DE1508455B2 - Process for making low carbon steel sheets with improved drawability and yield strength - Google Patents

Description

The invention relates to a method for producing steel sheets with a low carbon content improved drawability and yield strength through crystallographic reorientation of the grain structure, whereby hot-rolled plates made of this steel with more than 0.02% carbon by cold reduction by 50 rolled up to 85% to sheet thickness and the cold rolled sheet in a hydrogen atmosphere at one temperature is annealed between 552 and 843 ° C.

Aluminum killed steels are known to have excellent drawability. Such steels, steels that are named "SK" -quenched and tempered steels according to the US standard are characterized by flattened or Cupcake-shaped ferrite grains, which are crystallographically oriented and therefore good drawability of the material. Such grains are box annealed in the finished steel sheet generates, meanwhile, a selective growth of the aluminum nitride precipitation favorably oriented grains is effected. However, aluminum killed steels are relatively expensive, not only because of the cost of the alloy, but also because of the low yield of the ingots and the high conditioning costs. It is now known (GB-PS 10 01 877), the annealing process in in a decarburizing atmosphere, although it is not specified from which gases this atmosphere should be composed.

It is also known to perform the annealing process first in a moist and then in a dry hydrogen atmosphere to take place (US-PS 32 15 566), with moist hydrogen for the main part of the glow time is used and only for a short afterglow time the use of dry hydrogen as After decarburizing gas is recommended to restore normal ductility of the steel the steel has become brittle as a result of annealing in a moist hydrogen atmosphere.

In another known method of this type (OE-PS 2 39 294) the annealing treatment of the Stahls in a hydrogen atmosphere, which must be moist for a certain period of time, in order to increase the decarburizing effect compared to that of a dry hydrogen atmosphere, whereby the low decarburization effect of dry hydrogen compared to that of moist hydrogen to a large extent is known (Engeneering magazine, January 22, 1932, p. 92).

The object of the invention is now to use its degree of moisture in the method of the type mentioned different hydrogen atmospheres, which adversely affect the yield strength of the steel influence, avoid and thereby reduce the procedural costs.

This object is achieved in that the annealing process for setting a Carbon content in the sheet of 0.004 to 0.02% extends over 10 hours and only in dry Hydrogen with a dew point of -34.4 ° C takes place. In particular, there is a glow time proven, which takes 15 to about 20 hours.

With the help of the novel process, a crystallographic texture with a high-grade (III) -orientation and a lower (100) -orientation is created. The low carbon steel treated in the manner according to the invention has a very favorable average i? Value, which thus indicates good drawability. This .R value indicates the degree of anisotropy of the sheet. The .R value is defined as the ratio of the percentage change in width (e _w , change in shape of the width) to the percentage change in thickness (e " change in shape of thickness), so it is

R = eje _t ,

R =

ln (WJW _f ) \ n (L, W _x / L ₁ W ₁

Here, W and L denote the width and length of the measuring section before (index i) and after the change in shape (index /). This expression is based on the assumption that the volume of the measuring section remains constant during the test and it avoids the direct measurement of the thickness which, because of its small value in the case of sheet material, leads to less accurate results.

To determine an average .R-value, several samples, which are in different Angles have been taken from the material, usually at about angles of 0, 45 and 90 ° to the Rolling direction ^ tensile tests undertaken. The average Ä-value of the sheet can then be like calculate as follows:

R = (K ₀ I + 2R ₄₅ o + i? ₉₀ o) / 4.

The K-value is therefore a useful parameter for indicating the degree of mechanical anisotropy of a material present. An isotropic sheet material has the R value 1. If R is less than 1, the sheet is thinning excessively and is therefore unsuitable for drawing processes. For deep drawing, the .R values should preferably be equal to or greater than about 1.5, but materials with lower K values are also suitable for some applications.

The difference between the individual /? Values

indicates the tendency of the sheet to form lugs during the drawing process. The greater the difference, the more the tendency to form earing is also stronger.

