CZ2010554A3 - Method of annealing steel half-finished product - Google Patents

Abstract

In a time-saving method of annealing a steel blank to obtain, by a defined method, annealed steel with favorable carbide morphology in the ferritic matrix, the steel blank is subjected to a basic heating rate of greater than 10 degC / s to a temperature in the range of 650 to 900 deg. C throughout the cross-section, then cooled to a maximum of 200 degC at a rate greater than 0.5 degC / s, which is carried out at least twice, and then cooled at a rate greater than 0.5 degC / s.

Description

Annealing of steel blank

Technical field

The invention is an energy and time-saving method of annealing a steel blank to produce a microstructure and soft annealed material properties or a microstructure with recrystallization or otherwise specifically annealed material properties.

BACKGROUND OF THE INVENTION

Known methods of soft annealing or recrystallization annealing are carried out in furnaces, whether continuous or chambered, in which the steel blanks remain for several tens of minutes to hours, depending on the size. Annealing is normally carried out by heating just below the temperature of _C1, with a different length of time, followed by slow cooling to room temperature. During the annealing process there is a stiffening process in the microstructure and recruiting processes are in progress. Usually, the morphology of carbides also changes. The strength and hardness of the steel blank decreases, while its ductility increases and the plastic properties are renewed.

The annealing process can be accelerated by varying the temperature around the Aci temperature. However, the available literary sources also assume a cycle of several hours. It is also possible to anneal above the temperature of A _C ia and then slowly cool down. In this way, the soft annealing process can be shortened, remaining in the order of several hours. This process of austenitization of perlite with limited homogenization of austenite provides a sufficient amount of nucleation, which on cooling produces fine globular carbides, which contribute to better formability of the material. Subsequently, the steel blank is continuously cooled, which guarantees a fine globular pearlite. The steroidized structure is associated with a decrease in the hardness and strength properties of the material. In addition to spheroidization of carbides, this decrease is also affected by the solidification processes in the ferritic matrix.

In the heat treatment method described in U.S. Patent 6,190,472 high carbon steel is annealed by a heat treatment at a temperature and _C1 - * 20 ° C or higher, followed by rapid cooling below the A and _C. The next step is heating to A _cl + 20 ° C or higher followed by cooling to 69 ° C at 3.5 ^? C / min. or less to room temperature. This procedure significantly reduces the annealing time and has been verified during the heat treatment of the tubes in a continuous annealing furnace.

Similarly, the acceleration of carbide spheroidization of chromium alloyed high carbon steel γ is described in JP 0411031715. The heat treatment consists of heating to a temperature of 780 to 820 ° C and subsequently cooling below the Aci temperature at a rate of less than 200 ° C / hfc4. Next, heating to a temperature in the range of A _C i to A _C1 4-40PC, cooling below the temperature of A _C i at a rate lower than

AND

2,000 £ C / h04, heating to Ao to _C1 4 · 40 ° C and cooling below Aci t »*>

at a rate of less than 75 ° C / h 2.

The annealing methods described above, which achieve the desired spheroidization of the carbides, are energy intensive because they require longer residence at the primary heating temperature.

The other processes described which also result in spheroidization of carbides also involve heating, but which is followed by plastic deformation. Here, too, the desired spheroidization of the carbides is achieved, but with simultaneous deformation, which may not be possible for each blank.

The acceleration of spheroidization by plastic deformation is described in Materials Science and Engineering A 432 (2006) "Effect of deformation on the evolution of spheroidization for ultra high carbon steel," pp.324 to 332. For high carbon steel (1.1% Mn, 0) , 7% Mn) was made heating at 10 ° C / s to a temperature of 1150 ° C with a holding time 1 minute, then cooling to 700 ^Q C at 20 ° C / s and at this temperature was carried out with deformation of varying intensity. The best results were obtained by ramming a cylindrical sample with a diameter of 10 mm and a height of 15 mm to 40% of the height. The deformation was followed by cooling of 0.2 ° C / s. A structure composed of a ferritic matrix and uniformly spaced globular carbides was achieved.

