DE2263431A1 - COLD-ROLLED STEEL SHEET FOR PRESS FORMING - Google Patents

Description

NIPPON STEEL CORPORATION, Tokyo, Japan Cold rolled steel sheet for press forming

The invention relates to a cold-rolled steel sheet for press forming, that is made by continuous annealing.

Cold rolled steel sheets are widely used for such automobile parts used, which are cold-formed by drawing processes, for example rear or front fenders and Doors for which excellent draw formability is required.

As cold-rolled steel sheets, which have such excellent draw formability, aluminum-calmed-

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te, low-carbon steels used, which have a carbon content of 0.03 to 0.07 percent by weight and 0.02 to 0.08 percent by weight of acid-soluble aluminum, and also cold-rolled titanium-containing extremely low-carbon steel sheets in which the drawing deformability is not through ^; the heating rate during annealing is influenced and which are superior to the aluminum-killed low-carbon steel sheets in that carbon and nitrogen are bound by titanium. However, this aluminum-calmed, low-carbon steel sheet is almost annealed in a bell-type annealing furnace as a fixed winding or as an open winding. However, such box annealing has the following disadvantages: A long time is required for annealing, the power is low and uniform heating of the entire winding or the interior of the furnace is not possible for structural reasons, so that a uniform material quality over the entire length of the winding is not possible can be achieved. In particular, when the annealing is performed with a tight winding, poor contact between the surface of the steel sheet and the gas atmosphere in the furnace results, and moreover, various kinds of disturbances in surface properties due to a lack of uniformity easily occur. A titanium-containing, extremely low-carbon steel sheet has the disadvantage that its production cost is higher than that of an aluminum-killed, low-carbon steel because the former contains about 0.1% by weight of titanium.

The above problems encountered when box annealing aluminum killed cold rolled steel sheet are avoided by continuous annealing carried out in a continuous annealing furnace. The continuous annealing generally has the following advantages: The efficiency is high for construction reasons, one obtains equal to- ¹

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shaped material qualities over a winding and between different windings and a uniform contact between the surface of the steel sheet and the gas atmosphere in the Furnace is secured. On the other hand, there is also a continuous _ A solid glow is not free from disadvantages. "With rapid heating and brief soaking generally deposited crystal grains. Due to a high cooling rate after the glow carbon contained in the steel is insufficient than Carbide precipitate, so that the continuously annealed steel a high yield stress compared to box-annealed steel material and high tensile strength, but low elongation at break and low η value, as well as high tempering having. Therefore, the former is inferior to the latter in press formability. The continuous annealing treatment is therefore usually not used for the production of cold rolled steel sheet for press forming purposes, but mainly for the production of sheet steel, that

is used for tinplate production.

The reasons that a cold rolled steel sheet is one is subjected to continuous annealing treatment, an unfavorable Has press deformability can be seen below. With a rapid heat-up and one that takes place in a short time It is difficult to grow the crystal bodies when glowing, but they are also easily deposited. When cooling down after After the annealing, the amount of firmly dissolved plastic is large, so that a hardening tempering is produced, which the steel material makes hard. Eventually, with rapid heating, a recrystallized whole structure will develop inhibited, especially with regard to the drawing deformability is desirable. The biggest problem encountered in the production of cold-rolled steel sheet for drawing purposes of grades of SPCD class according to the Japanese industrial standard (JlS)

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occurs, however, a hardening remuneration due to the Abachrek- kens.

In order to avoid a hardening of the steel material due to a Härtungsvergütung, a method has been used in which a steel sheet winding, which was quenched after the continuous annealing, gradually to a heat treatment (over-aging sbehand lung) is subjected, at a temperature between 300 ⁰ C and 500 ⁰ C is carried out in a continuous annealing furnace or in a hood annealing furnace. However, if the over-hardening treatment in a hood-type annealing furnace takes place on the basis of the continuous annealing, the efficiency of the annealing process is reduced and the original goal of carrying out the annealing process continuously has to be abandoned. Even if the over-hardening treatment is carried out continuously in a continuous annealing furnace, it is still difficult to get the steel material sufficiently soft when the over-hardening treatment is carried out on an economical or practical scale, e.g.