According to the method characterized above, it was found that by controlling the eventual existing carbon content and by preventing excessive decarburization a high yield point can be maintained and a low carbon steel can be produced which has both improved drawability as well as having a high tensile strength.

The crystallographic texture of a sample is usually made more complete with its construction Pole figures determined from X-ray intensity measurements. To find a slight deviation in the texture, however, a direct comparison of two pole figures can reveal exact differences in qualitative terms Respect not reveal. It has therefore proven to be the best way to use the integrated maximum intensities measure some reflections from the plane of the metal sheet and measure them in units of the appropriate Express the maximum intensities of any sample. The numerical values of the relative intensities obtained in this way

IO is directly proportional to the pole densities of a certain plane that is parallel to the plane of the sheet. Since the drawability of a sheet depends on the relative frequency of the special crystallographic planes in the plane of the sheet, this technique is very useful. The intensities of five different reflections, e.g. (110), (200), '(112), (310) and (222) are measured. The intensity of the (222) reflection represents the second order of the (III) reflection and therefore shows the proportion of the (111) texture. In the same way, the intensity of the (200) reflection determines the proportion of the (100) texture. These two textures and their counterparts therefore influence the drawability. The present test results showed a good agreement between the R values and the texture.

The invention is described in more detail below by means of examples. In the following illustration reference is made to three continuously cast steels with a low carbon content, starting with "A", "B" and "C" are designated, and the following compositions are given in percent by weight exhibit.

Table I.

Chemical composition of steels

Mn

Si

Alsol

A. 0.04 0.42 0.01 0.017 0.014 0.005 0.003 0.012 B. 0.05 0.45 0.03 0.018 0.014 0.006 0.003 0.011 C. 0.014 0.23 0.01 0.018 0.012 0.006 0.004 0.025

It can be seen that compositions A and B have the required carbon content of over 0.02% whereas Composition C had a lower starting carbon content. Everyone these steels were hot rolled in a rolling mill under the following conditions.

Table II

Hot rolling conditions

45

Crystallographic studies of the hot-rolled plate, the results of which are given in the following Table III shows only minor differences between the three steels, except the fact that the (110) intensity of the steel compound C is very high and the (222) intensity were lower than both compounds A and B. The texture of compounds A and B was practical the same.

Steel temperatures in ° C

Entry exit

reel

A 1060-1066
B 1060-1066
C 1260

871

621 2.44 621 2.44 582 2.18

Table III
Final thickness 50

(mm) Relative intensities of the selected
X-ray reflections from the hot rolled one

Plate, steels A, B and C

55

The fine structure of the plate hot-rolled from these steels consisted of unidirectional grains with an ASTM grain size of about 8. For compositions A and B, at the grain corners fine pearlitic inclusions found, while carbide plates are present at the grain boundaries was. In composition C the pearlite inclusions were only sparsely present, but occurred a few thin carbide plates at some grain boundaries.

60

(Π0)
(200)
(112)
(310)
(222)

0.80
1.59

1.23

1.16
1.57

0.89
1.70
1.26
1.10
1.34

In the case of aluminum killed steels, the crystallographic texture is created by precipitation of the aluminum nitride controlled. When treated according to the invention

The control of the crystallographic texture is done through the use of old, low-carbon steels of the cementite normally present. This can be done through various annealing treatments before cold rolling, for example solution treatments and tempering, done or by decarburization during of crystallization annealing, which causes the creation of the desired textures. It has found that excellent R values are obtained in low carbon steels can, the initial carbon content of which was above about 0.02%, as is the case with the compounds, for example A and B is done by having the annealing done in a dry hydrogen atmosphere with the carbon content reduced to about 0.004-0.02%. Also was found, as will be shown below, that such a treatment for compound C to does not give satisfactory results because the initial carbon content is already below the 0.02% limit was.

