US3929517A

US3929517A - Process for producing a steel having a superb combination of high strength and substantial toughness

Info

Publication number: US3929517A
Application number: US377634A
Authority: US
Inventors: Yoshimi Nakamura; Toshihiro Minami; Eizi Niina; Katuzi Mizutani
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1972-07-07
Filing date: 1973-07-09
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30

Abstract

A carbon steel or low alloy steel having a superb combination of high strength and substantial toughness is produced by a process in which the steel is subjected to cold working at a reductionof-area value of at least 10% to induce an initial strain therein, rapidly heating the steel to a temperature ranging from 250*C to the A1 transformation point, and rapidly cooling the steel after it has been held at the maximum temperature for less than 3 minutes. The strength and toughness as well as the surface appearance of the steel can be recovered or improved by subjecting the steel to repeated cycles of the treatment.

Description

United States Patent [1 1 Nakamura et al.

[ Dec. 30, 1975 PROCESS FOR PRODUCING A STEEL HAVING A SUPERB COMBINATION OF HIGH STRENGTH AND SUBSTANTIAL TOUGHNESS Inventors: Yoshimi Nakamura; Toshihiro Minami, both of Kobe; Eizi Niina, Nishinomiya; Katuzi Mizutani, Kobe, all of Japan Assignee: Kobe Steel Ltd., Kobe, Japan Filed: July 9, 1973 Appl. No.: 377,634

YIELD POINT TENSILE STRENGTH, kqjmm g a a 8 a a a s 2 5 I l 3,674,570 7/1972 Hallstrom etal. 148/12 Primary ExaminerW. Stallard Attorney, Agent, or FirmOblon, Fisher, Spivak McClelland & Maier [57] ABSTRACT A carbon steel or low alloy steel having a superb combination of high strength and substantial toughness is produced by a process in which the steel is subjected to cold working at a reduction-of-area value of at least 10% to induce an initial strain therein, rapidly heating the steel to a temperature ranging from 250C to the Al transformation point, and rapidly cooling the steel after it has been held at the maximum temperature for less than 3 minutes. The strength and toughness as well as the surface appearance of the steel can be recovered or improved by subjecting the steel to repeated cycles of the treatment.

9 Claims, 14 Drawing Figures REDUCTION-OF-AREA, lo

AS HOT THEN 20% 200% ROLLED DRAWN I l l 500C 600C 700C TEMPERATURES AT WHICH DRAWN STEELIS HEAT TREATED US. Patent A8 HOT ROLLED Dec. 30, 1975 Sheet 1 of 12 THEN 20% 800C 300C 400C 500C 600C 700g DRAWN v TEMPERATURES AT WHICH DRAWN STEELIS HEAT TREATED FIG.1

EFFECT OF THE TEMPERATURE OF THE TREATMENT PROCESS OF THE INVENTION US. Patent Dec. 30, 1975 Sheet 2 of 12 3,929,517

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20% DRAWING TREATMENT TREATMENT TREATMENT of the INVENTION of Ihe INVENTION of the INVENTION IONCEI (TWICE) (3 TIMES) EFFECT OF THE REPEATED TREATMENT ON STEEL OF THE PROCESS OF THE INVENTION IFOR AISI I043 STEEL AT 600C I US. Patent Dec. 30, 1975 Sheet4 of 12 3,929,517

ELECTRON MICROSCOPIC STRUCTURE OF ROLLED STEEL IAISI I027 STEEL) ELECTRON MICROSCOPIC STRUCTURE OF THE STEEL SUBJ ECTED TO THE TREATMENT OF THE INVENTION. (43% DRAWING,THEN RAPID HEATING AND COOLING) U.S. Patent Dec. 30, 1975 Sheet 5 of 12 3,929,517

ELONGATION,% REDUCTION-OF-AREARA 8 g 8 g 8 8-3 a 8 l I I5 THEN 20% DRAWN l l l l l l l STEEL IS QUENCHED 200C 300C 400C 500C 600C 700C J AT 870C AND v TEMPERED AT 570C TEMPERATURES AT WHICH DRAWN STEEL IS HEAT TREATED FIG.5

EFFECT OF Mn-Cr B ALLOY SUBJECTED TO THE TREATMENT OF THE INVENTION US. Patent Dec.30, 1975 Sheet6of12 3,929,517

