AU463543B2 - Heat treatment process for low-carbon steel - Google Patents

Description

2 8,2 93 This invention relates to a heat treatment process

for low-carbon structural non-alloyed steel or structural

steel (max. 0.26% C) with a maximum manganese alloy content

of the steel in the form of bars being continuously

passed through a quick-heating zone using e.g. electron rays, to but in particular induction, as a heat source to be heated

high temperatures and subsequently being quenched.

The physical properties of non-alloyed steel depend

first of all essentially on the carbon content, i.e. except

for the heat treatment condition or cold deformation degree. and With an increasing carbon content, the tensile strength

the elastic limit increase, while the breaking elongation

decreases (cf "Werkstoffhandbuch Stahl and Eisen", 4th edition,

1965, G 1, in particular pictures 1 and Increased

strength properties are obtained by hardening and tempering

in relationship to the carbon content. Generally, a minimum to carbon content of more than 0.30% by weight is considered

be necessary for conducting this heat treatment for non- thereby alloyed heat-treatable steel, but the weldability is

impaired or eliminated.

Attempts have been made to develop processes in order

to improve the physical material properties of non-hardenable

steel.

So for instance a process is known from "Stahl und alloy elements Eisen" 1949, pages 186-194, in which a saving in St 52, .was to be permitted for structural steel of the type St 52, in which steel with a carbon content in accord with for but with lesser cortents of manganese and other elements

increasing strength was after a warm rolling into sheets,

quenched in water from rolling heat.

2 463543

28.2 93ni 0.

For a steel having 0.18% C and/ an increase

in strength resulted to the desired degree but with a

concurrent decrease of the elongation. This process has

not been able.to.find practical acceptance, in particular

because the.values obtained were subject to fluctuations.

Furthermore, late in the fifties a heat treatment

process originating from laboratory tests became known, for a

low-carbon steel wire, the steel wire subsequently to be

further processed in a cold condition. According-to this

process steel wires of 8 down to 2 mms in diameter were

inductively heated to high temperatures and subsequently

quenched by water. With a rising quenching temperature, an

increase in the tensile strength and a decrease in the elongation

resulted. These tests have apparently not resulted in a

manufacturing-scale process.

Furthermore, a process for hardening and then tempering

articles throughout which are composed of non-hardenable steel

is known from Austrian Patent Specification 281,089, wherein

steel having a carbon content of about 0.10 to 0.23% is

treated as for surface hardening by means of gas/oxygen burners

or electrical heating devices, water showers being provided

downstream thereof in a continuous process maintaining a

minimum advance rate of about 100 to 200 mms per minute.

The heat treatment described there results in an increase in

hardness, the hardness differential between the surface and

-the core in the non-tempered condition being broadly balanced

in the subsequent tempering operation depending on the

strength required, so that reference can actually be had to a

tempering throughout.

463.543

28,2 93ni which Contrary to the above-mentioned prior art specific material exclusively results in the increase of a increase of property by sacrificing another, for instance, the elongation, the strength accompanied by a decrease of this conventional this invention is directed towards obviating

counter-action of specific material properties. The solution of this problem is, of particular since the safety of interest for concrete reinforcing steel, increased. A buildings could thereby be substantially strength properties, structural steel mat having conventional of area 6g) greatly but with its elongation before reduction give a much greater increased over the above prior art would safety. yieldability to the building along with a sufficient would be of On the other hand, a great strength increase elongation rates in interest along with still satisfactory

particular for prestressed concrete reinforcement. heat The present invention provides a continuous 0.26% C) non-alloy treatment process for low-carbon (max. manganese alloy content steel or structural steel with a maximum through of the steel in the form of a bar being passed high temperature and a quick-heating zone to be heated to a steel is heated in subsequently being quenched in which the a temperature of between a 10 to 70% cold deformed condition to such a way that the heat 600 to 13000°C in its outer region in the central region of introduced into the outer region heats 1000C/s to a temper- the bar at an average rate of at least Ac3; and the heated steel is ature within the range from 450 C to having undergone any phase quenched without the central region

transformation a-y--a quenching being at the earliest

4 463.543

28,2 93n started when the central region reaches 450 0 C and at the

latest prior to a reduction of the outer surface temperature

to below 555 0 C.

