EP2297367B1 - Method for producing a formed steel part having a predominantly ferritic-bainitic structure - Google Patents

Description

The invention relates to a method for producing a steel molding having a predominantly ferritic-bainitic structure.

In order to meet the existing in modern bodywork requirement for low weight while maximum strength and protective effect, nowadays in such areas of the body, which may be exposed to high loads in the event of a crash, hot-formed components are used, which are made of high-strength steels , Examples of such steel moldings include the A and B pillars, the bumpers and door impact beams of a passenger car.

In hot press hardening of steel blanks divided from cold or hot rolled steel strip, the sheet metal blanks concerned are heated to a deformation temperature generally above the austenitizing temperature of the respective steel and placed in the mold of a forming press in the heated state. In the course of the subsequent transformation undergoes the sheet metal blank or the molded part of it by the contact with the cool tool a rapid cooling, resulting in the component hardness structure. It may be sufficient if the component cools without active cooling alone by the contact with the tool. However, rapid cooling can also be supported by the fact that the tool itself is actively cooled.

As in the article " Potential for lightweight body construction ", published in the trade show newspaper of ThyssenKrupp Automotiv AG for the 61st International Motor Show in Frankfurt, 15-25 Sept. 2005 , hot press hardening is reported to be used in practice, in particular for the manufacture of high-strength boron-alloyed steel body components. A typical example of such a steel is the steel known under the name 22MnB5, which can be found in the steel key 2004 under the material number 1.5528.

A steel comparable to steel 22MnB5 is made of JP 2006104526 A known. This known steel contains in addition to Fe and unavoidable impurities (in wt .-%) 0.05 - 0.55% C, max. 2% Si, 0.1-3% Mn, max. 0.1% P and max. 0.03% S. To increase the hardness, additional amounts of 0.0002 - 0.005% B and 0.001 - 0.1% Ti can be added to the steel. The respective Ti content serves for setting the nitrogen present in the steel. In this way, the boron present in the steel can develop its strength-increasing effect as completely as possible.

According to the JP 2006104526 A are made of the composite steel so first sheets, the then preheated to a temperature above the Ac ₃ temperature, typically in the range of 850-950 ° C. In the subsequent cooling in the pressing tool, starting from this temperature range, the martensitic microstructure ensuring the desired high strengths is formed in the component molded from the respective sheet metal blank. Favorably, it has the effect that the sheet metal parts heated to the stated temperature level can be shaped to complex-shaped components at relatively low forming forces. This is especially true for such sheet metal parts, which are made of high-strength steel and provided with a corrosion protection coating.

The components produced from boron-alloyed steels in the above-described manner achieve strengths of more than 1,500 MPa. However, the required complete martensitic structure of the components has the result that the components have an insufficient residual elongation at break of 5-6% for many applications. The relatively low residual elongation at break is associated with a low toughness. In applications where good deformation behavior is required in the event of a crash, this results in components which are produced from boron-alloyed steels in the known manner often no longer meet these requirements. This applies in particular when the components to be produced are parts for an automobile body.

In the DE 10 2005 054 847 B3 has been proposed by a downstream heat treatment the To improve the crash behavior of steel components produced by hot press hardening, which in addition to iron and unavoidable impurities (in% by weight) 0.18 - 0.3% C, 0.1 - 0.7% Si, 1.0 - 2.50% Mn, max. 0.025% P, 0.1-0.8% Cr, 0.1-0.5% Mo, max. 0.01% S, 0.02-0.05% Ti, 0.002-0.005% B and 0.01-0.06% A1. In the course of the heat treatment, the hot-press hardened components are kept at 320 - 400 ° C. Apart from the fact that such a heat treatment step can be incorporated only with great effort in the established for the production of hot-pressed steel components process chain, practical investigations have shown that the elongation at break of thus heat-treated components deteriorates significantly.

