US3839021A

US3839021A - Heat-resisting steel

Info

Publication number: US3839021A
Application number: US00259541A
Authority: US
Inventors: Y Abe
Original assignee: Mitsubishi Steel Mfg Co Ltd
Current assignee: Mitsubishi Steel Mfg Co Ltd; Mitsubishi Steel KK
Priority date: 1971-07-20
Filing date: 1972-06-05
Publication date: 1974-10-01
Anticipated expiration: 1991-10-01
Also published as: DE2229759A1

Abstract

A heat-resisting steel with superb heat resistance, and yet with a low coefficient of heat expansion, against such hightemperature environments as above 900*C is obtained by uniformly dispersing 2 - 10 percent by weight of one or more carbonitrides selected from the group of those of titanium, zirconium, niobium, tantalum and vanadium into a steel containing 1 - 30 percent by weight of chromium.

Description

United States Patent Abe 1 Oct. 1, 1974 1 HEAT-RESISTING STEEL 2,080,001 5 1937 Becket 75/126 F 2,232,705 2/1941 Hull [75] Inventor: Yoshihiko Abe, Tokyo, Japan 2,268,427 12/1944 Schlumpt 73] Assignee; i i hi Seiko Kabushiki Kaisha 2,370,124 2/1945 Charlton 75/126 F Tokyo Japan 2,801,916 8/1957 Harris 2,848,323 8/1958 Harris 75/126 P [22] Filed: June 5, 1972 [21] Appl. No.: 259,541 Primary Examiner-Hyland Bizot Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [30] Foreign Application Priority Data July 20, 1971 Japan 46-53592 [57] ABSTRACT Mar. 6, 1972 Japan 47-22245 A heatqesisting Steel with Superb heat resistance and [52] US Cl. 175/124 75/125 75/l26 C yet with a low coefficient of heat expansion, against 35/126 i 75/126 such high-temperature environments as above 900C [51] Int CL 6 39/54 is obtained by uniformly dispersing 2 10 percent by [58] Field P 126 F weight of one or more carbonitrides selected from the group of those of titanium, zirconium, niobium, tanta- [56] References Cited lum and vanadium into a steel containing 1 30 per- UNITED STATES PATENTS cent by weight of chromium. 1,542,233 6/1925 Girin 75/128 T 1 Claim, 2 Drawing Figures TENSILE STRENGTH (Kg /mm G TENSILE STRENGTH INCREASE IN WEIGHT DUE TO OXIDATION (800Cx30hr) (rng/cm INCREASE IN WEIGHT DUE TO OXIDATION AMOUNT OF CARBONITRIDE (WEIGHT /o) HEAT-RESISTING STEEL BACKGROUND OF THE INVENTION The present invention relates to heat-resisting steels, and more particularly to steels with superb heat resistance, and yet with a low coefficient of heat expansion, against such high-temperature environments as above 900C, especially containing sulphur.

In general, heat-resistivity is an indispensable condition for steels to be used in high-temperature circumstances; and beside great strength and toughness not only against high temperatures, such steels are desired to possess great resistance against various hightemperature atmospheres, and yet with a low coefficient of heat expansion.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide a heat-resisting steel which satisfies all of the desirable characteristics abovementioned.

It is another object of the present invention to provide a heat-resisting steel which has a heat-resisting quality comparable to that of the conventional heatresisting steels and yet has such an excellent resistance to sulphur as not being possessed by the conventional heat-resisting steels.

It is a further object of the present invention to provide a heat-resisting steel which, in comparison with the orthodox ones, is superb in heat resistivity, and particularly so against sulphuric matters, while its coefficient of heat expansion is remarkably small.

Another object of the present invention is to provide a heat-resisting steel whose heat resistivity does not rely on such an expensive element as nickel and which chromium in a considerably small amount in comparison with its amount for the orthodox ones, still being provided with superb heat resistance.

A steel according to the present invention is characterized by a chromium containing steel having specific carbonitrides dispersed therein uniformly and finely beyond certain degree of concentration.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the present invention will become more apparent to those skilled in the art when considered in reference to the following detailed description in the light of the accompanying drawings wherein:

FIG. 1 is a photograph showing the microstructure of a steel made according to the present invention in which 4.7 percent by weight titanium carbonitride is dispersed into a steel mainly consisting of, by weight, 24.5 percent chromium, 3.5 percent molybdenum and 1.7 percent silicon; and

FIG. 2 is a diagram showing the tensile strength at 700C of various steels and also the weight increases due to oxidation when they are exposed in the air for 30 hours at 800C, whereby the steels are those in which, in to the steels mainly consisting of, by weight, 1.0 percent chromium and 0.25 percent molybdenum, various amounts of such sole or complex carbonitrides as titanium, titanium niobium or zirconium vanadium are dispersed.

