US3028270A

US3028270A - Production of high tensile strength, high notch toughness steel by low temperature anneal

Info

Publication number: US3028270A
Application number: US835948A
Authority: US
Inventors: Morita Sadayoshi; Sato Makoto
Original assignee: Yawata Iron and Steel Co Ltd
Current assignee: Yawata Iron and Steel Co Ltd
Priority date: 1958-08-25
Filing date: 1959-08-25
Publication date: 1962-04-03
Anticipated expiration: 1979-04-03

Description

Aprxl 3, 1962 sADAYosHl MoRlTA ETAL 3,028,270

PRODUCTION OF HIGH TENSILE STRENGTH, HIGH NOTCH ToUGHNEss STEEL EY Low TEMPERATURE ANNEAL INV ENTOR` AKUT@ $470 By M WMM/ AUM/verf April 3, 1962 SADAYOSHI MORITA ETAL PRODUCTION OF HIGH TENSILE STRENGTH, HIGH NOTCH TOUGHNESS STEEL BY LOW TEMPERATURE ANNEAL Filed Aug. 25, 1959 CM l.

20a son 400 no aw 6.

.Zin/2456 m m 5 Sheets-Shea?l 2 2M 3M 4M JM aw C. /l/mea/ Temperature Fig. 4-4

STEM 4 cnaf.

INVENTORS MDA YOSH/ MOR/TA By Kwe( Pm Anale/vm April 3, 1962 sADAYosHl MoRlTA ETAL 3,023,270

PRODUCTION OF HIGH TENSILE STRENGTH, HIGH NOTCH TOUGHNESS STEEL BY LOW TEMPERATURE ANNEAL Filed Aug. 25, 1959 5 Sheets-Sheet 3 S rEEA Alme/ Tempe/nm.

IN VENTORS A TToR/vers April 3, 1962 sADAYosHl MORITA ET AL PRODUCTION OF HIGH TENSILE STRENGTH, HIGH NOT ESS STEEL BY LOW TEMPERATURE ANNEAL 5 Sheets-Sheet 4 TOUGHN Filed Aug. 25, 1959 STEEL 7 IN VENTORS April 3, 1962 SADAYOSHI MORITA ET Al. PRODUCTION OF HIGH TENSILE STRENGTH 3,028,270 HIGH NOTCH URE ANNEAL TOUGHNESS STEEL BY LOW TEMPERAT Filed Aug. 25, 1959 5 Sheets-Sheet 5 Annea/ Temperature Acre( (sul,

INVENToRs YA/JA YUSH/ MOR/r4 5 y WMM United States Patent' Office 3,023,2 Patented Apr. 3, 1902 3,020,270 PRODUCTEGN 0F HHGH TENSILE STRENGTH, HIGH NTCH TUGHNESS STEEL BY LQW TEMPERATURE ANNEAL Sadayoshi Merita and Makoto Sato, Yawata City, Japan, assignors to Yawata Iron and Steel Co., Ltd., Tokyo, Japan, a corporation of Japan Filed Aug. 25, 1959, Ser. No. 835,948 Claims priority, application Japan Aug. 25, 1958 7 Claims. (Cl. 148-143) The present invention relates to a high tensile strength, high notch toughness steel, and more particularly, to a process for improving the resistance of rolled structural steels to the initiation and propagation of Susceptibility to brittle fracture.

It is known that the notch toughness of rolled structural steels has been considered as one of the most important mechanical properties required for steel structures, and more particularly for welded steel fabrications.

This application is an improvement of the copending application Serial No. 808,313, Production of High Tensile Strength, High Notch Toughness Steel, tiled by Morita et al. on April 23, 1959, now abandoned.

In the aforementioned copendiug application, reference is made to the improvement 4in physical `and mechanical properties of steel by the process which comprises subjecting a rolled steel which has been either hot rolled or reheated, such as, by normalizing, to accelerated quenching by a temperature interval of more than 100 degrees centigrade and preferably 200 degrees centigrade within the temperature range of 1000 to 600 C. at a cooling rate of about 1 to 10 degrees centigrade, and preferably 3 degrees centigrade per second in order to enhance the notch toughness.

