US3795550A - Heat treatment process for non-alloyed low-carbon structural steel - Google Patents

Abstract

A LOW CARBON STEEL CAN BE HEAT TREATED TO IMPROVE EITHER ITS PHYSICAL PROPERTIES OR ITS ELONGATION WITHOUT DETRIMENT TO THE OTHER BY HEATING A SHELL REGION OF THE STEEL QUICKLY AND THEN ALLOWING HEAT TRANSFER TO TAKE PLACE UNTILE THE SHELL TEMPERATURE PREFERABLY DROPS TO THE VALUE OF THE CORE TEMPERATURE WHEREUPON THE STEEL IS INTENSIVELY QUENCHED IN WATER.

Description

March 5, 1974 TE RE ET AL 3,795,550

HEAT TREATMENT} PROCESS FOR NON-ALLOYED LOW-CARBON- S'IRUCTURAL STEEL Filed April 29, 1971 Tns [Joli United States Patent O 3,795,550 HEAT TREATMENT PROCESS FOR NON-ALLOYED LOW-CARBON STRUCTURAL STEEL Ludwig Ettenreich, Wien, Austria, and Otto Reimann, Dusseldorf-Oberkassel, and Klaus Greulich, Erkrath, near Dusseldorf, Germany, assignors t Bau-Stahlgewebe G.m.b.H.

Filed Apr. 29, 1971, Ser. No. 138,657 Claims priority, application Germany, Apr. 30, 1970, P 20 21 245.7;Mar. 13, 1971,? 21 12 103.9

Int. Cl. C21d 1/18 US. Cl. 14812.4 19 Claims ABSTRACT OF THE DISCLOSURE A low carbon steel can be heat treated to improve either its physical properties or its elongation without detriment to the other by heating a shell region of the steel quickly and then allowing heat transfer to take place until the shell temperature preferably drops to the value of the core temperature whereupon the steel is intensively quenched in water.

BRIEF SUMMARY OF THE INVENTION This invention relates to the heat treatment of nonalloyed low-carbon structural steel (max. 0.26% C) for improving the physical properties thereof. The steel is in the form of bars continuously passed through a rapidheating zone, and is heated to a high temperature and subsequently quenched.

The physical properties of non-alloyed steel depend essentially on the carbon content (neglecting heat treatment or mechanical working operations). With increasing carbon content, the tensile strength and the elastic limit increase, while the elongation decreases (cf. Werkstotfhandbuch Stahl un Eisen, 4th edition, 1965, G 1, in particular pictures 1 and 2). Increased strength properties are obtained by hardening and annealing in relationship to the carbon content. Generally, a minimum carbon content of more than 0.30% by weight is considered to be necessary for conducting this heat treatment for nonalloyed heat-treatable steel, but the weldability is thereby impaired or rendered impossible.

Attempts have been repeatedly made to develop processes in order to improve the physical properties of nonhardenable steel.

Thus, for instance a process is known from Stahl und Eisen 1949, pages 186-194, in which conventional strength-increasing alloy elements were reduced in structural steel of the type St 52 in order to save such alloy elements. Therein steel with a carbon content in accord with St 52, but with lesser contents of manganese and other strength-increasing elements, was increased in strength by quenching sheets in Water which were rolled and thereby heated.

For steel having a content of 0.18% C and 0.46% Mn, an increase in strength was obtained in the desired degree but with a concurrent decrease in elongation. This process is not acceptable in practice because the values obtained were subject to high fluctuations.

In the late nineteen-fifties a heat treatment process originating from laboratory tests became known for low carbon steel wire, the steel wire subsequently being further processed in cold condition. According to this process, steel wires of 2-8 mm. in diameter were inductively heated to high temperatures and subsequently quenched in water. With a rising quenching temperature, an increase in the tensile strength and a decrease in the elongation resulted. These laboratory tests have not resulted in a manufacturing-scale process.

