DE2511805B2 - Manufacture of high-strength, cold-rolled deep-drawing steels using a continuous heat treatment process - Google Patents

Description

The invention relates to a method for producing high-strength, cold-rolled deep-drawing steels, in which the steel after cold rolling at a heating rate of at least 200 "C / min heated to 650 to 900 ° C, 10 to 120 seconds at this Annealed temperature and then cooled at a cooling rate that is at least corresponds to the usual air cooling.

With regard to measures to ensure the safety of a motor vehicle and similar vehicles In recent years, the need for highly ductile, cold-reduced steel sheets has grown. However The research and development for this type of steel was not always sufficient to to meet this need. The current situation is that there is little or no effect there are appropriate facilities for this. Given the current situation the industry has been found that a conventional practice for manufacturing the above high strength cold reduced steel is that expensive elements such as Mn, Ti, Nb or V are added and the steel through a Bundle annealing process is annealed. However, such a method is associated with the following disadvantages:

1. The addition of expensive elements in large quantities causes an increase in costs,

2. the cold reduction process becomes difficult due to the hard quality,

3. The unevenness of the strength of the steel is inevitable.

Many various improvements have been proposed and put into practice to improve the Addition of these special items to replace. The representative examples of these procedures are reported in Japanese Patent Specification 40-3020 (known in the UK as the BISRA process) and in Japanese Patent Nos. 46-9541 and 46-9542 (in known in the USA as the domestic steel process). With these techniques, the BISRA procedure is a cold-reduced one Steel strip heated to 740 to 850 ° C, quenched to 150 to 250 ° C, the quenched strip immediately reeled up and accelerated aging by self-starting. However, it must be the carbon content can be greatly increased so that the required strength is achieved by such a method will. Of course, this increased carbon content results in a decrease in the ductility of the steel. That The process is not fully continuous, as self-starting is essential in the federal government and productivity

ίο decreased. In the above-mentioned inland steel process, a cold-reduced steel strip is heated to a temperature above the A3 transformation point and quenched in water to increase its strength. The steel sheet obtained by this method, however, has a weakness in that its strength is ^{lowered by about 15 kgf / mm 2 in} post-press molding, while its ductility is inferior in strength-elongation ratio. In other words, the steel is hard when it is to be subjected to press forming and becomes soft when it is processed into a finished product. Such a steel sheet is very difficult to treat and requires greater improvement in raising the safety of an automobile, etc. However, no proposal has been made to manufacture an inexpensive steel sheet which does not lower the strength after baking without adding special elements .

From the German Offenlegungsschrift 24 35 927 a method of the type described at the outset is available known. In the known method, a steel with a content of 0.04 to 0.12% carbon, 0.10 up to 1.60% manganese and 0.01 to 0.20% acid-soluble aluminum are used. The resulting steel is however, with regard to its mechanical properties, especially its ductility and strength, in particular with regard to the requirement for stove enamelling after compression molding in motor vehicle construction not yet fully satisfactory.

The present invention is based on the object of a simple and effective method for To create a cold-reduced steel that does not have the disadvantages of the known method has and in particular the resulting steel with good deformability even after one on the Press molding following stoving has a high strength.

This object is achieved according to the invention according to claim 1, characterized in that the method of the initially The type described is applied to a steel containing 0.04 to 0.12% carbon, 0.1 to 1.6% manganese and Contains 0.5 to 2.0% silicon.

According to a preferred embodiment of the invention, this method is applied to the steel according to claim 1 with the proviso that the cooling takes place in a water jet and the strip is then tempered for 10 to 300 seconds at 300 to 500 ° C.

In a further advantageous embodiment of the invention, this method is used the steel according to claim 1 with the proviso that the annealing temperature is 700 to 900 ° C, the strip then cooled in a jet of water and tempered for 4 to 300 seconds at 180 to 300 ° C.

The invention makes a high-strength, cold-reduced one when a continuous annealing process is used Steel created a surge, at least

the same level of initial strength of the steel even after being press formed to a required level has the following stove enamel finish and which without the addition of special elements despite the Increased strength, good deformability ϊ The steel obtained according to the invention is therefore particularly well suited, in particular, to guarantee the required safety of motor vehicles. The marked improvement achieved by the invention the mechanical properties of steel, in particular the increase in tensile strength, is due to the achieved synergistic effect, which is due to the high silicon content selected according to the invention in Connection with the applied special heating cycle of the continuous annealing process will.

