EP1939308A1 - Method for manufacturing a component through hot press hardening and highly rigid component with improved breaking strain - Google Patents

Abstract

Production of a steel component comprises heating a sheet metal part to a temperature lying between the Ac1 and the Ac3 temperature of the steel and hot press quenching the heated sheet metal part so that the component has a structure in which the components are present as ferrite and martensite. The steel of the sheet metal part contains (in wt.%) 0.10-0.45 carbon, 0.05-0.50 silicon, 0.8-1.7 manganese, 0.05-0.6 chromium, maximum 0.015 phosphorus and maximum 0.003 sulfur. An independent claim is also included for a steel component produced by the above process.

Description

The invention relates to a method for producing a high-strength component with improved elongation at break by hot press hardening and a high-strength component with improved elongation at break and toughness.

In hot press hardening of sheet metal blanks divided from cold or hot rolled steel strip, the blanks are heated to a forming temperature and placed in the heated condition in the tool of a forming press. In the course of the subsequently carried out deformation, the sheet metal blank or the component formed from it undergoes a rapid cooling due to the contact with the cool tool, resulting in hardened structure in the component. The rapid cooling can be assisted by actively cooling the tool itself.

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 September 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 that under the name 22MnB5 known steel, 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 be produced from the composite steel thus first sheets, which are then preheated to above the Ac ₃ temperature, typically in the range of 850 - 950 ° C, lying temperature. 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.

Achieve the components produced from boron-alloyed steels in the manner explained above Strengths of over 1500 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 leads to components which are manufactured from the known boron-alloyed steels in the known manner often no longer being able to meet this requirement. This applies in particular when the components to be produced are parts for an automobile body.

The invention was therefore based on the object to provide a method which allows to produce complex shaped steel components, on the one hand have a sufficiently high strength and good elongation at break and on the other hand, a sufficient toughness, in order to meet the demands placed on them in practice to suffice. In addition, a component should be specified whose range of properties on the one hand under normal conditions a high dimensional stability and on the other hand in an accident ensures a high energy absorption capacity.

With regard to the method, the object stated above has been achieved by the subject matter of claim 1. Advantageous embodiments of this method are specified in the dependent claims on claim 1.

With regard to the component, the object stated above has been achieved by the subject matter of claim 8. Advantageous embodiments of this component are specified in the dependent claims on claim 8.

The invention is based on the basic idea of avoiding complete martensite hardening of the component during press hardening. Instead, according to the invention, a mixed structure is produced in the component to be produced according to the invention, which on the one hand ensures surprisingly high strength and, on the other hand, high elongation values and a toughness of the component according to the invention which is adequate for the respective application.

In order to achieve this, according to the invention sheet metal parts are hot press-hardened, which are produced from a steel which is known per se with regard to its composition. Its alloy contents (C: 0.10-0.45%, Si: 0.05-0.50%, Mn: 0.8-1.7%, Cr: 0.05-0.6%, P: max 0.015%, S: 0.003% maximum, and optionally one or more of the following alloying elements Al: 0.01-0.05%, N: 0.002-0.005%, Ti: 0.01-0.1%, B: 0.0008 - 0.008%, remainder iron and unavoidable impurities, data in wt .-%) are coordinated so that it inherently has a high strength potential.

Unlike in the prior art, however, the sheet metal part made of this steel according to the invention is heated only to a temperature at which the austenitization is not yet completed. Rather, a heating temperature is selected according to the invention for the press hardening, which is between the for the Ac ₁ and Ac ₃ temperatures determined for each steel (Ac ₁ temperature: temperature at which the transformation into austenite begins; Ac ₃ temperature: temperature at which the transformation to austenite is completed). At these heating temperatures, the sheet processed in each case has a mixture of austenite and ferrite.

According to the invention, therefore, only a "partial austenitization" of the sheet metal part takes place, in which the existing prior to heating ferrite structure of each processed sheet is retained to a decisive for the properties of the component obtained share.

In the course of inventively made partial austenitization of the respective sheet metal part thus only the structural components of the carbon-rich phases such as perlite, bainite, martensite and cementite are converted into austenite. Depending on the selected temperature level of the partial austenitization, a part of the ferrite originally present in the sheet metal part also converts to austenite.

The heating of the sheet metal parts according to the invention, however, always controlled so that present after heating in the sheet metal part ferrite and austenite side by side. The respective size of the proportions of ferrite and austenite can be controlled via the heating temperature.