The hot rolled samples of compositions A, B and C were reduced by 70% by cold rolling their thickness, namely from 2.44 mm to 0.74 mm, as far as connections A and B are concerned, reduced, while compound C was reduced from 2.18 mm to 0.66 mm. The samples were then at 716 ° C annealed in dry hydrogen, the dew point of which was -67.8 ° C. They were on it for 20 hours kept at annealing temperature and then allowed to cool in the oven. The texture of the samples is related with that of a typical aluminum-killed (SK quality) deep-drawing steel in Table IV reproduced.

Table IV

Relative intensities of selective X-ray reflections from under dry hydrogen
Annealed low carbon steels
and a typical SK steel

A. B. C. SK (110) 0.09 0.17 0.25 0.25 (200) 0.17 0.23 0.54 0.38 (112) 1.18 1.12 1.42 2.04 (310) 0.24 0.27 0.45 0.26 (222) 6.00 5.80 4.70 4.90

35

40

45

From this table it can be seen that the unfavorable texture components, i.e. (200) and (110) of compounds A and B are considerably below those of SK steel, whereas the very cheap one Texture component (222) is larger than the SK steel. On the other hand are the (HO) and (222) components of compound C is the same as that of SK steel, while the (200) and (310) components are noticeably higher. The (112) component of all three low carbon steels is below those of SK steel.

The R values, which are indicative of relative drawability, yield strength and grain size, which were also tested, are shown in Table V as obtained from duplicate samples. The corresponding values for the SK quality steel are also given for comparison.

Table V

The R-values, the yield strength and grain size
from annealed in a dry hydrogen atmosphere
Low carbon steels compared to the corresponding values of a typical
SK steel

A. B. 1.77 C. 1.25 SK Ro » 1.82 1.16 0.91 1.73 R450 1.70 2.30 1.93 1.31 R ₉ O = 2.48 1.60 1.25 2.22 R. 1.93 16.2 21.9 1.64 Stretch limit 13.1 16.3 (kp / mm ² ) 6.0 9.0 ASTM 5.5 6.5 Grain size

As can be seen from the table, the R values of alloys A and B are those of SK steel is the same or superior, while the R values of compound C are unsatisfactory. The grains are in the same direction in all three carbon steels in contrast to SK steels.

The degree of decarburization depends to a certain extent on the flow rate or speed of the hydrogen-containing atmosphere and the surface of the sample exposed to this atmosphere. With a flow rate of approximately 60 to 80 cm ³ / min. the carbon content can be reduced to about 0.004% if the sheets are carefully arranged. If the sheets are arranged so that they are loosely in contact with one another, the carbon content can be maintained at about 0.016%. Extensive tests have shown that a fundamentally good texture, high R values and a high yield point are always obtained when the carbon content in the annealed steel strip is reduced to about 0.02%. On the other hand, it has been found that the texture and the .R values of the annealed steel strip are always poor when the carbon content present at the end is greater than 0.02%. It can be seen from the results of many tests that the desired crystallographic texture is developed during annealing in a dry hydrogen atmosphere through the influence of the precipitation of iron carbide on the growth properties of the grains.

The effect of the heating time and the holding time on the ratio of permanent deformation to stress is shown in Table VI. These results show that with a constant soak time, the average R values increase slightly with increasing heating time and with constant heating time, they decrease with decreasing soaking time. The residual carbon content affects the R values in such a way that low R values are associated with high carbon contents, ie with carbon contents greater than 0.02%. However, the residual carbon content also indicates that the most effective decarburization takes place mainly during the soaking period of the annealing treatment.

Table VI

Effect of heating rate or holding time on the average ratio of permanent change in shape to tension, to the K-value and the residual carbon content

Heating rate

(Time up to 716 ⁰ C)

Hold time R

(At A
716 ⁰ C)

AWAY

6 min.
30 min.
360 min.
900 min.
900 min.
900 min.
900 min.