In the publication “Transactions of the American Society of Metals ", Vol. 43 (1951), pp. 1130/1132 Although basically made an indication that an increased silicon content the mechanical properties of steel is supposed to increase. For this purpose, an equation for the Tensile strength given taking into account the silicon content. With given silicon contents However, using this equation results in significantly smaller increases in strength than with the steel produced according to the invention with corresponding silicon contents. The cause of that by the Invention achieved markedly improved result lies in the synergistic effect, which according to the invention brought about by the high silicon content in combination with the selected heating cycle. On the other hand, there is no reference to the heating conditions in the cited literature. This literature reference could therefore not contribute anything to the present invention.

Furthermore, the German Offenlegungsschrift 23 30 123 describes a method in which a cold-rolled Steel strip heated for several hours to 670 ° up to the A3 transformation point and with less than ^ o lOOVh, in particular 25 ° / h., Is cooled. if In the case of this reference, too, it concerns the production of a steel strip for motor vehicle bodies is and the ranges of C, Mn and Si according to this reference with the corresponding Overlap content ranges according to the present invention, this reference for do not contribute anything to the present invention. Namely, the present invention is different from that known method already in that the cold-rolled steel strip, which the usual components without the addition of expensive special elements, has a very special heating cycle is subjected to the steel properties in combination with the silicon content selected according to the invention to improve in the continuous annealing process to an extent as in the known Procedure is not achieved. Furthermore, in the known method, the silicon is also used, among other things used as a deoxidizer, while in the present invention the deoxidation alone with Aluminum is carried out. By contrast, the addition of silicon in the invention shows this Sub-structure in the cold-rolled state is an idiomorphic type, which is only found in the case of the one used in accordance with the invention specific heating cycle is effective. Thus, the silicon is added according to the invention to the to obtain a fine-grain recrystallization structure through the synergistic effect. Otherwise relates this reference on steel with a maximum of 0.2% C content. In the known annealing process, so is as much C as possible during the heating hold stage solved on what the precipitation hardening of the fine carbon in the following stage depends, and this large amount of dissolved C becomes a solid solution in the rapid cooling stage. In contrast, at of the present invention, the sub-structure of the cold rolled steel after cold rolling by the Silicon addition is the idiomorphic kind, and the fine-grained recrystallization structure is in that Heating cycle of the continuous annealing process reached.

The invention is described below in conjunction with the description of exemplary embodiments explained in more detail with the drawing

The drawing shows three courses used in the method according to the present invention Heating cycles.

The composition of a steel effective for the purpose of the present invention is at for steel production are dimensioned as follows:

C: 0.04 to 0.12%
Si: 0.5 to 2.0%
Mn: O.10 to 1.6%.

This composition and other elements are therefore chosen for the following reasons:

C: The lower limit of 0.04% is set to the critical limit in oxygen freshening in an industrial-scale LD converter, while the upper limit is 0.12% is set so as not to impair the ductility and spot weldability of steel.

Si: This element is one of the most important requirements for the present invention. If Si is less than 0.5%, hardly any effect of the present invention appears, while more than 2.00% Si die Deformability drops drastically and makes the cold reduction process almost impossible. The Si effects should be such that the machinability is not impaired and the σ;, value is improved while the Variation in strength is kept to a minimum considering the balance between the Tensile strength and elongation, d. H. the (σ /, + O) value. When a strip is subjected to a fully continuous annealing process, it occurs differently than that Solution hardening the Si effects are much more evident than with other processes. As the cause of this phenomenon was investigated, it was found that an intermediate structure, in particular a dislocation structure, in the cold reduced state by "the above-mentioned addition of Si a peculiar kind shows, which in connection with the rapid heating brings about numerous and small grains, which will be followed later is yet to be referred to. Furthermore, the grain growth is inhibited because of the short dwell time, which will also be referred to later. Such an effect of grain refinement increases the tensile strength and improves the elongation. At the same time, there is a very narrow spread of strength Indication of the formation of a hard quench structure, which is even then very low due to the addition of Si is held when the heating temperature is higher. Thus the Si plays an incomparable role in the continuous annealing process and can be a really effective and appropriate element in achieving

of the objects of the present invention. Specifically, a Si content of 0.7 to 1.5% is considered to be am most preferred area recommended.

