US3620717A

US3620717A - Weldable, ultrahigh tensile steel having an excellent toughness

Info

Publication number: US3620717A
Application number: US761474A
Authority: US
Inventors: Shozo Sekino; Toshiyuki Fujishima; Hiroshi Yada
Original assignee: Yawata Iron and Steel Co Ltd
Current assignee: Yawata Iron and Steel Co Ltd
Priority date: 1967-09-27
Filing date: 1968-09-23
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16
Also published as: GB1241964A; FR1602700A

Abstract

A weldable, ultrahigh tensile steel prepared by adding V and Cu to a low-carbon steel containing Ni and Mo, to which there may be further added W, Nb and Ta, characterized by being able to stand a low temperature and possessing a good weldability and an excellent toughness at a very high level of the strength, particularly the toughness in the welding heat-affected zone, said properties being particularly imparted by the addition of V and Cu.

Description

United States atent Inventors Shozo Seklno;

Toshiyuki Fujishima; Hlroshi Yada, all of Kltakyushu, Japan Appl. No. 761,474 Filed Sept. 23, 1968 Patented Nov. 16, 1971 Assignee Yawata Iron & Steel Co., Ltd.

Tokyo, Japan Priority Sept. 27, 1967 Japan 42/621 25 WELDABLE, ULTRAHIGH TENSILE STEEL HAVING AN EXCELLENT TOUGHNESS 10 Claims, 1 Drawing Fig.

[1.5. CI 75/125, 75/123 J, 75/123 K, 75/128 V, 75/128 W, 75/128 G, 75/124, 148/31, 148/36 Int. Cl ..C22c 39/50, C22c 39/54 Field of Search 75/123,

w mews? Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Joseph E. Legru Attorney-Wenderoth, Lind & Ponack ABSTRACT: A weldable, ultrahigh tensile steel prepared by adding V and Cu to a low-carbon steel containing Ni and Mo, to which there may be further added W, Nb and Ta, characterized by being able to stand a low temperature and possessing a good weldability and an excellent toughness at a very high level of the strength, particularly the toughness in the welding heat-affected zone, said properties being particulariy imparted by the addition ofV and Cu.

Yleld strength -Oll V- notch Impact value (at- 80C) WELDABLE. ULTRAIIIGH TENSILE STEEL HAVING AN EXCELLENT TOUGHNFSS BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to a high-toughness, ultrahigh tensile steel having an excellent weldability.

2. Description of the Prior Art l-leretofore, it was known that a steel having a yield strength of above 90 lcg/mm. and at the same time a relatively good toughness could be obtained, when adding a carbide-forming element to steel containing 4 to 8.5 percent Ni and then subjecting the resultant steel to heat treatments such as quenching and tempering, but it was difficult to obtain a steel which could fulfill the conditions as mentioned in the following, because of the toughness of the welding heat-affected zone being lowered with an increase in the yield strength, that is, for steels to be placed at the service as materials for use in deep sea constructions, particularly such as an outer hull plate of submarine, the following conditions are to be regarded as indispensable:

i. The toughness should be so high as it may stop a further enlargement of cracks of any length occurred at the minimum temperature practically applied and ii. a welding should be easily feasible and the toughness as above mentioned should be imparted also to the weld.

SUMMARY OF THE INVENTION The main object of the present invention is to provide a steel which sufficiently fulfills the conditions as above mentioned, that is, a structural steel material which can well stand a low temperature and has an excellent toughness at a very high level of strength.

Another object of the present invention is to provide a structural steel material which has an excellent weldability and also an excellent toughness particularly of the weld.

And still another object of the present invention will become clear from the following disclosure, making reference to the accompanying drawing, which shows the relations between the Cu content of the steel of the present invention and the yield strength (A), the 2 mm. V-notch Charpy impact value (B) at 80 C. and the 2 mm. V-notch Charpy impact value (C) of a welding thermal cycle reproducing test piece.

The composition ranges of the steel of the present invention, by which the abovementioned objects can be obtained, are as follows:

C 0.04 to 0.15% Si 0.05 to 0.5% Mn 0.05 to 1.5%

Ni 4 to 8.5%

Mo 0.25 to 1.5%

Cu 04 to 3%, preferably 0.5

V 0.04 to 0.I5%

Al an amount usually contained in a killed steel the balance being Fe and unavoidable impurities.