As a result of numerous attempts by the inventors of the present application, which related to the chemical composition of the steel sheets, in order to achieve the To accelerate the phenomenon of over-hardening, it was found that a common aluminum-killed low-carbon Steel, which contains a corresponding amount of silicon, causes an acceleration of the over-hardening rate. Through this the steel material becomes sufficiently soft, even if an over-hardening treatment has been carried out for a short time, and a cold-rolled steel sheet which can be used for press forming is obtained.

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It has also been found by the same inventors of the present application that when vacuum degassing is carried out on the above-described aluminum killed low-carbon steel sheet to reduce the carbon content to 0.001-0.009%, a cold-rolled steel sheet can be obtained in which the draw formability and the The ductility is in the same range as that of titanium-containing, extremely low-carbon steel sheets and that the ductility is not influenced by the heating rate of the annealing. ^;

The invention therefore provides a possibility of producing cold-rolled steel sheet which is excellent in press formability by subjecting it to a continuous annealing process. , '■

The invention also provides a cold-rolled steel sheet excellent in press formability by an over-the-counter treatment of a continuous Applying glow system. .-j

The invention also provides a cold-rolled steel sheet which is always excellent in draw formability without being influenced by the cooling rate following the hot whelling and a winding temperature by subjecting the steel to vacuum degassing treatment to increase the carbon content to 0.001-0.009% to reduce.

Further advantages of the present invention will become more detailed from the following description with reference to the accompanying drawings explained.

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The drawing shows the influence of the silicon content on changes in the properties of steels when aluminum-killed, low-carbon steel sheets with a different silicon content are subjected to over-hardening during continuous annealing. ^:

The steel sheet according to the invention is a cold-rolled steel sheet with excellent press formability, the steel made of less than 0.08 wt% carbon, 0.05 to 0.4 wt% Manganese, 0.015 to 0.10% by weight silicon, 0.01 to 0.1% by weight acid-soluble aluminum, 0.003 to 0.015% by weight nitrogen and the remainder of iron and unavoidable impurities consists.

First, the constituent parts of the steel sheet according to the present invention will be explained.

The carbon content of the steel sheet according to the invention corresponds to that of the cold-rolled low-carbon steel of SPCC class (general cold-rolled steel sheet) or the SPCD class (cold rolled steel sheet for drawing purposes) accordingly G 31 41 of the JIS. It is not strictly necessary that To reduce carbon content in order to lower the hardening compensation. However, if the same characteristics as of Titanium-containing extremely low-carbon steel sheet for the inventive device sheet steel will be needed, its carbon content will be decreased to 0.001 to 0.009 wt% by subjecting the steel to a vacuum degassing treatment.

To avoid brittle fractures at the time of hot rolling caused by sulfur, it must contain manganese. In view of the fact that an aluminum killed steel, which is made by melting in a converter, usually sulfur in an amount greater than 0.05%

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contains, the manganese content must be above 0. -05% so that the above-mentioned sulfur content is fixed. However, there is a The larger proportion of manganese results in a deterioration in the workability of the steel Limit for this 0.4%.

Silicon is normally contained in an amount of less than 0.01% in a cold rolled steel sheet that is in has been refurbished in a steel melting furnace in accordance with the provision of G 3i 41 of the JIS. In the steel according to the invention however, the silicon content must make up an amount of more than 0.015%, preferably more than 0.02%, so that Properties of a steel sheet for press forming (SPGD class) with a continuous anneal which is given an over-aging treatment contains, without the conditions of the manufacturing stages carried out before cold rolling, for example, the conditions for hot rolling.

However, if the silicon content is greater than 0.1%, it increases the strength of the steel sheet increases to a value higher than necessary, so that the machinability is deteriorated. the therefore the upper limit is 0.1%.

In connection with the silicon content mentioned above, it must be mentioned that there is a mutual interaction between the silicon component and the carbon component. It was found by the inventors of the present application that in the case of a steel according to the invention which has a Contains silicon content in a range from 0.015 "to 0.1%, preferably the carbon content in a range of is reduced below 0.009% by subjecting the steel to a vacuum degassing treatment to reduce the r-value of the Relief of the heating rate of the annealing.

