EP1929055B1 - Method for treating a steel strip - Google Patents

Abstract

The present invention relates to a method for treating a steel strip, particularly a steel strip made of highly resistant heat-treated steel, by cold rolling. The inventive method comprises at least a first step of heat treatment consisting of recrystallization annealing and at least another subsequent step of heat treatment consisting of hardening. Said method is characterized in that the time-temperature curve during the first step of heat treatment is chosen in such a way that only a partial recrystallization takes place and that the elongated structure produced by the previous strain-hardening is partially conserved. The inventive method enables reducing significantly the annealing time and annealing temperature during recrystallization annealing while at the same time reducing the annealing time during shape hardening. The processing chain can be reduced by more rapid annealing cycles, the reduction of the annealing temperature leading to significant energy savings. Said method allows producing e.g. semifinished sheet metal products for vehicle body components having an optimized strength/elongation ratio.

Description

The present invention relates to a method of treating steel strip after cold rolling with at least a first heat treatment comprising recrystallization annealing and at least one further subsequent heat treatment comprising hardening. The present invention further provides a sheet metal semifinished product intended for further processing, which was produced by this process, and motor vehicle components, in particular bodywork components of motor vehicles, which were produced from such a semifinished sheet metal product.

In the process chain of steel production, a heat treatment is usually required after reducing cold rolling. Due to the reducing cold rolling, the steel sheet undergoes considerable strain hardening, which necessitates thermally induced structure formation, since otherwise further processing by means of conventional cold forming technology would not be sufficiently possible. After the cold rolling, therefore, a final heat treatment called recrystallization annealing is required. This recrystallization annealing can be done in a hood or in a continuous furnace. In the conventional method, a complete recrystallization is sought and achieved. The steel strip is usually present as a coil during recrystallization annealing. This step of the heat treatment is generally carried out at the steel manufacturer prior to delivery of the semi-finished sheet metal to the processor. As antioxidant protection, the steel strip is usually coated, wherein the coating can take place in combination with the annealing process or as a decoupled process step. The user himself then takes the desired board cut.

Processes for recrystallization annealing of cold rolled steel sheet are well known in the art. For example, this describes DE-PS 1 168 462 such a method in which cold-rolled sheet of mild steel is annealed in an oven at temperatures of 600 ° C - 700 ° C for a period of 40 hours, wherein before the recrystallization annealing still a several hours of annealing is provided.

The DE 34 06 792 A1 describes a process for recrystallization annealing of cold rolled steel strip in a hood furnace under inert gas.

The DE 698 15 943 T2 describes a continuous process for continuous annealing of steel sheet under reduced pressure whereby a cold plasma is generated in a gas atmosphere and annealed at about 700 ° C.

All of the abovementioned processes known from the prior art have in common that the heat treatment is intended to achieve complete recrystallization and is accompanied by complete degradation of the fine-grained, longitudinally-elongated microstructure in the rolling direction.

The JP 62 093 341 A discloses a method of producing a steel strip after cold rolling in which only partial recrystallization occurs in the subsequent heat treatment.

In the automotive industry, for example, for demanding applications in the field of bodywork tempered steels, preferably high-strength heat-treated steels are used, which are characterized in particular by a very high strength. After the blank has been cut, the user is again subjected to a heat treatment with heating of the blanks to the austenitizing temperature. Thereafter, accelerated cooling (equivalent to quenching) follows to set a hardened microstructure state. This measure can be done for example as a combined forming and quenching in a press tool, which corresponds to the so-called form hardening. According to the state of the art, the final component geometry and the material strength or toughness of the metallic structure are set by the form hardening. The accelerated heat dissipation within the forming tool leads by the initiation of a phase transformation to the hardening of the component and thus to an increase in the strength.

Since according to previously described process chain in the context of the hot forming process of mold hardening at the user heat treatment takes place, from his point of view, the heat treatment in the sheet semifinished production (recrystallization) technologically not required, possibly even unfavorable in view of the adjusted mechanical end properties of the component.

This is where the present invention starts. The object of the invention is to provide a method for the treatment of steel strip after the cold rolling of the aforementioned type available, which under principle retention of the previous Prozessroute semi-finished sheet supplies, which allows the production of components with improved properties.

The solution to this problem provides a method for the treatment of steel strip of the aforementioned type with the characterizing features of the main claim.