Among the abovementioned components Mo may be replaced by a double amount of W. Further, to the steel having the composition ranges as above-mentioned there may be added at least an element selected from the group consisting of less than 2 percent Cr, less than 0.10 percent Nb and less than 0.10 percent Ta.

As a rule, the steel of the present invention having the composition as above mentioned is placed at practical services after it has been subjected to the following heat treatments: Immediately after a hot-working or after the hot-working followed by a normalizing at a temperature of 800 to l,100 C. the steel is quenched from a temperature ranging from 750 to 950 C. and then tempered at a temperature of 500 to 650 C. for 0.5 to 3 hours. Depending upon the composition series it is also possible to omit the quenching.

As above mentioned, in ,a steel containing 4.0 to 8.5 percent Ni, there occurs a deterioration of the toughness, particularly in the welding heat-affected zone in proportion to an increase in the yield strength. But, with the composition ranges of the present invention, which include especially 0.4 to 3 percent (preferably 0.5 to 1.5 percent) Cu and further 0.04 to 0.15 percent V added together with Cu, the inventors of the present'invention succeeded in elevating the strength of the material tempered after quenching and at the same time in remarkably improving the toughness of the welding heat-affected zone thereof. The relations between the Cu content and mechanical properties of the steel will be clearly understood from the attached drawing and the following disclosure.

BRIEF DESCRIPTION OF THE DRAWING The drawing involves changes in mechanical properties made in proportion to varying amounts of Cu added to the steel consisting of 0.1 percent C, 0.25 percent Si, 0.2 percent Mn, 6 percent Ni, 0.35 percent M0, 0.1 percent V, 0.03 percent Al and the balance being Fe and unavoidable impurities, wherein the yield strength indicated by a solid line (A) shows a sudden elevation in the value from about 0.4 percent on and begins to show a lowering tendency after 3 percent, and the 2mm. V-notch impact value at C. indicated by a dotted line (B) shows a very high value within the range of Cu, covering 0.5 to 1.5 percent and particularly so high as above 18 kg.- mo./cm. at 1 percent of Cu, or even within the ranges of 0.4 to 3.0 percent, the value above 10 l g.-m./cm., as usually required, which designates an excellent toughness of the steel of the present invention. Further, also the 2 mm. V-notch Charpy impact value at 80 C. indicated by a chain line (C) for test pieces subjected to a welding thermal cycle (with a heat input of 18,000 Jule/cm.) reproducing test shows an increasing value in proportion to an increasing addition of Cu. It is, however, to note that in the case of adding no V neither such high strength nor high toughness of the weld as above mentioned can be obtained, which is clearly seen from a reference steel No. E in the tables 3 and 4 to be shown later indicating the strength and toughness of a steel of the same composition series as that of the steel shown in the drawing, but without the addition of V.

Thus, in the present invention to a steel 4.0 to 8.5 percent- Ni steel there are added Cu and V and further M0 or W which may be added to replace a part or a whole of M0. The steel of the present invention thus obtained by adding the elements as above mentioned is characterized by displaying an excellent toughness at a yield stress of more than kg./mm. due to a secondary hardening by means of the addition of V and Mo or W in a material a welding heat-affected zone by a mutual function of Cu and V.

In the steel of the present invention C is necessary for a strengthening of the steel due to the formation of carbide and for maintaining the proper hardenability. In an amount of iess than 0.04 percent the efiect of C cannot be displayed, but in an amount of more than 0.15 percent a reduction in the toughness in a welding heat-affected zone is unavoidable.

Therefore, the C content is specified to be within the range of 0.04 to 0.15 percent.

Si and Mn are necessary as a deoxidizing agent in making the steel of the present invention. However, Si and Mn an amount of less than 0.05 percent respectively are insufficient to perform the deoxidizing. However, with an amount of more than 0.5 percent Si the toughness is lowered. With an increasing amount of Mn the strength is elevated to some degree and the toughness in a welding heat-affected zone is a little improved, but if above 1.5 percent, there appears remarkably a tendency of deteriorating the toughness of the material tempered after quenching. On the basis of the facts above-mentioned Si is specified to 0.05 to 0.5 percent and Mn 0.05 to 1.5

percent.