In accordance with the present invention, the steel is subjected to the low temperature anneal which comprises holding it at a temperature of 350 to 650 C. for a time period of 3,0 to 90 minutes either immediately after it has been subjected to accelerated. cooling or cooled to room temperature, and thereafter to cooling in the air. By this low temperature anneal, excellent properties of tre steel in notch toughness as well as in ductility heretofore unable to attain by the'process of accelerated cooling either Vafterrolling or normalizing can be obtained.

Recently, I. H. Gross, E. H. Kottcamp, and R. D. Stout report in Welding Journal, 37 (1958), No. 4, 16C-S, and A. l. Rubin, l. H. Gross, and R. D. Stout in Welding Journal, 38 (1959), No. 4, 182-S, that they propose the use of spray quenching for heavy-section pressure-vessel steels in order to increase the cooling rate, and they can improve the strength and particularly the notch toughness of the pressure-vessel 4-in steel plate by accelerated cooling, provided only that stress relieving or tempering at a temperature above 1l50 F. or 620 C. followed the treatment. However, the above treatment lies in the application of spray quenching within the operating temperatures from austenitizing to room temperatures, and the structure of the steel thus obtained is not always of pearlite only, which is different from the result of the present invention, but of a hardened stmcture in the case of alloy steels, therefore the notch toughness of the steel is improved by the mechanism of hardening and tempering. In addition, the above treatment contemplates only the cooling rate below the temperature as low as 1300 F. or 705 C. because of the spray quench-` ing down to room temperature, and reference is not made to the cooling rate at a relatively elevated temperature contemplated by the present invention. Accordingly, in the cooling process of the instant invention, the cooling rate at an elevated temperature is so important that the operating temperature range of ferrite and pearlite trans- L formations is quickly cooled by accelerated cooling with the formation of a structure Yconsisting of either fine ferrite and pearlite or ferrite, pearlite and some upper bainite. Thereafter the carbides of this structure are made to spheroidize by the treatment of a low temperature anneal.

In the conventional heat treatment, such as, martempering and austempering, the cooling rate at the time of quenching yis considerably so much faster than that of the present invention that such heat treatment cannot be applied for processing large structural steels and high tensile steels; In austempering, the structure of the steel immediately after the hardened state is still of austenite which is to Vbe transformed gradually into bainite by the subsequent holding temperature, and its final objective is directed to the formation of a structure consisting chiefly of bainite. Further, in martempering, the steel is directly quenched to a temperature below the Ms. temperature to transform into martensite at the constant temperature, and then cooled to room temperature with the completion ofthe martensite formation by the transformation at the constant temperature.

The heat treatment called, marquenching, is a process which comprises holding the steel at a temperature immediately above Ms. until the entire section thereof reaches the same temperature, then `air cooling it to proceed its Ar transformation gradually. Hence the structure of the steel thus treated is of martensite. A heat treatment added with a subsequent tempering process is called, marquench temper treatment.

Accordingly, it seems clear that the above heat treatments of prior art are fundamentally dilferent from the heat treatment of the instant invention. The effects produced by the new heat treating technique comprising a low temperature anneal after the accelerated cooling are probably due to relieving of cooling stress produced by the accelerated cooling, line pearlitic structure, spheroidizing of upper bainite structure, phenomenon of ferrite restoration, and precipitation of nitrides and carbides. The range of accelerated cooling temperatures is preferred between A3 and A1 transformations. However, the effects` of the invention are attained when the steel is subjected to water quenching from an elevated temperature above A3 if the temperature 0f final rolling or heating is high. However, from a commercial point of view, water quenching from an elevated temperature above 1200 C. is hardly feasible. Further, the advantages of the invention are scarcely achieved if the steel is notv subjected to accelerated cooling by a temperature interval of at least degrees centigrade within the temperature range of A3 to A1.

