DE1508455C3 - Process for the production of steel sheets with low carbon content, improved ductility 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, 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 T from 0.004 to 0.02% extends over 10 hours and exclusively in dry hydrogen with a dew point of -34.4 ° C takes place. An annealing time that is more than 15 to about 20 hours has proven particularly useful.

With the help of the novel process, a crystallographic texture with a high-grade (111) -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 R 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 the thickness), so it is

= eje "
In (W, /

R =

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, tensile tests are carried out on several samples taken from the material at different angles, usually at angles of 0, 45 and 90 ° to the rolling direction. The average .R value of the sheet can then be calculated as follows:

R = (R _0. + 2i? ₄₅ o +

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

The difference between the individual R values

shows the inclination of the sheet metal during the drawing process To form lobes. 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 both an improved drawbar wedge as well as having a high tensile strength.

The crystallographic texture of a sample is usually determined by constructing complete pole figures from X-ray intensity measurements. To find a slight deviation in texture, however, a direct comparison of two pole figures cannot reveal exact differences in qualitative terms. It has therefore proven to be the best way to measure the integrated maximum intensities of some reflections from the plane of the metal sheet and express them in units of the corresponding maximum intensities of any sample. The numerical values of the relative intensities obtained in this way are directly proportional to the pole densities of a certain plane which 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 (111) reflection and therefore shows the proportion of the (l 11) texture. In the same way, the intensity of the (200) reflection represents 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 illustrated below by means of examples described in more detail. The following illustration is based on three continuously cast steels with low Carbon content referred to as "A", "B" and "C" and the following in Have percent by weight specified compositions.

Table I.

Chemical composition of steels

Mn

Si

AIsol

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

Steel temperatures in ⁰ C
entry

Exit reel

A. 1060-1066 B. 1060-1066 C. 1260

871

Final thickness
(mm)

621 2.44 621 2.44 582 2.18

45

50 Crystal graphic examinations of the hot rolled plate, the results of which are given in Table III below, show only minor differences between the three steels, except for the fact that the (HO) intensity of the steel compound C is very high and the (222) intensity is lower were than in both compounds A and B. The texture of compounds A and B was practically the same.

Table III

Relative intensities of the selected
X-ray reflections from the hot rolled 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 (HO)
(200)
(Π2)
(310)
(222)

65 0.80
1.59
1.23
1.16
1.57

0.89
1.70
1.26
1.10
1.34

1.12 1.78 1.14 1.10 1.14

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

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

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 again seven.

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 (Π0) 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

given. The corresponding values for the SK-Gülestahl are also given for comparison.

Table V

The R values. the yield strength and grain size
of annealed in a dry hydrogen atmosphere
Low carbon steels compared to the equivalent values of a typical
SK-Slahls

SK

35

40

45

50

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 favorable one Texture component (222) is larger than the SK steel. On the other hand are the (110) 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 i? Values, which indicate the relative drawability, the yield strength and the grain size, also are as they were obtained from duplicate samples, in Table V again-

R ₀ . 1.82 1.77 1.25 1.73 R ₄₅ 1.70 1.16 0.91 1.31 R ₉₀ 2.48 2.30 1.93 2.22 R. 1.93 1.60 1.25 1.64 Stretch limit 13.1 16.2 21.9 16.3 (kp / mm ² ) ASTM 5.5 6.0 9.0 6.5 Grain size

As can be seen from the table, the R values of alloys A and B are equal to or superior to those of SK steel, while the R values of compound C are unsatisfactory. In contrast to the SK steels, the grains are aligned in all three carbon 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 i? 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 experiments 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, while they decrease with decreasing soaking time. The residual carbon content affects the λ 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
Hold time to the average ratio of
permanent change in shape to tension on the
i? value and the residual carbon content

Heating rate

(Time up to 716 ⁰ C)

Hold time R

(At A
716 ° C)

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 residual carbon content and / or the tendency to form longer grains, which is awakened by annealing in a wet hydrogen atmosphere. The chemical composition of the samples A and B annealed in a dry water-substance atmosphere is given in Table VI11. As can be seen, the nitrogen content has decreased significantly. Since the unbound nitrogen is a major cause of stress aging in steels with a low carbon content, the undesirable effects of aging on the mechanical properties can be kept to a minimum by this treatment.

Table VIII

Chemical composition of steel after annealing in dry hydrogen

20 steel Si

AWAY

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 for 30 Table IX Using wet hydrogen results in excessive decarburization and excessive grain growth adjust, although acceptable R-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.

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.

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

35

Table VII Drier 1.90 Water- 1.68 Wet hydrogen B. material 1.76 1.21 1.54 A. 2.55 B. 2.26 A. 1.20 1.99 1.59 1.82 2.08 R ₀ O 12.5 16.3 1.43 1.50 R45 ⁰ 2.25 6.9 R ₉₀ O 6.0 7.0 1.73 R. 7.0 7.0 Stretch limit 0.007 0.004 (kp / mm ² ) 6 to 1 0.0015 to ASTM 0.0014 Grain size 0.0015 carbon salary (%)

40

55

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 R values obtained are generally satisfactory. The reason for this difference can be found in the low 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 compared to the stress aging index the well-known box-annealed edge steels, which are usually between 20 and 25%.

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 is reduced to about 0.016-0.04%. The effect of this treatment is to that the drawability of the material is improved without the yield strength being deteriorated. The It can also be seen from the above that excessive decarburization results in excessive grain growth As a result, the drawability and the yield strength of the steel sheet are adversely affected. In addition, the product is due to the fact that with the help of dry hydrogen at Annealing also achieves a significant reduction in nitrogen content, for example to about 0.001% becomes, relatively age-resistant and deep-drawable.

609 621/14

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 time period door the annealing process is more than 15 hours to about 20 hours.