In the subsequent press-hardening of the sheet heated in the manner according to the invention, the austenitic microstructure converts to martensite. This Martensitanteil ensures in the finished component for the high Strength. By contrast, the ferrite content present in the metal sheet after heating is still retained in the component even after the press-hardening and ensures the desired high elongation at break and improved toughness.

Since the carbon content in austenite at Teilaustenitisierung invention is each significantly higher than the total carbon content of the steel (for example, in a microstructure with 50% each austenite and 50% ferrite, the C content of austenite about twice as high as the C content of Total of steel), the austenite remains substantially stable until press hardening. This enables safe process control during press hardening.

In the case of the procedure according to the invention, consequently, a microstructure which is similar to that of dual-phase steels is set in a targeted manner in the respectively obtained component. By way of the size of the proportions of martensite and ferrite, it is possible to set directly whether the strength, the elongation at break and / or the toughness are in the foreground of the component produced according to the invention.

In the procedure according to the invention, the formation of the proportions of martensite is influenced directly by the choice of a suitable heating temperature. Heating temperatures lying close to the Ac ₃ temperature lead in the finished component to larger martensite proportions and thus higher strength, while particularly good elongation values can be achieved by lower heating temperatures close to the Ac ₁ temperature.

The elongation characteristics of a component produced according to the invention are always greater than the corresponding components Values that can be determined for a manufactured in a known manner component with vollmartenitisiertem microstructure. Thus, in accordance with the invention, high-strength components can easily and purposefully be produced whose elongation at break is up to twice greater than the breaking elongation of a component produced in a known manner.

The invention thus provides a possibility for the practical use of which machines are required that are less expensive than the hitherto known variants of hot forming and thus trigger lower procurement and maintenance costs. The savings are achieved not only by the significantly reduced demands on the heat source required for the heating of the sheet metal part, but also by the lower requirements that are placed on the forming tool. By inventively lower temperatures of the cooling effort in the tool is minimized. In addition, components produced according to the invention may be trimmed with conventional cutting tools depending on the strength class. The expensive laser cutting of the components obtained can be omitted.

A particular advantage of the invention consists in the fact that it is possible to produce components in the procedure according to the invention which have an optimum combination of strength and ductility. Compared to the known measures for setting a certain strength and Dehnbarkeitsniveaus, such as the subsequent tempering of conventional press-hardened components, the invention offers a further potential savings. Thus achieve the invention hot-press hardened components have a strength level equivalent to that of conventionally press-hardened and subsequently annealed at temperatures of 300-400 ° C with fully martensitic structure. At the same time, however, the elongation values of components produced according to the invention are significantly higher than those of the conventionally press-hardened and tempered components. Therefore, with the product produced according to the invention, with a minimized production outlay, a component is available that possesses a combination of high strengths and very good elongation properties that is superior to the state of the art.

The inventive method is suitable for processing sheet metal parts, which are obtained from hot or cold rolled steel strips. If necessary, the steel strips in question may be subjected to a surface refinement before the hot press hardening carried out according to the invention. In the course of this surface refinement, they can be, for example, hot-dip aluminized (eg coating with an AlSi (AS) or AlZn layer (AZ)), hot-dip galvanized (eg coating with a simple zinc (Z), a zinc iron ( ZF) or a zinc aluminum layer (ZA)) or with an organic-inorganic coating (eg a lacquer coating which on the one hand provides corrosion protection and on the other hand improves the tribological properties of the blank during press hardening). The use of surface-treated sheet-metal parts has the advantage, especially when carrying out hot-pressing curing, that the coating applied to the base substrate prevents excessive scale formation on the sheet-metal part, by which the result of the press molding would be degraded.

As long as a sufficiently rapid cooling is ensured in the hot-pressing tool, the success achieved by the invention is independent of how the hot press hardening itself is carried out. Accordingly, in the context of the invention, the hot molding press hardening can be carried out directly or indirectly. In direct hot molding press hardening, a board is used as the sheet metal part, which is heated in the manner according to the invention and from which then directly, d. H. Without further deformation steps, the respective component is formed. In the case of indirect hot-pressing hardening, on the other hand, an arbitrarily shaped sheet-metal part is preformed in at least one step from a sheet metal blank. This preformed sheet metal part is then heated in accordance with the invention and receives its final shape during the subsequent hot-press forming step.

To the inventively in the range of Ac ₁ - to Ac ₃ temperatures carried out heating of each thermosetting sheet metal part may be followed by cooling to the entry into the hot press tool so that with the onset of the forming process in the sheet metal part in each case with regard to the respective work result optimal temperature is. This cooling can take place, for example, in the course of workpiece transfer from the heating device to the pressing tool in air or under a protective gas, by which a scaling of the surface of the sheet metal part to be prevented.

