DE2108788A1 - Process for the production of low carbon steel of unusually high drawability - Google Patents

Description

Process for making low carbon steel of unusually high drawability.

The invention relates to a method for the production of sheet steel with improved deep-drawing properties Appropriate control of both the variable composition factors and the procedure.

There are currently two basic types of sheet steel with low carbon in the deep drawing market used. These two types are the low-carbon, unkilled steels and the aluminum-killed steels Steels. Low carbon, unkilled steels are more economical and have better surface properties

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Patent attorneys Dipl.-Ing. Martin Licht, Uipl.-Wirtsch.-Ing. Axel Hansmann, Dipl.-Phys. Sebastian Herrmann

as aluminum killed steels, but do not have the very good drawing properties of aluminum killed steels, required for extreme drawing operations. Other elements, particularly titanium, have a high affinity to it Oxygen are also often used in the manufacture of killed steels, either alone or in conjunction with conventional reducing agents such as aluminum. Aluminum-titanium or aluminum / titanium-calmed Steels, however, appear to be less suitable for continuous casting, although they have outstanding deep-drawing properties. These steels are due to the high Costs of reducing agent additives and the low yield of the molds as well as the high conditioning costs also expensive. It is therefore desired to develop a new deep drawing steel that is relatively inexpensive and is nevertheless suitable for processing by conventional or continuous casting techniques.

The drawability of sheet material can be determined by simple stress tests. When a strip-shaped sample is stretched to a greater length, its width and thickness decrease. The plastic stress or flow ratio can serve as a measure of the degree of mechanical anisotropy of the material. This ratio is referred to as the "R value" and is defined as the ratio of the percentage change in width (e = width _{tension) to the percentage change in thickness (e t} = thickness tension), ie

or R = In

m (L

where V and L ti · width or length «de« M »» eab »ohnittes and the indices i and f are the initial and final

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Measured values (before and after the application of voltage) of these Dimensions mean. This expression is based on the assumption that the volume of the measuring section during the The experiment remains constant and eliminates the direct measurement of the thickness, which is due to its low value in the case of sheet metal leads to inaccurate results.

The R-value is therefore a common parameter for the indication of the degree of mechanical anisotropy of a given material. For an isotropic sheet is the R value is 1. If R is less than 1, the sheet becomes inappropriately thin and is therefore suitable for drawing operations undesirable. For very strong deep drawing, the R values should be equal to or greater than 1.5 and preferably greater than 1.7.

In order to achieve an average R-value, tensile tests must be carried out on several samples at different angles, usually under O'k5 and 90 to the rolling direction. An average R-value of the sheet can then be determined as follows:

R = R _o ° + 2Rj ₁₅ O + R ₉₀ O) A

The difference between the individual R values shows the tendency of the sheet to form lugs during the drawing process. A uniform value for R is extremely desirable. The fluctuations in the R-value in the plane of the sheet are often denoted by Δ R, where Δ R is defined as follows:

^{Δ R = (R} 90 ° ♦ 2 ♦ have been taken

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Although Δ R s = O does not necessarily indicate that none If earing has occurred, this value is useful for describing the more planar anisotropy of the sheet.

In recent years, the drawing properties of steel sheets have been associated with crystallographic textures brought. Good drawability and high R-values are part of it to the cubic corner-centered Testur or (ill) planes parallel to the plane of the sheet. The next most preferred orientation for good deformation properties is the (112). Lower R values belong to both the cubic face-centered (100) texture as well as the cubic edge-centered (HO) texture. Thus, the proportion of less preferred textures should be used for good drawing properties or structure must be limited to a minimum. For optimal Drawability, the ideal textures consist of arbitrary orientations of (ill) planes in the sheet metal plane, i.e. from a (ill) fiber texture, where the sheet plane is normal like the fiber axis is arranged.

The crystallographic texture of a sample is usually made up of complete pole figures X-ray intensity measurements determined. However, a direct comparison of two pole figures can be used to determine a small variation in texture does not reveal the detailed differences in quantitative terms. It has therefore proven best to use the integrated peak intensities of various reflections from to measure the plane of the sheet and express it in units of corresponding peak intensities of an arbitrary sample. The numerical values of these relative intensities obtained in this way are directly proportional to the Poldiobtm special plane that is parallel to the plane of the sheet ·. Because the drawability of a sheet of the relative occupation special crystallographic planes in the bleo plane

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depends, this technique has proven to be very useful. The intensities of 5 different reflections, i.e. (IiO), (200), (112), (310) and (222) are measured. The intensity of the (222) reflection, which corresponds to the reflection of the (lli) second order, therefore represents the proportion of the (lii) texture. Similarly, the intensity of the (200) reflection represents the portion of the (100) texture In the correlation between the R values and the texture As the actual test results have shown, there is very good agreement.