The neat content is introduced into the outer region

through the adjustable parameters of the quick heating source

(such as power density and penetration depth), the adjustable 2 parameters for induction are, e.g. power density in W/cm

and frequency.

The time.at which the heated steel is quenched will

be described hereinafter with reference to a temperature

balancing range (as hereinafter defined as the range

tl t:.t 3 according to the Figure), the time initiation value

(as hereinafter defined t-tl according to the Figure) of which

range is at the core temperature of 450 0 C."

Preferably the quenching operation is effected with

water, in particular water jets.

The cold deformation degree of the treated steel is

preferably from 30 to

Preferably the heat treatment is conducted on steel

having from 0.06 to 0.20% carbon, preferably up to 0.12% carbon,

in particular from 0.08 to 0.10%, the steel having the conven-

tional contents of impurities such as silicon manganese etc.

or alloy contents such as occur in the so-called high strength

structural steel.

For particular results, a cold deformation degree

of the steel from 20 to 45% is preferred. Preferably, the

central region is heated at an average rate of about 700 C/s.

The specified averaga value can be determined by calculation

by dividing the desired maximum value of the core temperature

5 463,543

28,293m

for this (time from the intro- by the total of time required or other heating zone to the duction into the induction coil This heat treatment begining of the quenching operation). steel having is advantageously applied for non-alloyed 0.26%, in particular from 0.08 a carbon content from 0.06 to conventional contents of silicon and 0.22%, the steel having the and manganese. It can be an advantage to increase steel to 0.8 up to 1.8%. manganese content of the treated for a preferred increase In a particular embodiment, (610, the gauge length of a of the elongation properties its diameter, and 6 while test piece which is ten times or even improving the practically maintaining acceptable is heated to at least 700°C strength properties, the steel the temperature balancing range in its outer region, and for region a Figure) in the central (tlt<t3 according to the is and of a maximum of 750C temperature of at least 600°C surface temperature of at the caused to result for an outer in particular of 600 to 750oC most slightly above Ar 3 but whereupon from that condition (t t 2 according to the Figure), in the any phase transformation quenching is effected before a cooling rate (average central region has occurred with 0 It should be noted that although value) of at least 800 C/s. the attained in the central region high temperatures may be is so short that the kinetic time elapsing before quenching transformation prevent any factors involved in a phase The taking place in that time. phase transformation from to a period of the cooling rate relates specified average value has been chilled to a temperature of time in which the surface operation, below 150°C. At the beginning of the quenching

-6 463.543

28,293rn

a significantly higher value is obtained. At the beginning

of the quenching operation a cooling rate between 1200 and

1500 C/s is preferred. The upper limit of this process

is, particularly for the lower outer region temperatures

that quenching is effected prior to a reduction in the outer

surface temperature (t t 3 according to Figure 1) to below 0 550 C of the conventional cooling fluids, a shower treatment

with water at a pressure of 3 to 5 atmospheres (42.7 to 71.1

psi) is preferred. In this treatment, in particular a

water amount between 6 and 15 litres per kg steel is used.

A water amount of 6 to 10 litres per kg is preferably used.

The specification of kg steel corresponds to the amount of

bar steel conveyed into the quencher zone. The last mentioned

amounts of the quenching fluid water in pressure and quantities

are particularly preferred in the last"mentioned process.

Another preferred upper time limitation for initiation

of quenching is determined by a reduction of the outer

surface temperature (t t 3 according to Figure 1) below 700 0 C. Preferably, quenching is effected at the higher cooling rate

by spraying on water at a pressure of 7 to 12 atmospheres

(99.6 to 170.6 psi). The water is preferably used at rates

of 10 to 30'litres per kg steel. A water amount of 20 to

litres per kg steel is preferred.

It can be an advantage after any of these heat

treatments to effect a slight cold deformation to the extent

of a straightening after the quenching for specific

intended uses of the steel.