:The temperature at which the accelerated cooling procedure is finished should be usually preferred about 700 C. However, in the case of a heavy section steel plate, the temperature gradient takes place between the surface and core thereof. For example, in a steel plate 50 mm. thick, the temperature of the surface thereof is higher than that of the core by a temperature of 100 to C. It follows from the above that cooling the surface of the steel plate down to the temperature as low as 600 C. is required in order to cool the core thereof to the temperature of 700 C. Probably, it may be diiicult to perform a straightening operation on a steel plate if the temperature of the surface thereof reached 600 C., but, as a matter of fact, the surface is reheated due to the high temperature of its core, hence the difliculty of the straightening operation will be obviated.

Referring to the temperature range of the low temperature anneal, the tempering temperature proposed by J. H. Gross, E. H. Kottcamp and R. D. Stout in the article mentioned hereinbefore is an elevated temperature', such as, either 1150 F. (620 C.) or l350 F. (730 C.), which is entirely different from the heat treating technique of the invention. It seems evident that they expect the eects of quenching and tempering. HOW- ever, in the heat treatment of the invention, good results are not attained because the steel becomes brittle at such high temperature. Thus, the notch toughness of steel is not improved by the treatment at a temperature either above 650 C. or below 350 C. Particularly, at a temperature above 650 C., the static strength of steel is lowered. Accordingly, the temperature range of 350 to 650 C. is specied in the low temperature anneal in accordance with the present invention.

The heat treating process of the invention can be applied eectively to almost every kind of structural steels, and, from the nature of heat treatment, more particularly to the steel structure consisting of more than 50% pearlite plus ferrite (added with a very small amount of upper bainite). Accordingly, the chemical composition of steel may be limited as follows: rC is less than 0.30%, Si less than 0.50%, Mn less than 1.50%, and other alloying elements as desired. Further, if the heat treatment of the invention is applied to the high tensile steel of the improved Mn-Si type favored with a tensile strength of more than 60 kg./mm.2, the elects produced thereby are astounding. This heat treatment can improve the notch toughness as well as the ductility of steel as shown by the reduction of area and elongation to a remarkable degree. Further, an increase of yield ratio resulting from the rise of yield point is also astonishing. In accordance with the low temperature anneal as selected, a commercial production of structural steels having excellent mechanical and physical properties is possible and practical.

The elects and advantages of this invention may be better understood from the following description of illustrative examples taken in connection with the accompanying diagrams, in which:

FIGS. 1-6, inclusive, show the diagrams when the invention is applied to low-alloy structural steels.

FIG. 1 shows the elects of the low temperature anneal (hold one hour) of rolled steel subjected tothe accelerated cooling (900-700 C., 3 degrees centigrade per 1, 2, 3 and 4 subjected to the above heat treatments over the fibrous fracture, respectively.

FIGS. 6-1, 6-2, 6-3 and 6-4 show the elects of the low temperature anneal (hold one hour) of rolled

steels

1, 2, 3 and 4 subjected to the above heat treatments over the Tr. 15 and Trs. transition temperatures, respectively.

FIGS. 7-12, inclusive, show the diagrams when the invention is applied to common structural steels.

FIGS. 7-1 and 7-2 shows the effects of the low temperature anneal (500 C., hold one hour, then cool in the air) of rolled steel subjected to the normalizing heating (910 C., one hour) and then to the accelerated cooling (900-700 C., 3 degrees centigrade per second) over the Kommerel and Kinzel tests, respectively.

FIGS. 8-1, 8-2 and 8-3 show the effects of the low temperature anneal (hold one hour) of rolled

steels

5, 6 and 7 subjected to the accelerated cooling over the static mechanical properties, respectively.

FIGS. 9-1 and 9-2 show the effects of the low temerature anneal (hold one hour) of rolled

steels

6 and 7 subjected to the accelerated cooling over the notch toughness, respectively. t

FIG. 10 shows the effects of the low temperature anneal of the rolled steel 8 subjected to the normalizing heating (910 C., one hour) and then to the accelerated cooling (900-700 C., 3 degrees centigrade per second) over the static mechanical properties.

FIGS. ll-l and 11-2 show the eiects of the low temf perature anneal (hold one hour) of rolled steel 8 sub-- I. APPLICATION OF THE INVENTION TO LOW- ALLOY STRUCTURAL S T E E L S INCLUDING HIGH TENSILE STEELS When the heat treating technique of the invention is applied to low-alloy structural steels including high tensile steels, much more excellent results are obtained than when applied to common structural steels described hereinafter.