Another possibility of producing a hardened metallic component is from the DE 102 08 216 C1 known. In this known method, a board or a preformed mold component, each consisting of a steel of the above type, heated in a heating device to an austenitizing and then fed via a transport path to a curing process. During transport, portions of the first type of board or die which are intended to have higher ductility properties in the final component are quenched from a predetermined cooling start temperature which is above the γ-α transformation temperature. This quenching is terminated when a predetermined quench stop temperature is reached, before conversion to ferrite and / or perlite has taken place or after a low conversion in ferrite and / or perlite. Subsequently, the board or the respective molded part is held isothermally to convert the austenite into ferrite and / or perlite. Meanwhile, in the areas of the second kind, which should have relatively lower ductility properties in the final component, the hardening temperature is kept just high enough for sufficient martensite formation to take place in the areas of the second type during a hardening process. Finally, then the cooling is carried out. For this purpose, the resulting molded part is immersed in a separate operation in a quenching tank or the like to form the desired martensitic hardness structure. This procedure also requires a process management that can be integrated into a modern production plant only with great effort. In addition, there is also the problem with the components produced by this known method, that while they have a high strength, they are at the same time so brittle that they do not meet the requirements for their deformability which are set in practice.

In addition to the above-discussed prior art is from the EP-A-1 767 659 a process for the production of steel moldings is known, which is as a preform or board in the two-phase region, ie at a temperature between the Ac1 and the Ac3, is heated, and then formed in a (cooled) pressing tool and quenched at the same time. The board preheated in each case to a temperature which is preheated in the two-phase mixing area in the thermoforming tool itself is cooled directly and so rapidly that after the Cooling in the component a multi-phase structure with a ferrite content is present.

In the article "Status and innovation trends in hot stamping of USIBOR 1500 P" by Hein Philipp et al., Published in STEEL RESEARCH INTERNATIONAL, Verlag Stahleisen GmbH, Dusseldorf, DE, (20080201), Vol. 79, No. 2, ISSN 1611-3683, pages 85-91 is generally reported on the state of steel components by hot press forming of 22MnB5 steel. It is assumed that blanks made from this steel are austenitized prior to thermoforming by heating them to a temperature of 900-940 ° C. Likewise, approaches for adjusting different microstructures in a monolithic steel product formed from such a heated board are explained by different heat treatments performed during and after hot working. In this context, the possibility of influencing the microstructure of the component to be produced in a warm tool is also addressed. However, the options mentioned there always assume that the steel product is first completely austenitized. In the part of the steel product covered by the hot tool, a bainitic microstructure then sets in.

The of Lenze F.-J. et al. authored article "Manufacture of Body Components from Hot Formed High Strength Steel Materials", EFB Tagungsband, European Research Society for Sheet Metal Working, (20050101), Vol. 25 , gives an overview of the conventional Hot forming and press hardening of blanks made of steel 22MnB5 and a steel known as "CP-W 800" and presents results on so called "hot forging". In the warm forging process, steel products are heated to a temperature below the Ac1 (here 650 ° C) temperature of the steel under investigation and formed at this low temperature. This temperature is below the recrystallization temperature at which, for example, in the case of the components consisting of the steel 22MnB5, an overall structural change occurs.

Against the background of the prior art described above, the object of the invention was to provide a method with which it is possible to produce steel moldings in a process-technically simple manner, in which a high strength is combined with a good residual elongation at break.

This object has been achieved by the method specified in claim 1. Advantageous embodiments of this method are specified in the dependent claims on claim 1.

According to the invention, a steel molding is produced with a predominantly ferritic-bainitic structure.

For this purpose, a starting material in the form of a steel plate or a preformed steel part is provided. If a hitherto undeformed steel plate is processed as a starting material, the entire process is referred to as a "one-step" process. In contrast, a preformed Processed steel part, one speaks of a two-stage process, whereby in the first stage, a previously undeformed board is deformed so that the resulting steel component has not yet reached its final shape.

The respective starting material according to the invention consists of a steel of a known composition, which in addition to iron and unavoidable production-related impurities (in wt .-%) C: 0.02 - 0.6%, Mn: 0.5 - 2.0%, Al : 0.01 - 0.06%, Si: up to 0.4%, Cr: up to 1.2%, P: up to 0.035%, S: up to 0.035% and optionally one or more elements from the group "Ti, B, Mo, Ni, Cu, N", wherein - if present - Ti in a content of up to 0.05%, Cu in a content of up to 0.01%, B in contents of 0 , 0008 - 0.005%, Mo in contents of up to 0.3%, Ni in contents of up to 0.4%, N in contents of up to 0.01%. Particular importance with regard to the strength inventively produced components comes here to the respective C content, whereas in particular the contents of Si, Mn, Cr and B are adjusted so that the formation of bainite promoted and the formation of larger amounts of martensite in the structure of the component be avoided.

