US3418110A - Hardenable steel material containing aluminum - Google Patents

Description

Dec. 24, 1968 sus'uMu GODA ETAL 3,418,110

HARDENABLE STEEL MATERIAL CONTAINING ALUMINUM Filed Oct. 31, 1967 & NL WW Mm & a u

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m mm F -IIR KT MX S GMOTO FIGZ ATTORNEYS United States Patent 1 Claim. ci. 75-124 ABSTRACT OF THE DISCLOSURE The present invention is to provide an alloyed steel having excellent mechanical properties and high hardenability obtained by adding Al in an amount of 0.06 to 0.15 wt. percent to a steel consisting of 0.25 to 0.65 wt. percent C, 0.005 to 0.30 wt. percent Si, 0.20 to 2.0 wt. percent Mn, 0.03 to 1.50 Wt. percent Cr, 0.03 to 0.50 wt. percent Mo, 0.005 to 0.30 wt. percent V, 0.0005 to 0.003 Wt. percent B and balance being Fe and unavoidable impurities.

This application is a continuation-in-part of application Ser. No. 426,231 filed Jan. 18, 1965, now abandoned.

This invention relates to steel material having a high hardenability obtained by largely improving the hardenability of steel by the addition of small amount of aluminum.

It is well lmown that a small amount of aluminum may be added to an ordinary carbon steel or magnetic steel with the following purposes: in the case of an ordinary carbon steel with the purpose or fixing detrimental elements such as nitrogen by causing them to combine with the added aluminum and making the crystalline grains fine, and in the case of a magnetic steel with the purposes of improving the magnetic properties of the steel. The amount of aluminum to be contained in the magnetic steel is published to be usually 0.01 to 0.50 wt. percent. But, in this case it is contemplated to obtain the improvement of the magnetic properties by the collaboration with silicium contained in the steel in a range of from 2 to 5%.

Besides, heretofore it has been also known to cause a steel to contain several percents of aluminum in order to improve the heat-resistance, acid-resistance and alkaliresistance of the steel. However, there has never been made reference to the effect of improving the hardenability by the addition of aluminum.

When carrying out the hardening of a steel material, there may be enumerated several factors which determine a the hardening efficiency. The factor to be mentioned at first is the composition of austenite, that is, the amounts of carbon and other alloying elements contained in the steel. It is well known that the hardenability of steel may be improved, if the contents of carbon and other alloying elements are great. However, in the case of structural steel and the like it is not desirable to increase the content of carbon in view of the weldability and toughness of the steel. Further, it is economically disadvantageous to add various kinds of alloying elements for the purpose of improving the hardenability of steel.

The hardenability of steel is also influenced by the austenitic grain size. It is said that the hardenability may be improved by enlarging the austenitic grain size. Further, the amount or the state of distribution of carbide particles in the austenite, or the existance of non-metallic impurities such as oxide or sulfide in the steel are said to be the factors exerting influences on the hardenability of the steel.

In general, the hardenability of steel does not come so much in question when heat-treating a steel material of relatively small diameter, but in the case of heat-treating a massive steel material there occur often troubles of the steel material lacking in uniformity in quality or of the desired strength or toughness being not obtained on account of the shortage of hardening due to the low hardenability of the steel material.

On the other hand, the high hardenability of the steel material bring the advantages of being able to easily stabilize the quality of the steel material at the desired state and of extremely simplifying the heat-treatment of the material.

Heretofore, the most effective method of obtaining a high hardenability of steel material, which brings advantages as above mentioned, was said to be the addition of alloying metals such as manganese, chromium, molybdenum, nickel, boron and the like to a steel. Further, as the enlarging of the austenitic crystalline grain size was also known to be a method of improving the hardenability of the steel, it is possible to carry out the hardening 0 process while heating the material at a possibly high temperature. However, this method is not recommendable, because the toughness of steel will be thereby largely impaired.

As to the effect of aluminum of influencing the hardenability of steel material there are already some reports in a few documents. According to the reports of these prior documents the following three kinds of effects of aluminum are referred to. The first of them is the effect which is said to relate to the size of crystalline grains and to be the strongest amount other effects: that is, aluminum would make the crystalline grains fine and would largely detriorate the hardenability of steel material. The second effect of aluminum is said to facilitate the transformation of a steel and thereby to reduce the hardenability of the steel, though not certain, because aluminum oxide would lie scattered in the form of fine particles in the steel, which would accelerate the transformation as a nucleus of effecting the transformation when carrying out the hardening process. Thirdly, aluminum is said to possess also a positive effect of improving the hardenability of steel material.