7 463,543

28,293m for the elevation For other purposes, in particular for that of the creep of the elasticity limit (as well as after any of these strength), it may be of an advantage at temperatures between treatments to conduct a heat treatment around 340°C, in 100 and 380oC after quenching, preferably times in which the particular in the range of retention (for instance a retention elasticity limit greatly increases

time of 20 to. 30 minutes). advantageously Bars treated according to this method 36 mms, in particular 6 to 16 mms. have a diameter between 4 and particular significance This heat treatment may afford cold deformed bars of structural and advantages in its use for for prestressed or slack steel, in particular concrete steel bars for welded structural reinforcements, in particular for

steel mats. the silicon content Within the specified analysis limits of the manganese content is normally limited to a maximum

to a maximum of 0.8%. to quench It can for other purposes be advantageous region in the upper within the temperature of the central preferably 450 to 550°C and range of the crystal recovery Ar 3 for increasing the elongatio .with surface temperatures above maintaining acceptable properties (610, 6g) while practically The quenching of or even improving the strength properties. effected to a temperature below the outer region is preferably to a temperature the crystal recovery range, in particular cooled to this around 2000C. After the outer region has been balancing is conducted temperature, the remaining temperature readily recovering temperatlu in air, the quenched outer region

by the heat leaving the central region.

463.543

28,2.93n

Particular erodiments of the invention are explained

hereinafter, in particular the technical measures being

discussed in detail.

Several terms used in the specification and in the

claims are firstly to be defined

"Form of a bar": This is understood to cover dimensions which

normal to their longitudinal axis essentially always have the samc

cross-sectional area, but in particular also dimensions which

furthermore have a uniform surface contour.

"Outer region" of the bar: This is understood to be the region

of the bar disposed radially outwardly in relationship to

the central region of the bar and is that region that is

heated in the quick-heating zone to a high temperature. As

disclosed by the paragraph to follow, the thickness of the

outer region in relationship to the volume of the central

region in a particular case is selected in dependence on the

desired conditions aimed at. Technically, the thickness of

the desired outer region to be heated in the quick-heating

zone is determined by the predetermined frequency when using

the induction method (and in this method is referred to as

the "shell" of the bar while the central region is referred

to as the "core" of the bar); the thickness of the shell

is greater; the lesser the frequency selected. The amount

of heat transmitted is controlled by the coil dimension and

the power density, The higher the power density selected, the

quicker and the higher the shell (of given thickness) heats

up. Thus, for a particular frequency by relating the advance

rate of the bar to the coil dimension and the power density

(retention tirte of the bar in the induction zone), the desired

9 46 3543

28.29 3n Relating of temperature in the shell can be adjusted. ratio of outer region the required power density to the effected according to the volume to central region volume is balancing range. desired temperature of the temperature term "temperature For a better understanding of the to the Figure. In the balancing range" reference is made of an inductive Figure the time/temperature relationship with water has been heating without a subsequent quenching in air by radiation). The plotted diagrammatically (cooling corresponds to the diagrammatic illustration approximately a diameter of 8 mms temperature course of a wire having frequency primarily had been which as a result of a specific a thickness of about 0.8 mms. heated in an outer region having flux to the central As shown in the illustration, a heat degree during the region already takes place to a minor the induction coil. In the illustrated retention time in flux occurs from the hot example, the major part of the heat cold central region after outer region to the relatively To s in Figure). leaving the induction coil (the point with time over the bar cross This balancing of heat occurring balancing section is covered by the term of the temperature t For accomplishing the range At that is between tI and 3 water quenching effects of this invention, the subsequent period of time in the must be effected within a specific the outer region and the course of the heat balancing between central region has undergone central region, i.e. before the during the treatment, and any phase transformation temperature drops below at the latezt before, the surface is'started is 550C. The actual time at which quenching

-n -63.543

28,293nm

chosen for any spezific case.