The thus composed starting material (steel plate or preformed steel part) is so warmed at a lying between the Ac1 and the Ac3 temperature of the steel heating temperature that incomplete austenitization of the starting material occurs. Accordingly, at the end of the austenitizing phase, the microstructure of the starting material consists of ferrite and austenite.

Subsequently, the starting material is placed in a press mold and formed therein to the steel molding. The press-hardening takes place in a temperature range in which the microstructure of the primary material in the two-phase region of ferrite and austenite is.

Essential to the invention is now that the steel molding is brought to a bainite formation temperature which is above the martensite start temperature but below the pearlite transformation temperature of the steel from which the steel sheet or the preformed steel part are respectively made.

It is equally important that, as soon as this bainite formation temperature is reached, the steel molded part is kept isothermally substantially isothermal at the bainite formation temperature over a bainitization time, until a structure consisting predominantly of ferrite and bainite has formed in the steel molded part. The bainitization temperature to be set in each case depends on the bainite transformation temperature, which is differentiated upward in each case according to the chemical composition of the enriched austenite by the martensite start temperature and perlite transition temperature.

The cooling rate during press hardening is significantly influenced by the austenitizing and mold temperature. This must be so fast that the board cooled without conversion to the Bainitumwandlungstemperatur and kept constant at this temperature becomes. By this procedure it is achieved that at the end of the bainitization time in the steel mold part there is a structure which, in addition to the ferritic and bainitic parts of the structure, has minor amounts of retained austenite and possibly less than 5% of martensite. The residual austenite contents in the resulting component, which are essentially determined by the carbon content, can be up to 10%.

After the end of the bainitization time, the steel mold is cooled to room temperature.

According to the invention, therefore, the temperature control with respect to the austenitizing process and the subsequent press hardening is controlled so that adjusts a mixed structure of ferrite, bainite and a proportion of retained austenite in the component. The method according to the invention thus provides a steel component whose microstructure is characterized by a ferritic-bainitic microstructure. This bainitic microstructure gives a component produced according to the invention improved deformation properties, in particular an improved residual elongation at break. Along with this, steel moldings produced according to the invention have an improved crash behavior, without the need for separate tempering treatment, since bainite can be regarded as a type of tempered martensite.

Moreover, the method according to the invention makes it possible to cool the steel component more slowly than in the conventional methods in which the cooling takes place in the tool with the aim of martensitic To produce hardness structure. Therefore, in a method according to the invention the risk of the formation of component distortion is minimized and the components produced according to the invention are characterized by a particularly high dimensional accuracy. In order to ensure a slow cooling of the steel component, the pressing tool can also be specifically heated to carry out the method according to the invention.

In addition to the advantages mentioned above, there are further advantages of the invention in the energy savings possible by the comparatively low oven temperature during austenitization, in the reduced thermal load of the optionally present surface coating, in which due to the lowered furnace temperature during austenitization possible use of Zn-coated starting material and in that it is possible in accordance with the method according to the invention to variably set the mechanical characteristics according to the component requirement by varying the austenitizing and mold temperature. Finally, steel moldings produced according to the invention are also characterized by a high bake hardening potential after press hardening.

To be able to use the advantageous properties achieved with the invention particularly safely, the ferrite and bainite in the structure of the steel molding at the end of Bainitisierungszeit should be at least 90%, the ferrite and bainite each should be at least 30%.

Since the martensite formation is prevented as completely as possible according to the invention, it is fundamentally advantageous if, at the end of the bainitization time, the martensite portion of the steel molding is less than 1%, in particular limited to only traces.

