DE3042067A1 - METHOD FOR MELTING STEELS - Google Patents

Description

3042087

The invention relates to a method for soft annealing of steels to improve their formability and machinability.

Soft annealing is a well-known technique for the treatment of steel «, It gets in the main used to soften the steel so that it can be machined or cut into parts in an economical manner can be shaped in a certain shape. Annealing is generally carried out by turning the steel in one The furnace, which is kept at the austenitizing temperature, is heated and it is cooled in a controlled manner outside the furnace. The steel is heated to a temperature above the austenitizing temperature, the so-called temperature below A3 heated and then cooled in such a way that it contains the so-called higher conversion products in its microstructure contains, namely perlite, block ferrite, spheroid carbides and combinations thereof *. The higher ones Transformation products are to be distinguished from the likewise well-known lower transformation products, namely Bainite, acicular ferrite and martensite. the higher conversion products are generally softer and more ductile than the lower conversion products. At the Annealing to improve machinability is therefore the goal of forming higher conversion products and substantially exclude lower conversion products.

Hot rolled steels are often used in large annealing furnaces annealed. The space and investment requirements for such large ovens are often disadvantageous for their use in the way. In addition, the use of such large ovens results in other well-known ones

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share. First of all is the thermal efficiency such furnaces are very low, making increasing fuel costs a more efficient method of heating the steel make appear desirable- Perner happens that Heating the steel in a furnace by radiation, conduction and convention, 30 that long times are required are in order to evenly treat a batch brought into the furnace »Such long times are already disadvantageous in itself, since the high temperatures used require the use of a non-oxidizing atmosphere, i.e. one? Require a protective gas atmosphere or a vacuum, the maintenance of which requires additional energy.

It is therefore desirable to use large annealing furnaces to avoid provided the physical properties of the annealed steel are then within acceptable Limits are «. In U.S. Patents 3,908 4-31, 4,040,872 and 4,088,511 have been proposed to apply steels in various thermal cycles handle electrical resistance heating. Such methods have the advantage that workpieces from Steel with high thermal efficiency very quickly and evenly over its said cross-sectional area let heat.

Electrical resistance heating has also been used in soft annealing processes, as in US Pat. No. 3,855 described. In this process, a cold-worked steel is rapidly heated by electrical resistance heated to a temperature above the A1 temperature, so that the ferrite present in the steel is converted into austenite. The carbides present become not dissolved in the resulting austenite, but are retained as a separate phase. The steel is then quenched down to room temperature to get the

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Austenite to be converted back into ferrite, so that only the Work hardening of the steel is eliminated and the carbides remain unchanged.

To obtain steels with improved ductility. ". and toughness, the annealing process can be modified in such a way that the steel quickly becomes over its austenitizing stone temperature lying temperature is heated, the heating rate so is chosen so that most of the carbides are dissolved in the resulting austenite, small carbide particles however, remain in undissolved form. The remaining carbide particles are sufficient Amount that they can serve as cores for the growth of higher conversion products on cooling, overall, the annealing process is shortened or accelerated «

One aim of the invention is thus to create an improved process for the soft annealing of steels.

A particular object of the invention is to provide an improved process for the soft annealing of steels which a steel is heated so quickly to a temperature above its A3 temperature that small Carbide particles remain in undissolved Porm and can serve as cores for the growth of higher 'transformation products, so that a steel of higher Maintains ductility and toughness.

Another object of the invention is to provide improved steels that have been annealed to achieve high ductility, formability and toughness »

The following are embodiments of the invention

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explained with reference to the drawing. Show it:

1 shows a graphic representation of the temperature profile over time when heating and cooling two steels to demonstrate the temperature dependence in an accelerated annealing process according to the invention,

2 shows a graphic representation of the temperature profile over time when heating and cooling various steels to demonstrate the time dependency the method according to the invention,

3A shows a photomicrograph of the structure of a 4-142 steel prior to treatment according to FIG Invention,

I ¹ Xg. 3B is a photomicrograph of the steel according to FIG. 3A after heating to 768 ^° C. and subsequent quenching,

30 shows a photomicrograph of the steel according to FIG. 3A after heating to 768 ^° C. and cooling in the air,

Figure 4 is a graphical representation of physical Properties (hardness and tensile strength) of an 8640 steel as a function of the austenitizing temperature,

Fig. 5 is a schematic representation of an apparatus for annealing of steels according to the invention using the Method E _t

6A is a photomicrograph of the microstructure of a 4140 steel prior to treatment according to FIG Invention,

Fig. 6B see a microfotografii of the steel according to FIG. 6A after annealing in the furnace,

6C shows a micro-photographic image of the steel. 6A after annealing using the method according to the invention,

ORiGENAL

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Figure 7 is a graph of impact strength one furnace annealed and one annealed using the method according to the invention 4140 steel as a function of temperature,

! ig. 8 is a graphical representation of the machinability of oven-annealed and using the invention Process annealed 4140 steels,

Prop. 9A is a photomicrograph of the microstructure a 4140 steel prior to treatment according to the invention,