The graph in the Figure shows the temperature only the course without the subsequent water quenching, for the water lower and the upper time having been plotted quenching to be effected. For the lower time limit be (abscissa t1 it is necessary that the core temperature

above Tj K (450 0 For maintaining the upper time limit to (abscissa t 3 the surface temperature is not allowed

drop below T3S(5500C). The temperature values specified regard specify these in the claims and in the examples in this and of the central border conditions (temperature of the surface

region). It is clear that for the case of extraordinarily profile across quick heating and cooling the temperature surface and the central the bar cross-section between the outer t 2 on the Figure. region be non-uniform even at the time't may already be For instance the outer region temperature is rising. falling whilst the central region temperature When conducting a process within this invention, the appropriate the person skilled in the art has in choosing aspects conditions to consider further the following courses A deviation can result between the curve representing which can be simulated by means of computers and the actual the surface and the central region temperature because in the temperature courses obtained in practice from the surface to computer programme the radiation losses coil have not been atmosphere after leaving the induction person considered. On the other hand, it is easier for the the surface skilled in the art in practice to determine pyrometer. temperature by means of a disappearing filament

463,543

28.293nM

values specified in the For this reason the temperature concerning the outer surface claims and in the specification the time limit (abscissa t 3 of which characterise the upper the measured pyrometer temperature balancing range are limit of temperatures. It is added that the upper time is not preferred in practice, the temperature balancing range serves to limit the process rather this specification perhaps time limit for the water of this invention for the upper quenching quenching operation. In practice preferably for the selected within the specified time that point of time is region at which increasing central temperature balancing range surface temperature curves temperature and the decreasing

intersect each other )t t 2 in Figure). can readily The time range of this intersection means of a pyrometer measurement, be determined in practice by by conduction toward the central since after the heat balancing the capable of being measured at region the temperature drop slower than before, since heat outer surface occurs markedly is only further effected by emission from the outer region

radiation to the atmosphere. in detail on the It is to be explained hereinafter can be taken in practice to basis of examples what steps specified in the claims. fulfil the border conditions the process as defined According to one feature of region is heated to a temperature in the main claim, the outer 0 such a way that the heat content "between 600 and 1300 C in region heats the central region introduced into the outer 100 0 C/s, preferably at least at an average rate of at least to within the range from 450°C 300 0 C/s, to a temperature

Ac 3

-12 463,543

28.293/

As shown by the following examples, a central region temperature of 700 0 C is preferred. It is calculated

from the preferred heating-up rate of an average of 3000C/s 0 and a central region temperature of 700°C that 2.33 sec.

elapses from entering the quick-heating zone to the time of the

water quenching operation. In setting out from the specified

border conditions and a bar diameter of 8 mms, it can be calculated

that for a retention time of 1.3 sec in .the coil frequency

of 485 kcps and a-power density of 850 W/cm 2 are required to

attain these conditions. These conditions can just as well

be accomplished with a lesser frequency (greater shell thick-

ness) and a lesser power density. An alternative is to heat

a thin shell to a high temperature, the core still remaining

relatively cold.

According to this process, this short-time heating

is effected only in the outer region "shell". Subsequently,

a portion of the heat stored in the outer region is radiated

to air, while the major portion is transmitted into the central

region by heat transmission according to the conventional heat

transmission equation. In this regard, the heat content

stored in the outer region for a preferred increase in the

.obtained elongation properties is to sufficient in order to

heat the cetral region at least to temperatures in the upper

crystal rhcovery range (no structure variations having yet

occurred), preferably at least 450 to 55 (Figure abscissa tl).

The temperature thereat still initially remains above Ar 3 in

the outer region. In the then following expedited quenching I by means of water, the outer region is cooled usually to

temperatures below 200 C while the remaining balance can be

effected in air, the quenched outer region recovering by the

heat transmitted from the central region.

13 463,543

28.2 9 3 /71n,

As shown by the following first e:xample an increase

in the tensile strength B the breaking (represented by oa with a strength and oS the yield point) can be obtained by a great increase of the elongation with a steel treated

process within this invention. Table 1 shows the technological As a standard initial values and the final values achieved. to which the value in column 1 the temperature is specified short time. outer region (shell) has been heated for a

TABLE 1 steel with 0.12% Analysis: Non-Alloyed, low carbon concrete 6.5 mm in the cold deformed condition. carbon, diameter B B Elongation Heating of the outer region 2 2 2 to C kp/imn kp/mm kp/mm2 in

Starting material deformation 8% 60.0 55.0 5.0 degree 64.7 56.0 8.0 13% 800°C 67.7 56.5 11.5 900°C 72.0 56.0 16.0 11% 1000°C

in As shown by the table, the greatest increase increase in the tensile .the elongation is from 8 to 15% with an 2 2 the outer region strength from 60 kg/mm to 67.7 kg/mm when is heated to 900°C.