The alloy of steel constituting the primary material to be processed in accordance with the present invention includes conventional MnB steels and temper steels alike. In addition to iron and unavoidable impurities (in% by weight), a heat-treatment steel particularly suitable for carrying out the process according to the invention has C: 0.25-0.6%, Si: up to 0.4%, Mn: 0.5-2 , 0%, Cr: up to 0.6%, P: up to 0.02%, S: up to 0.01%, Al: 0.01-0.06%, Ti: up to 0.05% , Cu: up to 0.1% and B: 0.008 - 0.005%. On the other hand, MnB steels which are suitable for the process according to the invention have C: 0.25-0.6%, Si: up to 0.4%, Mn: 0.5-2.0%, Cr: up to 1.2 %, P: up to 0.035%, S: up to 0.035%, Mo: up to 0.3%, Ni: up to 0.4% and Al: 0.01-0.06%.

Typically, the austenitizing temperature of the steels from which the starting material processed according to the invention is made is in the range of 750-810 ° C. The heating time provided for the heating at the heating temperature is usually in the range of 6 to 15 minutes.

In particular, in the production of steel moldings, which are intended for the construction of bodies for vehicles, especially automobiles, it is advantageous if the starting material with a protective against corrosion metallic coating is provided. This coating also protects the respective primary material (steel plate, preformed steel part) from the press mold during transport from the oven, where it is preheated to the austenitizing temperature. The corrosion protection coating can be designed so that it protects an oxidation of the hot steel substrate with the ambient oxygen even when transported in air.

A particularly practical variant of the method according to the invention is characterized in that the press forming and the bainitization of the steel component produced in the course of the press molding takes place in the press forming tool. Accordingly, a particularly advantageous variant of the invention provides that after the compression molding of the starting material, the steel mold part then obtained remains in the compression mold and brought there to the Bainitbildungstemperatur and held for the Bainitisierungszeit. In this case, the press-forming tool is preferably tempered such that the starting material, starting from a temperature above the bainitizing temperature, is already cooled to the bainitizing temperature during its compression deformation to the steel component. The tool closing time of the pressing tool, within which the shaping, cooling and bainitization of the steel molding takes place, is in this case usually 5 to 60 seconds, in particular 20 to 60 seconds.

In the case that the cooling on the Bainitisierungstemperatur and the Bainitisierung in a Tool be completed, the bainitization time is shorter by the time duration than the tool closing time, which is required to bring the respective starting material to the Bainitisierungstemperatur.

As an alternative to bainitization in the press-forming tool, it is also conceivable, after the press-forming process, to remove the shaped steel part molded from the starting material from the press mold and to bring it to the bainite-forming temperature in a separate operation and to maintain it over the bainitization time. Such a procedure can be displayed if a corresponding system technology is available. Thus, such an approach can be used, for example, when a salt bath or a lead bath is available for heating up and holding at the bainitization temperature, into which the steel component can be brought after press molding.

The typical range of the bainitization temperature at which the baintization according to the invention is preferably carried out with the aim of forming a ferritic / bainitic structure is typically limited downwards by the martensite start temperature of the respective steel composition of the starting material, while being set lower than 500 ° C at the top can be used to avoid the formation of pearlite.

The technical complexity associated with carrying out the method according to the invention can also be reduced to a minimum by the fact that after the end of the Bainitisierungszeit the cooling of the resulting steel molding is carried out in a simple manner in air.

For the implementation of the method according to the invention are suitable steel blanks, which have been divided from a hot-rolled or cold-rolled flat product, such as tape or sheet. It is also possible to apply the method according to the invention to a steel part which has been preformed in a previous work step. The latter is useful, for example, when the shape of the steel component to be produced is so complex that several shaping steps are required for its production.

Due to their property profile, steel components produced according to the invention are particularly suitable for use as crash-relevant parts of an automobile body. The inventive method is particularly suitable for the production of longitudinal and floor cross members, which should have a particularly good energy absorption capacity in practice.

The invention will be explained in more detail by means of exemplary embodiments.

In Fig. 1 For example, a typical course of the temperature T during the performance of a method according to the invention is recorded over the time t. Accordingly, as a starting material to be deformed in each case to a steel component, for example, with a prior to corrosion heated AlSi coating steel plate initially heated to an austenitizing temperature TA, which is below the Ac3 temperature but above the Ac1 temperature of the steel from which the steel plate is made in each case. At this austenitizing temperature TA, the steel plate is held for a time tA until the steel plate is completely heated so that there is a mixed structure of austenite and ferrite therein. The area in which the steel has a structure is in Fig. 1 marked with A, while the area of the mixed structure of ferrite and austenite is marked with "A + F".