Mn: The lower limit of 0.10% is set in view of the hot brittleness during which The upper limit is set at 1.6% in consideration of the limitations on steelmaking operations by a Siemens-Martin furnace or converter. The preferred range is 0.10 to 1.00 percent on the side a still low Mn content. Is the Mn content above 1.00%, a two-phase structure is easily formed with the hard quench structure and a soft one Ferrite structure, which causes a decrease in ductility and an increase in the influence of heating temperature on strength, and a Increase in the possibility of a greater variation in material quality, especially tensile strength, results.

Al: This is only added to adjust the degree of deoxidation and can be 0.002 to 0.2%.

S, P and O: The content of these elements should be in Be lower in terms of deformability.

A steel strip composed of the above composition should not have a special manufacturing process require. Any conventional procedure will suffice, such as in the following way:

Steel production by Siemens-Martin furnace or converter

Ingot Production> Flat Rolling - ^

continuous pouring ^

► Pickling> Reduce cold.

Of course, in the hot rolling step, in order to obtain sufficient strength, a high final rolling temperature is used and a low coiling temperature is preferred, while in order to achieve good deformability, the but is somewhat inferior, a high finish rolling temperature and a high coiling temperature are preferred. That Ribbon is then subjected to one of the following three types of continuous annealing (see figure):

Cycle I.

In the first stage of a fully continuous annealing process, the cold-reduced strip must be subjected to rapid heating of at least 200 ° C./min. This is done to obtain a fine-grain recrystallization structure by the synergistic effect of the above-mentioned idiomorphic structure caused by the addition of 0.50 to 2.00% Si and the above-mentioned rapid heating. The annealing temperature and the holding time are set to 650 to 900 ⁰ C and 10 to 120 seconds. 650 ° C. or 10 seconds is the lower limit for achieving an essentially perfect recrystallization structure. 900 ° C or 120 seconds is the upper limit for preventing the decrease in strength and deformability caused by grain growth. If the belt stays longer than 120 seconds, this reduces the speed of the line, hence productivity, which results in industrial disadvantages. The cooling following the above stage can be carried out in a conventional manner. The cooling by normal air or compressed air is cheap and easy depending on the system or the work process. A modified method is to cool the tape to about 400 ° C by the forced air cooling to allow the tape to dwell for about 300 seconds and then to cool it to room temperature for reeling. The steel thus obtained shows an increase in its tensile strength of about 8 kgf / mm ^{2 as} compared with that of a steel having the same composition as a conventional one

ίο has been subjected to insert annealing and has a Deformability similar to that of case-annealed steel with the same strength level is. This demonstrates the possibility for the manufacture of a high strength cold reduced steel without any addition of expensive special elements and without any interference with cold reduction. This is the effect of Si addition that is characteristic of the fully continuous annealing process due to the method according to the present

2 (i invention.

Cycle II

The solidification mechanism in Cycle II stems from the precipitation hardening of fine carbides as well also the above-mentioned hardening by the grain refinement. So the requirements for that are Heating and dwell the same as those for cycle I, taking special care strict adherence to the lower limit for the

jo temperature and time is dedicated. The aul The upper limit set at 900 ° C / 120 seconds is in addition to inhibiting recrystallization grain growth required for the following reasons. If it exceeds the upper limit, the dei

J5 The difference in hardness between the ferrite matrix forming the basic phase and the secondary quenching structure forming the secondary phase is greater and the deformability is impaired. The tape is then quenched from the above heating temperature and dwell time in a jet of water to room temperature. Such a jet of cooling water is used to remove the vapor film which forms on the surface of the moving belt, and consequently to achieve a very high cooling rate. Dissolved carbon is not excreted. A commonly used method is to spray a large amount of cooling water onto the belt moving in air or water. As mentioned above, the moving belt is quenched to room temperature. This process plays a very remarkable role in the subsequent stage of the carbide precipitation process. The quenched to room temperature strip is tempered to a Temperatui between 300 and 500 ⁰ C. In this tempering step, precipitation nuclei are formed, from which dissolved carbon is precipitated as fine carbides. These fine carbides act to increase strength and play the main role in restraining to a minimum the strength reduction caused by the complete precipitation of dissolved carbon. This tempering temperature is maintained for a period of 10 to 30 seconds. The lower limit before 300 ° C / 10 seconds is the possible minimum speed for precipitating the dissolved carbon. The upper limit is set at 500 ° C / 300 seconds to prevent the coarsening of the fine carbides and to reduce the contribution of these carbides

To ensure strength. If the dwell time exceeds 300 seconds, the speed becomes the Plant will slow down, and its productivity will be low. Such an elimination treatment (a low temperature tempering treatment) d. i.e., the treatment to eliminate the dissolved carbon as fine carbides before the above quenching is very effective while preventing the lowering of the strength, which leads to the excretion of a large amount of solute Carbon accompanies the stoving finish after press molding. This excretory treatment is also a requirement along with the above quick heating treatment in cycle II. The tape treated in this way is finally cooled to room temperature and wound up. However, there is no restriction on the cooling, and normal one using compressed air Cooling proves to be industrially advantageous.