Thermomechanical treatment of carbon steel with 0.67% C is described in Materials Science and Engineering A 527 (2010), “Effect of deformation and annealing temperatures on ultrafine microstructure development ...”, pp. 1926-1932. temperature 600fC. Heating and cooling always proceed at a rate of 5 ° C / min. On samples with dimensions 5 x 5 x • λ

Oinm was first heated to a temperature of 600 ° C, followed by a holding time of 1 Oin and a nominal deformation of 0.7. After a 1 minute hold, cooling to / followed by a temperature of Λ Second heating to 600 | ^o C was carried out without the deformation ^Λ ;

with a 12 minute hold followed by cooling to (room temperature) A structure composed of uniformly dispersed globular carbides in a ferritic matrix was achieved.

Technology enabling rapid spheroidization of carbides is described in patent CZ 301 ^ 718. This process is based on heating the steel blank to a maximum temperature of A _C i, followed by mechanical forming and cooling of the steel blank. This procedure results in perfect spheroidization of the carbides. The whole process is very short and the spheroidization of carbides is progressive. A common drawback is also the need to include the deformation process as a necessary operation to obtain the desired structure in the technology chain. The deformation must be performed in a non-standard temperature interval.

SUMMARY OF THE INVENTION

The invention is a method of annealing a steel blank, the object of which is to obtain in a defined manner annealed steel with a favorable morphology of carbides in a ferritic matrix. The steel blank is subjected to a basic heating at a rate greater than 10 ° C / s to a temperature in the range of 650 ° to 900 ° C throughout the cross-section. Subsequently, it is cooled by a maximum of 200 ° C at a rate higher than 0.5 ° C / s, which is carried out at least twice, and then cooled at a rate higher than 0.5 ° C / s. In this way, the microstructural state and the mechanical properties required after soft annealing, recrystallization annealing or other, for example, normalization annealing are achieved. The heat treated steel blank is either cooled to room temperature or cooled to less than 750 ° C, then heated at a rate greater than KIC / s to a maximum of 900 ° C, at least once, followed by cooling to room temperature. wcbwíta rate higher than 0 _f 5? C / s.

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In order to achieve the desired properties of the steel blank, it has been found suitable to precede the basic heating by heating the steel blank at a rate greater than 10 ° C / s to a temperature in the range 650 ° C to 90 ° C throughout the cross-section. fC / s. Also suitable for some materials when heating the steel blank at a speed greater than Lo ^ C / s to a temperature in the interval 650 ^{9 until}

900 ° C over the entire cross-section that precedes the basic heating is delayed.

.'PepiW pictures in drawing?

Fig. 1 is a diagram showing a method of annealing a steel billet having undergone a basic heating to 740 ° C throughout its cross-section with intermediate cooling to 680 ° C repeatedly; Fig. 2 shows a method of annealing a billet billet having undergone a basic heating to 720 ° C. 3 shows a diagram of the annealing process, where the first heating of the steel blank is at a temperature of 830 ° C over the entire cross-section, followed by intermediate cooling to 800 ° C repeatedly, followed by cooling to temperature 650 ° C repeated heating to 740 ° C, in Fig. 4 is shown the method of annealing similar to Fig. 3, but at 900 ° C the heating preceding the basic heating is delayed, in Fig. 5 is the method of annealing in which it is continuously transferred from one heating to 830 ° C to the other basic heating shown in Fig. 1. Fig. 6 shows a diagram of the annealing process, wherein the first heating of the steel blank is at a temperature of 890 ° C over the entire cross-section, followed by cooling to 875 ° C repeatedly, followed by cooling to 35 ° C, followed by reheating to 740 ° C followed by cooling in air.

Examples of implementation? Γηα <ι · * <· <.

Example 1

Bar cold formed of 15 mm diameter steel with a chemical composition of C 0,12% by weight, Mn 0,39i% by weight, Si 0,09% by weight, P 0,011;% by weight, S0,01% by weight, Cr 0 , 02% by weight, Ni 0.0 li% by weight, Cu 0.02% by weight, N ₂ 0.006%

A 3 With weight. Its notch toughness KCV mini = 64J / cm ² , contractional yield strength Rpo, 2 ⁼ 570 MPa, breaking strength Rm = 599 MPa, ductility A ₅ = 13.5%, hardness 180 HV30. The steel blank was induced to 74 ° C for 9 seconds by induction as shown in Figure 1, then cooled freely to 680 ° C. This cycle was repeated three times and then the steel blank was freely cooled in air. This procedure spheroidized the pearlitic lamellar colonies to globular carbides and achieved the following mechanical values, KCV mini = tenlJ / cm, lower yield strength R _eL - 239 MPa, upper yield strength R _eH = 269 MPa, yield strength R _m = 391 MPa, ductility A ₅ = 40% and hardness 102 HV30.