U P O O mme mmwnqx TWICE SAMPLE QUENCHING (870C) TEM PERING (570C) o cE U.S. Patent Dec.30, 1975 Sheet70f 12 3,929,517

MICROSCOPIC STRUCTURE OF A STEEL SUBJECTED TO QUENCHING AND TEMPERING IAISI I027 STEEL) MICROSCOPIC STRUCTURE OF A STEEL SUBJ ECTED TO THE TREATMENT OF THE INVENTION IAISI I027 STEEL) US. Patent Dec. 30, 1975 ELONGATION,% REDUCTION -OF-AREA,% YIELD POINT TENSILE STRENGTH kqJmm aaaa Sheet 8 of 12 3,929,517

AS HOT THEN ROLLED THEN 20% DRA WN QUENCHED AT 880C AND TEMPERED AT 400C r TEMPERATURES AT WHICH DRAWN FIG.8

EFFECT OF HEATING TEMPERATURE ON A Mn-Cr-B ALLOY IN THE TREATMENT OF THE INVENTION STEEL IS HEAT TREATED US. Patent Dec. 30, 1975 Sheet 9 of 12 3,929,517

ELONGATION,% REDUCTiON-OF-AREA,% YIELD POINT THEN 20% DRAWN I I I I l I AS HOT THEN 300C 350C 400C 450C ROLLED QUENCHED Y AT 880C AND TEMPERATURES AT WHICH DRAWN TEMPERED STEEL IS HEAT TREATED AT 500C EFFECT OF HEATING TEMPERATURE ON A Mn-Cr-B ALLOY IN THE TREATMENT OF THE INVENTION US. Patent Dec. 30, 1975 Sheet 10 of 12 3,929,517

T. w w 0 $292625 TwIcE 3 TIMES QUENCHING ONCE TwIcE 3 TIMEs TREATMENT OF THE INvENTIoN TggEfiJN TREATMENT OFTHEINVENTION (300C) (350C) QUENCHING ONCE TEMPERING (400C) FIG. 10 0 EFFECTS OF REPEATED TREATMENT OF THE INVENTION (Mn-Cr-B ALLOY) USO Patent ELONGATION, REDUCTION OF-AREA YTELD POINT TENSILE STRENGTH, kg/mm Dec. 30, 1975 Sheet 11 of 12 l l l QUENCHING ONCE TWICE 3TIMES TEMPERING TREATMENTOFTHEINVENTION QUENCHING TWICE 3TIMES TEMPERING TR A'FMENTOF THE INVENTION F|G.10b

EFFECTSOF REPEATED TREATMENT OF THE INVENTION (Mn Cr-B ALLOY) US. Patent Dec. 30, 1975 Sheet 12 of 12 3,929,517

400C 500C Y TEMPERATURES AT WHICH DRAWN STEEL IS HEAT TREATED THEN 200C 300C 20% DRAWN AS HOT ROLLED 4 r r I 1 u m J m a m L w m u m m n D X k yr P t mmmwmmmwmw mwmw wmw E? 665 559 E8 3w; wm -Lo-zO sDSm $29223 RELATIONSHIP OF MECHANICAL PROPERTIES OF DIFFERENT PORTIONS OFA COIL 0F AISI I043 STEELTO THE TREATMENT OF THE INVENTION PROCESS FOR PRODUCING A STEEL HAVING A SUPERB COMBINATION OF HIGH STRENGTH AND SUBSTANTIAL TOUGHNESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for producing a carbon steel or low alloy steel which has a superb combination of high strength and substantial toughness. More particularly, the invention is concerned with a process in which a steel is initially cold worked then rapidly heated and finally cooled.

2. Description of the Prior Art Generally, steels which are hot rolled have low tensile strength and a low yield point. Thus, in order to obtain steels with sufficient stength, various metal working and heat treatments have been necessary. For instance, it has been attempted to strengthen and toughen steel by refining the steel to high levels. It has also been attempted to attain these characteristics by cold reducing steel to the extent of 60-80% in area using a wire drawing mill in order to improve the strength of the steel and then subject the cold reduced steel to a bluing treatment. However, these procedures suffer from the disadvantage that the increase in steel strength is almost always accompanied by a reduction in the toughness of the steel.

A need, therefore, exists for a process for producing carbon steel or low alloy steel by a process in which the strength and toughness of steel can both be maintained or improved without a decrease in either of the important steel qualities.