The quality of the steel obtained in the above manner shows a further improvement over that of cycle I. The increase in tensile strength by 3 to 14 kp / mm ² over that of cycle I is confirmed. The steel also has better ductility compared to the coil annealed material, which has the same level of strength. Needless to say, this is caused by the synergistic effect of grain refinement by Si addition produced by a continuous annealing process and precipitation hardening by fine carbides.

Cycle III

The cycle III differs from the above cycle II in that a part of the dissolved carbon in the tempering treatment carried out at a low temperature remains in solution. For this purpose it should be tempering after quenching within 180 to 300 ° C for 4 to 300 seconds. If the starting is below 180 ° C / 4 seconds takes place, the dissolved Carbon does not precipitate in such a heat treatment of the continuous annealing process. However, if the tempering exceeds 300 ° C / 300 seconds, complete excretion of dissolved carbide. The reason that the starting process is so limited in cycle III is that the so-called stoving effects in the stoving stage after compression molding cannot be anticipated. In other words, the one after the elimination treatment partially residual dissolved carbon is found as fine carbides in the post-press baking paint step precipitated, whereby these carbides bring about the increase in strength (bake-hardening effects). This steel is relatively soft at the time of press forming and, as a finished product, shows a higher strength than the original one Steel, which is of great benefit for the safety of automobiles, etc.

In order to further promote the effect of the carbon precipitation treatment during tempering, the The amount of dissolved carbon produced by the quenching increased more than that in cycle II

ίο be. Accordingly, the heating temperature should be after the cold reducing can be set to at least 700 ° C or more. This takes the loosened Carbon increases in the tempering step after quenching. As a result, sufficient bake hardening effects easily become achieved.

The requirements of the carbon precipitation treatment are the same as those in cycle II, with the exception of the heating temperature after cold reduction. The winding obtained in this way is re-rolled at 0.5 to 2.0% and shipped to the users. If the user is an automobile manufacturer, the vehicle body is completed by press molding, assembling, painting and baking. The baking process is usually carried out at 100 to 180 ° C for 10 to 60 minutes while fully exerting the baking hardening effects of the steel. It is possible to increase the yield strength of the steel by over 10 kp / mm ² and its tensile strength by 5 to 30 kp / mm ² over that of cycle I, while the ductility is better than a coil annealed steel with the same strength level.

The drawing shows a course of the above three heating cycles operating with continuous annealing. These cycles should be specific depending on the intended use of the steel and the desired strength level can be selected. Even if you choose any cycle, it can be a high strength cold reduced steel can be easily manufactured with high productivity and low cost, the obtained steel is excellent compared with a steel made according to the usual case annealing Has deformability and strength.

In Table I, there are shown examples showing the effects by the method of the present invention.

The manufacturing requirements not shown in Table I are as follows:

Final hot rolling temperature 830 to 880 ° C

Coil temperature 550 to 620 ° C

Final thickness of the hot-rolled strip 2.8 mm

Final thickness of the cold-rolled strip 0.8 mm

Degree of cold deformation 0.8 to 1.5%

Baking of the paint 180 ° C / 30 min

Table II shows the standardized strength values of the usual types of material annealed in the bundle mechanical properties.

Table I.

stole 1 purpose Composition,% Si Mn Al cycle Heat treatment Annealing temperature 800 ° C / 3 h ο ₂ C. Heating speed and time 800 ° C / 60 seconds ° 3 1.02 0.35 0.027 100 ° C / h 800 ° C / 60 seconds ⁰ 4 Influence 0.08 1.02 0.35 0.027 Insert annealing 800 ° C / min 800 ° C / 60 seconds of heating 0.08 1.02 0.35 0.027 Cycle I. 800 ° C / min cycle 0.08 1.02 0.35 0.027 Cycle Il 800 ° C / min 0.08 Cycle III