! ig. 9B shows a photomicrograph of the steel ! ig. 9-A- after glowing in the furnace,

! ig. 9C shows a photomicrograph of the steel ! ig. 9A after annealing using the inventive Procedure,

! ig. Figure 10 is a graph showing the impact strength of one furnace annealed and one in use annealed by the method according to the invention 4140 steel as a function of temperature,

! ig. 11 is a graphical representation of the machining properties of one furnace annealed and one in use 4140 steel annealed using the method according to the invention,

! ig. 12A is a photomicrograph of the insert of 8640 steel before treatment,

! ig. Figure 12B shows a photomicrograph of the steel. 12A after annealing in the furnace,

! ig. 120 is a photomicrograph of the steel after Hg "12A after annealing, using the Verfa invention hr ens,

! ig. Figure 13 is a graph showing the impact strength of an oven annealed and an in-use one of the process according to the invention annealed 8640 steel as a function of the temperature,

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14A is a photomicrograph of the fine structure of a 6150 steel before treatment;

Figure 14-B is a photomicrograph of the steel 14A after annealing in the furnace,

14C is a photomicrograph of the steel 14A after annealing using the method according to the invention,

Figure 15 is a graph of impact strength one furnace annealed and one annealed using the method according to the invention 6150 steel as a function of temperature,

16A is a photomicrograph of the fine structure of a 1144 steel before a treatment,

16B is a photomicrograph of a steel according to FIG. 16A after annealing in the furnace;

16C is a photomicrograph of the steel according to Fig. 6 after annealing using the method according to the invention,

17A is a photomicrograph of the fine structure of an 86L20 steel before treatment,

Figure 17B is a photomicrograph of the steel 17A after annealing in the furnace and

17c is a photomicrograph of the steel 17A after annealing using the method according to the invention.

The invention is based on the knowledge that in hypereutectic steels by annealing a high degree of Having ductility and toughness achieved by quickly bringing the steel to a temperature above its upper transition temperature Heated lying temperature, so that austenite and carbides contained therein form, the heating rate is chosen so that most carbides are dissolved in austenite and small, usually approximately spherical carbide particles are obtained in undissolved form stay. After heating, the steel is

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chen speed cooled that doing the in unsolved Form retained in austenite carbide particles serve as cores for the growth of higher transformation products, especially pearlite and Block ferrite and iron carbide in the form of small balls. It has been shown that in this way Carbides formed when cooled have considerably smaller particle sizes than in a furnace annealing process formed, with the result that the finer microstructure of the steel annealed in this way compared to an oven-annealed steel Steel has vastly improved ductility, formability, and toughness.

The inventive method is for hypereutectic steels with a carbon content of up to ₅ O 7 wt.%, In particular from 0.1 to 0.7 wt.% Applicable. Such steels can contain relatively small amounts of common alloying elements such as chromium, molybdenum, nickel and / or manganese. A steel with a proportion of less than 5% by weight of such alloy elements is usually referred to as "low-alloy steel". Examples of hypereutectic steels which can be used in the context of the invention are given in Table I below:

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φ KN O 4- I. O I. O OJ VD O O KN J O V Φ O O O I. O r * d O CO O tr * O I. CTv O 4th IfN V " O O CM ί O O V O CM OO O I. O Ph O CTv O v- CO I. O I. KN I "I CO ΙΓ \ CO O CTv ITS VD O CT> O CTv CTv O O O ■ · O VD O O V O ITS O 4 · co ν- O KN V " LfN _ο V- O V- O D- CTv O V ia co O O D- V " O ITN O D- OJ VD CTv CM O D- CVl KN U OJ ο CM OJ O V LTN ο O O O V " O O O _j. O CM OJ O O OO V- KN • Η O O O 4 · OJ ■ = * & «Η O O O O O O O O O V- O OJ D- LfN • Η V O V- ■ V- O OJ O KN CM 03 O ο KN O ο KN O" O CM O O KN KN O OJ VD CO V " V " ί O CQ ON he" O O V " O O O O V O O KN O O V ν i- V CM
4 " V CO O Ph O O O O VD 6th co O
LfN co D- O ο O ο

O

KN CM CM O

O O

LTs CM CM VD

CM CVi VD VD O

IfN D-KN

V Q ο v ~ Q V V V "
V C.
V CVJ VD
ω V O CM O W. O
rvi ! V- O ¥ KD sample 86L? Ct <i sample sample sample co H sample sample sample sample sample

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3Ό42067

In the preferred application of the invention, the steel is in the form of a workpiece, which for itself can be heated alone, so that the progress of the heating can be precisely controlled. To this .Ατιförderung suffice, the workpiece is preferably one with a continuously constant cross-section, e.g. around a bar, a rod, a pipe and the like .. · ■ '.

In a preferred embodiment, the individual Workpiece, preferably while monitoring the temperature by means of a sensor device, by direct electrical Resistance heating or heated quickly by electrical induction heating. The rapid heating allows the economical handling of large quantities of workpieces and ensures rapid progress the austenite transformation »for carrying out the invention The method is particularly suitable for direct electrical resistance heating, according to the Description of US-PS 3,908 4-31, which here in this respect Is referred to. With this type of heating, an electric current is passed through the steel workpiece passed through. The electrical resistance of the workpiece causes it to heat up rapidly evenly over the entire cross-section.