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The Γ value is an average of the plastic deformation ratio (plastic strain ratio), which serves as a guide for the drawability of the steel. If, however, the silicon content is below the above-mentioned range, the r-value cannot depend on its dependence on the heating rate of the Annealing can be exempted even if the carbon content is in a range of 0.001 to 0.009% as in the present one Invention is listed in detail.

If the silicon content is more than 0.015%, the Draw formability independent of the winding temperature of the annealing process. This shows that there is a correlation between the carbon content and the silicon content.

In order to achieve the object according to the invention, a content of acid-soluble aluminum in an amount of more than 0.01% is necessary. The lower limit of the acid-soluble aluminum, which is necessary to avoid the occurrence of elongation deformations caused by oxides contained in the steel is preferably set to 0.02%. On the other hand, when the acid-soluble aluminum is 0.1% exceeds, the workability of the steel is deteriorated. The upper limit is therefore set at 0.1%.

To reduce hardening due to the hardening remuneration, the cold-rolled steel sheet is subjected to a continuous annealing treatment so that the over-hardening speed is accelerated; the coexistence of the three components, i.e. silicon, aluminum and nitrogen, are more effective than silicon alone. For this reason, nitrogen must be in be contained in an amount greater than 0.003%. However, when the nitrogen content is over 0.015%, the workability becomes worsened. The upper limit is therefore set at 0.015%. From the above it can be seen that the most important ι

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Conditions for solving the problem according to the invention Limitation of the four essential chemical components Carbon, silicon, aluminum and nitrogen are for the reasons mentioned above.

The preparation of the inventive steel sheet by the method steps of the refining of molten steel in a steel melting furnace such as a converter, _s an open vessel furnace, an electric furnace, the production of an ingot, plate 'by blooming or continuous casting and subsequent hot rolling. The production temperature of the hot rolling process is preferably kept higher than 800 ^° C. There is no limit to the winding temperature. This means that in the case of an aluminum-killed low-carbon steel, the drawing deformability of the steel cannot be achieved unless the steel is heated to a grain enlargement point as the finishing temperature of the hot rolling process and is rapidly cooled after the hot rolling process, because the drawing deformability, because of the Deposition of AlN at: the grain boundaries is decreased, or when the winding occurs at a temperature close to the Al transformation point. In contrast to this, in the present invention, neither the finishing temperature during V / arm rolling nor the winding temperature has an influence on the effective components of the steel according to the invention from promoting over-hardening. The hot-rolled coil obtained in this way is then subjected to a cold rolling process after it has been annealed. The reduction percentage in cold rolling must be greater than 30%, preferably 60% to 85%, with a view to deformability. The cold-rolled winding is then annealed in a continuous annealing furnace of a suitable heating system.

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The annealing temperature is higher than 680 ⁰ C, but lower than 900 ⁰ C, and the time required for the recovery time is less than 10 minutes. The annealed coil is then subjected to an over-aging treatment in a device for an over-aging treatment, the temperature conditions of 300 ⁰ C to 500 ⁰ C and a time of less than 10 minutes, preferably 2 to 5 minutes, are observed.

In the case of the steel according to the invention, the cooling rate also has from the annealing temperature to the temperature of the over-hardening treatment has no influence on the effect that is achieved by the over-compensation treatment.

After the over-aging treatment is carried out, the steel sheet becomes subjected to a suitable method such as fine rolling or passing through a straightening machine to the shape to normalize and to rule out the expansion of the yield point. The invention provides a cold rolled steel sheet with excellent press formability, consisting of less than 0.08% by weight of carbon, 0.05 to 0.4% by weight of manganese, 0.015 to 0.10 wt% silicon, 0.01 to 0.10 wt% acid soluble Aluminum, 0.003 to 0.015% by weight nitrogen and the remainder iron and unavoidable impurities. This cold rolled Steel sheet is subjected to a process stage of continuous annealing with or without an overaging treatment manufactured.

The invention is explained in more detail below with the aid of examples.

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Example 1 ' . ·

Table 1 shows the chemical components of various types Steels to illustrate examples of the present

Invention.