A component according to the invention is characterized in that it consists of a steel which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.45%, Si: 0.05-0.50%, Mn: 0, 8-1.7%, Cr: 0.05-0.6%, P: max. 0.015%, S: max. 0.003%, and optionally one or more of the following alloying elements A1: 0.01-0.05%, N: 0.002-0.005%, Ti: 0.01-0.1%, B: 0.0008-0.008% , wherein the component has a mixed structure with proportions of ferrite and martensite and at a breaking elongation A80 of 6 - 14% has a tensile strength of 800 - 1500 MPa.

Preferably, the proportions of martensite and ferrite in the structure of the component according to the invention are coordinated so that it has a tensile strength of 800 - 1100 MPa. Such a component fulfills most of the requirements in practice in terms of its strength and at the same time has an optimized toughness.

An inventive component optimally adjusted by adjusting the corresponding proportions of martensite and ferrite in accordance with the invention is characterized in that its breaking elongation A80 is in the range of 10-14%.

Due to their special property profile, components according to the invention are suitable in particular for use as parts of vehicle bodies. Thus, components according to the invention can be easily formed so that they can be used as crash-relevant components in an automobile body, on the one hand ensure sufficient rigidity of the respective body, but at the same time in a position should be to convert the sudden kinetic energy in an accident safely into deformation energy. Accordingly, can be in the inventive. Way preferably bumper reinforcements, A-, B-, C- and D-pillars as well as for these columns certain reinforcements, sills, roof frames, side members, tunnel reinforcements, side impact absorbers and mounting plates for automobile bodies produce.

Of course, components according to the invention are particularly preferably produced by the process according to the invention. This not only allows the simple adaptation of the property profile of the respective component to its intended use, but allows the sheet metal parts before their hot press hardening according to the invention only to a low compared to the prior art heating temperature, a particularly cost-effective production of high-strength components with a toughness , which is clearly superior to that of conventionally produced, purely martensitic components. It shows that the components produced according to the invention also have a high bake hardening potential due to the composition of the steel used according to the invention and the special processing path, which enables an additional increase in the strength of components according to the invention.

The invention will be explained with reference to an embodiment.

In the attached diagram is schematically the underlying principle of the inventive method shown. On the left side of the diagram, a section of the iron-carbon diagram prepared for a manganese-boron steel is shown, while on the right side of the diagram, the temperature profile over the respective steps of the method according to the invention can be seen. Accordingly, the respective sheet metal part is first heated within a time t1 to a heating temperature TE lying between the Ac ₁ and the Ac ₃ temperature of the carbon content X-containing steel from which it is produced. Subsequently, it is kept at the heating temperature TE over a time t2 sufficient for the desired partial austenitization. This is followed within a time t3, the transfer to the hot press tool, within which the sheet metal part cools in air to a still lying above the Ac ₁ temperature deformation temperature TV. With this temperature, the sheet metal part is placed in the hot pressing tool in which it is then cooled within a short time t4 quickly to a temperature of less than 100 ° C, converting the austenite formed during partial austenitization in martensite.

• Stahlerzeugung mit Konverter;
• Vergießen des Stahls durch konventionellen Strangguss oder in einer Gießwalzanlage zu einem Vormaterial, wie Bramme oder Dünnbramme, wobei der Einsatz einer Gießwalzanlage bevorzugt ist;
• Warmwalzen bei einer 850-900°C betragenden Warmwalzendtemperatur;
• Haspeln des erhaltenen Warmbands bei einer 560 - 600 °C betragenden Haspeltemperatur;
• erforderlichenfalls Beizen des Warmbands.

A typical process involving the invention of manufacturing components, which are particularly suitable for the construction of automobiles, can be described as follows:

• Steelmaking with converter;
• Casting the steel by conventional continuous casting or in a cast roll mill to a starting material, such as slab or thin slab, wherein the use of a casting and rolling plant is preferred;
• Hot rolling at a hot rolling end temperature of 850-900 ° C;
• Coiling the resulting hot strip at a coiler temperature of 560-600 ° C;
• if necessary, pickling of the hot strip.

If the hot-rolled strip already has sufficient thickness and quality for the subsequent use of the component to be produced from it, the hot-rolled strip can be produced by fire aluminizing (AS, AZ), hot-dip galvanizing (Z, ZF, ZA) and / or organic-inorganic coating with a be provided against corrosion protective coating or oxidation.

