in improving the strength at 600 to 700° C. This action is stably manifested from 0.01%, so the lower limit is preferably made 0.01%.
  • the upper limit is made 0.3%. Furthermore, if considering the formation of surface defects and the weldability and manufacturability, the lower limit is preferably made 0.01% and the upper limit is preferably made 0.07%.
  • the B is effective for immobilizing the N which is harmful to the workability and for improving the secondary workability. It is added as needed in 0.0003% or more. Further, even if added in over 0.0020%, the effect becomes saturated. The B causes a deterioration in the workability and a drop in the corrosion resistance, so this is added in 0.0003 to 0.002%. If considering the workability and the manufacturing cost, the lower limit, is preferably made 0.0005% and the upper limit is preferably made 0.0015%.
  • N is effective for improvement of the high-temperature strength and is added as needed in 0.011 or more. Further, if added in over 0.20%, the solution strengthening becomes too great and the mechanical properties fall, so 0.01 to 0.20% is added. If considering the manufacturing cost and the toughness of hot rolled sheet, the lower limit is preferably made 0.02% and the upper limit is preferably made 0.15%.
  • Zr like Nb, Ti, etc., forms carbonjtrides to suppress the formation of Cr carbonitrides and improve the corrosion resistance, so is added as needed in 0.01% or more. Further, even if added in over 0.20%, the effect becomes saturated and formation of large oxides causes surface defects, so and it is added in 0.01 to 0.20%. Compared with Ti and Nb, this is an expensive element, so if considering the manufacturing cost, the lower limit is preferably made 0.02% and the upper limit is preferably made 0.05%.
  • Sb is effective for improvement of the resistance to sulfuric acid and is added as needed in 0.001% or more. Further, even if added in over 0.5%, the effect becomes saturated and embrittlement occurs due to grain boundary segregation of Sb, so 0.001 to 0.20% is added. If considering the workability and manufacturing cost, the lower limit is preferably made 0.002% and the upper limit is preferably made 0.05%.
  • Co is effective for improvement of the wear resistance and improvement of the high-temperature strength and is added as needed in 0.01% or more Further, even if added over 0.5%, the effect becomes saturated and the mechanical properties are degraded due to solution strengthening, so 0.01 to 0.5% is added. From the manufacturing cost and stability of high-temperature strength, the lower limit is preferably made 0.05% and the upper limit is preferably made 0.20%.
  • Ca is an important desulfurizing element in the steelmaking process and also has a deoxidizing effect, so is added as needed in 0.0003% or more. Further, even if added over 0.01%, the effect becomes saturated and a drop in corrosion resistance due to Ca granules or deterioration of workability due to oxides occurs, so this is added in 0.0003 to 0.01%. If considering the slag treatment and other aspects of manufacturability, the lower limit is preferably made 0.0005% and the upper limit is preferably made 0.0015%.
  • Mg is an element which is effective for refining the solidified structure in the steelmaking process and is added as needed in 0.0003% or more. Further, even if added in over 0.01%, the effect becomes saturated and a drop in corrosion resistance due to the sulfides or oxides of Mg easily occurs, so this is added in 0.0003 to 0.01%. Addition of Mg in the steelmaking process results in violent combustion by oxidation of Mg and lower yield. If considering the large increase in cost, the lower limit is preferably made 0.0005% and the upper limit is preferably made 0.0015%.
  • a REM is effective for improvement of the oxidation resistance and is added as needed in 0.001% or more. Further, even if added in over 0.1%, the effect becomes saturated and granules of REM cause a drop in corrosion resistance, so 0.001 to 0.1% is added. If considering the workability of the products and the manufacturing cost, the lower limit is preferably made 0.002% and the upper limit is preferably made 0.05%.
  • the grain size number after cold rolling and annealing is made 5.0 to 9.0.
  • the grain size number has to be made 5 or more. However, if the grain size number is made too large, grain refinement will, cause the mechanical properties to change to a low ductility and high strength, so the size is made 5.0 to 9.0. If considering optimization of the Lankford value, which governs improvement of deep drawability, and reduction of the skin roughness at the time of working, the size is preferably made 6.0 to 8.5.