These steels were cold-rolled to a sheet thickness of 0.8 mm. Through the usual process steps of producing cold-rolled steel sheets under hot-rolling and cold-rolling conditions as shown in FIG. The steels were then ^{annealed at 700 °} C. for 3 minutes in a continuous annealing furnace, after a heating time of one minute. After annealing, the steels were cooled to 370 ° C. at cooling rates as shown in Table 3. are posed. Subsequently, they were subjected to, over-aging treatment at 43o ⁰ C for 5 minutes, subjected to "Further, each steel finely rolled at a reduction percentage of 1.5%. The mechanical properties of the cold-rolled steel sheets obtained by the above-mentioned process steps are shown in Table 4 «,

Cold rolled steel sheets containing acid-soluble aluminum, nitrogen and silicon in amounts that are within of the ranges according to the invention are generally softer than those steels which contain these components in quantities that are outside the respective ranges. They are also excellent in all mechanical properties, like elongation at break, η-value and r-value. It is therefore understandable that the steel sheets according to the invention are very have good press deformability. At the same time must be established v / ground that the mechanical properties of the steels according to the invention depend only slightly on the winding temperature (coiling temperature) during hot rolling and the cooling rate can be influenced after annealing.

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Example 2

For the purpose of studying the influences of silicon on the mode of operation of the over-remuneration treatment were aluminum-calmed Low-carbon steels with different silicon contents are subjected to continuous annealing. Afterward some of them have undergone over-reward treatment, while others have did not take place. The mechanical properties of the steels were compared in both cases. The results of this Investigations are shown in the figure.

The composition of the steel samples examined and the treatment conditions are the following: ■

1. Composition:

C 0.04 - 0.07%

Mn j 0.18 - 0.30%

Solubility Al '■ 0.04 - 0.06%

N 0.0045 - 0.0060%

2. Conditions for V / arm rolling:

Finishing temperature 890 + 10 ⁰ C, winding temperature 630 + 10 ⁰ C

3. Conditions for cold rolling

Reduction percentage 73%

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4. Conditions for the continuous glow

Equilibrium conditions 700 ° C χ 90 sec.

Heating speed '60 sec. Up to

700 ⁰ C

Average cooling speed

22 ° c / sec.

Over-aging treatment 350 C χ 5 min.

5 «fine rollers

Reduction percentage 1.5%

As can be seen from the accompanying drawing, steel sheets which contain silicon in an amount of more than 0.01%, in the case of over-hardening with the same conditions, it is considerably softer, which means that it is press-deformable is so surprisingly increased that the material properties obtained in this way are very close to those of the SPCD class of JIS that are box annealed.

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Table-1: Chemical composition

CO O (D OO U) CJ

ti
C. invention tfick-
Lung
No. C. 0 Mn Si P. S. Sol.
Al N O ü
1 Invent φ 0.05 0
0 .15- 0.032 0.02 0.01 0.053 0.0038 0.005 in the area] U
φ B.
C. 0.03
0.06 0
0 .33
.20 0.020
0.047 0.02
0.03 0.02
0.02 0.040
0.032 0.0048
0.0058 0.008 ^
0.007 llplate φ D.
E. 0.05
0.02 0 .23
.29 0.017
0.080 0.01
0.04 0.01
0.02 0.080
0.057 0.0059
0.0045 0.004
0.005 3
cn • of the area 0.06 0 .19 0.023 0.02 0.01 0.045 0.0056 0.010 I.
U CD
Λ G 0.05 0
0
0 .32 0.009 0.01 0.01 0.053 0.0049 0.005 Outer plate H
I.
J 0.04
0.05
0.07 0 .28
.31
.32 0.035
0.010
0.024 0.02
- ■ -0.02
0.01 0.02
0.02
0.02 0.006
0.002
0.045 0.0029
0.0019
0.0024 0.018
0.032
0.011 -μ
co . K 0.04 .30 .0.010 0.02 0.02 0.054 0.0058 0.006

Table 2: Rolling record

Wick
lung
No. Conditions for the (° _c \
Wn-rmx ^ izen Wrap temp
rature Kaltre-
duction
percent-
rate {%) A ^ Finished
working temp. 560 73 . B. 890 560 73 C. 880 560 • 73 D. 850 .680 73 E. 870 680 73 F. 900 680 73 G 890 680 73 H 865 560 . ■ ⁷³ I. 890 680 73 J 880 680 '.: ⁷³ K 890 560 73 880