continuation purpose 25th Mn 11 805 cycle 10 Annealing temperature 700 ° C / 3 h stole speed and time 720 ° C / 60 seconds 0.48 40 ^o C / h 720 ° C / 60 seconds 9 Influence of Composition,% 0.48 Insert annealing 800 ° C / min 720 ° C / 60 seconds 5 Si content c; 0.31 Al Cycle II 800 ° C / min - 6th 0.27 the same Heat treatment 800 ^o C / min 800 ° C / 60 seconds ° 7 0.07 ( 0.47 0.033 the same Heating - 800 ° C / 60 seconds ° 8 0.07 ( 0.35 0.033 - 800 ° C / min 800 ° C / 40 seconds 9 Influence of 0.06 ( (1.42 0.012 Cycle II 800 ° C / min 800 ° C / 40 seconds ° 10 C content 0.08 0.35 0.004 the same 600 ° C / min 800 ° C / 40 seconds 11 Influence of 0.08: 0.99 0.072 Cycle III 600 ° C / min 720 ° C / 120 seconds ο J ₂ Mn content 0.08 1.58 0.027 the same 600 ° C / min 720 ° C / 120 sec ° 13 0.15 ( (1.35 0.046 the same 10 ° C / min 720 ° C / 120 sec ° 14 Influence of 0.08 0.35 0.027 Cycle I. 600 ° C / min 600 ° C / 90 sec 15th Heating 0.06 ( 0.35 0.012 the same 4200 ° C / min 70O ° C / 90 sec ° 16 speed 0.07 0.35 0.051 the same 600 ° C / min 800 ° C / 90 sec ° 17 Influence of 0.08 0.35 0.027 Cycle II 600 ° C / min 920-C / 90 sec 18th Heating 0.08 0.35 0.027 the same 600 ° C / min 800 ° C / 60 sec ° 19 temperature 0.08 0.35 0.027 the same 600 ° C / min 800 ° C / 60 sec ° 20 0.08 0.35 0.027 the same 4200 ° C / min 800 ° C / 60 sec 21 Influence of 0.08 0.35 0.027 Cycle III 4200 ° C / min 800 ° C / 60 sec 22nd rapid cooling 0.08 0.35 0.027 the same 4200 ° C / min 800 ° C / 60 sec ° 23 Influence of 0.08 0.35 0.027 Cycle II 4200 ° C / min 800 ° C / 60 sec 24 Reason 0.08 0.35 0.027 the same 4200 ° C / min 800 ° C / 60 sec ° 25 treatments 0.08 0.35 0.027 the same 4200 ° C / min 700 ° C / 60 sec 26th 0.08 0.52 0.027 the same 4200 ° C / min 800 ° C / 60 sec 27 Material without 0.08 1.21 0.027 the same 4200 ° C / min 900 ° C / 60 sec 28 Si addition 0.08 1.21 0.027 the same 4200 ° C / min 29 0.08 1.21 0.027 the same 4200 ° C / min 30th 0.15 0.013 the same 31 0.06 ί 0.036 0.06 0.036 0.06 J 0.036 Si ), 09 ), 09 ), 75 , 39 2.51 , 02 ), 92 , 02 ), 96 , 11 , 02 , 02 , 02 , 02 , 02 , 02 , 02 , 02 , 02 1.02 1.02 , 02 , 02 traces traces traces traces

° Steels according to the invention

Table I (continued)

stole 1 Heat treatment requirements Reason To 1% cold deformation δ Ob + δ According to ι IF Increase in treatment the Yield strengths ° 3 % Burn-in kp / strength ° 4 Scare off painting mm ² Δο ₅ 5 38.6 76.9 Os 38.1 6th - kp / kp / 33.8 84.0 50.5 kp / mm ² ° 7 400 ° C / 60 seconds mm ² mm ² 26.7 84.8 kp / 58.2 ⁰ 8
9 250 ° C / 60sec 27.5 38.3 24.5 86.8 mm ² 62.5 ° 10 - 38.6 50.2 46.0 79.1 27.7 33.0 + 0.2 11 in the water jet 350 ° C / 60 seconds 42.2 58.1 35.0 77.2 38.8 42.3 +4.3 ο ₁₂ the same 350 ° C / 60sec 47.2 62.3 29.2 84.9 44.5 55.9 + 10.4 ⁰ 13 350 ° C / 60 seconds 19.2 33.1 25.2 85.5 58.6 60.7 ° 14 in the water jet 400-C / 60 sec 30.1 42.2 26.7 84.8 19.0 58.2 15th the same 400 ° C / 60 sec 40.2 55.7 12.5 81.8 31.1 69.2 ⁰ 16 the same 250 ° C / 120 sec 49.2 60.3 25.9 87.2 45.2 63.2 ⁰ 17 in the water jet 250 ° C / 120 sec 42.2 58.1 22.1 96.7 54.2 75.9 the same 250 ° C / 120 sec 55.1 69.3 16.3 93.6 44.5 77.5 the same 47.2 61.3 40.0 81.2 58.1 41.2 + 10.5 the same - 55.0 74.6 34.1 83.9 57.7 49.6 + 12.0 the same 56.3 77.3 34.0 85.0 67.0 51.3 + 12.6 28.6 41.2 68.9 _ 37.2 49.8 29.9 40.0 51.0 39.2 39.8