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A further process for thorough annealing (hardening and tempering) of articles composed of non-hardenable steel is known from Austrian patent specification 281,089, wherein steel articles having a carbon content of about 0.10 to 0.23% are treated in a manner similar to surface hardening by means of gas/oxygen burners or electrical heating devices and subsequent water sprays, the articles being advanced at a minimum rate of travel of about to 200 mms. per minute. The heat treatment results in an increase in hardness, the hardness diiferential between the surface and the core in the non-annealed condition being broadly balanced in the subsequent annealing operation depending on the strength required, so that reference can actually be had to an annealing (tempering) throughout.

Contrary to the cited prior art which exclusively results in the increase of one specific material property at the sacrifice of another (e.g. increase of strength accompa nied by a decrease of elongation), the present invention seeks to provide a method in which one property can be improved While the others are at least retained but preferably also increased.

The solution of this objective is particularly applicable to reinforcing steel for concrete, since thereby the safety of buildings can be substantially increased. A structural steel mat having conventional strength properties, but a greatly increased elongation gives a much greater yieldability to the building along with increased safety. On the other hand, a great strength increase with conventional elongation rates is particularly applicable for prestressed concrete reinforcement.

The objects of the invention are achieved by the following measures:

(1) The steel is heat-treated in a cold-deformed condition and (2) Is heated to a temperature of between 600 and 1300 C. only in a circumferential shell region; the total heat content introduced into the shell region being controlled by adjustable parameters of power density and penetration depth (e.g. for induction heating: power density in w./cm. and frequency) such that the heat content serves to heat the core at a rate of at least 100 C./sec. and preferably at least 300 C./sec. to raise the temperature of the core between 450 C. and Ac and (3) The heated steel is quenched within a temperature balancing range (t t t according to FIG. 1), the time initiation value (t-=t according to FIG. 1) at which the core temperature is 450 C.

Preferably, the quenching operation is effected with water, particularly in the form of water jets.

The degree of cold deformation of the treated steel is between 10 to 70%, and preferably between 20 and 45%.

Preferably, the described heat treatment is conducted for steel having a carbon content of 0.06 to 0.26% carbon, and particularly 0.08 to 0.22%, the steel having conventional contents of silicon, manganese etc. or alloy contents as occur in so-called high-strength structural steel.

Preferably, the core is heated at an average rate of about 700 C./sec. The specified average value can be determined by dividing the desired maximum value of the core temperature by the total time required to achieve this (time from the introduction into the induction coil to the beginning of the quenching operation).

The manganese content of the treated steel can be advantageously increased from 0.8 to 1.8%.

When it is desired to increase the elongation properties (6 6 while maintaining or even increasing the strength properties, the shell region of the steel is heated to at least 700 C., and for the temperature balancing range (t stsn according to FIG. 1) in the core a temperature of at least 600 C. is produced while the core is at a maximum of 750 C. and no more than a value slightly above Ar (t=t according to FIG. 1). Thereupon a quenching is effected at an average cooling rate of at least 800 C./sec. The specified average value of the cooling rate relates to the period of time in which the surface has been cooled to a temperature below 150 C. At the begining of the quenching operation, a significantly higher value is obtained.

At the beginning of the quenching operation a cooling rate between 1200 and 1500 C./sec. is preferred.

The upper limit of the process of this invention is characterized in that prior to reduction of the temperature of the shell region (t=t according to FIG. 1) to a value .below 550 C., the quenching is effected.

Of the conventional cooling fluids, a spray treatment with water at a pressure of 3 to 5 atmospheres is preferred. In this treatment, Water in an amount between 6 and liters per kg. steel and preferably between 6 to 10 liters per kg. steel is used. The weight of steel corresponds to the amount of bar steel conveyed into the quenching zone.

At the beginning of the quenching operation, the quenching is preferably effected at a cooling rate of 1450 to l700 C./sec.

The quenching operation is effected before the temperature of the shell region (t=t according to FIG. 1) is below 700 C.

Preferably, the quenching is effected at the higher cooling rate by spraying water on the steel member at a pressure of 7 to 12 atmospheres. The amount of water is at the rate of 10 to 30 liters per kg. steel, and preferably between to 30 liters per kg. steel.

It is advantageous to subject the steel member to a slight cold deformation to the extent of a straightening, after the quenching for specific uses.