It is known that a steel sheet with an R value above about 1.20 has superior drawability, and known deep-drawn sheets generally have maximum R values which, for example, approximately correspond to the latter value, as described in US Pat. No. 3,2kk 565 is. Higher II values are known and listed, for example, in US patent specification 3 kOk 047. Such high R values have hitherto been associated with very low carbon contents, as are achieved, for example, by excessively long decarburization annealing treatments in the manner described in the two above-mentioned patents.

The exponent η for plastic work hardening is a second fundamental factor in deep-drawability in connection with the R-values. The exponent η becomes experimentally as the slope of the curve from the logarithm of the true stress versus the logarithm of the true strain im Area of uniform expansion determined. High n values Like high R values, they are beneficial to improved drawability, although the η value is much more remarkable depends on microstructural factors than on the crystallographic Texture. PUr steels with a low carbon content, i.e. low-carbon steels, so η varies between 0.2 and about 0.3 f Ir .1.U «. st «, *. 10 9 8 3 7/1131

The object of the invention is therefore to provide a steel sheet that has a crystallographic Texture with an emphasis on (ili) and (112) planes in the plane of the sheet. A further aim is to create a steel composition which, when used with produced by the process of the invention results in extremely high R values, i.e. values greater than about 1.5 and preferably greater than 1.7, in conjunction with good n-values.

In addition, the steel connection to be created should only have a moderately low carbon content, which enables a combination of high R-values and low & R-values without affecting the yield point strength is adversely affected, and without the need for the sheets of this material to undergo an expensive decarburization treatment to undergo.

Finally, a method for producing sheets in the above steel composition is to be provided with the help of which the maximum E-value potential can achieve.

According to the invention, this object is achieved by a method for the production of sheet steel with an extraordinarily high drawability, which is composed of the following process steps:

a) adjusting the composition of a steel melt in such a way that they in wt -.% is made up of 0.005 to 0.15% manganese, 0.03 to 0.1% carbon, 0.004 to 0.03% sulfur, less than 0.015% Oxygen, a maximum of 0.06 % silicon, a maximum of 0.02% phosphorus and the remainder iron with the usual impurities that occur in steel production {

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b) hot rolling a slab produced from the melt at a temperature which is sufficiently high, so that the formation of pre-eutectoid ferrite is prevented;

c) cold reduction of the resulting sheet to 60 to 80%; and

d) warming the cold-reduced sheet over a Time from 12 to 30 hours at a temperature of 6 ^ 9 to 732 C to give the sheet a crystallographic texture with predominantly (111) and (112) levels in the sheet metal level.

It has been found that a number of compound and method related variables are important in establishing maximum R values. In order to obtain the necessary high yield strengths, the carbon content of deep-drawing steels is generally in the range from 0.06 to 0.1%. If a melt of such a steel is adjusted so that the manganese content remains below about 0.15% and the oxygen content below about 0.015 % , the subsequent careful mechanical treatments and heat treatments result in a steel with the desired high R values. It is essential that the cast slab having a composition which is in the above range is hot-rolled at a temperature sufficiently high to ensure that the final hot-rolling continuous pass takes place above the temperature at which pre-eutectoids occur Ferrite forms. After such hot rolling the steel between 60 and 80% must be cold reduced and annealed by heating for a period of at least 12 hours at a temperature of 649-732 ⁰ C. It has now been found that a steel composition requires the carefully measured amounts of carbon, hangan, and oxygen

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along with controlled maximum amounts of sulfur, phosphorus and silicon it contains, to form deep-drawing steel sheets when subjected to the above treatment results in unusually high R-values above 1.5 and good n-values having. The composition of these steels is shown in the following table.

Element Composition, wt%

Broadly preferred

Carbon 0.03 to 0.10 0.03 to 0.075

Manganese 0.005 to 0.15 0.005 to 0.07

Silicon 0.06 max. 0.06 max.

Sulfur 0.004 to 0.03 0.01 to 0.2 *

Phosphorus 0.02 max. 0.01 max.

Oxygen 0.015 max. 0.01 max

Impurities occurring in steel production.

* * t

The upper limit of 0.02% sulfur results from the

Need to control hot cracking. If the tendency to hot cracking is caused by the addition of other elements, for example aluminum or titanium, is reduced, then somewhat higher are used to limit the more planar anisotropy Values welcome.