Table 1.-Chemz'cal Analysis of Low Alloy Structural Steels To Be Tested Steel plate 20 mm. C Mn Si P S Cu Ti Cr Nl Mo V Al A l inthlck soluble soluble l 0.15 1. 20 0. 35 0. 009 0.017 0, 08 0. 20 0. 53 0.020 0.009 2 0.13 1.08 0. Q 0. 009 0.011 0.07 0. 014 0. 24 0. l2 0.16 0.09 0. 034 0. 006 3 0.13 1.11 0. 34 0.010 0.011 0.07 0. 009 0.03 0. 61 0.12 0.12 0.040 0.010 4 0.17 1.16 0. 43 0.016 0. 007 0.07 0. 006 0.21 0.61 0. 09 0.09 0.050 0.017

second) over the static mechanical properties thereof.

FIGS. 2-1 and 2-2 shows the edects of the low temperature anneal (hold one hour) of rolled steels subjected to the accelerated cooling (900-700 C., 3 degrees centigrade per second) over the Charpy impact and brous fracture, respectively.

FIGS. 3-1, 32, 3-3, and 3-4 show the effects ot the low temperature anneal (hold one hour) of rolled

steels

1, 2, 3 and 4 subjected to the normalizing heating (900 C., one hour) and also to the accelerated cooling (900- 700 C., 3 degrees centigrade per second) over the static mechanical properties, respectively.

FIGS. 4-1, 42, 4-3, and 4-4 show the effects of the low temperature anneal (hold one hour) of rolled

steels

1, 2, 3 and 4 subjected to the normalizing heating (900 C., one hour) and later to the accelerated cooling (900- 700 C., 3 degrees centigrade per second) over the notch toughness, respectively.

FIGS. 5-1, 5-2, 5-3 and 5-4 show the effects of the low temperature anneal (hold one hour) of rolled steels Table 2.-Mechanical Properties of Low Alloy Structural Steels T o Be Tested When the low temperature anneal is applied to the

steel plates

1, 2, 3 and 4 having such chemical analyses and mechanical properties as listed in Tables l and 2 without recourse to the accelerated cooling after the iinal rolling procedure, an increase in the notch toughness is not noticed. However, when such steels are subjected to the accelerated cooling (900-700 C., about 3 degrecs centigrade per second) after the rolling step and then to the low temperature anneal, the yield point and the elongation of the steel l are improved as shown in fFIG. 1. It is also shown that the transition diagrams of the steel after the low temperature anneal are transferred to the side of the low temperature. As shown in FIGS. 2 1 and 2 2, Charpy impact values are improved with the rise of anneal temperatures, and at the same time the fibrous fractures tend to increase `as the impact values do. Thus, an increase in the notch toughness of the steel subjected to the accelerated cooling after rolling and then to the low temperature anneal in accordance with the invention is so remarkable -that the heat treating technique of this invention can be advantageously applied to the production of high tensile steels. However, in this test, an hour is adopted as a holding time of period for anneal so that the selection of a suitable time of period for anneal depending7 on a high or low temperature should be effected. In addition, a relatively low temperature anneal for an extended period of time is not desired from an economical point of view, and, further, is not practical.