-14 463,543

28.2 9 3 n

As mentioned previously, the preferred tilae for

quenching is selected after the heat balancing when the

temperature of the outer surface drops below that of the

central region (t t 2 according to the Figure). This time

can easily be determined in practice by a pyrometer measure-

ment described previously. For a specific bar diameter,

this value can be accomplished in the case of induction heating

by the correlation of the advance rate, frequency and power

density within the limits specified in the claims by means of

various procedures. So for instance a very thin shell (as

compared with the core volume) can be heated to a very high

temperature in the coil (for above Ac 3 or possibly also a

very thick shell to a temperature which is not very much above

to 200 0 C) the temperature resulting at the time t 2

(according to the Figure).

Hereinafter, three further examples will be described

with reference to the results in Tables 2, 3 and 4.

Tables 2, 3 and 4 respectively show the variations

in the physical material properties after the present heat

treatment for different bar cross-sections 8 and 12 mm

diameters). The quenching was initiated on reaching the

moment in time within the temperature balancing range (Figure

tl<t<t 3 at which the increasing central region temperature

intersects the decreasing outer surface temperature (t t2

The first line value A of the Tables 2, 3 and 4 represents

in each case the physical values of the starting material

(36% cold deformed).

The analysis of the steel was

0.19' C, 0.16% Si, 0.61% Mn

15 463.543

28.293/71

conditions For the induction heating the following

were selected the coil 1 sec. Retention times in 485 kcps Frequency

Total period of time to beginning sec. of quenching

the cooling zone 0.5 sec. Retention time in

For quenching two different intensities were selected (water quenching) S4 Atmospheres (56.9 psi); quantity 8 litres/kg steel

II 8 Atmospheres (113.8 psi); quantity 26 litres/kg steel

about 60 C at The bar still had a temperatufe of the cooling zone for the the surface shortly after leaving

quenching intensity I. II belong to the The cooling conditions listed under This range is character- range of intense cooling conditions. of 7 to 12 atmospheres ised by spraying water at a pressure 20 to 30, litres/kg steel. in amounts from 10 to 30, preferably prior to a The quenching is effected preferably (t t3 reduction in the decreasing outer surface temperature o in the Figure) below 700 C. Under these conditions the value of at least 850°C/s. cooling rate effected has an average 1450 rate lies in the range between A typical preferred cooling and 17000C/s. steel bar of 6 mms Table 2 shows the values for the

diameter.

-16 463543

28,293/'71 TABLE 2

T C (at t=t 2 aB 2 kgm/nif2 Fig. 1) 2 kg/mm2 aS 0 g A

36% cold deformed 61.4 55.0 8.3

650 66.0 58.6 13.7 7.3

cooling intensity I-700 60.0 5i.5 15.2 13.0

7.50 79.0 42.0 10.0 5.7

As shown in particular in Table 2, a substantial

increase in the elongation properties results with accompanying

satisfactory strength values as compared with the cold deformed

starting condition. A using the lesser quenching intensity I, 0 in particular'for a selected "quenching. temperature" of 700 C.

It is to be pointed out now that for steel bars having

larger cross-sections, which will be described later, the

preferred value in the temperature balancing range for the time 0 t t2 is lower, i.e. is 650 C for bar cross-sections of .8 and

12 nms (700 C for 6 mms).

If following a heat treatment with a view to

increasing the elasticity limit a heat treatment after quenching 0 is conducted at about 340 C for 30 minutes, then an increase

of the elasticity limit of up to 50% may be achieved. The

yield limit is maintained, while the tensile strength slightly decreases.