In summarizing these prior opinions on the effects of aluminum the prevailing opinion is that the effect of aluminum on the hardenability of steel material would be insignificant, or rather of a negative nature. According to the results of various experiments the hardening multiplying factor of aluminum is reported to be below 1.30 when causing a steel to contain 0.10 wt. percent. Therefore, no effect of aluminum of improving the hardenability of steel material has been expected.

In contrast with the prior opinions on the negative effect of aluminum on the hardenability of steel material the inventors have discovered an entirely unexpected novel fact that the hardenability of steel material may be remarkably improved by causing the steel to contain aluminum in a range of 0.06 to 0.15 wt. percent.

A further object of the present invention is to provide a low alloyed steel having a high hardenability at a low price.

A still further object of the present invention is to provide a steel high in hardenability rendered by subsequently adding a specified amount of aluminum to the steel material of such a composition as being able to cause the ideal critical diameter, that is, the maximum diameter of an infinitely long cylinder which in an ideal quench will just transform to given specific mirco-structure, referring to the hardenability of steel, to reach more than 1.5 inches.

Other objects of the present invention will be made clear by making reference to the following description and the attached drawing, in which;

FIG. 1 represents the effect of improving the hardenability of steel obtained by the addition of small amount of aluminum in correlation with the ideal critical diameter D FIG. 2 is a diagram showing a relation between an amount of Al and the hardening multiplying factor of Al (MFAl).

In the following the features of the present invention will be explained with reference to a steel, in which the ideal critical diameter is more than 1.5 inches. However, it is, of course, possible and effective to apply the idea of the present invention to a steel having the ideal critical diameter of less than 1.5 inches.

The said ideal critical diameter D may be given by multiplying the basic hardenability (D which is again determined by the carbon content in steel and the crystalline grain size, by the hardenability multiplying factors of various alloying elements, as shown by the following formula:

XMF XMF XMF (l) in which C D (1nch)= -(lJO-ODQN) in which C=Carbon content in steel (wt. percent) N=Grain size number of austenitic crystalline grains MF =0.70-Si+1.00

MF =3.33-Mn+1.00 (in case of Mng1.2%) MF =5.10-(Mn-1.2)+5.00 (in case of Mn l.2%) MF ,=2.16-Cr+1.00

MF =3.00-Mo+1.00

MF =200.O-B+1.00 (in case of B 0.003%) In the above formula the amounts of the elements Si, Mn, Cr, Mo, V and B contained in the steel are represented by weight percent respectively.

The idea of the present invention is to remarkably improve the hardenability of a steel by adding an amount of aluminum more than 0.06 wt. percent to the steel material, in which D shown in the above formula is more than 1.5 inches.

In case that a steel shows the ideal critical diameter D in the above Formula 1 of less than 1.0 inch, the steel corresponds to an ordinary carbon steel, and the steel showing D more than 1.0 inch indicates a low alloyed steel. The effect of the present invention of improving a hardenability of steel is particularly remarkable at the low alloyed steel, particularly for use in machine construction, to which various alloying elements are added so that the ideal critical diameter may become more than 1.5 inches. The steel of the present invention, in which D is more than 1.5 inches, is that which contains as alloying elements 0.25 to 0.65 wt. percent, C, 0.005 to 0.30 wt. percent Si and 0.20 to 2.00 wt. percent Mn 0.3 to 1.5 wt. percent Cr, 0.03 to 0.50 wt. percent Mo, 0.005 to 0.30 wt. percent V, 0.0005 to 0.003 wt. percent B and 0.06 to 0.15 wt. percent Al. These amounts of the above enumerated components to be contained in the steel of the present. invention have been calculated according to the above Formula 1 so that the ideal critical diameter D; of more than 1.5 inches may be obtained.

The present invention is to provide a steel high in hardenability by adding 0.06 to 0.15 wt. percent aluminum to the low alloyed steel material of such a composition as being able to obtain the ideal critical diameter D of more than 1.5 inches.

The reasons of specifying the content of each alloying element are as follows:

Among the basic elements, carbon, silicium and manganese, which are inevitably to be added, if the content of carbon is less than 0.25 wt. percent, the steel will lose the required strength and hardenability. But, if above 0.65 wt. percent, cracks are formed in the material during the hardening and sufficient toughness can not be obtained. It is very difficult in the steel-making and consequently unpractical to reduce the content of silicon to less than 0.005 wt. percent. On the other hand, the content of silicon up to circa 0.30 wt. percent is in general an amount required for the steel-making, and in the present invention, if the silicon content exceeds 0.30 wt. percent, the hardening of steel becomes difficult on account of an increase in macro streak flaws due to decarburization products and a rise in the transformation point. Manganese is also difiicult like silicium in the steel-making to reduce its content to below 0.20 wt. percent. But, if the content of manganese is forcibly reduced to below 0.20 wt. percent, the hot-workability will be deteriorated. But, if exceeding 2.00 wt. percent, the material will become brittle, though the strength may be improved as in the case of silicium.