SUMMARY OF THE INVENTION Accordingly one object of this invention is to provide a process for producing carbon steel or low alloy steel which is characterized by a superb combination of high strength and high toughness.

Another object of the invention is to provide a process for producing a carbon steel or low alloy steel wherein'the toughness is greater than that obtainable by cold working of the steel alone.

Still another object of the invention is to provide a process for producing a high strength carbon steel or low alloy steel which has a better surface smoothness or appearance than those steels which have been subjected to prior art heat treatment procedures.

Briefly, these objects and other objects of the invention as hereinafter will become more readily apparent can be attained by producing a carbon steel or low alloy steel, by subjecting the steel to cold working wherein the reduction-in-area is not less than The cold working induces a preliminary strain. The steel is then rapidly heated to a temperature of from 250C to the Al transformation point, and rapidly cooled after it had been held at the maximum temperature for less than 3 minutes. The strength and toughness as well as the surface appearance of the steel can be improved by subjecting the steel to sequential cold working heating procedures.

In the heat treatment, a heating rate of over 50C/min with a holding or dwelling time of no more than 3 minutes at the maximum heat treatment temperature and cooling rates, not less than 50C/min are used, while reduction-of-area values of 10 to 50% are maintained. The temperature used in the heat treatment may be divided into two ranges: the range of 250 which-are only hot rolled, andthe range of 450C to the A transformation point wherein no sacrifice in toughness occurs but high strengths are produced.

According to another aspect of this invention, a process is provided for producing a steel of the type described, which further comprisesan additional step of cold working to achieve a reduction-in-area of no more than 50% which improves the strength of the steel although the toughness decreases somewhat. (Hereinafter throughout the specification the cycle of cold working, rapid heating to a predetermined temperature while the temperature is held for several minutes, and rapid cooling will be referred to as the treatment of this invention) BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a graph of the mechanical properties of a series of hot rolled AISI 1043 steels which, by the process of the invention, have been subjected to 20% cold drawing, and thereafter have been rapidly heated to 200C, 300C, 400C, 500C, 600C and 700C, followed by rapid cooling.

FIG. 2 is a graph of the mechanical properties of AISI 1043 steels after repeating the cycle of 20% cold drawing, rapid heating to 600C and then rapid cooling wherein the individual steps are varied;

FIG. 3 is a graph of the NAKAMURA rotary bending fatigue characteristics of AISI 1027 hot rolled steel wires which have been subjected to 20% cold drawing, then rapid heating at a heating rate of 240C/min to a temperature of 550C which has been held for 15 seconds and rapid cooling;

FIG. 4 is two electron microscopic photographs (a) and (b) taken at a magnification of 22,500 times, wherein (a shows the structure of AISI 1027 steel prior to the application of the process, of the invention, while (b) shows the structure of AISI 1027 steel after it has been subjected to the process of the present invention, i.e., 43% cold working, rapid heating to 600C and then rapid cooling;

FIG. 5 is a graph of the mechanical properties of AISI 1027 steels which have been subjected to 20% cold drawing, rapid heating to a temperature ranging from 200C to 700C and then rapid cooling;

FIG. 6 is a graph showing the mechanical properties of AISI 1027 steels, after the cycle of cold working and heat treatment of the present invention has been repeated several times,

FIG. 7 shows two electron microscopic photographs (a and (b) taken at a magnification of 1000 X, wherein (a) shows the structure-of a steel sample prior to the application of the process of the invention, while (b) shows the steel after it has been subjected to the process of the invention;

FIG. 8 is a graph of the mechanical properties of Mn-Cr-B alloys which have been subjected to water quenching from a temperature of 880C, tempering at 400C, 20% cold drawing, rapid heating to a temperature of from 300C to 450C and then rapid cooling;

FIG. 9 is a graph of the mechanical properties of Mn-Cr-B alloys which have been subjected to water quenching from a temperature of 880C, tempering at 450C, cold working, rapid heating to a temperature of from 300C to 450C and then rapid cooling;

FIGS. 10a and 10b are graphs of the mechanical properties of Mn-Cr-B alloys after the alloys have been subjected to from one to three cycles of a metal treatment process wherein each cycle consists of water quenching the steel from 880C, tempering at C, 20% cold drawing, rapid heating to a temperature ranging from 300C to 450C and then rapid cooling; and