When a workpiece is heated according to the method of the above-mentioned US Pat both ends connected to an electrical power source, so that the electrical current is complete flowing through it. Thanks to the steady flow of current through it, usually in the form of a bar or Rod present -Workstück results in a uniform temperature rise of the same in both axial and also in radio direction. So this is the interior of the workpiece and its outer areas at the same time

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heated so that no thermal stresses can develop. In contrast, in a conventional Oven heats the outer areas of the oven charge much faster than the inner area, causing the steel to enter the. The outer areas of the batch are completely converted into austenite, while the inner areas may, under certain circumstances, has not yet been converted to austenite. In furnace annealing it is extremely difficult, if not impossible, to to control the heating rate so that austenite transformation with a solution of only one part the carbides can be achieved. During furnace annealing there is a tendency that all of the carbides go into solution, so that no small carbide particles remain in undissolved form, which then become cores when cooling could serve for the formation of the higher conversion products.

In practicing the invention, a steel workpiece is heated very rapidly, usually within a: time between 1 second and 10 minutes, to a temperature above the upper transition temperature or A3 temperature. The heating can be controlled within relatively narrow limits by taking advantage of the well-known endothermic nature of austenite transformation. As is now generally known, the temperature of a workpiece remains constant when the austenite transformation starts or even decreases over a period of time between a few seconds and a few minutes, which to a certain extent depends on the heating rate. A typical heating curve to austenite transformation is shown in FIG. 1 for a 4-14-2 steel indicated in Table 1 as sample A. As can be seen in this figure, two steel workpieces 1 and 2 were rapidly heated to a temperature of 766 ⁰ C and 8-4-3 ° C. The heating curve shows that the solution of the carbides

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takes place when the rate of temperature rise remains constant, i.e. in the area of the so-called heat transition point. When this point is reached care must be taken that not all of the carbides go into solution in the austenite produced. It's an essential one Feature of the invention that sufficient amounts of carbides are retained in undissolved form, which can serve as cores for the precipitation of the higher conversion products when the workpiece is cooled.

The heating to the austenite transformation temperature is so only continued for a short period of time after reaching the heat transition point »During this period most of the structure is converted to austenite, so that which is present at the highest temperature The structure consists essentially of austenite and undissolved carbide particles * Bach's austenite uranium transformation the workpiece is allowed to cool in the air in an uncontrolled manner.

As can be seen in FIG. 1, the cooling of steel samples 1 and 2 is interrupted at 660 ° C. and / or. 471 ⁰ C. At this point the precipitation of conversion products begins. The interruption point of the cooling is therefore determined by the annealing temperature used. In the case of the sample 1, the austenite transformation temperature xrarde so gewä hl t that on cooling caused mainly higher conversion products while originated 2 lower transformation products during the cooling of the heated to a higher Temperatui ^ sample.

The full results of this study are summarized in Table 2 below:

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sample hardness
Brinell
(Ehn) Untreated at 311 Zugfe
sturdiness
kp / cm Flow
grenza
kp / cm strain
% reduction
% Annealed 3 min 212 10 756 5 434 14.1 37.2 766 ⁰ C, 7 761 4,492 24.0 59.7 36 sec at Annealed 3 min 235 776 ⁰ C, 8 935 4,949 18.5 4-1.6 58 sec at Annealed 4 min 266 804 ⁰ C, 9 504 5 223 17.9 40.2 26 sec at Annealed 5 min 282 843 ⁰ C, 10 355 6 741 16.1 42.2 2 sec

The data show an increase in hardness accompanied by a decrease in ductility with increasing annealing temperature.

From the above data it can be seen further that the accelerated annealing is the invention dependent on the annealing temperature according to "If the temperature is too high _v so the entire carbide goes into solution, so that then no seeds are present, the Ausfällurg the higher Accelerate conversion products.

It was also found that the time also the annealing process according to the invention affects _s because longer heating times cause all existing carbide particles go into solution. This phenomenon is shown in FIG. 2 on the basis of several samples which were kept at the annealing temperature for different periods of time before being cooled in air. Fig. 2 shows

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only the temperature profile during heating and cooling in the range above 593 ° C, within that the higher timeshift products are formed. In addition, the figure also shows the hardness values of each Rehearse.

How to do in ELg. 2 recognizes, the interruption point of the cooling decreases with an increase in the time span over which the samples are kept at the annealing temperature. This results in the tendency that instead of the According to the invention aimed at higher uranium conversion products to an increasing extent lower conversion products form, so that an increase in the annealing time thus results in an increase in hardness. After a glow period of 12 min arises within the in lig. 2 no interruption point in the cooling process so that the specimens in question have the greatest hardness.