Core of the present invention is a modified heat treatment after the reducing cold rolling, are controlled by the thermally induced microstructural changes so that the occurred in the previously desired complete recrystallization complete degradation of the very fine grain in the longitudinal direction elongated microstructure is deliberately avoided. For this purpose, only such a proportion of recrystallized microstructure and therefore just as much further cold workability is set by the choice of the time-temperature run that a handling of the coils, that is, a suppression of the so-called clock spring effect is ensured and a straightening over, for example, roller-based stretch levelers is ensured for setting flat board blanks for the particular forming technology further processing. According to the invention, in the first heat treatment, a recrystallization annealing is carried out in such a way that a recrystallized fraction in the microstructure of from about 15% to about 45%, preferably from about 20% to about 40%, is achieved.

The remnants of the finely grained deformation structure still present due to the partial recrystallization according to the invention also have a positive effect on the subsequent processing step of hardening by the user. For example, it comes through the preheating of the boards for the mold hardening to another structural change that is to ensure a sufficient hot forming capacity in the balance between the interaction of temperature and structure dependence of the yield stress. By adapting the time-temperature guidance to the changed structure formation as a result of the modified first heat treatment, the preheating during mold hardening can additionally be optimized by reducing the previously very high temperatures (above A _C3 in the pure austenite range). According to the invention in particular the goal is in the foreground, remains of the remaining fine-grained elongated deformation structure of the to obtain reducing cold rolling. These structural components may lead to increased strength due to the fine grain for the form-hardened component, but also due to the remaining deformation structure shares also increased ductility with an immediate positive effect z. B. on the crash behavior when manufactured from the semi-finished sheet metal body according to the invention body parts.

The further subsequent heat treatment used in the process of the invention may be a final heat treatment, i. final heat treatment of the component to the user. This final heat treatment may include forming to provide a desired component geometry, e.g. B. a form hardening as mentioned above. However, the further heat treatment does not necessarily have to be accompanied by a forming. Preferably, however, the further, usually final heat treatment comprises shaping in a mold-forming tool. However, this also includes the case in which the component has already received the final shape and the mold-forming tool serves to ensure that the component retains this shape during the heat treatment (avoidance of distortion, etc.) Cooling provided via a cooling medium or via a mold-forming tool.

The method according to the invention is preferably used for the heat treatment of steel strip made of heat-treatable steel, in particular steel strip made of ultra-high-strength steels is heat-treated. In this case, particular preference is given to tempering steels which contain manganese and / or boron as alloying element. As an example, the steel grade 22MnB5 is called. The alloying elements manganese and boron promote a rapid structural change, which is advantageous in particular for mold hardening.

A further advantage of the method according to the invention lies in the fact that the flow behavior of the material can be controlled during a subsequent transformation by way of direction-dependent structural components which have been retained during the first heat treatment.

The experiments have further shown that preferably the recrystallization annealing at a temperature below the recrystallization temperature, preferably about 1% to about 10%, more preferably about 2% to about 6% below the Recrystallization takes place. At the recrystallization temperature recrystallization is clearly the fastest. As the difference between the annealing temperature and the recrystallization temperature increases, the parameter window becomes larger. The solution according to the invention thus makes it possible to significantly lower the annealing temperature during the first heat treatment. Since only a partial recrystallization is sought, depending on the process route, the annealing time in the first heat treatment compared to conventional methods can be significantly shortened. This allows quite significant energy savings.

In the subsequent heat treatment, that is, for example, the molding, a shortening of the annealing time is also possible. Utilizing the retained directionality of the material, the forming process can be more flexibly controlled. Since the annealing cycles can thus be shortened in both annealing treatments, this leads to a significant shortening of the overall process chain.

When using the method according to the invention and maintaining the austenitizing conditions customary hitherto, it is possible to achieve equally good or even slightly improved tensile strengths, in particular when treating very high-strength tempered steels. The values for the elongation at break can be increased significantly. This results in optimized strength / elongation ratios. This is due to the fact that after the first heat treatment, the orientation is partly retained and, after the further heat treatment (in particular, the mold hardening), even a finer grain structure is optionally obtained. Thus, significantly better material properties can be achieved while maintaining the conventional annealing times.

However, according to the present invention, the austenitizing conditions can be changed also in comparison with the conventional methods, in particular, the austenitizing time can be shortened. With such a modified further heat treatment, a better strength / elongation ratio is still obtained than after the conventional process route. Here, the technological progress is clear, which brings the inventive method with it.