DETAILED DESCRIPTION OF INVENTION In the manufacture of the heat-resisting steels of the present invention, it is necessary, firstly, to add carbonitride-forming elements to a molten steel prepared beforehand containing a remarkably excessive amount of carbon in comparison with the amounts of oxygen, nitrogen, and sulphur, so as to cause a preferential reaction between the carbonitride-forming elements and the carbon, and then to uniformly disperse and precipitate the thus-obtained solid carbide together with the nitride, and lastly it is necessary for the resulting dispersed carbonitrides to be stable even though it is subject to high temperatures.

As a carbonitride-forming agent, which achieves such auniform dispersion and thermostability of the solid carbonitride in the microstructure of the steel, a carbonitride may be named with such a composition of one or more elements selected from among such elements as titanium, zirconium, niobium, tantalum and vanadium.

In these carbonitride dispersing agents, the size, uniformity, and quantity of a dispersant are important factors to determine their characteristics, so it has been confirmed that a preferential nitridation of the boron in the molten steel is desirable for this reason.

From the explanation above, it is easily understandable that the steels which can meet the requirements of the present invention can be manufactured by an industrial mass production system.

FIG. 1 shows a microstructure of an example of the heat-resisting alloy steels according to the present invention which clearly shows the unifrom distribution of the carbonitrides in the steel.

The effect of a carbonitride uniformly dispersed in the steel on its quality at high temperatures will be explained in reference to FIG. 2. There are shown in it the variations of the steels high-temperature-proof strength (at 700C) and high-temperature oxidation resistance (800C X 30 hr) in the case of the various carbonitrides composed solely or complexly of titanium, or titanium-niobium, or zirconium-vanadium dispersed in a range of up to 10 percent by weight. Irrespective of the kind of the dispersing carbonitride, the respective characteristic curves can be plotted by an identical curve. However, with the amount of the carbonitride being below 2 percent by weight its contribution toward the heat-resistivity of the steel is almost zero; however, if the its content surpasses that value, it sharply increases the heat-resistivity of the steel. In the case of the content of the carbonitride being above 7 percent by weight, its effect remains nearly unchanged, so it is understandable that its preferable content should be limited to less than 10 percent by weight, viewed from the aspects of manufacturing technique and economy.

In this case, the components of the matrix having the dispersion of carbonitrides cannot be ignored. Table I indicates the comparative effects of the components of a matrix of steel having the dispersion of titanium or zirconium carbonitrides in 4 percent by weight or so, particularly the comparative effect of the amount of chromium dispersed in the steel, in terms of vanadium attack resistance and oxidation resistance. As is apparent from Table I, the steels which contain no carbonitride and that in which the carbonitride is dispersed into the matrix belonging to the carbon steel (the sample Nos. 1 3) have practically no vanadium attack resistivity (refer to the column of the V-attack resistance") and a little oxidation resistivity (also refer to Oxidation resistance,) while the steels according to the present invention (sample Nos. 4 and in which chromium is contained in the matrix clearly show that In the steels according to the present invention, their corrosion resistivity at high temperatures are remarkably improved by the existence of one or more such elements contained in the matrix as silicon, molybdea high temperatures their resistance to corrosion 5 num, boron, copper and aluminum as well as the carsharply increases. From this result, it is concluded that bonitride-forming elements such as titanium, zircoin a steel to be made a matrix having the dispersion of nium, niobium, tantalum and vanadium. In order to a carbonitride, it is essential to add chromium. Thus it show this fact, in Table 3, there is shown the influence can be sa1d that the feature of the present invention lies of the chemical compositions of the steel according to 1n making advantage of the synergistic effect on the the present invention on its oxidation resistivity at high heat res1st1vity of the steel of the dispersed particles of temperatures. In Table 3, the steels belonging to the carbonitrides and chromium contained in the matrix. group of samples marked A are those consisting mainly Table 1 Dispersed carbonitride Main alloy V-attack Oxidation components resistance resistance Sample in the matrix (loss in (increase in No. Kind Amount(weight weight; weight:

mg/cm lhr) mg/cmlhr) 1 l.0%Cr-0.25%Mo 195 1.01 2 240 1.30 3 Ti 4.6 232 0.90 4 Ti 4.3 1.1%Cr 132 0.26 5 Zr 3.8 1.0%Cr-0.5%Mo 145 0.18

Oxidation resistance The test piece is exposed in the air at 800%. for hours.