The above-described IT tests with 414-2 steel samples show that the results obtained by accelerated annealing according to the invention both from the? Both the annealing temperature and the annealing time are dependent. To speed up the operations. To determine annealing, the fine structure of the steel was examined before annealing, at the annealing temperature and after cooling in the air. The structure of the steel in the unannealed and cooled state could be examined using conventional metallographic methods. A classic metallurgical quenching process was used to observe the austenite state at the annealing temperature. For this purpose, a steel sample A was quickly ^{heated to 768 °} C. and quenched in moving water. As a result, parts of the structure that were present as austenite were converted into martensite before quenching. Using an etching agent that does not attack the martensite

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₁₉ '3Ü42067

by means of the austenite structure could then be at room temperature can be observed using conventional metallographic techniques.

In! "Ig. $ A, 3B and 5C, the structure of a steel is the Sample A in the unannealed condition, in the annealed and quenched condition or in the annealed and air-cooled condition State shown. Microphotography was used to show the extremely fine structure of the samples a scanning electron microscope (SEM) is used.

The microphotographs clearly show that the structure that was present before the treatment was converted into austenite by the rapid heating, with some carbide particles in undissolved form retained in austenite. Since condensation nuclei were already present in the austenite structure, there was one longer maintenance of the annealing temperature for the Core formation through higher conversion products is not required. When the temperature drops below the Al temperature the retained carbide particles began to grow, with pearlite then attaching to the carbide cores formed. Correspondingly, the time required to anneal the steel has been reduced considerably «For similar Investigations of various other types of steel were found in each case that the austenite structure consisted of austenite with small spheroidal carbide particles. The lag caused by the rapid heating of undissolved carbide particles in the austenite structure can be the cause of the with the erfindungsgeraäße Procedure achieved beneficial results are considered.

The retention of undissolved carbide particles in the austenite structure of steel has already been mentioned in the literature. With slow heating, the amount of austenitic

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^: 3Ü42067

However, the structure of the remaining carbide particles is low. One slowly heated to the austenite transformation stone temperature Steel therefore hardly shows the properties that result from the rapid annealing. In different Experiments have shown that the phenomena occurring during accelerated annealing in Furnace annealing does not occur.

In one such experiment, bars of 8640 steel were used of sample B annealed at different temperatures in the furnace and then cooled in air. Afterward rods of the same sample were annealed by electrical resistance heating and also cooled in air. The mechanical properties resulting from these types of treatment are shown in Fig. 4-. the Oven-annealed samples showed a relative appearance after cooling large beards on »whereas in the faster heated samples there were clear differences in hardness in the air-cooled state are recognizable. The mechanical properties of some samples from this test are summarized in Table 5. The furnace-annealed steel has mechanical properties very similar to those of the unannealed steel, while the quick annealed sample is considerably softer. So the steel speaks of the accelerated Anneal in the same way as the 4142 steel examined above. The one from the accelerated glow caused phenomena occur with 8640 steel even more clearly because this steel has a lower hardenability than the 414-2 steels

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sample
8640 steel

Table 5

Brinell hardness Tensile strength-p Flow- (Ehn) speed kp / cm limit

kp / cm

Elongation reduction %%

Untreated 256

Furnace annealed at 815 ^, 254 air-cooled time: 1 h

Rapid annealed at 815 O, 205 air-cooled, Time: 4- min

9 329 6 629 15.1 39.1 9 287 6 763 15.5 4-7.0

7 810 4- 998 20.0 50.8

The investigations show that the result of the accelerated annealing is dependent on the annealing temperature, because at temperatures above 843 ⁰ C schnellgeglühte samples did not get the desired properties. The investigations also show that the result of the accelerated annealing also depends on the annealing time, since none of the oven-annealed samples obtained the desired properties on cooling in air, regardless of the annealing temperature. The furnace annealing is simple, too slow for the phenomena caused by the accelerated annealing to occur. Maintaining the austenite transformation temperature for a relatively long time leads to the fact that the carbide present goes into solution or the particle size of the carbide is reduced to such an extent that a sufficient quantity of growth nuclei is no longer present during the subsequent cooling.

In one embodiment of the invention, it may be desirable when machining large workpieces Series to ensure a uniform progress of cooling. For example, steel rods are heated and then simply stacked in a rack to cool down,

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so the bars stacked first could cool down considerably faster than the bars stacked last, so that the individual rods of such a series receive different properties. To avoid this, you can use For cooling the workpiece, an insulated drain rack of the type shown in FIG. 5 can be used. the Annealed rods pass through such a frame with a dwell time of, for example, 10 mine Such a device does not need any external heat supply, so that no energy is consumed will. Investigations carried out with regard to the uniformity of the mechanical properties have shown that the cooling in the insulated drain rack gives good results.

In the wake of the above explanation of the basic principle of the invention are hereinafter practical embodiments of the invention as applied to soft annealing each of a length of approx. 2.1 described in having steel rods. In each example, the Steel examined in three states, namely in the untreated State, in the oven-annealed state and in the state annealed using the invention, wherein In each case, comparisons are drawn between the furnace-annealed steel and the steel annealed according to the invention.

example 1

This example concerns the annealing of a 4-140 steel of sample C according to table Ίο

Twenty bars of steel 414-0 of the sample C were annealed in a roller hearth kiln, "The annealing temperature was 84-3 ⁰ C and the treatment time was a total of 16 h.