The features mentioned in the dependent claims relate to preferred developments of the task solution according to the invention. Further advantages of the invention will become apparent from the following detailed description.

• Fig. 1 ein Diagramm zur Erläuterung der Rekristallisationskinetik bei der erfindungsgemäßen ersten Wärmebehandlung (Rekristallisationsglühen), wobei der rekristallisierte Anteil RX in % in Abhängigkeit von der Glühzeit in Minuten aufgetragen ist;
• Fig. 2 eine Graphik zur Erläuterung der mechanischen Eigenschaften von Stahlband nach dem Rekristallisationsglühen, abhängig vom rekristallisierten Anteil RX, wobei Zugfestigkeit und Bruchdehnung gegen den rekristallisierten Anteil RX in % aufgetragen wurden;
• Fig.3 aus experimentellen Daten ermittelte Kurvenverläufe für Zugfestigkeit und Bruchdehnung nach der weiteren Wärmebehandlung (dem Formhärten), wobei auf der Ordinate links die Werte für die Zugfestigkeit und rechts diejenigen für die Bruchdehnung wiedergegeben sind, aufgetragen jeweils gegen den rekristallisierten Anteil RX in % (von links nach rechts zunehmend);
• Fig. 4a eine Graphik, die den Einfluss der Austenitisierungszeit beim Formhärten verdeutlicht, wobei bei zwei verschiedenen Glühzeiten jeweils der Verlauf der Zugfestigkeit in Abhängigkeit von dem rekristallisierten Anteil RX in % wiedergegeben ist;
• Fig. 4b eine Graphik, die den Einfluss der Austenitisierungszeit beim Formhärten verdeutlicht, wobei bei zwei verschiedenen Glühzeiten jeweils der Verlauf der Bruchdehnung in Abhängigkeit von dem rekristallisierten Anteil RX in % wiedergegeben ist;

Hereinafter, the present invention will be explained in more detail by means of embodiments with reference to the accompanying drawings.
Showing:

• Fig. 1 a diagram for explaining the recrystallization kinetics in the first heat treatment according to the invention (recrystallization annealing), wherein the recrystallized fraction RX in%, as a function of the annealing time in minutes is plotted;
• Fig. 2 a graph illustrating the mechanical properties of steel strip after recrystallization annealing, depending on the recrystallized fraction RX, wherein tensile strength and elongation at break against the recrystallized fraction RX in% were plotted;
• Figure 3 from experimental data, curves for tensile strength and elongation at break after further heat treatment (form hardening), the ordinate on the left representing the values for the tensile strength and the right those for the elongation at break, plotted against the recrystallized fraction RX in% (from left to increasing right);
• Fig. 4a a graph illustrating the influence of Austenitisierungszeit in the molding, wherein in two different annealing times in each case the curve of the tensile strength as a function of the recrystallized fraction RX in% is reproduced;
• Fig. 4b a graph illustrating the influence of Austenitisierungszeit in the molding, wherein in two different annealing times in each case the course of the elongation at break in dependence on the recrystallized fraction RX in% is reproduced;

Experimental Procedure

The starting material used was uncoated cold strip made of the highest strength tempered 22MnB5 (composition according to Table 1 below) with a thickness of d = 1.75 mm in each case. <b> Table 1: </ b> Melt Analysis of a 22MnB5 (1.5528) C Si Mn P S al Cr tl B 0.19 to 0.25 from 0.15 to 0.40 1.10-1.30 max.0,025 max.0,015 from 0.020 to 0.060 0.15-0.35 0.020 to 0.050 0.0008 to 0.0050

The industrial cold rolled on a tandem rolling mill and supplied in the form of panels test material was cut into 600 mm long and 20 mm wide strips.

a) Experiments for the first heat treatment according to the invention (recrystallization annealing)