Then, in reference to Table 2, an explanation will be given on the effect of the quantity of chromium to be added into the steel of the present invention. In reference to steels containing chromium in different amounts ranging 1 to percent by weight having a dispersion of 5 to 6 percent by weight titanium-tantalum carbonitride, their oxidation resistivity and sulphurousacid-gas-proofness were compared at various temperaweight chromium for 1,100C to endure the effect of the present invention, and it can be understood that if the chromium concentration is ensured so much as that, the steels of the present invention have a phenomeral heat-resistivity.

of 25 percent by weight chromium and 8 percent by weight nickel having a dispersion of the carbonitride of titanium and it is understood that their oxidation resistivity is remarkably improved by the composite addition of molybdenum, boron and aluminum. Then, the 'steels belonging to the group of samples marked B are those consisting mainly of 18 percent by weight chromium with an addition of one selected from among mo lybdenum, aluminum and copper with carbonitrideforming element added above the concentrations necessary for the formation of the dispersed particles so that they are also dissolved in the matrix, and they show that their oxidation resistivity is remarkably improved. The steels belonging to the group of samples marked C, which consists principally of chromium, are shown to compare the effect of the addition of the elements abovesaid with the effect of addition of silicon, and, it is understood that the addition of these elements has the effect to reduce the necessary amount of chromium which is determined by the kind and the temperature of the environment gases, and silicon is especially effective with respect to this point.

Table 2 Increase in weight Increase in weight s l by nxidatinn (mg/cm bv sulphurous acid as m cm Dispersing amounts Amount of 900CX10 hr 1.100CX10 hr 800C l0hr 910C 10hr 1.000CX5hr chromium of Ti-carbonitride in the matrix (in the air) (in the air) (in 1OO%SO (in %SO,) (in 100%SO weight I Table 3 Chemical composition Oxidation resistance Sample Dispersed mark particle Matrix Oxidation rate (mglcm /hr) Kind Ci NI Si Mo Al Cu B others 1200C 1100C 1000C 1 Ti 4 5 25.2 8 3 10.8 2.52 0.13 A 2 Ti 5 25.5 8 0 2.1 0.015 1.0 0.22 0.04 3 Ti 4 1 25.4 8 2.0 1.0 0.4 0.10 0.04 1 Ti 50 18.1 1.31 1.05

B 2 Ti+Nb 6.7 18.3 2.1 1.05 0.42

Zr 3 Zr 54 17.6 3.8 3.7 0.20 0.057

Ta 4 Ti+Ta 3.8 17.3 1.3 1.2 0.03 0.030

V 5 Ti+V 4.3 18.1 1.9 2.6 0.15 0.082 1 Ti 4.5 26.9 3.10 0.52 0.17

Ti 2 Ti 4.3 24.5 0.8 0.35 0.17 0.09

Ti C 3 Ti 4.1 20.0 4.2 2.1 0.20 0.10 0.05

Ti 4 T1 3.8 18.2 1.1 3.1 1l 4 0.27 0.10 0.027

Ti 6 Ti 4.1 18.0 1.7 1.0 1.0 0.32 0.12 0.07

In this connection, it will be appreciated that the kinds of the additional elements vary depending on the objects of use of the steels, and, in the case where the high strength at high temperatures is not so much needed, even steels in which ferrite constitutes its matrix can do well, and as for the amounts of the elements to be added into the steel, when, at relatively low temperatures, the total amount is 0.2 percent by weight or more, its effect comes to display. However, as the temperature rises, the total amount needs in increase.

When 1,000C is made the object of the steels highheat resistivity, the total amount must be made 8 percent by weight or so as shown in Table 3.