When the steels are subjected to the accelerated cooling after the normalizing heating and then to the low temperature anneal, an increase in the yield point, that is, an increase in the yield ratio, and also an increase in the elongation and in the reduction of area by the low temperature anneal as shown in FIGS. 3 1, 3 2, 3 3 and 3 4 are characteristically appreciated, which clearly show that excellent values of static mechanical properties result from the low temperature anneal in accordance with the teachings of this invention. ln reference to the change of the notch. toughness represented by the Charpy impact value in connection with FIGS. 4 1, 4 2, 4 3 and 4 4, `an impact value, about 5 lig-m. per sq. cm. at the temperature of 1C. as subjected to the accelerated cooling, has increased t-o more than 20 kg.m. per sq. cm. by the low temperature anneal, which indicates an unexpected improvement. Thus, the effect of either accelerated cooling or low temperature anneal separately is not noticeably appreciated, but the effect produced by the combined accelerated cooling and low temperature anneal together is markedly signicant. There is also a similar tendency in the relation between fibrous fracture and anneal temperature as shown in FIGS. 5 1, 5 2, 5 3 and 5 4. Further, Tr. 15 and Trs. transition temperatures at which Charpy impact value lowers below ft-lb.=2.6 kg.rn. per sq. cm. and fibrous fracture lowers below 50%, respectively, of these steels are illustrated in FIGS. 6 1, 6 2, 6 3 and 6 4, which show clearly that the notch toughness is improved by the low temperature anneal in accordance with this invention.

Now, an application of this new heat treatment to high tensile steels will be considered. These steels have tensile strength of 60 kg. per sq. mm., and yield point of approximately 40 kg. per sq. mm., respectively, all of which meet the requirements of the high tensile steel specification or standard. However, the Charpy impact value of these steels is several .kg-m. per sq. cm., which is not satisfactory, but the impact value is considerably enhanced by this low temperature anneal of the invention in order to obtain a high notch toughness steel which superior mechanical properties.

By the way, there is a report on Weldability of High Tensile Structural Steels, by L. Reeve, British metallurgist, appeared in Transactions of the Institute of Welding, December 1953, pp. 154-166, in which tempering at the temperature of 650 C. after rolling or normalizing is carried out in the production of a high tensile steel with the tensile strength of approximately 60 kg. per sq. mm. However, the heat treatment in the above report is `a combination of heat treating techniques of prior art while, on the other hand, the present invention contemplates the effects of the accelerated cooling at an intermediate cooling rate, in other words, the improvement of vnotch toughness Yand ductility resulting from the formation of tine ferrite and pearlite, the effects of restoration phenomenon and stress relieving clue to the low temperature anneal, and other excellent etlects produced by the transformation of mico-structures.

Thus, a steel of high notch toughness heretofore unable to attain lby mere tempering after rolling or normalizing can be produced by, the new heat treating technique of the invention. Steels l, 2, 3 and 4 are several embodiments of the invention, and such mechanical properties as imparted to these steels have heretofore been achieved =by hardening and tempering treatments. In view vof the above advantages, it is understood that the heat treatment of the invention may impart a surprising eiect to the manufacture of a Ahigh tensile steelV having a tensile strength of more than 50 kg. per sq. mm.

In reference to the weldability of the steel heat treated by the invention, no cracking takes place at the temperature of 60 C. in Kommerel test as shown inFIG. 7 1 while horizontal contraction indicates approximately 1% of the temperature of 60 C. in Kinzel test.

T able 3.--Test Results of I.I.W. Weld Maximum H ardness Maximum hardness at heat Matrix hardaffected section HV ness HV, normalized, Steel accelerated Normalized, Normalized cooling and accelerated and as annealed cooling and accelerated annealed cooling 500 O.,1 hr.

Test results of I.I.W. weld maximum hardness conducted on the

steels

2, 3, and 4 are shown in Table 3, which indicates that the maximum hardness at the heat affected weld of the steel subjected tothe low temperature anneal is lower than that of the steel subjected to accelerated cooling only, which also proves that the weldability of the former is better than that of the latter. In addition, the maximum hardness even in the steel 4 having the tensile strength of approximately kg. per sq. mm. is 376. Accordingly, the weldability of these steels is so excellent that it is worth while appreciating this if the mechanical properties thereof are taken into account. Thus, it is clear from the test results of weldability conducted on the steel that the heat treatment of the invention is very eiective for the manufacture of a high tensile strength and high notch toughness steel.