20th
20th
20th
20th
2
10
20th

1.65
1.75
1.74
1.97
1.15
1.35
1.97

1.45 1.59 1.53 1.60 1.26 1.39 1.60

0.011 0.011 0.007 0.029 0.024 0.007

0.011 0.010 0.004 0.034 0.023 0.004

To show the importance of using dry hydrogen, a series of tests was carried out, in which the individual experiments differed only in that in some dry hydrogen and the others used wet hydrogen. Although, as is known, wet hydrogen has a greater effect on decarburization than dry hydrogen, it has been found that when used wet hydrogen results in excessive decarburization and excessive grain growth set, although acceptable jR values can be obtained. In Table VII are the results of four experiments with alloys A and B, two of which with dry hydrogen in the glow process and two with wet hydrogen in the glow.

Table VII

Dry hydrogen

Wet hydrogen

40

45

R ₀ o 1.90 1.68 1.82 1.54 R ₄₅ o 1.76 1.21 1.43 1.20 KgQo 2.55 2.26 2.25 2.08 R. 1.99 1.59 1.73 1.50 Stretch limit 12.5 16.3 7.0 6.9 (kp / mm ² ) ASTM 6.0 7.0 6 to 1 7.0 Grain size carbon 0.007 0.004 0.0015 0.0015 to salary (%) 0.0014

It can be seen from the results shown in Table VII that, as indicated above, the Yield strength when annealing in a wet hydrogen atmosphere is only about half as high as when Annealing in a dry hydrogen atmosphere, although the K values obtained are generally satisfactory. The cause of this difference may be in the low

Residual carbon content and / or the tendency to form longer grains, which are in a wet hydrogen atmosphere is awakened. The chemical composition of the substance atmosphere in dry water Annealed Samples A and B are given in Table VIII. As can be seen, has the nitrogen content is significantly reduced. Because the unbound nitrogen is a major cause of stress aging Low carbon steels can reduce the undesirable effects the aging on the mechanical properties by this treatment to a minimum.

Table VIII

Chemical composition of the steel after
Glow in dry hydrogen

stole

Si

0.007
0.004

0.02
0.03

0.016
0.016

0.001
0.001

0.009
0.008

The stress aging index of products that have been annealed in a dry hydrogen atmosphere is given in Table IX.

Table IX

Stress aging index *) from under dry
Hydrogen annealed steels (pre-stretched 8%,
aged 4 hours at 100 ° C and measured again

Percentage increase in the yield strength 12.7 12.4 (Stretching stress aging index)

*) Percentage increase in the yield point if the sample is pre-stretched by at least 5% or stretched beyond the discontinuous stretch range, then ^{aged for 4 hours at 100 °} C. and then checked.

The above values are in comparison to the stress aging index of the known box-annealed ones Edge steels, which are usually between 20 and 25%, are cheap.

It is essential for the success of the novel process that the initial carbon content of the treated Steels is above 0.02% and that also the carbon content during the annealing process is below dry hydrogen reduced to about 0.016-0.04% will. The effect of this treatment is that the drawability of the material is improved, without the yield point being worsened. From the above it can also be seen that Excessive decarburization leads to excessive grain growth that the drawability and the The yield strength of the steel sheet is adversely affected. In addition, due to the fact that with the help of the dry hydrogen during annealing there is also a considerable reduction in the nitrogen content, for example up to about 0.001%, is achieved, relatively aging-resistant and deep-drawable.

509542/145

Claims (2)

Patent claims: 1. Method of making low carbon steel sheets with improved drawability and yield strength through crystallographic reorientation of the grain structure, being hot-rolled Plates made from a carbon steel with more than 0.02% carbon by cold reduction rolled by 50 to 85% to sheet thickness and the cold rolled sheet in a hydrogen atmosphere is annealed at a temperature between 552 and 843 ° C, characterized in that that the annealing process to set a carbon content in the sheet of 0.004 to 0.02% extends over 10 hours and exclusively in dry hydrogen with a dew point of takes place below - 34.4 ° C. 2. The method according to claim 1, characterized in that the period of time for the annealing process is more than 15 hours to about 20 hours.