After the end of the austenitizing time tA, the steel plate is transported to a press forming tool. The transfer time required to close the press tool is in Fig. 1 denoted by tT. The temperature TW at which the steel plate enters the die is still within the temperature range Ac3 - Ac1.

The press mold is equipped with a tempering device which keeps it at a constant temperature corresponding to the bainitization temperature TB. The formed from the steel plate, with the press mold directly coming into contact steel mold part is cooled correspondingly over a cooling time tK to the bainitization temperature TB. The bainitization temperature TB is above the martensite start temperature Ms but below the pearlite transformation temperature. The area where perlite is formed is in Fig. 1 with P. characterized. Additionally is in Fig. 1 where F is the area in which pure ferrite is present and M is the area in which martensite is present.

Once the bainitizing temperature TB is reached, the steel component still sitting in the die is kept isothermal at the bainitizing temperature TB for a bainitizing time tB. The bainitization time t B is dimensioned so that at its end the structure of the steel component is essentially completely bainitic.

The cooling of the steel plate in the tempered pressing tool takes place within the cooling time tK so fast that the steel passes through the two-phase mixing A + F and a conversion in martensite M and perlite P is prevented, the martensite is avoided as completely as possible.

After reaching the end of the bainitization time tB, the tool is opened and the steel component is cooled to room temperature in still air. The tool closing time tW, which comprises the cooling time tK and the bainitization time tB, is 5-60 seconds, depending on the complexity of the shaping of the steel component to be produced and the sheet thickness of the respectively processed steel plate.

For two tests, two 1.5 - 2 mm thick steel plates SP1, SP2 were produced from a hot strip of 3 - 4 mm thickness by cold rolling, which consisted of a 27MnCrB5-2 steel with the composition given in% by weight in Table 1 ,

The first steel plate SP1 was then heated to an austenitizing temperature TA of 780 ° C and held at this temperature TA for an austenitizing time tA of 6 minutes. Table 1 Remaining iron and unavoidable impurities C Si Mn P S 0.294 0.24 1.13 0,017 0,002 al N Cr Ti B 0,035 0.0038 0.43 0.033 0.0010

Subsequently, the steel plate SP1 has been transported in air for 6 to 12 s transfer time tT in air in a press mold, which has been heated to a bainitization temperature TB of 400 ° C and kept constant at this temperature TB. In the pressing tool, the steel plate SP1 was then press-formed over a tool closing time tW of 40 seconds. The total pressing time included the cooling time tK in which the steel board SP1 was cooled from the tool inlet temperature TW to the bainitizing temperature TB, and the bainitizing time tB in which the bainite structure was formed in the steel component thermoformed in the press forming tool. Subsequently, the pressing tool has been opened and the steel component has been cooled to room temperature in still air.

The structure of the resulting steel molding had a ferrite content of 50%, a bainite content of 40%, a residual austenite content of 6% and a martensite content of 4%.

In the second experiment, the second steel plate SP2 was so thoroughly heated at an austenitizing temperature TA of 800 ° C. that it, too, was only partially austenitized. After this partial austenitization, the second steel plate SP2 has undergone the same process steps as the first steel plate SP1.

The properties of the steel moldings produced from the steel blanks SP1, SP2 in the manner described above are shown in Table 2. Table 2 circuit board TA [° C] Rp0.2 [MPa] Rm [MPa] Ag [%] A80 [%] SP1 780 374 759 12.7 19.7 SP2 800 464 802 11.4 19.0

Finally, for comparison, a steel plate also consisting of the 27MnCrB5-2 steel has been press-hardened martensitic in a conventional manner to give a steel shaped part. The residual breaking strain A80 was only about 6% in the component thus obtained. According to the invented method, the residual breaking strain A80 of the same quality is about 19%.

The bainitic press hardening according to the invention is therefore a process for hot press hardening instead of the usually produced martensite structure, a structure consisting predominantly of ferrite and bainite is set by an isothermal conversion during press hardening on the respectively press-formed steel component. The resulting ferritic / bainitic structure has improved residual elongation at high strength compared to martensite.