Ni is an element necessary to impart a good toughness to a steel material at the high level of strength. In the steel of the present invention an addition of more than 4 percent Ni is necessary in order to keep the toughness which is more than 90 kg./mm. and is to fulfill the conditions as above mentioned. But, with an addition of more than 8.5 percent a specially remarkable improvement in the toughness can not be expected. Therefore. the addition of Ni is limited to 4 to 8.5 percent.

Mo and V are a carbide-forming element respectively which has a function of effecting a remarkable secondary hardening by a tempering treatment within a temperature range of 500- to 650 C. and further V has the function of improving the toughness in the welding heat-affected zone under the coexistence with Cu. In the steel of the present invention a sufficient strength can not be obtained by the addition of M in an amount of less than 0.25 percent and V in an amount of less than 0.04 percent. On the other hand, if the addition of Mo and V exceeds 1.5 percent and 0.15 percent respectively, the effects thereof reach the saturation and there appear rather tendencies of the steel being softened and the weldability being deteriorated. Therefore, M0 is defined to be in the range of 0.25 to 1.5 percent and V'in the range of 0.04 to 0.15 percent. Further, a part or whole of Mo may be replaced by W.

Cu shows a remarkable effect of preventing the embrittlement of the welding heat-affected zone, particularly by the mutual function with V in the Ni-steel, the strength of which has been increased by carbides of Mo and V as above mentioned. in the steel of the present invention an addition of at least 0.4 percent Cu is absolutely necessary. That is, with an increasing amount of Cu above 0.4 percent its effect of preventing the embrittlement of the welding heat-affected zone becomes remarkable, simultaneously accompanied with the effect of increasing the strength of the steel to some degree. However, if the amount of Cu exceeds 1.0 percent, there appears a tendency of lowering the toughness of the material tempered after quenching, and particularly, above 3 percent, the said tendency becomes notable. Therefore, the amount of Cu is specified to the range of 0.4 to 3.0 percent, preferably 0.5 to L5 percent.

Cr has an effect as carbide-forming element of elevating the steel by causing fine carbonitride to precipitate mainly during a hot-working andof refining austenitic crystal grains thereby. However, if the addition of Nb+Ta exceeds 0. l0 percent, the toughness is rather deteriorated because of large precipitates of carbonitride being formed in grain boundaries. Thus, the addition of N b+Ta is limited to less than 0.10 percent.

W has substantially the same effect as M0 in forming carbide. In the steel of the present invention a part or whole of Mo may be replaced by a double amount of W without producing any difference in characteristics of the steel.

Al has an effect of refining crystal grains even with an addition of such a small amount, as is to be added to the steel as a deoxidizer. However, in the steel of the present invention the addition of Al may be dispensed with. However, if it is to be added, it may be added in an amount ofless than 0.07 percent, that is, an amount usually contained in a killed steel.

The steel of the present invention shall be explained more in detail on the basis of examples.

Alloy steel samples Nos. 1 to 15 having the chemical compositions as shown in table 1 within the ranges specified by the present invention were prepared by melting and then subjected to a slabbing and a hot-rolling to make steel plates having a thickness of 13 mm. respectively therefrom. Each steel plate was then normalized at a temperature of 850 C. and thereupon was quenched in water after heated to a temperature of 800to 900 C. The quenching was followed by a tempering at a temperature of 600 C. or 625 C. The mechanical properties of each steel plate thus quenched and tempered were shown in table 2. As the results of various investigations on said samples to measure small-sized 2 mm. V-notch impact values on conditions as shown in table 2, (l) and (2) it has been confirmed that in the case of l) the impact value is satisfactory, if it exceeds 8.0 kg.-m./cm. at a temperature lower by more than 20 C. than the minimum temperature practically applied, and in the case of 2) the impact value is satisfactory if it exceeds 4.0 kg.-m./cm. at a temperature lower by 20 C. than the minimum temperature practically applied to test pieces subjected to a welding thermal cycle reproduction test.