For other purposes, particularly for the elevation of the elasticity limit (as well as the creep strength), it is of advantage to effect a heat treatment at temperatures between 100 and 380 C. after quenching, preferably around 340 C., over a period of time in which the elastic limit greatly increases. An example of such a retention time is 20 to 30 minutes.

The bars treated according to this invention advantageously have a diameter of 4 to 36 mms., and preferably between 6 to 16 mms.

The heat treatment of this invention affords particular significance and advantages in its use for cold deformed bars of structural steel, in particular steel for prestressed or unstressed reinforcement members and particularly bars for welded structural steel mats.

Within the specified analysis limits the silicon content is normally limited to a maximum of 0.5%, the manganese content to a maximum of 0.8%

It can be advantageous to quench shortly before reaching the temperature balancing range (t gtst according to FIG. 1) with core temperatures in the upper range of crystal recovery, preferably 450 to 550 C., and with shell surface temperatures above Ar for increasing the elongation properties (5 6 While practically maintaining or even improving the strength properties. The quenching of the shell surface region is then preferably effected to a temperature below the crystal recovery range, in par ticular to a temperature around 200 C. After the surface region has been coded to this temperature, the remaining temperature balancing is conducted in air, the quenched surface region readily recovering heat from the core region.

The particular advantage of the process of this invention is that, in starting with a low-carbon non-alloyed structural steel having a specific cold deformation degree (e.g. 36%) which in this state has specific strength values and elongation values, improved properties can be obtained by a differentiated heat treatment in which uniform annealing is not permitted over the entire bar crosssection. As compared with the initial values in the cold deformed state on the one hand steel having greatly increased elongation properties While maintaining or even improving the strength properties can be obtained whereas, on the other hand, steel having greatly increased strength properties while maintaining the elongation properties can be produced. This result is in contradistinction t0 the present state of the art Where it is believed that strength values and elongation values can only be individually increased to the detriment of the other.

By the term cold deformation degree referred to above is meant any cold mechanical working to the extent of reduction of the cross section as indicated. Specifically a cold drawing of 36% is contemplated.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing variation of temperature with respect to time for a shell region and a core region of a steel member.

DETAILED DESCRIPTION In the outset, several terms used in the specification and in the claims are to be defined:

The steel member is described with reference to an elongated member in the form of a bar and such memher is intended to have the same cross-sectional shape and area in planes perpendicular to the longitudinal axis of the member for the length of the member. The bars may be circular or of any polygonal shape.

The bar is referred to as having a Shell region and this is understood to be the circumferential layer surrounding a core region of the bar, As will be disclosed later, the thickness of the shell, i.e. more particularly the volume of the shell in relation to the volume of the core, is determined by the desired core temperature. Technically, the thickness of the desired shell is determined by the frequency of an inductor coil as employed when heating by the induction method. The thickness of the shell increases, the lower the frequency. The amount of heat transmitted is controlled by the coil dimension and the power density. The higher the power density, the quicker the shell heats and the higher the temperature it reaches. Thus, by regulating the frequency and the rate of advance of the bar relative to the coil dimension and the powed density (retention time of the bar in the induction zone), a desired temperature in the shell is obtained. The relating of the required power density to the ratio of shell volume to core volume is effected according to the desired temperature of the temperature balancing range.

For a better understanding of the term temperature balancing range reference is next made to FIG. 1.

In FIG. 1 there is diagrammatically shown the relationship of temperature and time in the shell region and core region of the bar by an inductive heating without a subsequent quenching with Water and with cooling in air by radiation. The diagrammatic illustration shows the approximate temperature variation of a wire having a diameter of 8 mm. which as a result of a specific frequency primarily has been heated to a depth (shell thickness) of about 0.8 mm. by controlling the frequency of an induction source. As shown in the illustration, the core is also heated to a minor degree during the retention time in the induction coil.

In the illustrated example, the major part of the heat content in the shell region is transferred to the relatively cold core after the bar leaves the induction coil (T in FIG. 1). This balance of heat over the bar cross-sec-- tion is effected during the temperature balancing range At. For accomplishing the effects of this invention, the subsequent water quenching must be effected Within a specific period of time in the course of the heat balanc ing operation between the shell region and the core region.