FIG. 1 l is a graph of the mechanical properties, such as elongation, percent reduction-of-area, yield point and tensile strength of a heat portion and an intermediate portion of a coil of hot rolled AISI 1043 stell versus the working conditions which are: as hot rolled, 20% cold drawing, and 20% cold drawing followed by rapid heating and rapid cooling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before a thorough discussion of the embodiments of the present invention, is presented, certain fundamental factors upon which the invention is based must be elucidated.

i. Reduction-of-area value The percent area reduction, which is achieved by a cold drawing or stretching a steel wire, should be limited to a range of from 10% to 50%. Percent area reduction of not less thanv 10% is preferred for steels which have been hot rolled and normalized or annealed, while percent area reductions ranging from 10 to 50% are preferred for steels which have been quenched and tempered. For hot rolled steels, at values less than 10% the rate at which the strength of the steel increases declines, while for quenched and tempered steel appreciable reduction 'in the toughness of the steels is observed at values over 50%, which can result in difficulties in cold working the steel. However, the upper limits of the present area reduction depends upon the toughness of the steel. On the other hand, steels which have been hot rolled, normalized or annealed have better cold workability in contrast to steels which have been hardened and tempered, which promotes percent area reductions of over 50% by cold working procedures. For this reason, the upper limit is not specifically established for hardened and tempered steels.

ii. Heat treatment:

a. Heat and cooling rate:

The second step of the process of the invention is a heat treatment step. Although the heat treatment conditions depend on the dimensions or shape of the steel, slow or gradual heating will result in diminished strength of the steel. Thus, heating rates of at least over 50C/min. are preferable for the purposes of this invention. In addition since slow cooling of the steel leads to a decreased rate of strength of the steel, cooling rates of over 50C/min are preferred to rapidly cool the steel in an oil or water medium.

b. Holding or dwelling time at the heat treatment temperature:

Holding times of no more than 3 minutes are preferred. Although the holding time of steels at high temperatures depends upon the dimensions of the steel, and the heating rate and temperature to which the steel material is heated, holding times in excess of 3 minutes initiates annealing of the steel, whereby the desired strain which is introduced into the steel by the preliminary working procedures is impaired.

c. Heating temperature range:

The heating temperature range used in the process of the present invention is from 250C to the Al transformation point of the steel. Temperatures below 250C do not result in an increase in the strength of the steel, while temperatures above the Al transformation point degrade the effects established by the cold working of the steel and introduce the likelihood of martensite formation.

It must be understood that the process of the present invention differs in principle and objectively from conventional stress-relieving or annealing treatments of steel wherein the working strain is relieved after the cold working of a steel material. More specifically, one basic concept of the present invention is the discovery that when steel which has been subjected to rolling or quenching and tempering subsequent to rolling, is cold worked, rapidly heated, and then rapidly cooled, the steel will be strengthened without any sacrifice in toughness, yet a smooth skin or surface will be maintained due to the lower heating temperature and shorter dwelling time at the heating temperature in contrast to those steels which have been subjected to prior art heat treatments. By this procedure better surface conditions of the steel can be achieved when the cold working and heat treatment cycle of the invention is repeated more than once in order to increase the strength of the steel. One reason for repeating this cycle is that variations in the mechanical properties of hot rolled steels can be reduced resulting in steels of stable quality. Steels treated by the methods of this invention have mechanical properties which are essentially equivalent to those steels which have been subjected to hardening and tempering. Moreover, the steel of this invention have greater strength without sacrifice in toughness.

In another aspect of the invention, the steels which have been treated by one or more cycles of the invention, i.e., cold working and heat treatment are subjected to additional working wherein the area of the steel is reduced to no more than 50%. If the area of the steel is reduced more than 50%, the toughness, which has been retained by the sequential process of this invention will substantially decrease, thereby exerting an adverse effect on the subsequent working and quality of the final product. Furthermore, when the area of the steel is reduced by cold working up to 50%, not only is the strength of the steel improved and beautiful surfaces features obtained, but also the dimensional accuracy of the steel is improved.

Illustrative of the types of steel which are employable in the process of the present invention are carbon steels having a carbon content of no more than 0.6% or low alloy steels such as Cr steels, Cr-Mo steels, Ni-Cr-Mo steels, B steels and the like. Particular useful are fine grains having an austenitic-crystal grain-size of over ASTM No. 6 and which have high strength and sufficient toughness. Preferably, steels are used which have about the same toughness as the previously mentioned fine grain steels. I Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified.

Table 1 shows the chemical composition of samples of A181 1024, AlSl 1027, AISI 1043 and Mn-Cr-B steels.

6 250C to 450C which gives steels which exhibit a slight decrease in toughness and strength, and the range between 450C to the Al transformation point which TABLE 1 Type of C Si Mn P S C B Austenitic steel grain size AlSl 1024 0.22 0.27 1.46 0.017 0.015 7.0 A151 1027 0.25 0.23 1.50 0.017 0.018 7.0 A181 1043 0.43 0.28 0.75 0.020 0.018 6.5 Mn-G-B 0.24 0.35 1.06 0.018 0.015 0.84 0.011 9.0

FIG. 1 shows the mechanical properties of a hot rolled AlSI 1043 steel which has been subjected to 20% cold drawing, then is rapidly heated to a temperature of 200C to 700C in increments of 100C at a heating rate of 240C/min in an electric resistance furnace, where the temperature is held for seven seconds at each of the temperatures and rapidly cooled in oil.

As can be seen from FIG. 1 the A181 1043 steel which has been subjected to the cold working and heat treatment cycle of the invention exhibits greater strength than does the hot rolled steel. The increasing rate of strength of the AIS] steel which is established by the cycle of cold working and heat treatment of the invention is most prominent in the range between 300C and 400C. When the heat treatment temperature used in below 300C, the strength of the steel is equivalent to that of a cold drawn steel wire. If the temperature of the steel is over 400C, the strength diminishes, while the elongation and percent area reduction are maintained. At a heat treatment temperature of 600C,

gives steels which show great strength without sacrificing toughness.

Steels can be heated in the heat treatment step by any one of several suitable methods which include resistance heating, high frequency induction furnace heating, lead bath or salt bath immersion heating. However, the heating rate governs the effect of the treatment of 20 the present invention.

Table 11 shows the mechanical properties of a hot rolled AISI 1043 steel which has been subjected to 20% cold drawing, then heated from 250C to a target temperature of 600C at an average heating rate of 5C/min. to 240C/min where it is held for 7 seconds,

and rapidly cooled in oil. As can be seen from Table 11, the faster the heating rate, the greater the strength. Thus, it can be seen that heating rates less than 50C/min do not give the desired increase in the strength of the steel. The tensile strength data shows that at heating rates over C/min, steels are obtained which exhibit improved strength.

TABLE 11 Effect of heating rate on mechanical properties (A181 1043 Steel) Heating Yield Point Tensile Elongation Reduction of (kg/mm) strength area values C/s6C 77.5 92.5 23.5 54.0 240C/min 76.8 92.0 23.8 54.2 180C/min 75.5 90.0 23.5 53.5 120C/min 73.5 88.5 24.5 53.0 50C/min 71.0 87.0 26.5 53.3 20C/min 50.0 74.0 28.0 53.5 5C/min 42.0 71.0 28.0 53.5

average heating rate from 250C to 600C gauge length 4\/ A the steel shows a strength equivalent to that of the cold drawn steel with the accompanying increase in toughness. At the heat treatment temperature of 700C, the steel shows a toughness equivalent to that of hot rolled steel, and increased strength. For these reasons, the heat treatment temperature in the process of the present invention should preferably be in the range between 250C and the Al transformation point of the Table III shows the effect of holding time on steel samples at a target heat treatment termperature on the mechanical properties of steels which are treated by 50 the process of the present invention. The samples used herein were subjected to 20% cold drawing, then heated to a temperature of 600C at a heating rate of 240C/min and held at this temperature for less than 5 minutes, and rapidly cooled in oil.

TABLE 111 Effect of the holdin time on mechanical pro erties (A181 1043 Steel) steel. The temperature range is more efficiently used when it is divided into two ranges, i.e., the range from As is clear from Table III, the longer the holding time for the heat treatment of the present invention, the less will be the effect of the heat treatment and the accompanying decrease in strength of the steel. However, holding times up to about three minutes will not adversely affect the strength of the steel samples.

Table IV represents the effect of the cooling rate on the mechanical properties of the steel. The samples used were subjected to 20% cold working, then heated to 600C at a heating rate of 240C/min. and held at this temperature for 7 seconds, followed by cooling at various cooling rates by methods such as oil quenching l0 240C/min in an electric resistance heating furnace and furnace cooling.

TABLE IV while being held at each temperature interval for Effect of the cooling rate on mechanical properties (AISI 1043 Steel) gauge length 4% As is apparent from Table IV, cooling rates less than 50C/min impair the toughness of the steel product and decrease the strength. In this manner, steel which has been cold worked has superior strength when rapidly heated and rapidly cooled without any impairment in toughness. However, if further improvement is desired, the treatment process of the invention can be repeated.

FIG. 2 represents the mechanical properties versus the number of treatment cycles repeated, ranging from 1 to 3 times, in which hot rolled AISI 1043 steel has been subjected to cold working, heating to a temperature of 600C at a heating rate of 240C/min while being held at this temperature for 15 seconds and then rapid cooling. By repeating the cycle of cold working, rapid heating and cooling, the tensile strength of the steel may be substantially improved. Normally, only one treatment cycle will not substantially improve the tensile strength of the steel over that of a steel which has been cold worked. However, with more than one treatment cycle, the tensile strength of a steel sample can be substantially improved. The heat treatment temperature of 600C is a good temperature value to improve the strength of steels treated repeatedly with the process of this invention.

FIG. 3 is a plot which shows the results of a Nakamura rotary bending fatigue characteristics test on AISI 1027 hot rolled steel which has been subjected to 20% cold working, rapid heating to 550C at a heating rate of 240C/min where it was held for 15 seconds and then rapid cooling. As is clear from FIG. 3 the fatigue limit and fatigue limit ratio are increased by a factor of 1.3 times as great as those steels which have the same strength.

FIG. 4 shows two electron microscopic photographs (a) of the structure ofa hot rolled AISI steel and (b) of the same steel which has been subjected to 43% cold working, heating toa temperature of 600C which is held for 15 seconds at a heating .rate of 240C/min, followed by rapid cooling. As can be seen from FIG.

3(1)), the cementite contained in the pearlite of the steel which has been subjected to1the cyclic treatment of the invention has been cut into severalpieces. The

seconds, and then rapid cooling.

FIG. 5 shows that a 20% cold worked tempered martensitic steel when treated by the process of the invention has an increase in yield point as well as an increased tensile strength. The plot shows that when a cold worked steel is rapidly heated and cooled at a maximum heat treatment temperature of up to 400C, the yield point and tensile strength of the steel increases up to that point. Beyond 400C, both the yield point and tensile strength diminish. Meanwhile the elongation and reduction-of-area values remain essentially constant up to 400C. When heat treatment temperatures of over 400C are used, the tensile strength of the steel begins to decrease while the elongation and reduction-of-area values begin to increase. The data indicates that at temperatures between 250C and 450C in the heat treatment cycle of the process, the elongation and reduction-of-area values of the processed steel are about the same as conventional cold drawn steel wires, while the yield point and tensile strength of the processed steel are higher than the cold drawn wire. When heat treatment temperatures between 450C and the Al transformation point are used in the process of the invention, the steels which result have greater strength, tensile strength, elongation and reduction-of-area values compared to steels which have been hardened'and tempered. The two temperature ranges from 250C and 450C and from 450C to the A1 transformation point preferably should be used separately depending on the quality of the steel required. As has been described, improvements in the yield point and tensile strength of a steel may be achieved by subjecting the quenched and tempered steel to cold working, rapid heating and then rapid cooling. However, if further improvements in the steel is required, the treatment cycle of the invention may be repeated as desired.

FIG. 6 shows the mechanical properties of an AISI steel 1027 which has been subjected to repeated treatment cycles each consisting of the steps of 20% cold working, rapid heating to 600C at a heating rate of 240C/min in an electric resistance furnace, where the maximum temperature is held for 15 seconds and then rapid cooling in oil. The plots show the. data obtained for one, two and three cycles of the invention. The test results reveal that improvements in the tensile strength and yield point may be achieved without sacrificing toughness by repeating the treatment cycle of the. invention.

FIG. 7 showstwo electron microscopic photographs (a) and (b), wherein (a represents the structure of a steel which has been subjected to a quenching and tempering treatment, while (b) shows the structure of a steel which has been subjected to the treatment cycle of the invention at a maximum heat treatment temperature of 600C. The photographs show that the steel processed by the treatment of the present inventionhas a finer and more uniform structure compared to the conventinally treated steel.

FIGS. 8 and 9 show the results obtained for a -hot rolled Mn-Cr-B steel alloy which has been subjected to oil quenching from 880C, tempering. in a lead bath at 400C (FIG. 8) and at 500C (FIG. 9)'and then subjected to the treatment cycle of the invention. The data shows that when a Mn-Cr-B alloy has been subjected to a tempering treatment of 400C and then to the treatment cycle of the invention at a maximum heat treatment temperature of 300C, the tensile strength and yield point of the processed alloy are improved by about kg/mm compared with a steel in FIG. 8 which has been subjected to only a refining treatment. The elongation plot particularly shows that the processed steel of this invention has superior elongation properties over the 20% cold worked steel. FIG. 9 shows that a steel which has been subjected to a heat treatment temperature of 300C subsequent to the tempering treatment at 500C exhibits a tensile strength and yield point superior to a steel'which has been only'tempered by about 10 kg/mm while the elongation and reduction-of-area values remain essentially constant.

of 20% cold working, rapid heating to temperature of 300C, 350C, 400C and 450C wherein each temperature was held for 25 seconds, and rapid cooling while the numbers of cycles to which the steel was subjected varied l to 3 times. FIGS. 10a and 10b show that the steel samples which have been subjected to a heat treatment of 400C and under exhibit greatly improved strength while the elongation and reduction-of-area values have decreased somewhat. However, steel which hasbeen subjected to a heat treatment temperature of 450C has essentially the same elongation and reduction-of-area values as those of a steel which has been tempered with increased strength. Furthermore, as shown .in FIGS. 10a and 10b, all of the properties of the steel are improved when the steel is subjected to repeated cycles of the treatment process. The surface conditions of the steel subjected to the process of this invention is-better than the steel wire which has been subjected to quenching and tempering. Furthermore, the surface condition is gradually improved when the steel is subjected to repeated cycles of the treatment of the invention.

As has been mentioned, the surface condition of the steel which is treated by the process of the invention varies depending upon several factors which include the manner in which the steel is heated such as in a high frequency induction furnace, an electric resistance heating furnace, a lead bath, a salt bath or the like; the heating rate, and the time at which the steel is held at the heat treatment temperature.

Table IV shows data which presents a comparison of j the surface condition or roughness between two steel samples and an AlSl I027 steel which has'been subjected to the treatment of the invention in an electric resistance furnace.

' TABLE VI Surface roughness (u) Hot rolled Quenched Steel subjectedjto repeated cycles Table V compares the mechamcal propertles of steel and tempered of the treatment of the invention stee steel wire which has been processed by the treatment of, 40 once twice three times the invention to the properties of a p ano wire (SWP- V) which is used in the valve of springs of automobiles.

TABLE Type of Yield Tensile Elongation Reduction Relaxation steel point strength 4 V A of area value (kg/mm) (kg/mm) value (2) 100 hrs.

Steel wire 170.1 172.0 12.0 48.7 l.3 subjected to the treatment of the invention Piano wire I465 I713 69 45.5 3.6 for use in the valve springs of automobiles (l) 350C X 15 minutes, from the collection ofNlPPON BANE KOGYO KAI (Japan Spring Industrial Association). Vol. [3, I967 (2) The failure load is adopted at an initial load of 70%.

As is clear from Table V, the steel processed by the 60 As is clear from Table VI, the surface roughness .of the.

treatment of the invention shows a greater toughness and has a greater relaxation value than does the piano wire. According to the treatment process of the invention, even steel which has been subjected to a quenching and tempering treatment can exhibit increased strength while the toughnessremains essentially consteel which has been subjected to the treatment of the present invention is notas rough as hot rolled steel which has been quenched and tempered. As shown, the surface roughness may be further improved by repeating the treatment cycle of the invention. 7 v

Generally, themechanical properties of hot rolled steel wires vary depending upon the cooling rates used in the treatment of the wire and the areas of the wire tested such as the head, tail and intermediate portions of the wire.

A study of the relationship between the mechanical 1 1 propertiesof steel wire and the effect of the treatment of the invention on the wire wasatte'mpted in which the head and intermediate portions of a hot rolled coil of AISI 1043 steel were subjected to 20% cold drawing, rapid heating to temperatures of 200C, 300C, 400C and 500C at'a heating rate of 240C/min., wherein each temperatures was held for 15 secondsand rapid cooling inoiLThe results'of the tests are shown in FIG. 11. As can. beseen from FIG. 11, there is a difference of about '1cg/mm in the yield pointand tensile strengthbetween the'head and intermediateportions'of the hot rolled steel wire. However, after subjecting the wire to treatment cycles of the invention,'(20% cold drawing, rapid heating and cooling), the variations in the mechanical propertiesof the wire decrease.'