Table 4.- Chemz'cal analyses of WEL-TEN 60, Yawata Iron and Sleels Product Steel O Si Mn P S Cr Ni V Table 5,- Mechanfcal Properties of WEL-TEN 6c, Yawata Iron and Steels Product Yield Tensile Elongation V-notch Charpy point, strength, G.L.-200, impact value kg./mm.a Iig/mm.2 percent kg;-m./cm.2

Table 6.--T est Results of Mechanical Properties und Weldability of WEL-T EN 60, Yawata Iron and Steels Product Kcmmerel test +20 C. (-30 C.) Tekkentest, l Tensll Elongatlon V-notch Maximum cracking Steel Yield point strength percent charpy in1- hardness percent lig/mm.2 lig/rum.2 G.L.200 pact value LLW. HW, Angle of Angle of magnetic lrg.-n'1./cm.2 load 10 kg. crack inbending fracdetection itiation, ture, deg.

deg.

WEL-TEN 60(1) 49.2 60.9 23. 5 23. 5 339 120(90) 120( 120) 0 WEL-TEN 60(2).-- 52.9 63. 9 22.0 27. 2 336 120(80) 120( l20) 0 WEL-TEN 60(3) 56. 7 68. 5 19. 5 22.0 348 120(60) l20( 75) 0 An application of the new heat treatment of the invention to WEL-TEN 60, a high tensile steel of 60 lig/mm?, a new product of Yawata Iron and SteelV Company, Ltd., is described hereinbelow.

Table 7.--Chemcal Analyses of Common Structural Steels To Be Tested [Percent] A provisional specification for the chemical analyses and mechanical properties of WEL-TEN 60 is shown steel C Mn Si P s 1n Tables 4 and 5, respectively. Despite of small amounts of alloying elements, this steel has a tensile strength of 0.13 1.08 0.25 0. 012 0.016 more than 60 Jrg/mm? and also a superior weldability. gj (1)1 gg 81 glgi 81%? WEL-TEN 60 is manufactured by the heat treating 0.12 0.84 0.24 0.014 0.018 process which comprises subjecting va starting steel stock Table 8.-Accelerated Cooling Rates and Mechanical Properties of Common Structural Steels Treated Thereby Cooling temp., C. Charpy test, 0 C. Tensile test Plate Cooling Steel thickness, rate rum. Before After C/sec. Impact Fibrous Yield Tensile Elongacooling cooling value fracture, point, strength, tion,

kg.ml percent kgJmrn.2 kgJmm. percent 39. 5 895 640 2. 3 9. 03 11. 7 29. 9 46. 3 31. 5 36. 875 695 2. 1 7. 57 6. 7 31. 6 48. s 30. o 32. 910 749 1. 7 6.28 37. 0 2s. s 42. s 29. 5 31. s s60 685 1. 9 1o. 46 73. 3 26. 6 42. 5 30.0

to accelerated cooling Vat a cooling rate of about 5 C. per second within the temperature range of 900 to 650 C. after normalizing heating at the temperature of 95 0 C., then to air cooling, and finally to the low temperature anneal at the temperature of 450 C. Test results of mechanical properties and weldability of this treated steel are indicated in Table 6. As clearly shown in Table 6, despite of such a high tensile strength as more than 60 lig/nun.2 of WEL-TEN 60 heat treated by the process of the invention, its V-uotch Charpy impact value at the temperature of 0 C. indicates more than 20 kg.m../cm.2, which proves of high notch toughness. Such high notch toughness has been heretofore considered that it is achieved only by hardening and tempering. That such excellent mechanical properties have been imparted to this steel with a structure consisting of fine ferrite and pearlite may be thanks to the new heat treating technique of the present invention based on a novel idea.

When the low temperature anneal is applied to these steels, some increase in the elongation and also in the yield point is perceived, but any change in the reduction of area is hardly observed. The relation between the low temperature anneal and the static mechanical properties produced thereby is shown in FIGS. 8-1, 8-2 and 8-3, respectively.

The results of impact test at the temperature of 0 C. are shown in FIGS. 9-1 and 9-2, in which the impact values are somewhat improved by the low temperature anneal at the temperatures of either 400 C. or 500 C. It is also observed that brous fractures are somewhat improved by the heat treatment of the invention.