The diagram shown in FIG. 1 shows the temperature variation without subsequent water quenching and only the lower and the upper limits t and 23 have been shownim which water quenching is to be effected. At the lower time limit t it is necessary that the core temperature be above T (450 C.). At the upper time limit (t the shell surface temperature is not allowed to drop below T The temperature values specified refer to the limit conditions in the shell region and in the core. It is clear that for the extraordinarily quick heating and cooling phenomena, the differential temperature values between the shell surface and the core can differ therefrom, i.e. for instance are higher at the time t=t The process of this invention does provide a clear teaching, however of the limit conditions.

When following the teaching of this invention the subsequent aspects must be further considered:

A deviation can result between the temperature curves and these curves can be simulated by means of computers representing the shell and the core temperature relative to the actual temperatures obtained in practice because in the computer program the radiation losses from the surface to atmosphere after leaving the induction coil have not been considered. On the other hand, it is a simple matter for one skilled in the art to determine the surface temperature by means of a disappearing filament pyrometer. For this reason the temperature values specified for the shell region which characterize the upper time limit (t;,) of the temperature balancing range pertain to measured pyrometer temperatures. It is added that the upper time limit of the temperature balancing range is not preferred in practice and serves to limit the maximum time interval for the water quenching operation. In practice quenching is preferably effected Within the specified temperature balancing range when the increasing core temperature and the decreasing surface temperature intersect each other.

The time of this intersection can readily be determined in practice by means of a pyrometer measurement, since after the heat balancing of the core, the temperature drop at the shell surface is much reduced, as heat emission is only effected further by radiation to the atmosphere.

There will next be explained in detail the basis for establishing the limit conditions of the process of the invention.

The shell is heated to a temperature between 600 and 1300" C. with the condition that the heat content introduced into the shell is sufficient to heat the core at an average rate of at least 100 C./sec. and preferably at least 300 C./sec., to a temperature between 450 C. and Ac As disclosed by the following examples, a core temperature of 700 C. is preferred. It is calculated from the preferred heating rate of an average of 300 0/ sec. and a core temperature of 700 C. that 2.33 seconds elapse from the time the bar enters the coil induction to the time of the water quenching operation. Starting from the specified limit conditions and a bar diameter of 8 mm., it can be calculated that for a retention time of 1.3 sec. in the induction coil, a frequency of 485 kc.p.s. and a power density of 850 w./cm. are required. These conditions can just as well be accomplished with a lesser frequency (to provide a greater shell thickness) and a lesser power density. An alternative is to heat a thin shell region to a high temperature, the core still remaining relatively cold.

According to this invention, this short-time heating is effected only in the surface region or shell region. Subsequently, the heat content stored in the shell is emitted into the air, while the major portion is transmitted toward the core region. In this regard, the heat content stored in the surface region for the purpose of obtaining an increase in the elongation properties must be sufficient to heat the core at least to a temperature in the upper crystal recovery range (no structural variations yet), and preferably at least 450 to 550 C. (FIG. 1, abscissa t The temperature thereat still initially remains above Ar in the surface region. In the following quenching operation with water, the surface region is cooled to temperatures below 200 C., while the remaining temperature balance can be effected in air, the quenched surface region receiving the heat emitted from the core region.

As shown by the following first example, an increase in the tensile strength is obtained with a great increase of elongation in steel treated according to this invention. Table 1 shows the initial values and the final values achieved. Column 1 shows the temperature to which the surface region (shell) has been heated over a short time.

TABLE 1 [Analysis: Non-alloyed, low-carbon steel with 0.126) carbon, diameter 6.5 mm. in the cold deformed condition] Kp./mm. Elongation Heating of the surface region to C. 0'9, 0'5, rsk, percent Starting material 35% deformation degree 0 55.0 5.0 8

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

As alluded to initially, the time of the quenching is selected at which after heat balancing the temperature of the shell surface drops below that of the core (t=t This time can easily be determined in practice by a pyrometer measurement as previously described. For a specific bar diameter, this value can be established by the correlation of the advance rate, frequency and power density within the limits specified by means of various procedures. Thus, for instance, a very thin shell (as compared with the core volume) can be heated to a very high tecperature in the induction coil (far above Ac or a very thick shell can be heated to a temperature which is only 50 to 200 C. from the temperature at the time t;, (according to FIG. 1).