Besides, with high-heat-resisting steels, it is desirable to make its coefficient of heat-expansion small when exposed to a high-temperature environment; however, with the steels of the present invention, since the coefficient of heat-expansion of the carbonitride to be dispersed therein is hear half that of the steel of the matrix, the coefficient of heat-expansion of the steel of the present invention, in proportion thereto, is expected to become less in comparison with that of the orthodox steels. Table 4 shows the coefficients of heat-expansion at to 600C of the steels of the present invention in comparison with that of the orthodox steels, which reveals that with the steel of the present invention, its coefficient of heat expansion is lower than its calculated value predicable on the basis of the volume ratio of the matrix and the dispersed particles. That is, in the steels according to the present invention, the dispersed particles having a low coefficient of heat expansion control even the coefficient of heat expansion of the matrix and bring about an additional advantage that the overall coefficients of heat expansion of the steels are made further smaller than the calculated values.

Though the case where only chromium constitutes the principal alloy element of the matrix has been explained above, such an alloy steel is poor in thoughness at a room temperature and yet has not so high strength at a high temperature as compatible with the excellent resistivity to corrosion at high temperatures as stated above. Therefore, when a steel is required also to possess these qualities, the matrix is preferable to be made to be austenite dome with at least one element selected from the group of nickel and manganese, and, also it is effective that the matrix is made with at least one element selected from the group of silicon and molybdenum. Tables 5 and 6 show the effect of the above elements on strength at high temperatures, and Table 7 shows that the steels shown in Table 6 combines an excellent corrosion resistivity at high temperatures with excellent strength at high temperatures. Consequently, in order that the steel according to the present invention secures such high strength at high temperatures in addition to corrosion resistivity at high temperatures as not obtainable in the conventional steels it is required that those elements are added in the same total amount at the maximum as chromium to make the matrix an austenite structure, whereby their maximum amounts in total are 30 percent by weight for the steel containing 30 percent by weight chromium at the maximum.

TABLE 4 Range Coafiicient of 01 the heat-expansion Chemical composltion (percent) measured I tempara- Measured Calculated Sample C Si Mn N1 Or Mo carbonitride tures, 0. value value Invented steel I 0. 05 2.1 2.1 Ti-basel 3.2.. 25600 11.7)(10 12.3)(10- Invented steel II 0.0 1.7 2.2 Ti-base) 7.5.. 25-600 11.0X10- 12.0X10- 0. 1.7 3. 5 (Zr-base) 4.7.. 25-600 11-.0X10' 12.1X10' Table Sample Room temperature High temperature Kind and amount of Tensile Elong Tensile Elong- Composition dispersed particles strength ation Reduction strength ation Reduction Temp. of matrix (weight (kglmm of area(%) (kglmm (7c) of area(%) (C) 10%Cr (Zr Nb)base 62 98.6 8.8 18.5 9.6 41.0 81.6 10%Cr 3%MO 800 2%Si (Zr Nb)base 3.5 75.4 21.6 45.5 12.8 69.6 96.8 18%Cr (Ti)base 7.5 66.8 11.8 22.9 10.3 30.3 75.0 18%Cr 8%Ni 800 +1%Si (Ti)base 6.4 64.4 43.2 60.4 22.6 43.4 64.0 20%Cr (Ti v)base 4.7 70.3 10.3 21.0 3.4 93.0 97.9 1,000 20%Cr 20%Nl (Ti V)base 4.5 715 41.5 57.3 10.8 78.3 90.4

Table 6 Tensile strength( kg[mm"') Test Temperatures C) AlSl 202 Invented steel* 'Note:- AlS1 202 4.5; by weight of Ti-cnrhonitride sult of the comparison of sulphur attack, oxidation resistances and lead attack resistance, as apparent from Table 8, it should rather superior resistivities to the SUl-l 33B-steel, one of the best heat-resisting steels already known in the field of art. Moreover, from the economical point of view, since the present steel requires no valuable nickel and yet the chromium quantity is sufficient in less than one half of that contained in the SUH-33B, its manufacturing cost becomes less than Note:- AlSl-ZOZ 4.5% by weight of Ti-carbonitridc The following specific examples will serve to illustrate the excellence of the steels according to the present invention in their qualities and an economical adent invention, a heat-resisting steel having better heat resistivities can be obtained quite at a low price.