When the heat treatment of this invention is applied to the steel which has been subjected to normalizing heating and then to accelerated cooling, the results of tensile .test have been hardly affected as shown in FIG. 10. However, in reference to the impact value and the fibrous fracture, for example, at the temperature of -40 C., they are noticeably improved by the low temperature anneal at the temperature of 400 C. (hold one hour, then air cool) as clearly illustrated in FIGS. 11-1 and `11-2. Further, FIGS. 12-1 and 12-2 indicate the relation be- -tween Tr. l5 and Trs. transition temperatures and the low temperature auneal in the above case, which also 9 show that the transition temperatures at the temperature of 400 C. are the lowest.

As described in full details hereinabove, the novel heat treatment of the invention can improve the ductility as well as notch toughness of common structural steels just as elective as high tensile steels and other low alloy structural steels being treated by this invention. Accordingly, it is evident that the invention can be effective to the common steel, too.

We claim: Y

1. A process of producing a steel of high tensile strength and high notch toughness, which comprises quenching a rolled steel containing less than 0.30% C, less than 0.50% Si, and less 4than 1.50% Mn and other incidental impurities and which is at a temperature in the vicinity of a high A3 transformation temperature of said steel, from said high temperature down to a temperature higher than a low transformation temperature A1 -at a cooling rate between l and l degrees centigrade per second, said quenching being carried out at said cooling rate over a temperature range of more than 100 degrees centigrade within said temperature range of A3 to A1, subsequently holding said quenched steel at a temperature of 350 to 650 C. for a period of 30 to 90 minutes, and thereafter cooling said treated steel to room temperature.

2. A process as claimed in claim 1 in which temperature A3 is 1000 C. and temperature A1 is 600 C.

3. A process as claimed in claim 1 in which said steel contains at least one alloying element taken `from the group consisting of Ni, Cr, Mo, V, A1 and Ti other than C, Si and Mn.

4. A process as described in claim 1, wherein the starting material is a rolled steel consisting of more than 50% pearlite plus ferrite and containing carbon, silicon, and manganese below the said percentage limits.

5. A process as described in claim 1, wherein the starting material is a rolled improved manganese-silicon steel having a tensile strength of more than 60 kg./mm.2.

6. A process of producing a steel of high tensile strength and high notch toughness, which comprises heating a rolled steel consisting of more than pearlite plus ferrite and containing less than 0.3% C, less than 0.50% Si, and less than 1.50% Mn and other incidental impurities at about 910 C. for l hour to normalize the same, acceleratedly cooling said steel from about 900 C. to 700 C. at a rate of 3 degrees centigrade per second, subsequently holding the resulting quenched steel at a temperature of about 500 C. for 1 hour, and then cooling said treated steel in the air.

7. A process for producing a steel of high tensile strength and high notch toughness, which comprises holding, at a temperature in the range of 350 to 650 C. for a period of 30 to 90 minutes, steel obtained by quenching a rolled steel containing less Vthan 0.30% C, less than 0.50% Si, and less than 1.50% Mn and other incidental impurities from a high temperature in the vicinity of an A3 transformation temperature of said steel, of about 900 C. down to a temperature of about 700 C. at a,

cooling rate of about 3 degrees centigrate per second, which rate had been held over a temperature range of more than .100 degrees centigrade Within said temperature range from about 900 to 700 C.; and thereafter cooling the treated steel to room temperature.

Claims

1. A PROCESS OF PRODUCING A STEEL OF HIGH TENSILE STRENGTH AND HIGH NOTCH TOUGHNESS, WHICH COMPRISES QUENCHING A ROLLED STEEL CONTAINING LESS THAN 0.30% C, LESS THAN 0.50% SI, AND LESS THAN 1.50% MN AND OTHER INCIDENTAL IMPURITIES AND WHICH IS AT A TEMPERATURE IN THE VICINITY OF A HIGH A3 TRANSFORMATION TEMPERATURE OF SAID STEEL, FROM SAID HIGH TEMPERATURE DOWN TO A TEMPERATURE HIGHER THAN A LOW TRANSFORMATION TEMPERATURE A1 AT A COOLING RATE BETWEEN 1 AND 10 DEGREES CENTIGRADE PER SECOND, SAID QUENCHING BEING CARRIED OUT AT SAID COOLING RATE OVER