Otherwise, cold rolling is first carried out, which is preferably carried out without intermediate annealing, wherein the degree of cold rolling should be more than 40%. The resulting cold strip can then also be provided in the manner already described for the hot strip with a protective against corrosion or oxidation coating.

From the hot strip or cold strip then sheet metal blanks are divided. The sheet metal blanks are then heated to a heating temperature between the Ac ₁ and Ac ₃ temperatures.

After air cooling by a small amount of temperature in the course of the transfer of the sheet metal blank from the respective heating furnace into the die, the sheet metal blanks are placed in the hot pressing tool in which they are formed into the component and at the same time cooled so rapidly, that the austenite present after heating in its structure completely converts into martensite except for small residual austenite contents.

The inventive method has been tested on a sheet metal part, which has been divided as a flat-shaped board of a cold strip produced in the manner described above from the known 22MnB5 steel. The 22MnB5 steel typically contains (in wt%) 0.240% C, 1.3% Mn, 0.25% Si, max. 0.02% P, max. 0.005% S, 0.035% Al, 0.035% Ti, 0.155% Cr, 0.003% B and max. 0.1% of Mo, Cu and Ni, balance iron and unavoidable impurities.

The correspondingly assembled sheet metal part has been heated in an oven to a heating temperature of 780 ° C, in which there was a mixed structure of austenite / ferrite in the sheet metal part. Subsequently, the sheet metal part has been inserted by means of a conveyor in an actively cooled mold. During the transfer from the oven to the die, only a small amount of heat loss occurred, resulting in a temperature decrease of less than 5 ° C.

In the press forming tool, the sheet metal part has been deformed into a component intended for an automobile body. Due to the intense contact of the sheet metal part with the compression mold that occurred during the compression deformation, cooling was accompanied by the compression deformation, whose cooling rate was comparable to a quenching of the sheet metal part in an oil bath.

The automobile body component obtained according to the invention after hot press-hardening had a mixed structure consisting of 63% ferrite, 30% martensite and 7% retained austenite. The final strength determined for the component was 900 MPa, while its elongation at break A80 was 13%.

Claims (10)

A method of manufacturing a component by hot press hardening a sheet metal part made of a steel containing, in addition to iron and unavoidable impurities (in% by weight): C: 0.10 - 0.45% Si: 0.05 - 0.50% Mn: 0.8 - 1.7% Cr: 0.05 - 0.6% P: Max. 0.015% S: Max. 0.003% and optionally one or more of the following alloying elements al: 0.01 - 0.05% N: 0.002 - 0.005% Ti: 0.01 - 0.1% B: 0.0008 - 0.008% contains
characterized in that the sheet metal part for hot press hardening is heated to a temperature lying between the Ac ₁ and the Ac ₃ temperature of the steel and that the thus heated sheet metal part, if necessary after an intermediate cooling, is hot press cured, so that after hot pressing obtained component has a structure in which shares in ferrite and martensite are present. A method according to claim 1, characterized in that the sheet metal part is separated from a hot strip. A method according to claim 1, characterized in that the sheet metal part is divided from a cold strip. Method according to one of the preceding claims, characterized in that the cooling takes place after heating and before the hot press hardening in air. Method according to one of the preceding claims, characterized in that the sheet metal part is subjected to a surface finishing before the hot press hardening. Method according to one of the preceding claims, characterized in that the sheet metal part is hot-formed as a flat-shaped board in one step to the component. Method according to one of claims 1 to 5, characterized in that the sheet metal part is preformed in at least one step, and that the preformed sheet metal part is thermoformed to the component. Component consisting of a steel containing, in addition to iron and unavoidable impurities (in% by weight): C: 0.10 - 0.45% Si: 0.05 - 0.50% Mn: 0, 8 - 1.7% Cr: 0.05 - 0.6% P: Max. 0.015% S: Max. 0.003% and optionally one or more of the following alloying elements al: 0.01 - 0.05% N: 0.002 - 0.005% Ti: 0.01 - 0.1% B: 0.0008 - 0.008% contains, wherein the component has a mixed structure with fractions of ferrite and martensite and at a breaking elongation A ₈₀ of 6 - 14% has a tensile strength of 800 - 1500 MPa. Component according to claim 8, characterized in that it has a tensile strength of 800 - 1100 MPa. Component according to claim 8 or 9, characterized in that its breaking elongation A _{80 is} 10 - 14%.

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