The careful construction of the combined compound and the treatment steps will be apparent from the following Description of the drawing figures, in which show:

Fig. 1 is a graph from which the dependence of the R-value obtained on the warm whale temperature can be seen, and

Fig. 2 is a diagram showing the relationship between the measure of the new steel connections with the achievable S-values for sheets produced in the manner described here

t 0 9 i :: ν, ■ / ■ r j j

ORIGINAL INSPECTED

C. Mn TABLE I. S. Si : Composition of the steels examined (, G < <0.005 <0, , 005 Ni Cr N Old 5W ^mm jo) I. stole 0.05 0.46 0.011 0.052 'P Cu It , 005 0.015 0.011 0.003 <Ö "7002 TkTWn I. 0.07 0.005 0.006 0-.006 <0, It , 005 o, oo6 0.004 0.0006 <0.002 PPM I. II 0.05 0.05 0.005 0.0031 0.005 <o, ff M. 0.015 0.003 0.002 <0.002 20th te A. 0.06 0.10 0.004 0.04 η ft dd 0.002 0.003 dd 40 B. 0.06 0.14 0.004 0.04 η dd η 0.003 0.001 η 30th C. 0.04 0.20 0.004 0.04 H ti η 0.003 0.003 ti 20th D. 0.04 0.30 0.004 0.04 <o, oo5 <o. ^M. η 0.004 0.003 η 60 E. 0.05 0.38 0.004 0.04 ff fl 0.016 0.005 0.002 η 50 F. 0.04 0.56 0.004 0.04 dd , 005 0.016 0.006 0.001 dd 40 G 0.05 0.06 0.004 0.04 dd η 0.003 <0.002 0.001 0.002 30th H 0.05 0.06 0.0075 0.03 dd η 0.002 dd 0.001 η 45 I. 0.05 0.06 0.013 0.03 η ^w 0.003 η 0.001 ft 90 J 0.05 o, o6 0.016 0.03 <o, oo5 <o ^w 0.003 ti 0.001 η 90 K 0.05 0.06 0.020 0.03 dd It 0.003 dd 0.001 It 50 L. 0.05 0.06 0.029 0.03 η , 005 0.002 η 0.001 ff 65 M. 0.06 0.08 <0.004 0.03 η It 0.006 0.014 0.008 0.007 50 N 0.06 0.08 0.004 0.08 η 0.007 0.016 0.008 0.010 270 O 0.06 0.09 0.012 0.08 η 0.006 0.018 0.008 0.008 290 P. 0.06 0.10 0.024 0.08 0.007 0.019 0.007 0.004 350 Q 0.04 250

"* w o, o6 0.05 0.013 ο, οι <o, oo5 <o, oo5 0.02 0.002 0.0015 0.002 35

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Two compounds (Table I, Samples I and II) were hot rolled in six nearly equal reduction passes to a final thickness of 2.54 µl. After each pass, the panel was heated for 2 minutes to restore the temperature. The direction of rolling was reversed for each pass. The strips were air cooled, cold rolled to 70 "fo ", and dry annealed by heating to 710 ° C (at 20.3 ° C / hr) and held at that temperature for 20 hours (see U.S. Patent 3,404 047) The effect of the rolling temperature on the R value is shown in Figure 1. While the effect of the hot rolling temperature is impressive for both steels, it is even more emphasized for the low-manganese and low-oxygen steel (ii) maximum S value is achieved by the rolls immediately above the A_ temperature. B _e i tempera doors higher rolling takes this value from again, but not as rapidly as at lower temperatures.

The significance of the manganese content is recorded below in Figure 2 (Table II). A number of vacuum fused steels (Table I, Samples AG) were hot rolled at 927 ° C, cold rolled, and annealed for 20 hours at 704 ° C in an atmosphere of 656 H "and 94 % N" in six passes. To obtain an R value above 1.5, the steel should contain less than about 0.15% manganese. During production it will be very difficult to control the hot rolling temperature as precisely as was the case in the experiments described. Therefore, in order to ensure that an R-value above 1.5 is obtained, it will generally be necessary to use a much lower manganese content, ie a manganese content between about 0.005 to about 0.07%.

ORIGINAL INSPECTED 109837/1 131

- 11 TABLE II

Effect of manganese content on mechanical properties of sheet steel hit low carbon.