Table 3:

Cooling condition to 430 ⁰ C after annealing at

700 ° C

Cooling system Medium cool swin- / o _c / _sec )
thick I.
II
III 5.4 ("required time50 see.)
22 * - .; 5 ("time required0-2 seq.)
54.0 (required time: 5 see,)

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Table 4: Mechanical properties

I U
ι — Ι Φ Wick
lung
No. Kühlsy Stretch limit Tensile strength ■. fracture n-value 'r value 'Crystal- I. NJ XJXJ stem. (kg / mm ² ). speed strain size _ ^ NJ CD
• PÄ A. (kg / mm) \ / °) Wr. I. cn to ο
• H faO I. 22.5 33.2 44.0. 0.236 1.40 9.0 GJ ■ W φ £
φ t <3 B. -ρ φ »d II 21.3 32.5 44.3 0.240 1.42 8.4 co iH -H
^ · Η f- < C. O
co Φ UJ
• U f5 III 22.0 33.2 43.8 0.236 1.45 8.8 00 "KaI-
lead D. U) X!
CtJ £ -t E. I. 20.8 32.0 44.8 0.241 1.46 8.5 O -Pu Φ F. II 2.3.0 34.0 44.0 0.237 1.47 8.3 -J CO
03, £} CQ G III 19.5 31.2 45.7 0.240 1.51 8.0 Φ U Φ III 23.7 35.3 41.2 0.229 1.41 8.6 -P Φ Ä H N OT above
ι — I Ui · Η bO III 23.4 34.8 42.0 0.230 1.23 9.4 cd ρ φ c £ ca ^ 3 I. φ φ ΰ
fen ^^ ^^ ^ 2 III 27.3 36 ^J ; 5 40.1 0.225 1.08 10.2 + ^ Ο · Η H Φ W «Η
«3 H Φ ί ·· J III 24.0 36.0 42.2 0.228 1.25 9.0 K III 27.9 37.2 40.5 0.223 1.18 9.3

Example 3

Table 5 shows steel samples which are obtained when one Steels in a chemical composition as shown in Examples 1 and 2, a vacuum degassing treatment undergoes to reduce the carbon content to a range of 0.001 to 0.009 / 0.

These steels were hot-rolled and then cold-rolled under the usual hot-rolling and cold-rolling conditions as shown in Table 6 to obtain a steel sheet having a thickness of 0.8 mm. During this and afterwards, they were continuously annealed at 870 ° C. for four minutes. The steel sheets annealed in this way were then fine-rolled with a reduction percentage of Λ%. The mechanical properties of the steel sheets thus obtained through the above process steps are shown in Table 7.

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Table 5

co

(O OO U)

sample
No. LÖsl.
C Mn Si P Al ^N OS 'Al-calmed extremely low
carbon steel in the area
the invention A.
B.
C.
D. 0.007 0.15 0.032 0.02 0.043 0.0076 0.005 0.02
0.005 0.33 0.020 0.02 0.086 0.0103 0.008 '0.02
0.009 0.30 0.065 0.02 0.097 0.0138 0.006 0.03
0.004 0.10 0.145 0.01 0.027 0.0045 0.012 0.01 Al-killed steel except
half of the range of the inventor
manure E.
F. 0.013 0.28 0.030 0.02 0.043 0.0082 0.003 0.02
0.008 0.23 0.010 0.03 0.03 0.0056 0.005 0.02 Ti-containing, extremely low
carbon steel outside
• the scope of the invention G Ti
0.008 0.15 0.01 0.01 0.105 0.0045 0.003 0.03

CD

Table 6

Wick
lung
No. Conditions for the when- "
rolling (C) Winding temperature
rature Potash
duction- ..
percent
rate (%) A.
B.
C.
. "E.
* \
G Finishing
tunpstemp. 560
530
620
570
570
390
550 70
70
74
76
70
70
76 890
880
890
870
850
.840
900