TABLE 1 Sample N0. of the Composition (percent) steel of the present invention C Si Mn N1 M0 V Cu Sol. Al Others 0. 27 0.22 5. 94 0.81 0.11 0.42 0. 041 0.11 0.18 6.02 0.31 0.15 0.43 0.051 0.17 0.05 6.03 0.35 0.09 0.48 0.020 Cr, 1.01 0.22 1.36 5.41 0.50 0.10 1.44 0.034 Cr, 1.04 0. 24 0.47 4.29 0.54 0.10 0. 07 0.012 0.25 0. 22 8.13 0. 23 0.11 0.43 0.048 0.27 0.22 6.11 0.40 0.11 0. 42 0.042 0.28 0.24 4. 88 0.33 0.05 1.54 0.013 0. 25 0.10 5.05 0.35 0.10 2.86 0.002 0.42 0.18 6.02 0.36 0.15 0. 48 0.057 Cr, 1.08 0 20 0.25 4.80 0.35 0.10 0.51 0.036 gr, 1.45 r, 1.45 0.15 1.30 4. as 0.51 0.05 0. 51 0. 044 +1.8 M3 0. 27 0.22 6.17 0.30 0.11 0. 42 0.042 \1', 0.48 0.20 0. 22 6.05 0.11 0. 42 0. 046 W, 1.57 0.26 0.32 6.05 0.10 0.47 0.037 \V, 2.05

strength of the steel of the present invention. However, as TABLE 2 stronger carbide-forming elements such as Mo and V are used V-notch Charpy in the steel of the present invention, the effect of Cr as the car- 1139 a g z g bide-forming element is not so much expected. Moreover, as piece ubjigli d to a we 11g Cr has a tendency of deter orating the weldablltty, the addt Sample No notch Charm thermal cycle test tion of Cr may be dispensed with. But, also the addition of Cr of the steel of t Yifild impact age (Wil.]ll0ll?=ilt l/npug the present 5 reng at -8 01 15M 0 u 0 cm. is efi'ectwe in such a case, where a high hardenability 1s invention kgJmm, kgfim/cmfi required when the contents of such elements as C, Mn and n n 0 Mo, which elevate the hardenability of steel, are those near 3 the lower limits of the respective ranges within the composi- 1g; tion of the steel of the present invention. However, even in this 108 .3: 8 case, the addition of Cr is to be limited to less than 2.0 per- 18.? 1.} cent. as it has a tendency of lowering the toughness of the 96 :5 material tempered after quenching, because of a huge carbide g-g being formed thereby, if it is added in an amount more than 94 4,3 required. fg-g Nb and Ta are a strong carbonitride-forming element 122 9 7 2 112 10.1 7. 3

respectively, which has an effect of elevating the toughness of As is shown in table 2, all of the samples show the yield strength of more than 90 ltg./mm. and fulfill the conditions as above mentwned. Among these samples Nos. 1 and 4, which contain a smaller amount of C respectively and Nos. 1. 4. 5, 7.

many modifications and variations of the present invention are possible within the composition ranges above mentioned. It is therefore. to'be understood that within the scope of the composition ranges as above mentioned the invention may widely 14 and 15 which contain a larger amount of M (or W) be practiced otherwise thanasspecially described respectively show such very high impact values at -80 C. for What is claimed is: the test pieces subjected to a thermal cycle reproduction test 1. A weldable ultrahigh tensile steel having a yield strength asof more than 7 kg.-m./cm.. implying an excellent toughness of above 90 kgjmm? and at the same time an excellent of these samples This is one of the most characteristic featoughness a a low mp rature and good weldablllty compristures of the present invention. ng

At last, in order to evidence that the composition ranges of t0 5 pe cent y Weight C the steel of the present invention should absolutely by kept as 0105 i0 P t y ight Si specified by the present invention, there are shown in table 3 to p cent by W ight Mn some examples of steels having composition ranges different 4.0 to 8.5 percent by weight Ni from those of the steel of the present invention in that in one 0.25 to 3.0 percent by weight Mo of these examples there is lacking in one of elements essential 0.04 to 0.15 percent by weight V for the steel of the present invention and in another of them 0.4 to 3.0 percent by weight Cu there is added another of these elements too much or too litthe balance being Fe and unavoidable impurities. tle, and the results of the same tests as made in table 1 are 2. The steel claimed in claim 1, wherein Al is added in an shown in table 4. 2 amount of less than 0.07 percent by weight.