Furthermore, twenty rods of the? same sample under

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Applying the method according to the invention annealed. The annealing temperature was 788 ⁰ C, at a treatment duration of 33 see. The total treatment time for the twenty rods was less than 1 hour.

After annealing, the bars of both batches were cleaned, cold drawn and straightened. Then the Rods of both batches subjected to intensive tests. The results of these tests are summarized in Table 4- * The mechanical properties of the untreated and the annealed steel are for comparison purposes juxtaposed.

The steel annealed using the method according to the invention has a better combination of properties than the furnace-annealed steel. Although it has a somewhat greater hardness, the main difference between the two steels lies in the improved ductility of the steel annealed according to the invention of the steel was ofengeglühten after the treatment is 13.3% "that of the annealed steel of the invention, however, 17% _{t 5} 0 which is an improvement by 28%. Further, the cross-sectional reduction was at ofengeglühten steel 39 _s O% value and annealed steel according to the invention 59.3% * an improvement by 52% equals ". The elongation and cross-section reduction are a measure of the ductility of a steel. The improvements shown above compared to a furnace-annealed steel therefore indicate an obvious improvement in ductility and formability.

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Table 4

Brinell

Steel C
4140 hardness
(Bhn) Tensile strength-p
kp / cra Flow
border
kp / cm strain reduction Not annealed 315 10 615 8 471 13.0 39 Λ Furnace annealed 197 7 009 3 276 21.5 40.8 Annealed according to
invention 217 7 592 4 126 23.5 60.9 Furnace annealed +
cold drawn 226 7 838 5,708 13.3 39.0 Annealed according to
Invention and
cold drawn 238 8 393 6 601 17.0 59.3

The cause of the improved ductility of the steel annealed according to the invention can be clearly seen from the fine structure of the steel samples. 6A, 6B and 60 show the fine structure of the 4140 steel in three states, namely unannealed, furnace annealed and annealed according to FIG
Invention. In the unannealed. State consists of the structure of lower conversion products, namely higher bainite
and needle-shaped ferrite * The structure of the furnace-annealed
Steel consists of pearlite and ferrite «The structure of the
In contrast, according to the invention, annealed steel consists of ferrite, pearlite and small spheroidal carbide particles. The ferrite areas are not clearly delineated and contain small spheroidal carbide particles. Further
the steel annealed according to the invention has a smaller grain size «. This finer structure gives the steel improved ductility and formability compared to the coarser structure of furnace-annealed steel.

The finer microstructure gives the annealed steel
also increased toughness. In Fig. 7 is the
Notched impact strength determined by the Charpy method of bars removed from the two annealed batches is shown. The steel annealed according to the invention has

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has a lower transition temperature as well as an upper threshold energy which is nearly three times that of that of the furnace-annealed steel. The increased toughness is particularly valuable in those cases where a part is formed by cutting or non-cutting and then only surface-hardened »In these cases it is bestowed an increased toughness of the interior of the part improved breaking strength

To demonstrate that the improved toughness and ductility of the steel annealed according to the invention its machinability compared to that of an oven-annealed steel A comprehensive machinability test was carried out. The Steels machined with different types of tools. The results of the machinability test of the two Sample C 4140 steels are shown in FIG. In This test measures the increase in the diameter of the machined parts and as a function of time or the number of machined parts. Well-machinable steels show a relatively flat shape, close to the Time axis running growth curve »For poorly machinable In contrast, the curve rises steeply for steels »Die in Pig. 8 curves show * that the both annealed steels are about equally easy to machine. The steel annealed according to the invention is somewhat better machinable than the oven-annealed one, but the difference is insignificant.

Example 2

This example concerns the annealing of 4-140 steels of the Sample D in Table 1.

Twenty bars of 4-140 steel of the sample D were ofengeglüht h at 843 ⁰ C in a duty cycle of the sixteenth Further

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twenty bars of Sample D were annealed in accordance with the invention. This took place at a temperature of 815 ^° C. for about 36 seconds each time, so that the annealing of the twenty rods was completed in less than an hour.

The bars were then descaled, cold-drawn and straightened. The samples were extensively examined, and any leftover steel was used for a machinability test. The mechanical properties of the unannealed, the furnace annealed and the steel annealed according to the invention are summarized in Table 5. The steel annealed according to the invention again had a higher ductility and was somewhat harder than the furnace-annealed steel. Pig. 9A 9B and 90 show the _S 3? Turned add this steel in three states, namely, unannealed, according to the invention ofengeglüht and annealed. As in the previous example, the steel annealed according to the invention has a considerably finer microstructure than the furnace-annealed steel

Table 5

Brinell

Steel D hardness Tensile _p HLieß- 4140 strain reduction (Bhn) ness kp / cm cross% ©%

kp / cm

Unannealed 311 10 637 7 705 13.3 41.9 Furnace annealed + 231 8 373 7 192 12.1 42.8 cold drawn

Annealed to 241 8 956 7 444 13.8 53.1 Invention +
cold drawn

10 shows those determined by the Charpy impact method Toughness curves for the furnace-annealed steel and the steel annealed according to the invention. The superiority of the steel annealed according to the invention is also evident here. Its transition temperature is again low

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riger and its upper threshold energy higher than that of the furnace-annealed steel.