vantage as heat-resisting steels. 4O

EXAMPLE 1 EXAMPLE 2 h heat l y of the $1961 accordlng 9 Though the so-called 18-8 stainless steel is used in em mvemlon Whlch was Qbtamed y p R various fields as a heat-resisting stainless steel, the valuclhes 9 p y y g CaYbQmmde, combmed able nickel in it was replaced by 5 percent by weight with tltan um and niobium 1n the ratlo 0f 3 I to the low cost zirconium carbonitride, and the fine particles steel consisting mainly of, by weight, 20 percent silicon, of the carbonitride were uniformly dispersed in the 18 10 percent chromium and 3.0 percent molybdenum is percent-chromium steel. In this steel according to the shown in Table 8 in comparison with the SUl-l 33B- present invention the matrix was ferrite as shown in steel. Table 9, so it could eliminate the defects of the 18-8 Table 8 Test items Test condition Invented steel SUH 33B S-attacking rate 1n so 800C 0.05 0.03 (mg/cm /hr) do. 900C 0.17 0.25 Oxidation rate In the air 800C 0.00 0.00 (mglcm /hr) do. 900C 0.03 0.01 Pb-attacking rate In PbO 915C 950 780 (initially) (mg/cm /hr) do. 915C 440 (finally) Since the steel according to the present invention contained 10 percent by weight chromium, its usability covers a temperature range up to 900C, and as the restainless steel caused by its being an austenite steel, and at the same time it showed the extraordinarily excellent resistivity in the oxidation and the sulphur attack at a temperature above 900C. Accordingly, the present steel may be used, under some circumstances. as a superior and yet cheap heat-resisting stainless steel in place of a costly 18-8 stainless steel.

Table 9 Test item Test condition Invented steel 188 Stainless Steel observed cracking in lntergranular corrosion cracking By 118-4304 not recognized 3 pieces among 10 pieces Coefficient of heat expansion (X 10-) 25 600C 11.0 1708 V-attack resistance (loss in weight-mg/cm) 1n V205 800C X 20 hr 48 45 Pb-attack resistance (loss in weight-mg/cm") 1n PbO 915C x 1 hr 1.000 1,050 Oxidation resistance In the air 900C X 10 hr 0.0 0.0 (increase in weight-mg/cm-") do. 1,000C X 10 hr 2.1 23.4 S-attack resistance in 100% S 800C X hr 0.5 0.3 (increase in weight-mg/cm) do. 900C X 10 hr 1.2 23.6 do. 1,000"c x 10 hr 2.8 146 EXAMPLE 3 when immersed in lead oxide at 915C) A thermal reactor for the exhaust of automobiles demands a heat-resisting steel which can stand the high initially: 500 mglcmz/hr temperature gas above 1,000 C, but the hitherto subsequently: 15o mg/cm lhr known heat-resisting steels have not met this desire in such points as the resistivity against sulphur etc. Ac- D. The momentary tensile strength and elongation at cording to the present invention, a steel which can an- 1,000C: swer such a desire could be manufactured by dispersing 14.3 kg/mm and 64.0 percent, respectively. about 5 percent by weight titanium carbonitride into What is Claimed is: the steel consisting mainly of, by weight, 25 percent l. A heat-resisting nickel-free steel consisting essenchr i 20 r t ick l a d 2 percent l bd tially of a uniform dispersion of 2 percent to 10 percent num. The experimental results of the steel were as foly welght of 1110f? cafbonltl'ldes SeleCt6d f 1 the group consistmg of titamum, zlrconium, n1ob1um, A. The thickness of the oxidation layer when the steel tantamm, and Va nadlum Carbomtrides, P y was repeatedly subjected to the heating and the sudden Welght of Chromlum and to 30 Percent y Welghl of cooling cycle f LQOOOC X 1 hour in the one or more elements selected from the group conslstin the r Cycle 000038 mm ing of boron, silicon, molybdenum, manganese, copper in the respective subsequent cycle (on the average) and and the balance bemg and unavold' 0.00004 mm/cycle able lmpurltles- B. The thickness of the sulphurated layer when sub- 40

Claims

1. A HEAT-RESISTING NICKEL-FREE STEEL CONSISTING ESSENTIALLY OF A UNIFORM DISPERSION OF 2 PERCENT TO 10 PERCENT BY WEIGHT OF ONE ORE MORE CARBONITRIDES SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZICRONIUM, NIOBIUM, TANTALUM, AND VANADIUM CARBONITRIDES, 1-30 PERCENT BY WEIGHT OF CHROMIUM AD 0.2 TO 30 PERCENT BY WEIGHT OF ONE OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF BORON, SILICON, MOLYBDENUM, MANGANESE, COPPER AND ALUMINUM AND THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES.