DE2107640A1 - Continuous annealing process for steel with low yield strength, delayed aging properties and good drawability - Google Patents

Description

PATENT LAWYERS

TELEPHONE: COLLECTIVE NO. 22S341

TELEX 529979

TELEGRAMS: ZUMPAT CHECK ACCOUNT: MUNICH 91139

BANK ACCOUNT: BANK H. HOUSES

8 MUNICH 2,. BRÄUHAUSSTRASSE 4 / 1H

4 / Ma
P-2102-22

Nippon Kokan Kabushiki Kaisha, Tokyo / Japan

Continuous annealing process for steel with low yield strength, retarded aging properties and good drawability

The invention relates to a continuous annealing process of steels with good drawability, and in particular steels with low yield strength and delayed aging properties.

The material produced with the aid of known continuous annealing processes was mostly hardly used for drawing loads, since this material shows severe aging and a high degree of hardness. It is known that this is mainly due to the cooling process, which is much faster than the batch annealing process. It means that

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BATH ORIGINAL

rapid cooling process produces an increase in the solute Carbon in steel

Therefore, numerous studies have been carried out to improve the quality of the steels produced by this continuous Annealing process are produced. Typical examples of these investigations are the US steel process in the USA and called the Dortmund process in West Germany. That the former is said to be due to the storage treatment of the steel passing through and the latter to the aging treatment of the coiled steel. These methods are cited as a technique that improves workability made of hot-dip galvanized steel. However, it can be safely said that these improvements only lie in a decrease in the hardness of the steel. In particular, it is known that the rate of production in the latter Process is very low as it uses a batch annealing process. In any case, it shows no improvement the drawability and especially the aging properties of the steel.

The aim of the invention is therefore to improve the drawability and the alloying properties with the aid of a continuous annealing process which enables a high production rate. The characteristics of the invention are that is first the continuous strip to a temperature of above the Aj-point to below the A ~ point location, preferably heated to 750 to 800 ⁰ C, while the temperature of the above-mentioned US steelmaking lower than is the A. point. Secondly, this strip should be rapidly cooled from the temperature mentioned to less than 500 ^{° C.} In this case, the tape can of course also be cooled to room temperature. In any case, it is necessary that this be done quickly

BAD OBiGiNAL

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210764Q

The cooling rate is more than 50 ^° C. per second, and the higher this rate, the greater the improvement in quality. Third, should then a storage treatment at a temperature of 300 to 500 ^D C, preferably from 350 to 45.0 ⁰ C, for more than 10 seconds are carried out. Fourth, a high Wiekeltemperatur .for the. '"Continuous Warmwalz.schritt is recommended, which is over 630 ⁰ C, to make more effective to the above continuous annealing process.

According to the above-mentioned inventive method, the steel with low yield strength, with retarded Preserved aging and good drawability in a simple manner will.

The object of the invention is therefore a continuous annealing process of steels which exhibit good drawability, delayed aging and good press formability, which can be produced on a large scale.

In the following, the invention is based on examples and the attached drawing will be explained in more detail. It shows:

Fig. 1 shows an example of the continuous annealing cycle according to the invention in comparison with the conventional cycle.

Fig. 2 shows a modification of the cycle shown in Fig. Ϊ, the steel is cooled very quickly to room temperature.

Fig. 3 is a diagram showing the relationships between the yield strength or aging index and storage treatment temperature.

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In the method according to the invention it is desirable that the carbon content in the steel is limited to ίΤ0.06%. It is generally a fact that the yield strength has a strong dependence on the carbon content in the steel, particularly in a steel made by this continuous annealing process as compared to one made by the batch annealing process. For this reason, the carbon content is limited to the above range As found by the inventor, the yield strength of a steel having the above-mentioned carbon content can be effectively reduced as compared with other steel having the same level of carbon content and made by a conventional method when the steel is made with a carbon content of * 0.06% is wound at an elevated temperature of more than 630 ⁰ C · such a process is initiated to the grain growth and to increase the carbides in this invention of continuous annealing steps a result, it is confirmed that according to the inventive continuous annealing the yield strength is noticeably lowered.