The machinability test of the two annealed steels was carried out using a modified Taylor tool life method. In this process, a workpiece is used different speeds and feed speeds turned down until the turning tool fails, the times up failure of the tool at different speeds. in a logarithm! see coordinate system applied. This results in a straight line which shows the relationships between the working speed and represents the time to failure of the tool, lig. 11 shows the result of the machinability test of the two annealed 4140 steels of sample D. From this, that the two straight lines cross each other, it can be seen that there is a difference in machinability of the two steels. At low working speeds, as is usual for alloy steels the steel annealed according to the invention is somewhat better machinable. So although the annealed according to the invention Steel, on the other hand, is harder, tougher and more ductile than that oven-annealed, it is easier to machine than this one. These Differences in ductility, toughness and machinability together produce a significant improvement the mechanical properties of this steel.

Example 3.

Ten rods of a 4140 style from sample E in Table 1 were oven-annealed at 843 ° C in a working cycle of 16 hours. Another ten bars of the same steel were made according to the invention at a temperature of 787 ° C annealed. The individual rods were annealed in each case in 35 seconds, the total duration of the treatment was 45 minutes. The results of the treatment are summarized in Table 6.

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collected. The test pieces were not cold stretched after annealing. The rehearsals spoke essentially in the in the same manner to the annealing according to the invention as the samples examined above. From the in the In the data contained in Table 6, the improved ductility of the steel annealed according to the invention is evident recognizable.

Table 6

Brinell Annealed according to 194 Tensile strength o Flowing strain reduction Steel E. hardness invention speed kp / cm border
kp / cm % % 4140 (Ehn) 9 954 6,974 15.7 50.2 Not annealed 269 7 290 3 487 19.7 42.6 Furnace annealed 186 7 346 6 826 24.1 60.5 Example 4

JFive ten bars of sample J 4142 steel? were in a duty cycle of 16 h at 843 ⁰ C ofengeglüht. Another fifteen bars of the same steel were annealed in accordance with the invention. The annealing temperature was 787 ⁰ C for a duration of 60 see "The entire work cycle took less than an hour. Table 7 shows the mechanical properties of the steel in three states, namely unannealed, furnace annealed and annealed according to the invention. The steel annealed according to the invention in turn has an improved ductility compared to that of the furnace-annealed steel.

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Steel F
4142

Table 7

Brinell

Hardness. Tensile strength _p

(Bhn) speed kp / cm

Flow strain reduction limit%% kgf / cm

Annealed 268 9 912

Furnace annealed 194 7 156

Annealed according to

Invention 196 7 192

6,967

3,536

4,654

15.5 20.8

25.0

46.5 46.6

68.3

Example J?

Ten bars of the 8640 steel B according to Table 1 were ^{annealed in a working cycle of 16 h at 843 °} C. in a roller hearth furnace. Another ten bars of the same steel were annealed in accordance with the invention. The annealing temperature was 787 ⁰ C for a duration of in each case see 35th The entire working cycle lasted about 30 minutes.Table 8 shows the mechanical properties of the steel in three states, namely unannealed, furnace-annealed and annealed according to the invention. The steel annealed according to the invention in turn has an improved ductility compared to that of the furnace-annealed steel.

Table 8

Brinell

Steel B hardness tensile strain reduction _O flow 8640 (Bhn) ness kp / CNT cross% ©%

kp / cm

Annealed 258 9 286

Furnace annealed 176 6 953

Annealed according to

Invention 180 6 953

7 227

3,466,4246

16.7 22.8

28.1

46.9 47.0

64.6

The fine structure of the unannealed, the furnace-annealed and the steel annealed according to the invention is shown in FIG. 12A, 12B and 12C, respectively. The steel annealed according to the invention has a more spheroidal and somewhat

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finer structure than the furnace-annealed steel. There is therefore a similar difference in the IF-structure as in the examination of the 4-14-0 steel. The 8640 steel was also subjected to an Oharpy impact test toughness. The results of this test are in Pig. 13 shown. The steel annealed according to the invention again has a far superior toughness. It should be noted that the 864-0 steel was not cold-drawn before the test. It is therefore somewhat softer and tougher than the 4-140 steels described above.

Example 6

Twenty bars of the 6150-style G according to Table 1 were made in a working cycle of 16 h at 84-3 ° C in one Annealed roller hearth furnace »Another twenty bars of the same Steel were annealed according to the invention at a temperature of 815 ° C. The glow lasted for each stick 34- see, approx. 1 h in total. The mechanical properties of the unannealed, dee oven-annealed and des annealed according to the invention. Stahls are summarized in Table 9.