Table Vll'represents the mechanical properties of a hot rolled A181 1024 steel which has been subjected to 20% cold drawing, rapid heating at a heating rate of -240C/min wherein the maximum temperature was held for seconds, and rapid cooling.

TABLE VII 10 --subjecting said steel to cold working whereby the area of said steel is reduced to at least 10 rapidly heating said steel to a temperature in the range of 450C to the A,transformation point, and

rapidly cooling said steel wherein the period at 15 which 'said steel is held at the heating temperature is less than 3 minutes and wherein the heating rate 1 is at least 50C/min.

2. The process of claim 1, wherein said treated steel is further subjected to cold workingso that the final area is reduced up to 50% of the original'area.

A181 1024 Steel gauge length 4 VA As can be seen from Table VII, the treatment of the invention at ,a heattreatment temperature of 400C gives a steel with amine-reased strength of about 20 kg/mm while the toughness of the steel is decreased.

-3. The process of claim 1, wherein said steel has a ferrite and pearlite structure and has been hot rolled prior to said process. 7 I

.4..The process of claim 1, wherein said steel has a Table Shows'the mechanical Properties of a steel 40 ferrite andpearlite structure and has been hot rolled wire which has been subjected to the treatment of the invention followed by an additional drawing step and those of steel wires which have been subjected to cold drawing ranging from 10% to 60%.

and tempered prior to said process. I

5. The process of claim 1, wherein said steel has been hot rolled, quenched and then tempered before being gauge-length As shown in Table VIII, when the steel which has been subjected" to? :the treatment'of the invention is cold' drawn, the strength is increased while the toughness of the steel decreases somewhat. If the cold working rate or the r'eduction-of-area' value of the steel exceeds 60%, a significant decrease in toughness is observed.

Having now fully described this invention, it will be apparent to one or ordinaly skill in the art that many subjected to said process and said cold working is accomplished such that the area of said steel is reduced from 10 to a used to attain said temperature is not less than 50C/min. v I

7.'The process of claim- 1, wherein saidsteel is held at said temperature for no more than 3 minutes.

6. The process of claim 1, wherein the heating rate 8. The process of claim 1, wherein the cooling rate is 9. The process of claim 1, wherein the medium used used in said rapid cooling step is not less than for said rapid cooling step is oil or water. 50C/min. 1: =1:

Claims

1. A PROCESS FOR PRODUCING A CARBON STEEL OR LOW ALLOY STEEL HAVING A COMBINED FERRITE AND PEARLITE STRUCTURE, WHICH IS CHARACTERIZED BY HIGH STRENGTH AND TOUGHNESS, WHICH COMPRISES THE STEPS OF SUBJECTING SAID STEEL TO COLT WORKING WHEREBY THE AREA OF SAID STEEL IS REDUCED TO AT LEAST 10%. RAPIDLY HEATING SAID STEEL TO A TEMPERATURE IN THE RANGE OF 450*C TO THE A1 TRANSFORMATION POINT, AND RAPIDLY COOLING SAID STEEL WHEREIN THE PERIOD AT WHICH SAID STEEL IS HELD AT THE HEATING TEMPERATURE IS LESS THAN 3 MINUTES AND WHEREIN THE HEATING RATE IS AT LEAST 50*C/MIN.

2. The process of claim 1, wherein said treated steel is further subjected to cold working so that the final area is reduced up to 50% of the original area.

3. The process of claim 1, wherein said steel has a ferrite and pearlite structure and has been hot rolled prior to said process.

4. The process of claim 1, wherein said steel has a ferrite and pearlite structure and has been hot rolled and tempered prior to said process.

5. The process of claim 1, wherein said steel has been hot rolled, quenched and then tempered before being subjected to said process and said cold working is accomplished such that the area of said steel is reduced from 10 to 50%.

6. The process of claim 1, wherein the heating rate used to attain said temperature is not less than 50*C/min.

7. The process of claim 1, wherein said steel is held at said temperature for no more than 3 minutes.

8. The process of claim 1, wherein the cooling rate is used in said rapid cooling step is not less than 50*C/min.

9. The process of claim 1, wherein the medium used for said rapid cooling step is oil or water.