The method according to the invention can be used for all types of steel such as unskilled, aluminum-killed steel and the like, if the carbon content of these steels respectively * Is 0.06%. There are no restrictions on the components other than carbon. All known methods of obtaining of cold rolled steel strips can be used, i.e. steel making processes, ordinary casting processes, continuous Casting processes, slab making processes, common hot rolling processes and cold rolling processes.

The continuous annealing cycle according to the invention is shown in FIGS. In these figures, I shows this cycle and II a conventional cycle. Heating can be inventive

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must be carried out as in the ordinary cycle II, since the quality and aging properties of the steel are hardly affected by how the strip is heated. The Durchglühtemperatur is in the range from above the recrystallization temperature to about 800 ⁰ C, preferably at 710 ° to 750 ⁰ C. The annealing time is identical to the to the belt to heat the conventional cycle, which is sufficient uniformly left '. Then the high cooling rate of more than 50 ^° C. per second is required for the tape to reach the storage treatment temperature, as will be explained later, or room temperature. A cooling rate of less than 50 ^° C. per second is not desirable because the aging index of the steel becomes higher than that of the batch-annealed steel.

Fig. 3 shows the relationship between the storage treatment temperature and the aging index or the yield strength. Referring to FIG. 3, it is easy to see that the storage treatment temperature should be ^{in the range of about 300 °} C to 500 ^{° C.} In particular, the temperature of 350 ^° C. to 450 ^° C. results in the lowest aging index and the lowest yield point. Such properties are evident from curve III of the yield strength and curve IV of the aging index. If the tape is stored for at least 10 seconds, the above properties can be easily obtained. There is no problem in the way of cooling after this storage treatment.

Actual embodiments of the steel according to the invention are as follows:

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example 1

Steel making furnace: LD converter
Chemical composition:

Table I.

stole C. Mn P. S. N O 1 0.031 0.29 0.012 0.016 0.0012 0.056 2 0.045 0.34 0.013 0.019 0.0015 0.053 3 0.052 0.30 0.013 0.023 0.0015 0.041 4th 0.060 0.33 0.011 0.022 0.0013 0.033 S. 0.078 0.33 0.015 0.023 0.0016 0.031 6th 0.088 0.34 0.015 0.018 0.0015 0.039

Hot rolling conditions after the slab formation: finishing temperature: 865 ° C Bundwickeltemρeratür: 580 ⁰ C final thickness of 2.8 mm

Cold rolling conditions after pickling:

Deformation: 71.4% final thickness: 0.8 mm

Annealing Conditions:

Cycle A: according to the invention with continuous annealing cycle annealing temperature: 750 ⁰ C cooling rate: 120 ° C / sec. Storage treatment: 400 ° C χ 30 sec.

Cycle B: conventional continuous annealing process
Annealing temperature: 750 ⁰ C

Cycle C: conventional batch · ühvarfah ^ in annealing temperature: 700 ⁰ C

Glow duration:

6 hours

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Mechanical properties after the above-mentioned treatment see p

Table II

stole cycle Stretching
border
kg / well Tensile strength
ability ₂
kg / mm Total-
stretchable
% Aging
index
kg / mm Lank
ford - A. 21.5 33.0 46.9 3.8 1.26 1 B. 25.7 35.3 42; 3 7.2 1.27 C. 24.6 33.6 47.1 4.6 1.30 A. 22.8 33.4 46.5 4.1 1.30 2 B. 26.8 35.7 41.3 6.8 1.29 C. 23.6 33.8 46.1 4.9 1.33 A. 23.8 33.9 45.8 4.5 1.29 3 " B. 27.3 36.2 40.3 7.0 1.30 C. 23.3 34.1 44.9 5.3 1.36 A. 23.9 34.5 45.1 4.6 1.33 4th B. 28.6 36.9 40.0 7.3 1.26 C. 24.9 34.9 45.0 5.2 1.40 A. 25.9 35.0 43.9 4.3 1.31 5 B. 29.6 37.8 39.2 6.9 1.26 C. 25.9 34.9 44.1 5.2 1.30 A. 27.0 36.3 41.8 4.7 1.30 6th B. 31.5 37.9 38.1 7.1 1.25 C. 25.7 35.2 43.0 5.3 1.28