The various temperature-time cycles for varying the recrystallization conditions were run with a continuous test furnace. For this purpose, two strips per side are placed side by side on a conveyor belt in the oven. After the appropriate annealing time, the material was transported out of the oven and cooled in still air to room temperature. One of the two annealed strips was available for the subsequent mold hardness tests (further heat treatment according to the invention). From the second three samples for tensile tests according to DIN EN 10002 (initial measurement _length L ₀ = 50 mm, determination of the characteristic values R _p0,2 , R _m and A ₅₀ ) as well as a structural sample for metallographic examinations (determination of the recrystallized portion and the grain size) were taken ,

b) Form hardening the recrystallization samples

The second strip, after the recrystallization samples, was first austenitized. In addition, again in the experimental continuous furnace, a heating to 900 ° C took place with a holding time of 300 s. After these five minutes, the strip was transported via a chain transport to the form hardening tool. Until manual insertion of the strip in the tool passed about 5 s. The tool was closed by lowering a top plate and held in that position for 20 seconds. After driving up the tool, the sample material cooled down to approx. 70 ° C could be removed. Between each experiment, the tool repeatedly cooled, so that it had a maximum temperature of 40 ° C. The thermoset samples were also split into three tensile specimens and one metallographic specimen.

c) Variation of austenitizing conditions

After evaluation of the mechanical material testing and the metallographic investigations on the samples from the experiments a) and b), a precisely defined degree of recrystallization could be set on the basis of the results of various temperature-time cycles during the heat treatment. For each of the finely graded degrees of recrystallization, three strips each were annealed.

The variation of the austenitizing conditions was limited to an annealing time shortening to 200 s at 900 ° C (conventionally the annealing time is 300 s) and two temperature reductions to 850 ° C and 800 ° C at 300 s annealing time (the conventional annealing temperature is 900 ° C). For all recrystallization levels set, these three altered austenitizing conditions were performed with subsequent quenching in the tool. Tensile tests and metallographic investigations were also carried out on these conditions.

evaluation

In der

RX

der statisch rekristallisierte Gefügeanteil

a, b

Konstanten (werkstoff- und temperaturabhängig)

t_0,5

die Zeit, bei der 50% des Gefüges rekristallisiert sind, und

t

die Glühzeit ist.

On the basis of experimental data, the course of the statically recrystallized fraction was determined as a function of the annealing time. Recrystallization annealing follows the kinetics of the AVRAMI approach according to the formula: $$\mathrm{RX}\left[\mathrm{\%}\right]\mathrm{=}\left(\mathrm{1}\mathrm{-}\exp \left(\mathrm{-}\mathrm{a}\mathrm{*}{\left(\mathrm{t}\mathrm{/}{\mathrm{<figure-callout\; id="0"\; label="t"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}}_{\mathrm{0}\mathrm{.}\mathrm{5}}\right)}^{\mathrm{\wedge }}\mathrm{b}\right)\right)\mathrm{*}\mathrm{100}$$

In the

RX

the statically recrystallized part of the structure

a, b

Constants (depending on material and temperature)

t _0.5

the time at which 50% of the structure is recrystallized, and

t

the glow time is.

In the graphic according to Fig. 1 the recrystallized fraction RX in% is plotted against the annealing time in minutes. The curves are reported for four different annealing temperatures, namely T = 735 ° C (recrystallization temperature), T = 750 ° C (TR + 15 ° C), T = 715 ° C (TR - 20 ° C) and T = 700 ° C (TR - 35 ° C). From the illustration, it is clear that the recrystallization around the recrystallization temperature is the fastest. The recrystallization kinetics just below and just above the Recrystallization temperature has approximately the same course. The greater the difference between annealing temperature and recrystallization temperature (see curve at T = 700 ° C), the larger the parameter window, that is, the desired recrystallized portion RX of the structure can be best influenced by varying the annealing time. The graph shows that at this temperature after an annealing time of 20 minutes the recrystallized fraction RX is about 25%, while after an annealing time of about 30 minutes RX is already at about 80%.

In Fig. 2 The tensile strength (left) and the elongation at break (right) after recrystallization annealing are plotted as important mechanical properties as a function of the recrystallized fraction RX. The values result from experimentally determined data which were obtained after the first heat treatment according to the invention, that is to say after the recrystallization annealing. It can be seen that the tensile strength decreases as the recrystallized fraction RX increases, but at RX between about 20% and about 40%, sufficiently low values in the range of about 620 N / mm ² and 740 N / mm ^{2 are still} achieved. The elongation at break, however, increases with increasing recrystallized portion RX, where it can be seen that even with RX 20% to 40% good values are achieved and then increase only slightly with further increasing RX. The elongation at break A ₅₀ in% is shown.

On the basis of micrographs, it can be seen very clearly that parts of the fine-grained elongated deformation structure from the reducing cold rolling are partially retained after the first heat treatment. After the further heat treatment according to the invention (mold hardening), the same austenitizing conditions with a particle size of about 5 .mu.m even result in a finer microstructure than in the conventional process route with 100% recrystallization (particle size about 8... 10 .mu.m) in the first heat treatment.