TABLE 2 K mm. Pe nt T Q. (for t t2 g/ rce Vickers hardness m Fig. 1) an as 610 6; Surface Core A, 36% cold-deformation.- 61. 4 55. 0 8. 3 4. 0 260 260 650 66.0 58.6 13.7 7.3 240 250 .0 51. 5 15.2 13.0 220 250 0 42. 0 10. 0 5. 7 210 260 TABLE 3 Kg./mm. Percent Vickers hardness T 0. (t=tz) an as 610 5g Surface Core A, 36% co1d-deformation. 72 6. 9 2. 9 273 266 600 72 62 ll. 8 6. 8 268 257 72 61 13. 5 9. 0 271 277 76 58 10. 3 7. 3 315 274 78 56 8. 3 7. 3 423 273 TABLE 4 KgJmm. Percent Vickers hardness T C. (t=tz) (TB as 51 5,; Surface Core 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 (6, 8 and 12 mms. 0). The quenching was initiated on reaching the moment of time within the temperature balancing range (FIG. 1t t t at which the increasing core temperature intersect the decreasing shell surface temperature (FIG. 1t=t The first linevalue Aof the Tables 2, 3 and 4 represents in Retention time in the coil sec 1 Frequency k.c.p.s 485 Total period of time to beginning of quenching s c 2 Retention time in the cooling zone sec 0.5

For quenching two different intensities were selected (water quenching):

(I) 4 atmospheres (56.9 p.s.i.); quantity 8 litres/kg. steel (II) 8 atmospheres (113.8 p.s.i.); quantity 26 litres/kg.

steel The bar still had a temperature of about 60 C. at the surface shortly after leaving the cooling zone for the quenching intensity (I). The cooling conditions listed under (II) belong to the range of intense cooling conditions. This range is characterized by spraying water at a pressure of 7 to 12 atmospheres in amounts from 10 to 30, preferably 20 to 30 litres per kg. steel. The quenching is effected preferably prior to a reduction in the decreasing shell surface temperature (l=t according to FIG. 1) below 700 C. Under these conditions the cooling rate effected has an average value of at least 850 C./s. A typical, preferred cooling rate lies between 1450 and 1700 C.

As shown in Table 2, a substantial increase in the elongation properties is obtained with accompanying satisfactory strength values as compared with the cold deformed starting condition A for the lesser quenching intensity I for a selected quenching temperature of 700 C. But even quenching temperatures respectively 50 C. higher or lower still bring about physical values which are far above those previously known.

It is to be now noted that for steel bars having larger cross-sections which will be described later the preferred value of the temperature balancing range for the time t=t is lower, i.e. is 650 C. for bar cross-sections between 8 and 12 mm. (700 C. for 6 mm.).

If following the heat treatment of this invention, a subsequent heat treatment is conducted at 340 C. for 30 minutes, increases of the elastic limit of up to 50% are achieved.

An increase of the carbon content within the described limits requires a higher core temperature within the balancing range for maintaining the optimum conditions as compared with the case of a reduction of the carbon content, and vice versa. The temperature in the heated shell is adapted to the desired higher or lower core temperature.

In contradistinction to the prior art, the heat treatment process of this invention is successful in overcoming the conventional conception of antagonistic response of strength properties and elongation properties for cold deformed starting material. This surprising fact can possibly be explained when considering the Vickers hardeness values (Table 2) and associated textures (not shown). For the preferred improvement of the elongation properties sought according to this invention, the hardness values for the surface region of the bar are substantially below the values of the core. It is possible that just this distortion caused by the different structure in the surface and in the core region brings about the properties of this invention. The essential requirement is, however, that when employing the process of this invention, there is no uniform annealing. The core is to not undergo any phase conversion (u-y-u).

A bar of a diameter of 8 mm. (Table 3) shows just as good increases as the bar with diameter of 6 mm. The core was heated at an average rate in the range between 350 C./sec. and 420 C./sec. to the quenching time. The cooling rate from the beginning of entering the water spray to the end of the water spray was about 900 C./ sec. This corresponds to cooling intensity (II). This cooling intensity also brought about good results for a preferred alteration of the elongation properties. As shown in FIG. 3, the different hardness values of the shell and the core result in the high strength increases with accompanying good toughness properties.