Example 2

Cold rolled sheet of 0,6mm thickness, chemical composition C 0,042% by weight, Mn 0,202% by weight, Si 0,004% by weight,

P 0.005% by weight, S 0.009% by weight. Contractional yield strength in longitudinal direction R _p o.2 ⁼ 632MPa, in transverse direction Rpo.2 ^ 661 MPa, strength in longitudinal direction R _m = 652 MPa, in transverse direction R _m = 700 MPa, elongation in longitudinal direction A ₅₀ = 3,1%, in the transverse direction _{5 5} o = 2,5% and hardness 204HV1. By induction heating, as shown in FIG. 2, the steel blank was heated to 720 ° C in 6 seconds. Subsequently, cooling to 680 / C occurred in three seconds. Then the blank was reheated to a temperature 72 (TC for three seconds, and this cycle was then twice repeated. By this procedure, the recrystallization of the ferrite grains and the spheroidization of pearlitic lamellar colonies globule. Yield strength in the transverse direction of rolling reached R _p o, 2 = 329 MPa, transverse tensile strength R _m = 349 MPa, transverse tensile _{5 50} = 39% and hardness was 105HV1.

Example 3

Hot formed bar of 15 mm diameter and having a chemical composition of C 0,47;% by weight, Μη 0,708ώ by weight, Si 0,23% by weight, P 0,026% by weight, S 0,017% by weight, Cr 0,07%

SS η by weight, Ni 0.02% by weight, Mo <- weight by these mechanical properties. Notch toughness KCV mini 37J / cm ² , contractual yield strength Rpo, ₂ = 309 MPa, breaking strength R _m = 642 MPa, elongation A ₅ = 27%, hardness 178 HV30. The steel blank was inductively heated to 830 ° C for 14 seconds as shown in Figure 3, then cooled to 800 ° C in 5 seconds and this cycle was performed three times in total. Subsequently, the steel blank was freely cooled in air to 65 ° C. This was followed by heating to 740 ° C in 1 second and cooling to 680 ° C in 12 seconds, followed by heating to 74 ° C in 1 second. This was followed by cooling in air to a room temperature. This procedure resulted in a refinement of the ferritic grain from an average grain size of 30 < 5 > to 10 [mu] m and a partial spheroidization of the pearlitic lamellae into a globular form. The following mechanical values were achieved: • 7 toughness KCV mini 59J / cm, contractional yield strength R _{p0 2} = 433 MPa, with lower yield strength R _cL = 431 MPa, upper yield strength R _eH = 461 MPa, yield strength R _m = 666 MPa, elongation A ₅ = 30%, hardness 182 HV30.

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Example 4

Hot-formed semi-finished product of 15mm diameter steel with chemical composition C 0.47% by weight, Mn 0.70% by weight, Si 0.23% by weight, P 0.026% by weight, S 0.017% by weight, Cr 0.07% by weight Ni 0.02% by weight, Mo 0.007% by weight with mechanical properties: notch toughness KCV mini 37 J / cm2, yield strength Rp0.2 = 309 MPa, breaking strength Rm = 642 MPa, ductility A5 = 27%, hardness 178HV30 . The blank was subjected to induction heating to 830 ° C in 14 seconds, followed by a 15 second dwell at that temperature, as shown in Figure 4, followed by cooling in air to 650 ° C and then heating to 740 | ° C in 1 second. second and cooling to 680 ° C in 12 seconds and then heating again to 740 ° C in 1 second. Subsequent cooling to (room) temperature ¹ was performed in air. In this way, the ferritic grain was refined from an average grain size of 30 µm to 10 µmη and a partial spheroidization of the pearlitic lamellae into a globular form. The following mechanical properties of the processed material were achieved: notch toughness KCV mini 61 J / cm2, contractional yield strength Rp0.2 = 429MPa, lower yield strength ReL = 426 MPa and upper yield strength ReH = 458 µMPa, yield strength Rm = 670 MPa. Ductility A5 = 31% and hardness 181HV30.