  • the cold-rolled strip annealing temperature is made 850° C. or more where grain boundary segregation of Sn will not easily occur and is made 1100° C. or less where the grain size number will not easily coarsen.
  • the cooling rate 5° C./s or more in the 800 to 600° C. temperature range where grain boundary segregation of Sn proceeds in a short time.
  • the invention examples and comparative examples not containing Ti or Mo have contents of Ti and Mo shown by the symbols Further, in Table 1-1 and Table 1-2, the values of CI and GBSV of the invention examples and comparative examples not containing Ti or Nb were calculated based on the above-mentioned formula 1 and formula 2. Further, the values of CI and GBSV of the invention examples and comparative examples containing Ti and Mo were calculated based on the above-mentioned formula 1 and formula 2′.
  • the hot rolled coil was annealed at 900 to 1100° C. and was cooled down to ordinary temperature. At this time, the average cooling rate in the range of 800 to 550° C. was made 20° C./s or more
  • the of rolled, annealed sheet was pickled and cold-rolled to obtain sheet thickness 1.5 mm sheet, then the cold-rolled sheet was annealed and pickled to obtain a sheet product Nos. 1 to 34 in Table 1-1 are invention examples, while Nos. 35 to 56 in Table 1-2 are comparative examples.
  • the thus obtained hot rolled annealed sheet was heat treated at 700° C. for 1 hour (L-value: 19460), then was subjected to a Charpy impact test according to JIS Z 2242 and was measured for ductile brittle transition temperature (DBTT).
  • DBTT ductile brittle transition temperature
  • Table 2-1 and Table 2-2 The measurement results are shown in Table 2-1 and Table 2-2.
  • the test piece in this embodiment is a subsize test piece of the thickness of the hot rolled annealed sheet as is, and so the absorption energy was divided by the cross-sectional area (units: cm 2 ) to compare and evaluate the toughnesses of the hot rolled annealed sheets in the examples.
  • the criteria for evaluation of toughness was a ductility-brittleness transition temperature (DBTT) of 150° C. or less as being “good”.
  • test pieces for Auger electron spectroscopy were prepared. At the center parts of the test pieces in the longitudinal direction, notches of a depth of 1 mm and a width of 0.2 mm were formed. The test pieces were cooled by liquid nitrogen in the AES apparatus under super-high vacuum and struck to make them break, then measured for concentration of Sn at the grain boundary fracture surfaces. The measurement results are shown as “Grain boundary Sn concentration (at %)” in Tables 2-1 and 2-2.
  • a SAM-670 (made by PHI, Model FE) was used. The beam size was made 0.05 ⁇ m.
  • the concentration was calibrated so that the analysis value at the cleavage fracture surface becomes the same as the concentration of the base material. Auger electrons are emitted not only from the most superficial surface of the grain boundary fracture surface, but also from several nm deep. Therefore, with this method, while not the accurate concentration of Sn at the grain boundaries, as a general measurement value, using this technique, 2 at % or less was deemed as good.
  • the hot rolled annealed sheet was cold-rolled down to 1.5 mm, was annealed at 840 to 980° C. for 100 seconds, then was pickled.
  • the cold-rolled annealed sheet was welded by MIG bead-on-plate welding and was subjected to a sulfuric acid-copper sulfate corrosion test of stainless steel prescribed in JIS G 0575 to investigate the presence of any sensitization of the weld HAZ.
  • the sulfuric acid concentration was made 0.5% and the test time was made 24 hours. Sheets exhibiting grain boundary corrosion were deemed as failed in corrosion resistance. The results of evaluation are shown as “Improved Strauss test” in Tables 2-1 and 2-2.
  • a test tube of an outside diameter of 15 mm, a height of 100 mm, and a thickness of 0.8 mm was filled with the test solution to 10 ml.