The annealed bars were then cold drawn and straightened, as is the case with certain types of industrial ones Processing is happening. The properties of the cold drawn steel are also given in Table 9. The steel treated according to the invention is one of these Grade is slightly harder than furnace-annealed steel, but has better ductility. This occurs both before and after cold stretching.

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Table 9

Brinell
Steel G hardness
6150 (Ehn) Tensile strength-p '
speed kp / cm Flow
border
kp / cra strain
% reduction Annealed 299 10 615 7 845 12jO 37.8 Furnace annealed 195 7 086 3 726 20.7 44.1 Annealed according to
Invention 225 7 719 5,708 25.5 66.4 Furnace annealed +240 8 464 6 046 9.6 31.3 cold drawn Annealed in accordance with
Invention + 263
cold drawn 9 202 7 297 13.0 48.3

The structure of the unannealed, furnace-annealed and according to the invention annealed steel is shown in I ¹ Ig * 14A, 14B and 14C respectively. Two samples of the annealed steel were subjected to a Charpy impact test toughness. The results are in JTig. 15 shown. They relate to the 6150 steel after cold stretching * The steel annealed according to the invention again has a finer microstructure, improved ductility and higher toughness than the furnace-annealed steel.

Example 7

Some bars of the 1144 steel H according to Table 1 were furnace-annealed in a working cycle of 5 hours at a temperature of 843. A further five bars of the same steel were annealed according to the invention. The annealing temperature was 787 ⁰ C for a total treatment time of 20 minutes for the five bars "The treatment of the individual rods lasted see each 30th

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- 32 - '

Table 10 shows the mechanical properties of the unannealed, the furnace annealed and that according to the invention annealed steel. The hardness of the steel annealed according to the invention is here close to that of the Furnace Annealed Steel As in the previous examples, the steel annealed according to the invention has a superior one Ductility on. Figures 16A, 16B and 16C show the fine structure of the steel in unannealed, furnace-annealed or annealed according to the invention

Table 10

Brinell

Steel H hardness tensile strength _p flow elongation reduction 1144 (Bhn) speed kp / cm limit. %%

kp / cm

Not annealed
(hot rolled) 190 furnace annealed 162 annealed according to

Invention 165 6 264-4 035 25.0 4-8.5

Example 8

Several bars of 36L20 steel I according to Table 1 were oven-annealed at a temperature of 885 ° C. in a working cycle of 5 hours. More fifteen bars of the same steel were annealed according to the invention, the annealing temperature was 871 ⁰ C for a duration of each see the 31st The entire work cycle lasted 4-7 minutes «

6th 960 4- 281 19th , 5 41 , 4 6th 292 3 606 23 -Λ 41 ,1

Table 11 shows the mechanical properties of the unannealed, of the furnace annealed and the steel annealed according to the invention. With this type of steel, the Annealing according to the "method according to the invention" only one relatively small improvement in ductility achievable. In addition, the hardness is that annealed according to the invention Steel pretty high. The reason for these differences

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be seen from in Fig. 17A, 17 ^B and 17C of the recordings shown mikrofotografisehen microstructure of this steel. The steel annealed according to the invention has a much finer grain structure than the furnace-annealed steel. In the case of a low-carbon steel such as the 86L20 steel, the fine grain structure resulting from the process according to the invention is the dominant factor. The steel does not contain enough carbon for the carbides to play a dominant role. Due to the refinement of the grain structure, the steel annealed according to the invention is therefore somewhat harder than the furnace-annealed steel. Accordingly, only an extremely small improvement in ductility can be achieved when using the method according to the invention.

Table 11

• Brinell

Steel I _o hardness tensile strain reduction flow 86L20 (Bhn) ness kp / CNT limit%%

kp / cm

Unannealed 1? 2 5 849 3 7 * 7 25.2 62.4 Furnace annealed 141 5 265 3 269 29.7 62.1 Annealed according to

Invention 160 5 757 4-225 3O ₅ O 65.2

The above examples show that the inventive Process for the most varied of carbon and alloy steels is applicable- The investigated Eroben responded to the annealing method according to the invention in roughly the same way. In every alloy was creates a finer carbide structure, which gives the steel improved ductility, formability and toughness. These improved properties were noted without any loss in strength or machinability achieved. This combination of improved ductility, achieved without impairing machinability, Malleability and toughness is a surprising result.

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Usually causes an improvement in ductility and toughness, for a given hardness, a deterioration in machinability. When using the inventive However, the process creates a structure that does not follow this general trend.

From the foregoing it can be seen that the invention provides a significant improvement in the annealing of hypereutectic steels. By using direct electrical resistance heating, the invention ensures a very good degree of energy efficiency and at the same time eliminates the need for the ₉ long and controlled cooling periods required for furnace annealing of steels. In addition, the method according to the invention makes it unnecessary to use a non-oxidizing protective atmosphere that is necessary for the furnace annealing processes that have been used up to now *

The implementation of the method according to the invention and the facilities used for this can be carried out in various ways Modifications can be made without thereby departing from the scope of the invention as defined by the claims.