Fig. 3 shows the mechanical properties after the form hardening as a function of the recrystallized fraction RX before austenitizing. On the left, the tensile strength in N / mm ^{2 is again} indicated on the y-axis, and the elongation at break A ₅₀ in% on the right-hand y-axis. On the x-axis, the recrystallized fraction RX in% given from left to right increasingly. It can be seen that for an optimum combination of high strengths and high elongation values, an optimum range for RX is about 20% to about 40%. By means of the modified heat treatment according to the invention, an improvement in the elongation of at least 20% can be achieved.

The Fig. 4a and 4b illustrate the influence of austenitizing time, ie the duration of the second heat treatment on the mechanical properties after the form hardening. In Fig. 4a is the tensile strength in N / mm ² plotted against the recrystallized fraction RX in%, where two curves for each different annealing times are shown. The annealing time was 200 s or 300 s with the same annealing temperature of 900 ° C. It can be seen that the tensile strength values for the annealing time of 200 s are higher over the entire curve than for the longer annealing time of 300 s. The difference in the tensile strength values increases only slightly with increasing recrystallized fraction RX. In the Fig. 4b again for the two given annealing times at the same annealing temperature of 900 ° C determined curves for the elongation at break in% show that the values with increasing recrystallized fraction RX first again reach a maximum in the range between 20 and 40% and then decrease again. It is astonishing that despite the shorter annealing time of 200 s (lower curve) with a recrystallized fraction RX of about 40%, the fracture strength values are still better than the values with longer annealing times if a 100% recrystallization had previously been carried out. This means that the annealing time in the case hardening can be shortened, whereby higher tensile strength values are achieved than with previous 100% recrystallization according to the conventional process route. In addition, a shortening of the annealing time is also possible in recrystallization annealing itself in that the annealing process can be terminated earlier, namely when the desired degree of recrystallization of, for example, about 20% or about 40% is reached.

Claims (15)

1. Process for the treatment of steel strip after cold rolling, including at least a first heat treatment comprising recrystallization annealing and also including at least a further, subsequent heat treatment comprising hardening, characterized in that the time-temperature curve during the first heat treatment is selected such that recrystallization annealing is effected during the first heat treatment in such a manner that a recrystallized fraction of about 15% to about 45% is obtained in the microstructure, and the elongated microstructure produced by the preceding cold work-hardening is partially retained.
2. Process according to claim 1, characterized in that the further, subsequent heat treatment is a final heat treatment comprising press hardening.
3. Process according to claim 2, characterized in that the further, subsequent heat treatment comprises deformation.
4. Process according to claim 1 or 2, characterized in that the further, subsequent heat treatment comprises shaping using a shaping tool.
5. Process according to one of claims 1 to 4, characterized in that the further, subsequent heat treatment is followed by cooling using a cooling medium or using a shaping tool.
6. Process according to one of claims 1 to 5, characterized in that the further, subsequent heat treatment includes heating to austenitization temperature.
7. Process according to one of claims 1 to 6, characterized in that a steel strip made of a steel which can be quenched and tempered is heat treated.
8. Process according to one of claims 1 to 7, characterized in that a steel strip made of a super-high-strength steel is heat treated.
9. Process according to one of claims 1 to 8, characterized in that a steel strip made of a steel containing manganese and/or boron as alloying element is treated.
10. Process according to claim 9, characterized in that a steel strip made of 22MnB5 is heat treated.
11. Process according to one of the preceding claims, characterized in that the flow properties of the material during subsequent deformation are controlled by way of directional-dependent structural fractions obtained during the first heat treatment.
12. Process according to one of the preceding claims, characterized in that recrystallization annealing is effected during the first heat treatment in such a manner that a recrystallized fraction of about 20% to about 40% is obtained in the microstructure.
13. Process according to one of the preceding claims, characterized in that the recrystallization annealing is effected at a temperature below the recrystallization temperature, preferably about 1% to about 10%, particularly preferably about 2% to about 6%, below the recrystallization temperature.
14. Process according to one of the preceding claims, characterized in that the annealing time for the recrystallization annealing is shortened compared to conventional processes, and only partial recrystallization is thereby achieved.
15. Process according to one of the preceding claims, characterized in that the annealing time for the further, subsequent heat treatment is shortened compared to comparable, conventional processes.

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