An increase of the carbon content within the

maximum limit of 0. 26% requires a higher central region

temperature within the defined balancirg range for maintaining

-17 -463.543

28,293mn of the the optimum conditions than the case of a reduction

carbon content, and vice versa. The temperature in the

quick-heated outer region has to be adapted to the desired

higher or lower central region temperature.

In contradistinction to the described prior art,

a heat treatment process within this invention may overcome properties the conventional law of a counteraction of strength

and elongation properties on treatment of cold deformed that starting material." The essential thing is however, region annealing is limited to a short time and the central

is not to pass through any phase conversion Table 3 shows the values for a steel bar'of a diameter to above. This of 8 mms analysis being the same as referred a diameter bar has just as good increases as the bar having in the of 6 mms. The central region was heafed in an average 0 time. The range between 350 C/s and 420 C/s to the quenching shower to the cooling rate from beginning to enter the water 0 This corresponds end of the water shower was about 900 C/s.

to the cooling intensity II. This cooling intensity also of the brought about good results for a desirable alteration

elongation properties.

463,543

2, 2 9 3 /71 TABLE 3

0 T C(For t=t 2

2 2 kg/mm kg/mm

36%

cold deformed 6.9 2.9

cooling 600 11.8 6.8 intensity II 650 13.5

700 10.3 7.3

750 8.3 7.3

Table 4 (analysis again as set forth above) shows the property values for a steel bar of 12 nunmms diameter.

TABLE 4

T °C(t=t 2 o S SBkg/mm 2 kg/mm kg .mu kg.2mm

36% cold deformed 60 55 8.6 4.9 650 66 52.5 17.2 13.6

700 60.5 49.0 14.4 11.0

750 59.0 39.0 13.4

19 463.543

28,293nm

bars The specific advantages of the concrete for bars having high treated as described above are that a slack reinforcement elongation properties, buildings with as a result of the high are given a much higher safety rate while the high- proportion of proportionally elongation, for prestressed rein-. strength bars are particularly suited forcement. for the It is also essential that in particular achieved are not low-carbon grades the material properties spot welding. impaired for instance by resistance at the welded The examinations had been conducted mat. The examination of cross of a welded structural steel x 00.2 x the minimum shear force in a shear test (S=0.3 bar) brought about the cross-sectional area of the starting required minimum values.

The particular advantage of the above-described from a low-carbon non- processes is that, in setting out a specific cold deformation alloyed structural steel having strength degree 36%) which in this state has specific a differentiated heat treatment values and elongation values, by be effected throughout i.e. annealing not being permitted to when compared with the over the entire bar cross-section state there can be initial values in the cold deformed of steel having greatly produced on the one hand a grade in maintaining or even increased elongation properties and on the other hand a improving the strength properties strength properties in practi- grade having greatly increased properties. This result is cally maintaining the elongation the present opinion of the apparently in contradistinction to to which strength values people skilled in the art according only in a counter- and elongation values can be varied current tendency. 463.543

Claims (1)

1. 28,293/n THE CLAIMS DEFINING TP INVENTION ARE AS FOLLOWS

   1. A continuous heat =r,,atment process for low-carbon

   (max 0.26%C) non-alloy steel or structural steel with a

   manganese a'loy content up-to a maximum of the steel in

   the form of a bar being passed through a quick heating zone

   to be heated to a high temperature and subsequently being

   quenched, in which the steel is heated in a 10 to 70% cold-

   deformed condition in the quick-heating zone.to a temperature

   of between G60 to 1300°C in its outer region in such a way

   that the heat introduced into the outer region heats the central region of the bar at an average rate of at least 100°C/s to 00

   a temperature within the range from 4500C to Ac 3 and the

   heated steel is quenched without the central region having

   undergone any phase transformation quenching being

   at the earliest started when the central region reaches 0/ 450°C and at the latest prior to a reduction of the outer

   surface temperature to below 5500C.

   2. A process according to claim 1 wherein the quick-heating

   zone is an induction heating zone which heats the outer

   region directly to a shell depth which depends on the

   induction frequency.

   3. A process according to claim 1 or claim 2 wherein the

   quenching operation is effected with water, in particular

   water jets.