Example 5

Hot-formed semi-finished product of 15mm diameter steel with chemical composition C 0.47% by weight, Mn 0.70% by weight, Si 0.23% by weight, P 0.026% by weight, S 0.017% by weight, Cr 0.07% by weight , Ni 0,02% by weight, Mo 0,007% by weight of mechanical properties: notch toughness KCV mini 37 J7cm2,

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yield strength Rp0,2 = 309 MPa, breaking strength Rm = 642 MPa, ductility A5 = 27%, hardness 178HV30. The blank was subjected to induction heating to 83 ° C in 14 seconds as shown in Figure 5. This was followed by cooling in air to 650 ° C followed by heating to 74 (> ° C in 1 second and cooling to 68 ° C in 12 seconds and then reheated to 74 ° C for .ΤΤ / r · r / sec, cooling to room temperature in air to refine ferritic grain from an average grain size of 30; pml, to 17 pm and partial spheroidization of pearlitic lamellas into globular form The following mechanical properties of the processed material were achieved: notch toughness KCV mini 47 J / cm2, contractional yield strength Rp0.2 = 382MPa, lower yield strength ReL = 374 MPa and upper yield strength ReH = 389MPa , breaking strength Rm = 651 MPa, ductility A5 = 33% and hardness 179HV30.

Example 6

Bar cold formed of 15mm diameter steel with chemical composition C 0.12% by weight, Mn 0.39% by weight, Si 0.09% by weight, P 0.011% by weight, S 0.010% by weight, Cr 0.02% by weight , Ni 0,01% by weight, Cu 0,02% by weight, N2 0,006% by weight with mechanical properties: notch toughness KCV mini 64 J / cm2, yield strength Rp0,2 = 570 MPa, breaking strength Rm = 599 MPa, ductility A5 = 13.5%, hardness 180HV30. The blank is induced to 890 ° C for 16 seconds as shown in Figure 6. It is then freely cooled to 875 ° C in 3 seconds, then reheated to 890 ° C, cooled to 875 ° C, or. this cycle is repeated several times. Then, the blank is freely cooled in air to 350 ° C. Next, by induction, the sample is heated to 740 ° C in 5 seconds, freely cooled to 680 ° C in 3 seconds, further heated to 740 ° C, cooled to

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68 ° C. This cycle of heating and cooling between temperatures of 680 [deg.] And 740 [deg.] C. can be performed several times for greater process efficiency. Then the blank is freely cooled in air. This procedure partially spheroidized the pearlitic lamellas and refined the ferritic grain. Achieved properties of processed material: Notch toughness K.CV mini 95 J / cm2, lower yield strength ReL = 280 MPa, upper yield strength ReH = 292 MPa, yield strength Rm = 420 MPa, ductility A5 = 42%, hardness 113HV30.

Claims (5)

Patent claims 1. A method for annealing a steel blank, characterized in that the steel blank is subjected to a basic heating at a rate greater than 10 ° C / s to a temperature in the range 650 ° to 900 ° C throughout the cross section and subsequently cooled by a maximum of 200 ° C 0.5fC / s, which is done at least twice, then cooled at a rate greater than 0.5 ° C / s. X The method of annealing a steel blank according to claim 1, characterized in that the cooling is carried out at an oil temperature. The method of annealing a steel blank according to claim 1, characterized in that the cooling is carried out at a temperature of less than 750 ° C and is then heated at a rate of more than 10 ° C / s to a maximum of 900 ° C. , followed by cooling to room temperature / temperature greater than 0,5 ° C / s. . X Steel blank annealing method according to claims 1, 2 or 3, characterized in that the basic heating is preceded by heating the steel blank at a rate greater than 10fC / s to a temperature in the range 650 * to 900PC over the entire cross-section with subsequent cooling to maximum ambient temperature higher than 0.5 ° C / s. ¹ Λ The method of annealing a steel billet according to claim 4, wherein heating the steel billet at a temperature greater than 10 ⁹ C / s to a temperature between 650 ^and 900 ° C throughout the cross-section is delayed.