Table 7: Mechanical properties of annealed steels

sample
No. Stretch
limit ^
(kg / mm; Zugfe
sturdiness
(kg / mm) fracture
strain
(%) n-value r-value Crystal-
grain-
size
No. *1 A.
B.
C.
D. 14.3
15.0
15.1
15.3 32.6
32.0
32.0
32.5 49
■ i
■ 49
49
47 0 .259
0.250
0.255
0.254 1 "94
1.95
1.80
2.00 7.6
8.0
8.0
6.9 * 2 E.
F. 19.3
18.2 33.5
35.6 44
43 0.238
0.220 1.53
1.15 8.5
9.1 * 3 G 12.8 31.8 53 0.291 2.40 8.1

* 1: Steel within the scope of the invention * 2: Steel outside the scope of the invention

* 3: Ti-containing extremely low-carbon steel outside the Scope of the invention

* 4: 0.2% yield strength

Annealing system: Continuous annealing 870 ⁰ C χ 4 min.

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From the above, in particular with reference to Examples 1 and 2, the basic idea of the present invention can be seen. Its purpose is to use a continuous annealing process to create a steel sheet which has excellent press formability. The basic idea of the invention is based on the knowledge that a steel sheet can obtain excellent press formability in a continuous annealing treatment if main components of the steel are precisely prescribed, in particular silicon is added in an amount as prescribed by the present invention. The press formability of steel can, however, be improved even more remarkably by subjecting this steel to an over-hardening treatment, that is to say a kind of artificial hardening, as is sought in the softening of steels. : ·

With the present invention it has therefore been possible, in economic terms, to solve the greatest problem, one by one hardening refinement, conditional hardening, to overcome a problem encountered in the manufacture of a cold-rolled, low-carbon Steel sheet occurs by continuous annealing according to any conventional method. The reason for that The high rate of over-aging of the steel sheet according to the invention appears to be that contained in the steel Silicon promotes the precipitation of carbides. This also seems to be related to the fact that the The r value of the steel according to the invention, which is obtained after the continuous annealing, depends only slightly on the coiling temperature of the hot rolling step is affected because AlN precipitated during hot rolling is not the easy one Takes the form of AlN, but that a compound of Al-N-C fails. Hence, there is not much of a difference in the size of what has failed and how it is distributed from there.

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Table 3 also teaches that even without a Carrying out an over-hardening treatment at the end of the continuous annealing process on a steel according to the invention the press formability can be improved when the carbon content can be reduced to a range between 0.001 and 0.009% to make the steel softer, by subjecting the molten steel to a vacuum degassing treatment.

In this case it is also necessary to mention that the f- ¥ ert can also be freed from its dependence on the heating rate of annealing. In the case of steel with a carbon content of more than 0.01% changes in generally the dependence of the r-value on the heating rate of annealing as a function of the carbon content of the steel, as is typical for an aluminum killed cold rolled steel sheet is. By reducing the carbon content of the steel of the present invention to less than 0.09% by subjecting it to a vacuum degassing treatment Therefore, as mentioned above, this dependence on the r-value disappears. This shows. That the recrystallization structure of the aluminum-killed, cold-rolled steel sheet undergoes a change, from a certain limit of the carbon content, while it makes the carbon content of 0.01% a limit. From this it is understandable that the recrystallization structure of the aluminum-calmed, cold-rolled steel sheet with a carbon content of less than 0.01% is suitable for an annealing process, in which rapid heating is carried out.

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Claims (3)

Patent claims 1. Cold rolled steel sheet for press forming, characterized by subjection to a continuous annealing treatment during manufacture and a carbon content of less than 0.08% by weight, 0.05 to 0.4 Gev /% manganese, 0.015 to 0.10 Gev /% silicon, 0.01 up to 0.1% by weight nitrogen and a residual content of iron and unavoidable impurities. 2. A cold-rolled steel sheet according to claim 1, characterized in that the continuous annealing treatment contains an over-aging treatment at its end, said over-aging treatment at 300 ⁰ C to 500 ⁰ C over a period of less than 10 minutes, preferably, 2 to 5 minutes carried out, . 3. Cold rolled steel sheet according to claim 1, characterized in that the carbon content is reduced to a range of 0.001 to 0.009 Gevi% by subjecting the steel to a vacuum degassing treatment in the molten state. 309833/0772