   4. A process acccrding to any one of the preceding claims

   wherein quenching is initiated at tnhe moment in which the

   rising central region temperature inteisects the failing outer

   region surface temperature.

   21 463.5.43

   28,293n

   A process according to any one of the preceding claims

   wherein the central region is heated at a rate of at least

   300 C/s.

   6. A process according to claim 5 wherein the central region

   is heated at an average rate of about 700 C/s.

   7. A process according to any one of the preceding claims,

   wherein the cold deformation degree of the steel is 20 to

   8. A process according to any one of the preceding claims

   wherein for primarily increasing the elongation properties

   (610g while practically maintaining or improving on acceptable strength properties, the steel is heated to at

   least 700°C in its outer region and for the temperature

   balancing range a temperature in the central region of at

   least 600oC but not more than 750°C is caused to result for

   outer surface temperatures of at the-most slightly above

   Ar 3 but in particular from 600 to 7500C (t t 2 according to

   the Figure) whereupon from that condition quenching is effected

   with a cooling rate (average value) of at least 800 C/s.

   9. A process according to claim 8, wherein quenching is

   effected with an initial cooling rate from 1200 to 15000C/s.

   A process according to claim 8 or claim 9 wherein quenching

   is effected by spraying water onto the steel at a pressure

   of 3 to 5 atmospheres.

   22 463,543

   28,293n 11. A process accordinq to claim 10 wherein the amount of

   water is from 6 to 15 litres per kg steel.

   12. A process according to claim 11, wherein the amount of

   water is from 6 to 10 litres per kg steel.

   13. A process according to any one of the preceding claims

   wherein after the quenching a slight cold deformation

   (straightening) is effected.

   14. A process according to any one of the preceding claims

   wherein a heat treatment at a temperature from 100 to 380 0 C,

   in particular about 340 C is conducted after quenching with

   a retention time (in particular 20 to 30 minutes) in which

   the elasticity limit greatly increases.

   A process according to any one of the preceding claims

   wherein bars of steel treated have a diameter of from 4 to 36

   mms.

   16. A process according to claim 15, wherein the diameter

   *is from 6 to 16 mms.

   17. A process according to any one of the preceding claims

   wherein the silicon content has a maximum of 0.5 and the

   fianganese content has a maximum of 0.8%.

   18. A process according to any one of the preceding claims

   wherein the steel has a carbon content between 0.06 and 0.26%

   in particular 0.03 and 0.22% and may have conventional contents

   of silicon and manganese.

   23 463,543

   28,293rn

   19. A process according to claim i, claim 2 or claim 3,

   wherein the steel has 0.06 to 0.20% carbon, preferably 0.12%

   carbon, in particular 0.08 to 0.10%, the steel may have

   conventional contents of silicon, manganese or alloy contents

   as they occur in the so-called high strength structural

   steels.

   A process according to claim i, claim 2, or claim 3, or

   claim 19, wherein the cold deformation of the steel is to

   21. A process according to any one of the preceding claims,

   wherein the steel is cold deformed bars of structural concrete

   steel for prestressed or slack reinforcements, in particular

   for welded structural steel mats.

   22. A process according to claim i, wherein for increasing 1 the elongation properties (610,6 g6 while practically main-

   taining or improving afr acceptable strength properties, with

   the central region temperatures in the upper range of

   crystal recovery, preferably 450 to 550 C and with the outer

   surface temperatures above Ar 3 a quenching is effected.

   23. A process according to claim 22 wherein the quenching

   of the outer surface region is effected to a temperature

   below the crystal recovery range, preferably to a temperature

   around 200 0 C.

   24. A process for heat treating low-carbon steel according

   to claim i, substantially as any herein described and

   exemplificd. L A') A 1.

   28,293/71

   Low carbon steel v.'rinever treated by a process according

   to any one of the preceding claims.

   Dated this 10th day of June 1975.

   BAU--STAHLC-EhEBE G.mn.b.H.

   By its Patent Attorneys

   DAVIES COLLISON

   25463,5 43

   28.293m

   27 463.5 43