In general, chromium is existent in an amount up to 0.03 wt. percent as one of impurities in a steel, but an addition of chromium in a range of from 0.03 to 1.50 wt. percent is necessary for helping the effect of aluminum of improving the hardenability of steel, and for imparting to the steel the required strength.

Molybdenum has the same effect as chromium in helping the effect of aluminum of improving the hardenability of steel, if it is added in a range from 0.03 to 0.50 wt. percent. Further, the addition of molybdenum in the above range is also required for preventing the temper brittleness.

Vanadium is also added in a range of from 0.005 to 0.3 wt. percent alike chromium, molybdenum and nickel, as occasion calls. If the addition of vanadium is less than 0.005 wt. percent, the effect of the addition can not be materialized, but even if exceeding 0.3 wt. percent, the temper softening resistance can not be imparted and the effect of improving the strength, toughness and hardenability proportional to the addition are not achieved.

Boron may also be added to increase the effect of aluminum of improving the hardenability of steel, but in a range of from 0.0005 to 0.003 wt. percent. If below the above specified range, the effect of the addition of aluminum in improving the hardenability of steel can not be helped and promoted thereby, but more than the specified range, the effect corresponding to the increased addition can not be secured.

Thus, the steel according to the present invention is characterized by containing alloying elements such as chromium, molybdenum, vanadium, boron and in the range as specified above respectively, in addition to the basic elements including carbon, silicium and manganese, so that the ideal critical diameter D given by the Formula 1 may be more than 1.5 inches and further containing aluminum subsequently added in a range of from 0.06 to 0.15%.

In working the present invention, if the content of aluminum is less than 0.06 wt. percent, the sufficient hardening effect of aluminum is not displayed, but if it is more than 0.06 wt. percent, the hardenability of steel is rapidly improved by the addition of aluminum, and the greater the ideal critical diameter D of the steel is, the more rapidly the hardenability is improved. As evidently seen from the attached drawings, FIGS. 1 and 2, the highest hardenability is shown when the content of aluminum lies in a range of from 0.06 to 0.10 wt. percent. For instance, in the case of the steel material, in which the ideal critical diameter D is more than 3.0 inches,

the 50% martensite distance according to Iominy-test, that is, the distance from the quenched end of the Iominytest piece to the point, where the 50% martensite structure has been formed, in the length direction, reaches more than 40 mm., indicating a remarkable improvement of the hardenability of the steel material as compared with the conventional hardened steel material, in which the 50% martensite distance according to Jominy-test shows only 11 mm. In general, the greater the said 50% martensite distance according to Jominy-test is, the higher the hardenability of the steel material is. The above example relates to the advantages that the hardening by air cooling is feasible and consequently complicated device and process required for the hardening by liquid cooling will be superfluous.

In FIG. 1 the dotted line shows the hardenability of the conventional steel material. For instance, the ideal critical diameter of the medium carbon steel, containing 0.45 wt. percent C, 0.75 wt. percent Mn and 0.25 Wt. percent Si shows about 0.86 inch, supposing that the austenitic grain size be 8. The hardenability of this steel material is judged from the attached drawing to be only 4 mm. in the 50% martensite distance according to Jominy-test. In the steel material of this composition the 50% martensite distance may be improved only to 11 mm., even if the ideal critical diameter D is increased up to about 3.0 inches by supplementally adding any alloying element for accelerating the hardenability, for instance, 1.2 wt. percent in the case of chromium, 0.8 wt. percent in the case of molybdenum and more than 5 wt. percent in the case of nickel. Further, it is to note that it is practically difficult to use such an alloying element in a sufficient amount necessary for obtaining the sufiicient hardenability, because they are very expensive.

Further, the relationship between an amount of Al measured in the case of the alloyed steel manufactured according to the method of the present invention and MFAl (Multiplying Factor of Al) will be explained with reference to FIG. 2.

As above mentioned, in the steel of the present invention the content of Al thereof is limited to the range of from 0.06 to 0.15 wt. percent, which just corresponds toughness due to coarsening of grains, an increase in macto streak flaws and a promoted decarburization at a surface layer of steel. In this drawing also the MFA] values obtained by the experiments of M. A. Grossman are shown for reference, as are indicated by MAG (1) and MAG (2), wherein MAG (1) is an MFAl line obtained from FIG 23 in the article, Hardenability Calculated From Chemical Composition, by M. A. Grossman, page 249 in American Institute of Mining and Metallurgical Engineering Transactions, vol. 150, 1942, pages 227255 and MAG (2) is an MFAl line obtained from FIG. 19 in the article, Elements of Hardenability, by M. A. Grossman (ASM 1952). As is clearly demonstrated by the comparison with these reference values, the MFAl value of the steel of the present invention is much superior to these reference values. Further, it is to note that the present invention relates to a steel having D of more than 1.5 inches to which Al is added in the range as above specified. On the other hand, the steel shown in FIG. 23 in the article of M. A. Grossman, page 249, which contains maximum 0.08 wt. percent Al, has D of less than 1.5 inches, when calculated from the Formula 1. Such a striking hardening effect as is achieved by the present invention can never be expected for a steel having D of less than 1.5 inches.