TABLE 3 Composition (percent) Reference steel No. C S1 M11 Ni Mo V Cu Sol. Al Others 0.101 0.24 0.06 3. 75 0. 57 0.05 0.4 0.022 B. 0.005 0.24 2.48 4.65 0.30 0.14 0.5 0.05 C... 0.10 0.25 0.23 4.110 0.33 0.14 0.014 (3.1.05 1). 0.10 0.28 0.23 5.22 0.10 0.13 0. 51 0.0311 (r, 1.04 E. 0.10 0.20 0.25 4.1121 0.34 0.47 0.030 0.0.22

TABLE 4 3. The steel claimed in claim 1, wherein a part of Mo is t h Ch replaced by a double amount of W. imp gt cvaluo ia i. 4. The steel claimed in claim 1. wherem a whole of Mo 1s test re laced b adouble amount of W. 1 b' t d t P Y sil l i ng t ile rmigl 5. The steel claimed in cla1m 1. wherein the steel 1s 0 ole test (wit o o notch Charpy heat input quenched from a temperature ranging from 750 to 950 C R I t Yielld impact vagzv J01 1/8,00()l and then tempered at a temperature of 500 to 650 C 9 Hence 5 mm at u e 6 A weldable steel ultrahigh tensile strength having a yield tlN. 5 8e 0 Pg [mm 2 m [cm kg strength of above 90 leg/mm. and at the same time an excel 99 lent tou hness at a low temperature and good weldabilily. 103 5.11 2.6 102 11. 7 2.15 comprising g is 0.04 to 0.15 percent by weight 0.05 to 0.5 percent by weight S1 0.05 to 1.5 percent by weight Mn No. A contams a smaller amount of N1, No. B a larger 4D to 85 percent by weight Ni amount of Mn and No. C contains no Cu. All of these three 025 to 30 percent by weigh Mo samples show insufficient impact values of test pieces sub- 04 to 30 percent by weight Cu ected to a welding thermal cycle reproduction test. No. D has 0 04 to 0 15 percem by weight v a smaller amount of Mo and No. E contains no V. and both le'ss than'z 0 percent by weight Cr samples are not only low in the strength. as e1ther shows the the balance being Fe and unavoidable impuriies yield strength of less than 90 kgJmm. but also low 1n the v toughness, as are shown with the impact values of test pieces 7 the Steel cialmed m whefem Al is added m subjected to a welding thermal cycle reproduction test of ammlm Ofless than 007 ParcelIt y weight below 4 kg.'m./cm., which means that the coexistence of Cu 8. The steel claimed in claim 6, wherein a part of M0 is with V is indispensable. It is evident from the foregoing that 5 la d by a double amount of W. the composmon ranges f f j 5! pwduce a 9. The steel claimed in claim 6. wherein a whole of M0 is steel as that of the present 1nvent1on which 15 d1st1ngu1shed at replaced by a double amount the same time by an excellent toughness and weldability and particularly is fit for the uses which the present invention has The Steel claimed m claim the steeol object quenched from a temperature rangmg from 750:to 950 C.

The examples above mentioned do not designate those and tempewd atatemperamre ofsoo to 650 which limit the scope of the present invention. Obviously

Claims

2. The steel claimed in claim 1, wherein Al is added in an amount of less than 0.07 percent by weight.
3. The steel claimed in claim 1, wherein a part of Mo is replaced by a double amount of W.
4. The steel claimed in claim 1, wherein a whole of Mo is replaced by a double amount of W.
5. The steel claimed in claim 1, wherein the steel is quenched from a temperature ranging from 750* C. to 950* C. and then tempered at a temperature of 500* to 650* C.
6. A weldable steel ultrahigh tensile strength having a yield strength of above 90 kg./mm.2 and at the same time an excellent toughness at a low temperature and good weldability, comprising 0.04 to 0.15 percent by weight C 0.05 to 0.5 percent by weight Si 0.05 to 1.5 percent by weight Mn 4.0 to 8.5 percent by weight Ni 0.25 to 3.0 percent by weight Mo 0.4 to 3.0 percent by weight Cu 0.04 to 0.15 percent by weight V less than 2.0 percent by Weight Cr, the balance being Fe and unavoidable impurities.
7. the steel claimed in claim 6, wherein Al is added in an amount of less than 0.07 percent by weight.
8. The steel claimed in claim 6, wherein a part of Mo is replaced by a double amount of W.
9. The steel claimed in claim 6, wherein a whole of Mo is replaced by a double amount of W.
10. The steel claimed in claim 6, wherein the steel is quenched from a temperature ranging from 750*to 950* C. and then tempered at a temperature of 500* to 650* C.