Table 4 shows that for larger bars (12 mm.) the differences in the hardness are not so marked.

Based on micrographic study, the deformation texture in the core remains clearly visible.

The specific advantages of the bars made according to this invention are that for bars having high elongation properties, structures with unstressed reinforcement are given a much higher safety margin as a result of the high proportion of proportionality elongation, while the highstrength bars are particularly suited for prestressed reinforcement members.

It is also notable that for low-carbon steels, the material properties achieved with this invention are not impaired, for instance, by resistance spot welding.

Examinations have been conducted at the joints of a welded structural steel mat. The evaluation of the minimum shear force in a shear test (S=0.3 X0 x cross-sec tional area of the starting bar) showed that the steel mat satisfies the required minimum values.

What is claimed is:

1. A continuous heat treatment process for low-carbon steel (max. 0.26% C.) in bar-form 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 only in its shell to a temperature of between 600 and 1300 C. at a rate in which the core is heated at an average of at least C./sec., to a temperature within a temperature range from 450 C. to Ac and the heated steel is quenched during the temperature balancing between core and shell without the core having undergone any phase transformation (ony-u) quenching being at the earliest started when the core temperature reaches 450 C. and at the latent prior to reduction of the shell temperature to a value below 550 C.

2. A process as claimed in claim 1 wherein said rate of temperature increase is a least 300 C./ sec.

3. A process as claimed in claim 1 wherein the steel has a carbon content of 0.06 to 0.26% and silicon and manganese.

4. A process as claimed in claim 3 wherein the manganese content is between 0.8 and 1.8%.

5. A process as claimed in claim 1 wherein the degree of cold-deformation is 20 to 45% 6. A process as claimed in claim 1 wherein the steel member is heated to at least 700 C. in its shell region and the core reaches a temperature of between 600 C. and 750 C. before quenching, said quenching being effected at an average cooling rate of at least 800 C./sec. whereby the elongation properties of the steel member are increased while the strength properties are at least substantially maintained.

7. A process as claimed in claim 6 wherein the cooling rate is between 1200 and 1500 C./ sec.

8. A process as claimed in claim 6 wherein the shell surface temperature is at least 550 C. when quenching is effected.

9. A process as claimed in claim 6 wherein said quenching is effected by spraying water on the steel member at a pressure of 3 to 5 atmospheres.

10. A process as claimed in claim 9 wherein the water is sprayed in an amount between 6 to 15 liters per kg. steel.

11. A process as claimed in claim 1 comprising eifecting a slight cold deformation after quenching.

12. A process as claimed in claim 1 comprising heat treating said member for 20 to 30 minutes at a temperature between 100 and 380 C., after said quenching to increase the elastic limit.

13. A process as claimed in claim 3 wherein the silicon content is a maximum of 0.5% and the manganese content is a maximum of 0.8%

14. A process as claimed in claim 1 wherein quenching is effected with the core at a temperature in the upper range of crystal recovery, and with the temperature of the shell region above Ar whereby to obtain increased elongation properties while at least maintaining the strength properties.

15. A process as claimed in claim 14 wherein the temperature of the shell region is below the crystal recovery range after quenching.

16. A process as claimed in claim 1 wherein temperature of the core increases after the heating has stopped while the temperature of the shell region decreases and drops below the temperature of the core, said quenching being effected after the temperature of the core has reached the temperature of the shell region.

17. A process as claimed in claim 1 wherein said quenching of the member is effected with water jets.

10 18. A process as claimed in claim 1 wherein said steel member is circular and has a diameter of 4 to 16 mm.

19. A process as claimed in claim 16, wherein a very thick shell is heated to a temperature which is only 50 to 200 C. from the temperature at the time when the quenching is started.

References Cited UNITED STATES PATENTS 2,598,694 6/1952 Herbenar 148150 2,393,363 1/1946 Gold 61; a1. 148150 1,946,876 2/1934 Northrup 148150 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

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