continuation Heat treatment requirements Reason 25 1 1 805 ö Whether + d 12th <5 IF Increase in stole treatment Vo Yield strengths Vo To ; kp / strength 11 Scare off To mm ² Ao ₅ 450 ° C / 30 seconds _ _ _ 450-C / 30 see 29.2 84.4 55.5 kp / mm ² 450 ° C / 30 seconds O ₅ 27.6 85.9 the 58.2 in the water jet 450 ° C / 30 seconds 13.6 78.8 Burn-in 65.5 _ 18th the same 250 ° C / 60 seconds kp / 25.0 80.0 painting 54.7 ° 19 the same 250 ° C / 60 seconds mm ² 1% cold deformation 26.5 86.8 Os 61.7 ° 20 the same - 18.5 93.6 60.1 21 in calm water 350 ° C / 90 seconds 40.4 28.5 85.8 kp / 58.5 + 4.9 22nd in the water jet 600 ° C / 90 seconds 43.2 If 32.1 82.8 mm ² 50.5 + 10.9 ° 23 the same 350 ° C / 10 min 52.1 33.5 83.7 _ 50.7 -9.8 24 the same 35? ° C / 90 see 42.1 kp / 15.2 80.5 45.6 65.4 ° 25 the same 350 ° C / 90sec 46.3 mm ² 30.2 78.5 46.3 49.1 26th the same 350 ° C / 90 seconds 60.1 _ 18.6 83.7 53.0 65.0 27 the same 350 ° C / 90 seconds 42.5 55.2 15.3 91.0 47.0 75.2 28 the same 40.0 58.3 57.2 29 the same 39.5 65.2 50.3 30th the same composition 53.1 55.0 IF 45.9 31 according to the invention 37.2 60.3 kp / mm ² 41.1 ° steels II 50.5 75.1 40.0 Tabel Off setpoints; 55.3 57.3 54.2 Ob + δ 50.7 39.2 kp / mm ² +% 50.2 51.2 65.3 56.5 48.3 65.1 75.7

45 kp / mm ² variety
50 kp / mm ² variety
55 kp / mm ² variety
60 kp / mm ² variety
70 kp / mm ² variety

glow
thickness

C-Si-Mn-Nb

C-Si-Mn-Nb

C-Si-Mn-Nb-Ti

C-Si-Mn-Nb-Ti

C-Si-Mn-Nb-Ti

47.2
53.3
57.8
62.5
71.2

32.9
27.8
24.9
21.7
16.9

80.1
81.1
82.7
84.2
88.1

720 ° C / 3 h
0.8 mm

As for the mechanical properties of steel in general, its elongation tends to decrease as its tensile strength is increased. Accordingly, the deformability of steel should be expressed taking into account a change in its tensile strength together with a value for its elongation, since this can only be done insufficiently by the elongation alone. In the above Table I, a value of Ob + ö is used as a standard for representing the deformability. That is, the larger the value of Ob + ö becomes, the better the deformability becomes due to the strength. Table II shows the standardized parameters for the deformability of steel produced in a conventional coil annealing process, these parameters being expressed specifically with the abovementioned standard of Ob + δ . This table shows that steels with an Ob of 50 to 55 kp / mm ^{2 have} a value of Ob + <5 of about 82, while a steel of the 60 kp / mm ^{2 type has} that of 84 and a steel of the 70 kp / mm type mm ^{2 has} that of 88. By comparing the steels with the above standard, that is, a value of Ob + <3, the excellent formability of the steel according to the present invention can be clarified.