Steel % Hn R

45

90

Through annealed cut grain stretch size limit meshes / cm strength (kp / mm2)

A. 0.05 2.07 2.12 1.52 3.12 + 1.10 20.00 3.54 B. 0.10 1.71 1.96 1.10 2.67 +1.09 19.85 3.54 C. 0.14 1.60 1.86 1.08 2.43 +1.07 20.10 3.54 D. 0.20 1.36 1.74 0.80 2.10 +1.12 19.35 3.35 E. 0.30 1.17 1.43 0.75 1.72 +0.83 19.80 3.35 F. 0.38 1.05 1.08 0.67 1.78 +0.76 19.20 3.35

0.56 1.07 1.23 0.74 1.58 +0.67 19.55

2.56

The effect of the sulfur content on the development of the The texture and the S-value are shown in Table III. Cold rolling and annealing were essentially the same as above.

TABLE III

Effect of sulfur content on the mechanical properties of low carbon steel sheet.

Steel % SR

II I J K L M

45

0.004 1.85 1.96 1.42 0.0075 1.81 1.76 1.48 1.89 1.92 1.58 1.91 1.74 1.74 1.91 1.45 1.84

0.013 0.016 0.020 0.029

90

AR

Through- annealed section- grain size limit - meshes / cm strength

2761 +0 , 87 22.60 3.73 2.52 +0 , 66 23.35 3.93 2.47 +0 , 62 25.50 '•, 13 2.41 +0 , 34 25.80 4.13 2.50 +0 , 14 25.90 4.13 2.20 -O , 22 26.60 4.13

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The results show that, although for high R values a low sulfur content is not essential, but the possibility exists (in a low manganese content having steel) by sensible control of the sulfur content to produce a steel in which the planar Anisotropy, i.e. lower / \ R values, to a minimum is limited. The pole figures (not shown) generated from X-ray intensity measurements have made it clear that this change in the K values at 0 ° and 45 ° from the depends on the following crystallographic texture changes:

1) a somewhat more uniform (ili) fiber texture in W of the plane of the sheet and a larger (112) component, both of which lead to an increase in the R-value at 45 °,

2) an increase in the (200) component that decreases R for the 0 ° direction.

Thus, a range of 0.0i # to about 0.03 % sulfur is desirable, both to limit the planar anisotropy to a minimum and to increase the yield point strength (for example, steels N to Q). In the case of steels with a low manganese content, the upper limit is determined by the tendency towards heat resistance. Of course, if W uses additional elements such as Al or Ti to solve this problem, then slightly higher proportions of sulfur can be used.

Another connection mutable that has been found to be important for the purpose described here is the oxygen content, which should be low, so that it is warm of the steel is minimized or prevented during hot rolling. From the Table IV shows that if the oxygen content reaches a level of 250-300 ppm in the steel, one

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OftenWMNAL INSPECTED

harmful effect on the development of a steel with high B values can be expected.

TABLE IV

Positive stress ratio, ratio values for permanent Deformation of a steel with low carbon and low manganese content with oxygen levels of 250-350 ppm and variable sulfur content.

Steel wt

1 * ο

R.

90

Through- annealed section- grain size limit- Masohen / cm strength (k p / mm2)

N 0.027 1.57 1, 75 0.97 2.58 +1.20 20.30 3.54 0 0.029 1.53 1, 45 1.21 2.24 +0.64 22.40 3.73 P. 0.035 1.48 1, 30th 1.24 2.16 +0.49 25.50 3.93 Q 0.025 1.39 1, 00 1.21 1.15 +0.37 25.65 3.93

This adverse effect could occur due to the fact that the oxygen, which is in oxide form and / or is in the form of silicate inclusions, forms preferred nuclei for recrystallization. In order to obtain the desired R values, the The oxygen content of the steel has been reduced to around 150 ppm generally by a reduction in carbon under vacuum.

Although the winding temperature is in the usual temperature range from 538 ° - 6 ^ 9 ° C does not seem to be important, should For optimal properties, it is best not to exceed a temperature value of 621 ° C. To the winding process

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To imitate, Stahl ¥ was made in six passes at 932 ° C hot rolled to a thickness of 2.67m. Four separate sections were made in the following manner prior to the 7O £ Cold rolling and annealing as described above treated:

a) Cooled with air to ambient temperature,

b) air-cooled to 538 ° C, held for four hours, then air-cooled

c) air-cooled to 593 ° C, held for four hours, cools, then air-cooled,

d) air-cooled to 649 ° C, held for four hours, then air-cooled.

TABEL

Effect of the simulated winding temperature on the stress test values of the steel W.

RR ₀ R. - R _qo / \ R Through- annealed

cuts - grain size zenfestig- meshes / cm speed (kp / iim)

a 1.97 2.00 1.64 2.57 +0.65 24.40 3.93

b 2.11 1.92 1.87 2.79 +0.49 23.30 3.73 - 3.93

c 2.26 2.12 2.07 2.76 +0.37 23.60 3.93

d 1.92 1.91 1.55 2.68 +0.75 23.80 3.93

Work hardening exponent of the permanent deformation η and Toughness of steel c (above) (average of two measurements).

