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

Heat treatment process for non-alloyed low-carbon structural steel Download PDF

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Publication number
US3795550A
US3795550A US00138657A US3795550DA US3795550A US 3795550 A US3795550 A US 3795550A US 00138657 A US00138657 A US 00138657A US 3795550D A US3795550D A US 3795550DA US 3795550 A US3795550 A US 3795550A
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temperature
steel
core
shell
quenching
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L Ettenreich
O Reimann
K Greulich
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Baustahlgewebe GmbH
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Baustahlgewebe GmbH
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering

Definitions

  • 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.
  • 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.
  • non-alloyed steel 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.
  • 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.
  • 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.
  • a great strength increase with conventional elongation rates is particularly applicable for prestressed concrete reinforcement.
  • 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 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%.
  • 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.
  • 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%.
  • 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.
  • a cooling rate between 1200 and 1500 C./sec. is preferred.
  • a spray treatment with water at a pressure of 3 to 5 atmospheres is preferred.
  • 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.
  • the quenching is preferably effected at a cooling rate of 1450 to l700 C./sec.
  • 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.
  • 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.
  • the silicon content is normally limited to a maximum of 0.5%, the manganese content to a maximum of 0.8%
  • 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.
  • 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.
  • cold deformation degree 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.
  • 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.
  • 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,
  • 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.
  • 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.
  • 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.
  • the core is also heated to a minor degree during the retention time in the induction coil.
  • 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.
  • 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.
  • the core temperature At the lower time limit t it is necessary that the core temperature be above T (450 C.).
  • the shell surface temperature At the upper time limit (t the shell surface temperature is not allowed to drop below T
  • 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.
  • 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.
  • 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
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • t This time can easily be determined in practice by a pyrometer measurement as previously described.
  • 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.
  • 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).
  • 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.
  • a typical, preferred cooling rate lies between 1450 and 1700 C.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • 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.
  • a process as claimed in claim 1 comprising eifecting a slight cold deformation after quenching.
  • 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.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)
  • Heat Treatments In General, Especially Conveying And Cooling (AREA)
  • Heat Treatment Of Steel (AREA)
US00138657A 1970-04-30 1971-04-29 Heat treatment process for non-alloyed low-carbon structural steel Expired - Lifetime US3795550A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE2021245A DE2021245C3 (de) 1970-04-30 1970-04-30 Kontinuierliches Wärmebehandlungsverfahren an stabförmigen Baustählen
DE2112103 1971-03-13

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US (1) US3795550A (enrdf_load_stackoverflow)
AR (1) AR196395A1 (enrdf_load_stackoverflow)
AT (1) AT325085B (enrdf_load_stackoverflow)
BE (1) BE766405A (enrdf_load_stackoverflow)
BR (1) BR7102635D0 (enrdf_load_stackoverflow)
CA (1) CA946270A (enrdf_load_stackoverflow)
CH (1) CH552064A (enrdf_load_stackoverflow)
CS (1) CS179953B2 (enrdf_load_stackoverflow)
DK (1) DK130192B (enrdf_load_stackoverflow)
ES (1) ES390697A1 (enrdf_load_stackoverflow)
FR (1) FR2091027A5 (enrdf_load_stackoverflow)
GB (1) GB1345387A (enrdf_load_stackoverflow)
HU (1) HU167409B (enrdf_load_stackoverflow)
IE (1) IE35144B1 (enrdf_load_stackoverflow)
IL (1) IL36660A (enrdf_load_stackoverflow)
LU (1) LU63070A1 (enrdf_load_stackoverflow)
NL (1) NL7105932A (enrdf_load_stackoverflow)
NO (1) NO132964C (enrdf_load_stackoverflow)
PL (1) PL81634B1 (enrdf_load_stackoverflow)
RO (1) RO61536A (enrdf_load_stackoverflow)
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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3929524A (en) * 1973-07-26 1975-12-30 Nikolai Grigorievich Filatov Method of heat treating linear long-length steel articles, apparatus for effecting said method and articles produced thereby
US4248766A (en) * 1979-06-18 1981-02-03 The B. F. Goodrich Company Didodecylammonium beta-octamolybdate and composition containing same
US4336081A (en) * 1978-04-28 1982-06-22 Neturen Company, Ltd. Process of preparing steel coil spring
US4362578A (en) * 1980-10-16 1982-12-07 Teledyne Industries, Inc. Method of hot working metal with induction reheating
US4407683A (en) * 1978-04-28 1983-10-04 Neturen Company, Ltd. Steel for cold plastic working
US4793869A (en) * 1987-04-10 1988-12-27 Signode Corporation Continuous treatment of cold-rolled carbon manganese steel
AU607480B2 (en) * 1987-04-10 1991-03-07 Signode Corporation Continuous treatment of cold-rolled carbon manganese steel
CN112708742A (zh) * 2020-12-07 2021-04-27 江苏省镔鑫钢铁集团有限公司 一种高强度精轧螺纹钢筋连续感应热处理工艺及处理装置
CN113174466A (zh) * 2021-04-30 2021-07-27 洛阳Lyc轴承有限公司 40Cr15Mo2VN高氮不锈轴承钢感应淬火方法
CN113801979A (zh) * 2021-09-16 2021-12-17 青海大学 一种模拟火灾检测影响建筑物抗震性能钢筋极限强度的方法