  • the annealing conditions of the cold-rolled annealed sheet were changed to obtain 1.5 mm sheet products. These were subjected to aging treatment at 600° C. for 1 week, then were subjected to a V-notch Charpy impact test in that thickness as is. The results are shown in Table 4. At this time, the ductile brittle transition temperature becoming ⁇ 20° C. or less was made the passing condition.
  • Nos. 35, 39 to 41, 43, 44, 46, 49, and 50 had GBSV's larger than 0 and had amounts of grain boundary segregation of Sn after performing heat treatment at 700° C. for 1 hour larger than 2 at % by measurement by AES.
  • Nos. 43 to 45, and 47 to 49 had CI values of less than 8, so the grain boundary corrosion resistance evaluated by the improved Strauss test and the rust resistance evaluated by the salt spray test were inferior Nos. 36, 37, 38, 52, 53, and 51 were respectively high in Si, Mn, P, Ni, Cu, and Mo and were low in elongation due to solution strengthening, so were poor in mechanical properties.
  • No. 39 was high in S and No. 40 was low in Cr
  • No. 42 was low in Sn
  • No. 55 was high in B, so were poor in corrosion resistances evaluated by the salt spray test.
  • No. 42 was low in Sn, so was good in toughness even if GBSV was larger than 0.
  • No. 45 was high in Nb
  • Nos. 47, 45, and 50 were high in Ti
  • No. 54 was high in V, so defects occurred due to large inclusions and these were judged poor in quality.
  • No. 41 was high in Cr
  • No. 56 was high in Al and had surface defect, so were judged poor in quality.
  • references a1 to a3 all had grain boundary Sn concentrations of 2 at % or more after performing heat treatment giving L-values of 1.91 ⁇ 10 4 or more, and so all of a1 to a3 had DBTTs over 150° C. and had poor toughnesses. Further, as with a4, if the L-value is less than 1.91 ⁇ 10 4 , Sn does not segregate at the grain boundaries, so the DBTT is a low 80° C., but if the L-value becomes large, Sn segregates at the grain boundaries and the DBTT rises, so it was confirmed that it is necessary to evaluate the segregation of Sn at grain boundaries by an L-value of 1.91 ⁇ 10 4 or more.
  • steels in the range of the present invention all had maximum depths of corrosion of 50 ⁇ m or less. Note that, in the case of steel containing Ni or Cu in the range of the present invention, the maximum depth of corrosion was 20 ⁇ m or less or an extremely good result for corrosion resistance.
  • sheets having compositions of components, grain size numbers after cold-rolling and annealing, cold-rolled strip annealing temperatures, and cooling rates according to the present invention exhibited low ductile brittle transition temperatures and good toughness.
  • hi had a cold-rolled strip annealing temperature of 1100° C. or more, and a grain size number which is defined by the Microscope Type Test Method of Crystal Granularity of Steel set down in JIS G 0551 was less than 5.0. Therefore, the cooling rate at 800 to 500° C. was 20° C./s. However, the ductile brittle transition temperature was high.
  • the reference b2 had a cold-rolled strip annealing temperature of less than 850° C. and a grain size number of over 9.0, so the mechanical properties were poor.
  • b3 and b6 had cooling rates of less than 5° C./s at 800 to 500, and so the annealing temperature was suitable and the grain size number was also a suitable 8.0.
  • ductile brittle transition temperature was high.
  • b4 and b5 were compositions of the comparative examples, and so the cold-rolled strip annealing temperatures, cooling rates, and grain size numbers were in suitable ranges, but the ductile brittle transition temperatures were high.
  • the stabilizing elements Nb and Ti are optimized, so it becomes possible to produce stainless steel sheet which has little deterioration in toughness even if used at a high temperature and further is excellent in corrosion resistance of the sheet. Further, by applying the material according to the present invention to particularly the exhaust system parts of automobiles and motorcycles, it becomes possible to extend the service life of the parts and thereby raise the degree of contribution to society in general. That is, the present invention has sufficient applicability in industry.

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EP2915894A4 (en) 2016-10-26
KR20150056656A (ko) 2015-05-26
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JP6223351B2 (ja) 2017-11-08
EP2915894A1 (en) 2015-09-09
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