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Claims (18)

30420a? DIPL-ING. HANS H. HILGERS PATENTANWALT D-8000 MÜNCHEN Maximilianstrasse 43 Telephone (O 89) 222862 Telex 05-29 38O Fax (089) 222862 Postscheckkonto Munich No. 25571-8O9 Deutsche Bank Munich No. 46/29226 Your sign / Your Ref. Main sign / Our. Ref.Dalum / Date H-P 169? * November 1980 LaSaIIe Steel Company P.O. Box 6800-A, Chicago, Illinois 60680 USA Process for soft annealing of steels P_a t e s._t_a 5: _s_g_r_ü_c_h_ e 1. Process for soft annealing, of steel _v thereby marked that you can see a hypereutectic Steel to form austenite and carbides quickly to a temperature above its upper transformation temperature lying temperature, whereby the heating 21/0838 speed is chosen so that most of the carbides are dissolved in the austenite and small earbid particles in
one to form nuclei for the precipitation of
higher conversion products in sufficient quantity in
left unresolved lorm, and that you get the steel
cools to precipitate the higher conversion products
and an annealed steel of improved ductility,
Maintain malleability and toughness. 2. The method according to claim 1, characterized in that the hypereutectic steel up
Contains 0.7% by weight of carbon, 3. Method according to claim 1, characterized in that the steel contains 0.1 to 0.7% by weight of carbon contains. 4. The method according to claim 1, characterized in that the steel is less than 5 wt.% One Alloy element contains. 5. Method according to claim 4-, characterized in that the alloying element chromium _{5 is} molybdenum, nickel, manganese or a combination thereof « 6. The method according to claim 1, characterized in that the steel in less than 10 min
is heated to the upper transition temperature. 7 · The method according to claim 1, characterized in that the steel by direct electrical Resistance heating is heated. 8. The method according to claim 1, characterized in that the steel is in the form of a workpiece
is present with continuously the same cross-section. 130021/083 9. Procedure according to. Claim 1, characterized in that the steel in the absence of a inert atmosphere is heated » 10. A process for the soft annealing of steel, characterized in that one has a hypereutectic Steel to form austenite and carbides to above its upper transition temperature in less than 10 minutes heated lying temperature, so that most of the carbides are dissolved in the austenite and small Carbide particles in sufficient size to form nuclei for the precipitation of higher conversion products Amount remain in undissolved form, and that one cools the steel to precipitate the higher conversion products and an annealed steel of improved Maintain ductility, formability and toughness. 11. A method for annealing steel, characterized in that one has a hypereutectic Steel to form austenite and carbides evenly over its cross-sectional area, on one over Its upper transition temperature lying temperature is heated, the heating rate selected is that most of the carbides are dissolved in austenite and act as nuclei for the precipitation of higher transformation products suitable, small carbide particles are retained in undissolved form, and that the steel can be precipitated the higher conversion products cools down » 12. Process for soft annealing of steel with high thermal efficiency, characterized by g e k e η η, that you can use a hypereutectic steel to form austenite and carbides Passing electric current to a temperature above its upper conversion stage heated, the heating taking place in less than 10 minutes, so that most of the carbides dissolved in the austenite 130021/0838 3042087 and small carbide particles are obtained in undissolved form in an amount sufficient to form nuclei for the precipitation of higher conversion products stay, and that. the steel is cooled to precipitate the higher conversion products. 13. The method according to claim 12, characterized in that the steel is air-cooled. 14-. Process for the soft annealing of steel, characterized in that a hypereutectic Steel quickly to one above its own for a time sufficient to form austenite and carbides the upper transition temperature is heated, so that the carbides remain undissolved in the austenite and in one to form nuclei for the precipitation of higher conversion products are present in sufficient quantities, and that the steel is cooled to the higher Precipitate transformation products and obtain a calibrated steel. 15 · Method according to claim 14, characterized in that g e k e η η, that the steel is heated by direct electrical resistance heating in less than 10 minutes. 16. A method for annealing steel, characterized in that a ferrite and Hyporeutectic steel containing carbides for transformation of the ferrite in austenite is heated to a temperature above its upper transformation temperature, wherein the heating rate is chosen so that most of the carbides are dissolved in the austenite thus formed and small carbide particles in one to form cores for precipitation of higher conversion products sufficient amount is retained in "undissolved form, 130021/0838 BATH ORIGINAL and that the steel is cooled to produce the higher uranium conversion products precipitate and obtain a soft-annealed steel. 17. The method according to claim 16, characterized in that the soft-annealed steel is pearlite, Contains ferrite and spheroid carbides. 18. A method for annealing steel, characterized in that one uses a ferrite and Carbide-containing hypereutectic steel for converting the ferrite to austenite at a level above its upper Transformation temperature lying temperature heated whereby the heating takes place at such a rate that small carbide particles dissolved in the austenite are formed and the carbides in a product for the formation of cores to precipitate higher transformations sufficient amount are present, and that the steel is cooled to precipitate the higher conversion products. 130021/0838