Claims (15)

1. Method for producing a formed steel part having a predominantly ferritic-bainitic structure,

   - wherein a primary material is provided in the shape of a steel blank or a pre-formed steel part, which in each case is produced from a steel containing (in % by weight) C: 0.02 - 0.6%, Mn: 0.5 - 2.0%, Al: 0.01 - 0.06%, Si: max. 0.4%, Cr: max. 1.2%, P: max. 0.035%, S: max. 0.035%, and optionally one or more of the elements of the "Ti, Cu, B, Mo, Ni, N" group, with the proviso that Ti: max. 0.05%, Cu: max. 0.01%, B: 0.0008 - 0.005%, Mo: max. 0.3%, Ni: max. 0.4%, N: max. 0.01%, and the remainder as iron and unavoidable impurities,

   - wherein the primary material is heated through at a heating temperature (TA) lying between the Ac1 and the Ac3 temperature of the steel, such that at best incomplete austenitising of the primary material takes place,

   - wherein the primary material is placed into a press-form tool and formed therein into the formed steel part,

   - wherein the formed steel part is then heated to a bainite forming temperature (TB), which is above the martensite starting temperature (MS), however below the pearlite transformation temperature of the steel, from which the primary material is produced in each case,

   - wherein after cooling the formed steel part is maintained for an austempering period (tB) at the bainite forming temperature (TB) in a substantially isothermic manner, until the formed steel part has produced a structure consisting predominantly of ferrite and bainite, the martensite content thereof being less than 5%, wherein residual austenite contents of up to 10% may be present, and

   - wherein after the end of the austempering period (tB) the formed steel part is brought to room temperature.
2. Method according to claim 1, characterised in that the steel contains (in % by weight) C: 0.25 - 0.6%, Si: max. 0.4%, Mn: 0.5 - 2.0%, Cr: max. 0.6%, P: max. 0.02%, S: max. 0.01%, Al: 0.01 - 0.06%, Ti: max. 0.05%, Cu: max. 0.1%, B: 0.008 - 0.005% and the remainder as iron and unavoidable impurities.
3. Method according to claim 1, characterised in that the steel contains (in % by weight) C: 0.25 - 0.6%, Si: max. 0.4%, Mn: 0.5 - 2.0%, Cr: max. 1.2%, P: max. 0.035%, S: max. 0.035%, Mo: max. 0.3%, Ni: max. 0.4%, Al: 0.01 - 0.06%, and the remainder as iron and unavoidable impurities.
4. Method according to any one of the preceding claims, characterised in that the total of the ferrite and bainite portions in the structure of the formed steel part is at least 90% at the end of the austempering period (tB).
5. Method according to any one of the preceding claims, characterised in that at the end of the austempering period (tB) of the austempering period the martensite portion of the formed steel part is less than 1%, in particular is limited to traces.
6. Method according to any one of the preceding claims, characterised in that the austenitising temperature (TA) is 750 - 810°C.
7. Method according to any one of the preceding claims, characterised in that the heating period (tA) proposed for heating through at the heating temperature (TA) is 6 - 15 minutes.
8. Method according to any one of the preceding claims, characterised in that the primary material is provided with an anti-corrosion metal coating.
9. Method according to any one of the preceding claims, characterised in that, after the primary material has been press-formed, the formed steel part obtained in the press-form tool is brought to the bainite forming temperature (TB) and maintained for the austempering period (tB).
10. Method according to claim 9, characterised in that the tool closing time (tW) of the pressing tool is 5 - 60 seconds, in particular 20 - 60 seconds.
11. Method according to claim 10, characterised in that the austempering period (tB) is shorter than the tool closing time (tW).
12. Method according to any one of claims 1 to 8, characterised in that after press forming the steel part press-formed out of the primary material is removed from the mould and is brought in a separate process step to the bainite forming temperature (TB) and maintained for the austempering period (tB).
13. Method according to any one of the preceding claims, characterised in that the bainite forming temperature (TB) is higher than the martensite starting temperature of the respective primary material composition below 500°C.
14. Method according to any one of the preceding claims, characterised in that the formed steel part obtained is cooled down after the end of the austempering period (tB) in air.
15. Method according to any one of the preceding claims, characterised in that the formed steel part concerns a component of an automobile body.

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