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2124041C3 (de) * 1971-05-14 1981-05-07 Bau-Stahlgewebe GmbH, 4000 Düsseldorf Anwendung eines kontinuierlichen Wärmebehandlungsverfahrens auf stabförmige, untereutektoide Vergütungsstähle
FR2703069B1 (fr) * 1993-03-26 1995-07-07 Aciers Armature Beton Procédé de traitement thermique d'une armature par exemple pour béton armé et armature obtenue selon ce procédé .

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3929524A (en) * 1973-07-26 1975-12-30 Nikolai Grigorievich Filatov Method of heat treating linear long-length steel articles, apparatus for effecting said method and articles produced thereby
US4336081A (en) * 1978-04-28 1982-06-22 Neturen Company, Ltd. Process of preparing steel coil spring
US4407683A (en) * 1978-04-28 1983-10-04 Neturen Company, Ltd. Steel for cold plastic working
US4248766A (en) * 1979-06-18 1981-02-03 The B. F. Goodrich Company Didodecylammonium beta-octamolybdate and composition containing same
US4362578A (en) * 1980-10-16 1982-12-07 Teledyne Industries, Inc. Method of hot working metal with induction reheating
US4793869A (en) * 1987-04-10 1988-12-27 Signode Corporation Continuous treatment of cold-rolled carbon manganese steel
AU607480B2 (en) * 1987-04-10 1991-03-07 Signode Corporation Continuous treatment of cold-rolled carbon manganese steel
CN112708742A (zh) * 2020-12-07 2021-04-27 江苏省镔鑫钢铁集团有限公司 一种高强度精轧螺纹钢筋连续感应热处理工艺及处理装置
CN113174466A (zh) * 2021-04-30 2021-07-27 洛阳Lyc轴承有限公司 40Cr15Mo2VN高氮不锈轴承钢感应淬火方法
CN113174466B (zh) * 2021-04-30 2023-01-13 洛阳Lyc轴承有限公司 40Cr15Mo2VN高氮不锈轴承钢感应淬火方法
CN113801979A (zh) * 2021-09-16 2021-12-17 青海大学 一种模拟火灾检测影响建筑物抗震性能钢筋极限强度的方法

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Publication number Publication date
DK130192C (enrdf_load_stackoverflow) 1975-06-16
IL36660A0 (en) 1971-06-23
NO132964C (enrdf_load_stackoverflow) 1976-02-11
ES390697A1 (es) 1973-07-01
DK130192B (da) 1975-01-13
FR2091027A5 (enrdf_load_stackoverflow) 1972-01-14
GB1345387A (en) 1974-01-30
CS179953B2 (en) 1977-12-30
NL7105932A (enrdf_load_stackoverflow) 1971-11-02
LU63070A1 (enrdf_load_stackoverflow) 1972-06-28
NO132964B (enrdf_load_stackoverflow) 1975-11-03
BE766405A (fr) 1971-10-28
CH552064A (de) 1974-07-31
AR196395A1 (es) 1973-12-27
PL81634B1 (enrdf_load_stackoverflow) 1975-08-30
RO61536A (enrdf_load_stackoverflow) 1977-02-15
IE35144L (en) 1971-10-30
IE35144B1 (en) 1975-11-26
AT325085B (de) 1975-10-10
HU167409B (enrdf_load_stackoverflow) 1975-10-28
SE368838B (enrdf_load_stackoverflow) 1974-07-22
BR7102635D0 (pt) 1973-04-26
IL36660A (en) 1975-12-31
CA946270A (en) 1974-04-30

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