The examples of the present invention are summarized in the following table, showing a comparison of the steels of the compositions as specified by the present invention with the conventional steels in their hardenabilities.

As evidently seen from the following table, although all alloying elements excepting aluminum are substantially the same in both kinds of steels, the steels of the present invention show a marked superiority to the conventional steels, as indicated by the ditferences in the martensite distances of them. Further, the conventional steels No. 4 and No. 5 in the table are those, in which chromium is added in an increased amount, while reducing the amount of manganese, in order to improve the hardenability of the steel, but the hardenabilities of them are still by far inferior to those of the steels obtained by the addition of aluminum according to the present invention.

Chemical composition in wt. percent (balance Fe) Austenite J ominy-test Sort Sanipl grain size D1 inch martensite N o. C Si Mn P S Cr Mo Al B V N 0. distance,

Steel of the present 1 0. 43 O. 38 1. 56 0. 020 0. 012 0. 32 0. 04 7. 5 3. 29 75. 0 invention. 2 0. 47 0. 25 1. 48 0. 017 0. 007 0. l8 0. 02 7. 5 2. 71 24. 0 3 O. 46 0.27 1. 37 0.017 0.012 0.23 0. 16 7. 5 3. 20 25.0 4 0. 46 0. 26 1. 46 0. 012 0. 005 0. 21 0. 14 8. 0 3. 31 75. 0 5 0. 42 0. 23 1. 48 0. 012 0. 006 0. 18 0. 14 8.0 2. 87 46. 5 6 0. 39 0. 20 0. 71 0.011 0. 005 0.96 0.04 8. 5 3. 25.0 7 0. 19 0. 35 1. 41 0. 013 0. 006 0. 09 0. 02 8. 0 1. 67 16. 0 8 0. 44 0. 21 0. 83 0. 016 O. 010 0. 65 0. 02 8. 0 2. 45 I4. 0 9 0. 48 O. 17 l. 44 0. 018 0. 005 0. 10 0. O2 7. 5 2. 58 23. 5 10 0. 43 0. 21 0. 90 0.013 0. 011 1.16 0.02 8 0 3. 39. 0 11 0.53 0. 2O 0. 86 0.010 0. 006 0.02 0.49 8.0 2. 56 35. 0 Conventional steel. l 0. 43 0.38 1. 56 0. 021 0.012 0.32 0. 04 7. 5 3. 10 12.0 2 O. 47 0. 25 1. 46 0. 017 0. 007 0. 18 0. 02 7. 5 2. 58 8. 0 3 0. 47 0. 25 1. 37 0. 016 0. 011 0. 23 0 15 7. 5 3. 20 9. 0 4 0. 45 0. 29 0.58 0. 015 0.012 0. 67 7 0 1.82 6. 6 5 0. 40 0.28 0.75 0.016 0.013 1.15 7. 0 2.87 12.0

to the fact that MFAl shows such high values as above 1.4 which have never been achieved heretofore, as is evidently shown by a line A and a sphere enclosed with hatched lines a-b, 11-6 and cd in FIG. 2, when the content of Al lies within the above mentioned range. If the content of Al is lower than 0.06 wt. percent, MFAl shows values less than 1.4, as is shown by a sphere enclosed with lines hg and g-f. Thus, a remarkable effect of improving the hardenability by the addition of Al can not be expected. On the other hand, if the content of Al exceeds 0.15 wt. percent, there occur various undesirable phenomena such as a reduction of Having thus described the invention, what is claimed is:

1. A steel of excellent mechanical properties and high hardenability, which has an ideal critical diameter greater than 1.5 inches, consisting of 7 8 1 0.0005 to 0.003 Weight percent B and balance being 3,216,823 11/1965 Gulya 75-124 Fe and unavoidable impurities. 3,251,682 11/1966 Wada 75124 References Cited HYLAND BIZOT, Primary Examiner.

UNITED STATES PATENTS 5 US Cl. XRI 3,110,586 11/1963 Gulya 75-124 X 75 12g 3,155,495 11/1964 Nakarnura 75-124

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