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From the above tables it can easily be seen that the mechanical properties of the steels according to the invention, i. the steels 1-A, 2-A, 3-A. and 4-A, those of the conventionally continuously annealed steels, i.e. steels 1-B, 2-B, 3-B and 4-B are superior and those of conventionally batch annealed steels, ie. of steels 1-C, 2-C, .3-C and 4-C are similar when the carbon content of these steels is in the range according to the invention, i.e. ^ 0.06% as mentioned above. Needless to say, the mechanical properties of steels, their carbon content lies outside the range according to the invention, that is steels 5 and 6, are significantly worse than those of the steels according to the invention, even in the continuous annealing process of steels 5-A and 6-A.

Therefore, the reason is that the mechanical properties of the steel of the present invention are better than those of the others Steels in the limitation of carbon content, in the high Cooling rate and storage treatment temperature. That is, at such a high cooling rate, the saturated dissolved carbon is noticeably increased and in the subsequent Storage treatment step greatly accelerates the precipitation of such carbon. In accordance with the low yield strength, the properties of delayed aging, which is lower than that of in conventional Way, batch-annealed steel, can be obtained in a simple manner as shown in Table II is ·

Example II

The manufacturing conditions are the same as those in Example I, except for the hot rolling conditions.

Finishing temperature: 870 ^{° C}

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Bundle winding temperature: 7OO ° C Final thickness: 2.8 mm

Mechanical properties:

(cycle C shown in example I is excluded)

Table III

stole cycle Stretch
border
kg / mm " Tensile strength! G-
keitp
kg / mm Total-
stretchable
% Aging
index
kg / mm Lank
ford - 1 A.
B. 19.8
24.3 31.3
33.9 46.1
41.3 3.5
7.1 1.33
1.31 2 - A.
B. 20.4
25.2 31.9
34.0 45.3
40.6 3.8
7.1 1.26
1.26 3 A.
B. 21.0
26.8 31.8
35.6 45.2
40.1 3.9
6.8 1.31
1.25 4th A.
B. 20.7
27.3 32.0
36.0 45.0
40.4 4.1
6.8 ■ 1.29
1.28 5 A.
B. 22.9
28.7 33.9
3 7.5 "\ 44.1
40.0 4.3
6.9 1.23
1.24 6th A.
B. 24.5
29.8 34.3
38.1 42.5
38.0 4.1
7.0 1.21
1.25

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- ίο -

This embodiment differs from Example I in the coil winding temperature. That said, the steels will be at “The results show a greater improvement in mechanical properties than those in Example I. It should be particularly noted that

2
the yield strength is reduced by about 2 kg / mm compared to that of Example I.

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Claims (4)

210764Ö patent claims 1. Continuous annealing process of steels with low yield strength, with delayed aging properties and with good drawability, characterized in that the steel production is controlled so that the carbon content in the. Steel * 0.06% becomes that after the usual slab making, hot rolling and cold rolling a continuous
Annealing is performed in which the annealing temperature is in the range of and above the recrystallization temperature to about 800 ⁰ C then at a rate of more than 50 ° C / sec. up to 500 ⁰ C to 300 ⁰ C
is cooled, and that then a storage treatment is carried out in which the steel strip for at least 10
Seconds at a temperature of 300 ⁰ C to 500 ⁰ C is held. 2. Continuous annealing process according to claim 1, characterized in that the coil winding temperature of the hot rolling process is more than 630 ⁰ C · 3 · Continuous annealing process according to claim 1 and 2, characterized in that the storage treatment of the Stole! will. Steel strip in the range from 350 ⁰ C to 450 ⁰ C carried out 4. A continuous annealing process according to claim 1 to 3, characterized in that the Glühtemperatür is the steel strip in the range from 710 ⁰ C to 750 ⁰ C. 109840/1069 JJL Blank page