Steels 1 through 4 in Table I have been made basic in terms of one achieved by the heating cycles Effect observed. It has been found that the tensile strength of a steel with such a composition like that of the steel of the present invention, following a conventional coil annealing process

so (steel 1) is only that of a steel of about the grade 40 kg / mm ² , while the tensile strength of the above steel when treated with the cycle I of the present invention (steel 2) is more than 10 kg / mm ² is increased. This steel of the 40 kp / mm ^{2 type} was improved to a high-strength, cold-reduced steel of the 50 kp / mm ^{2 type.} At the same time, because of the strength, the deformability of steel 2 (ie σ /, + <5) was very good, ie 84.0. This value exceeds that of the steel of the grade

W) 50 kgf / mm ² of Table II, which expressively demonstrates the grain refining effect from the fully continuous annealing performed on the Si-added steel. The fact that the yield strength of steel after baking was +0.2 kgf / mm ² indicates that the grain refining effect is still maintained in a finished product and that the cycle I process is very useful with the Si-added steel related to high

Productivity, which depends on the fully continuous annealing process If the Zykius II according to the invention is applied to the above steel (steel 3) instead of the cycle I, the properties of the above steel can be further improved, which is clear from the above. That is, σ * this increases by about 8 kp / mm ² . As a result, the steel is improved to a high-strength cold-reduced steel grade 60 kp / mm ² of Table II, while its ductility (ub + δ) shows the same value of 84.8 as that of cycle I due to the above increase in strength. The fact that its Aa ₅ after stoving is +43 kgf / mm ² should be noted as a good stoving hardening effect. Needless to say, this measure is based on the synergistic effect of grain refinement by Si addition and precipitation of fine carbides II were applied, its mechanical properties are further improved than those in cycles I and II. These results are shown in Table I for steel 4. This means that the value of σ * + <5 is increased to 86.8 ^{despite the increased σ * of more than 62 kp / mm 2.} In particular, the increase in Ao _{s reached} a level of over +10 kp / mm ² , which means an excellent stoving-hardening effect and a counterpart for this value of the steel of the 60 kp / mm ^{2 type} from Table 11 with the system C-Si Is Mn-Nb-Ti. In addition, the steel 4 does not contain any special element such as Nb or Ti contained in the mentioned steel, which results in a low cost together with a high productivity of a fully continuous annealing process. The precipitation of remaining dissolved carbon on the dislocations results in an increase in tensile strength in the course of the stoving process. This phenomenon should be taken into account in demonstrating the usefulness of cycle III. Thus, each of the inventive cycles I, II and III is particularly effective as a safety measure in the motor vehicle, etc. A selection from these cycles naturally depends on the fully continuous annealing options to be used and on the strength level required for the intended use of the steel obtained. The influences of other requirements with the exception of those due to the above heating cycles are explained with reference to the examples, steel 5 and others.

Steels 5 to 9 were examined for the Si content. Steels 5 and 6 contain Si in one amount less than the range of the present invention. Even if the above heating cycle III was applied to such a low-Si steel (steel 6) resulted in full Continuous annealing process with the steel mixed with Si does not have a sufficient effect and also none Improvement of deformability. However, steels 7 and 8, which were sufficiently mixed with Si, showed an actually remarkable improvement in strength and ductility. If the Si, as in the case of steel 9, was increased to 2.51%, an edge break occurred during the cold reduction, which is a made further sample preparation impossible. The Si content should therefore be limited to below 2.00% will and, as in the case of steels 7 and 8, be in the range of 0.7 to 1.5%.

The steels 10 and 11 show an effect de; Carbon content. When the carbon content wi (in steel 11 is increased, the strength becomes au easily increased. However, as shown in Table 1, its deformability is strength inferior. In addition, the spot weldability is worse. Consequently, such a high carbon content should be avoided. The carbon content should preferably be limited to at most 0.12% will.

Steels 12 and 14 were observed for an influence of Mn. As de! In the Mn content, a large increase in the strength wedge was observed. However, when the Mn is 1.58% as in the case of steel 14, the value of Ob + i tends to become small in spite of the very excellent strength. The Aor value represents this bake hardening effect. The reason for this is the appearance of the quenching structure, ie martensite, when the Mn content becomes unnecessarily high. Accordingly, such a high Mn content should be avoided. The steel 13 containing about 1% Si and 1% Mn shows the most stable balanced properties and can be identified as the most preferred steel. The steel 13 shows a strength of about 70 kgf / mm ² and an excellent value of Ob + ö, namely 96.7, which far exceeds the values of the low-alloy steels shown in Table II. It is recommended that the Mn content be chosen on the lower side of 1.00%

3n is dependent on the required strength level.