Test η Uniform overall toughness

. . , Toughness in at break

direction 2.54 cm

0 ° 0.266 35%

45 ° 0.250 33%

90 ° 0.254 31 *

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As can be seen from the table above, a simulated cooling temperature of 649 *
ίϊ value and a higher / XR value /

simulated cooling temperature of 649 ⁰ C is a little lower

From the above information it is clear that a low-carbon steel sheet with S values greater than 1.7 and normal n-values can be established if within the composition range described and the process limiters mentioned. The extraordinary the good properties of this steel depend directly on it on the crystallographic texture, mainly a (lii) - fiber texture - in the plane of the sheet, which developed during the final box annealing process, The composition of the steel sheet can be used as a manufacturing method either the normal ingot process or the continuous casting process.

The product described here can be manufactured in the following manner. Z _u nearest the melt is melted until it has the prescribed composition * Then, the melt of a carbon reduction is subjected to vacuum in order to reduce in this way the oxygen content of the steel to 150 ppm or less, so that the resultant steel slab containing either can be produced by conventional ingot practice or by continuous casting, has a composition within the prescribed range. The slabs are then hot-rolled, if necessary, thereby re-heated, and at a temperature that is high enough (1232 ⁰ C) is to ensure that the final pass of hot rolling takes place at ei.ner temperature above that at which proeutektoldes ferrite. The steel is then wound up at around 53B - 621 ° C and cooled in air to room temperature. The hot-rolled skin is removed and the steel is

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Reduced cold by 60 to 80% to the final dimension. The desired Maximum R values outside of this range are not so easily obtained by cold reduction, so it is preferable is to adjust the hot-rolled thickness so that a Cold reduction of around 70% to the desired final size leads. The steel strip is then wound into an open spool and box annealed in a conventional manner, i. slowly warmed to soaking temperature, i.e. to approximately 704 ° C in at least 12 hours, with about 20 hours preferred to get the desired texture and dip to get high Π values.

Claims (11)

Claims 1. A method for producing sheet steel of unusually high drawability, characterized in that the composition of the steel melt is adjusted so that the melt in wt .-% 0.005 to 0.15% manganese, 0.03 to 0.1% carbon, 0.004 contains up to 0.03% sulfur, less than 0.015 % oxygen, a maximum of 0.06% silicon, a maximum of 0.02% phosphorus and the remainder iron with the usual impurities occurring in steel production that a slab produced from this melt is hot-rolled at one temperature high enough to prevent the formation of proeutectoid ferrite, that the resulting sheet is cold-reduced by 60 to 80%, and that the cold-reduced sheet is soaked at a temperature of 649 to 732 ° C for twelve to thirty hours, to give it a cristenographic texture old predominant (111) and (112) planes in the sheet metal plane. 2. The method according to claim 1, characterized in that the planar anisotropy of the sheet is limited to a minimum by setting the sulfur content of the melt to over 0.01%. 3. A method according to _n claim 1., characterized in that the manganese content is set to less than 0.07% in order to achieve by means of the ordinary ¥ armwalzverfahrens an H-value with certainty, which is greater than 1.5. 4. The method according to claim 1, characterized in that that the adjustment of the oxygen content takes place by carbon reduction under vacuum. i Ü ü ii;. '/' <Ί 3 1 5. The method according to claim k, characterized in that the final pass in the warning rolling process takes place at a temperature in the range of 927 to 999 ° C. 6. The method according to claim 5, characterized in that the cold reduction makes up about 70%, and that the Soaking temperature is about 704 ° C to thereby to maximize the It value of the sheet. 7. The method according to claim 5, characterized in that the melt for producing the slab is cast in a continuous casting process. 8. The method according to claim 1, characterized in that the composition of the melt is adjusted so that it consists essentially in% by weight of 0.03 to 0.075% Carbon, 0.005 to 0.07% manganese, maximum 0.06% silicon, 0.01 to 0.02% sulfur, maximum 0.01% phosphorus, maximum 0.01% oxygen and remainder iron with the usual at the Impurities occurring in steel production. 9. Sheet steel, produced according to one of the claims 1-8, characterized in that the sheet steel has an S value greater than 1.5. 10. Sheet steel according to claim 9, characterized in that that the Ε value is greater than 1.7 and that the manganese content is between 0.005 and 0.07%. 11. Sheet steel according to claim 10, characterized by duroh Stretch limit of over 0.01%. a yield strength of over 27 kp / mm and a sulfur content 1 U 9 ε. "? / 1 1 3 1