Steels 15 and 17 were examined with regard to the influence of the heating rate on the cold-reduced steel. This influence is a characteristic factor of the present invention. When the heating rate is slow as in the case of steel 17, the grain refining effect caused by the addition of Si, which does not appear in a coil annealing process, cannot be exerted and the strength obtained is only about 40 kg / mm ² . However, as the heating rate is increased, the strength of the steel is increased as in the case of steels 16 and 17, in which the strength is that of the 50 kgf / mm ² grade and the value of Ob + ö exceeds 83, which exceeds the values of low alloyed ones Steels in Table II and aptly demonstrates the utility of the present invention using Cycle I above alone.

; o Steels 18 through 21 were made in view of a Influence of the heating temperature on a cold-reduced steel investigated. If the temperature is the has a low altitude of 600 ° C as in the case of steel 18, the steel obtained shows no elongation whatsoever, and no voltage test was possible. In contrast, the steel 21 showed a drastic deterioration Deformability on with increasing strength and at the very high temperature of 920 ° C. This should can be avoided because of the quenching structure obtained by the rapid cooling in cycle II. the Steels 19 and 20, on the other hand, show that their strength and ductility are well balanced and that their Yield strength is considerably increased after baking (see Table I).

b5 Steels 22 and 23 were designed for a Influence of an applied quenching process observed. Quenching in calm water, like in the case of steel 22, makes the properties of cycle III

unclear, which particularly aims for a stoving hardening effect. That is, Aa ₅ is approximately +4.9 kp / mm ² , which does not differ much from that of steel 19 which is subjected to cycle II, which in particular does not have a stoving effect. On the other hand, Ao _{s of} the steel 23 quenched by a water jet has the great height of +10.9 kp / mm ² , while its value of o /, + δ due to its strength increased by 10.3 kp / mm ^{2 is 86.8} is very high. Thus, this quenching in calm water alone is undeniably insufficient to form the dissolved carbon in steel.

Steels 24 to 27 were examined for an influence of the tempering treatment. The steel 24 is an example in which no carbon precipitation treatment by low temperature tempering was carried out. This steel is very hard and shows an ο /, - value of 75 kp / mm ² , although the values for O ₄ and o /, are reduced due to the baking, namely by the amounts -9.8 and -15 kp, respectively / mm ² . These steels are of course unsuitable for all common purposes, let alone for motor vehicles that require a safety measure. On the other hand, the steel 26 having a high treatment temperature of 600 ° C. and the steel 27 having a long residence time of over 10 minutes tend to lower their strength and ductility. These are the undesirable influences caused by the coarsening of the fine carbide by the treatment with the above temperature and time. When the steels are within the restricted range of treatment temperature and time as in the case of steel 25, Ao _k and Aob do not decrease, and it is easy to maintain stable strength and ductility.

The steels 28 to 31 are the examples that increase their strength by other elements without any aim at excessive addition of Si. The steel 28 shows a sole increase in the carbon

Ki content and has a low value of 80.5 for Of, + ö with an o <, - value of 65.3 kp / mm ² . It is evident that this is well below the level of the same steel strength in Table II. Steels 29 to 31 show only an increase in the Mn content

r, on. These steels were heated (30 steel) or 900 ° C (steel 31) up to 700 ⁰ C (Steel 29), 800 ⁰ C. In each case, it has been found that their deformability is much lower due to their strength. The remarkable influence of the heating temperature on the strength should be emphasized here. That is, the variations in Ob corresponding to the heating temperature are very large when the variations in the heating temperature of the continuous annealing process are considered. When Si is added in the range of the present invention and the treatment requirements are in the limited range, such fluctuation as mentioned above cannot be seen and it is easy to obtain uniform material quality.

1 sheet of drawings

Claims (3)

Patent claims: 1. Application of the process for the production of high-strength, cold-rolled deep-drawing steels, in which the Steel after cold rolling at a heating rate heated from at least 200 ° C / min to 650 to 900 ° C, 10 to 120 seconds at this Annealed temperature and then cooled at a cooling rate that corresponds to at least the usual air cooling, on a steel that contains 0.04 to 0.12% carbon, 0.1 Contains up to 1.6% manganese and 0.5 to 2.0% silicon. 2. Application of the method according to spoke 1 with the proviso that the cooling takes place in a water jet and the strip is then ^{tempered for 10 to 300 seconds at 300 to 500 0} C, on the material according to claim 1. 3. Application of the method according to claim 1 with the proviso that the annealing temperature is j'00 to 900 ⁰ C, the strip is then cooled in a water jet and tempered for 4 to 300 seconds at 180 to 300 ° C, on the; Steel according to claim 1.