DE102006019567B3 - Method for producing formed steel components - Google Patents

Abstract

A method for producing a molded component from a semi-finished product of hardenable and hot-workable steel sheet, by hot forming with cold forming tools, wherein the hot working produces a hardened component having a martensitic and / or bainitic structure, wherein the hardened component is tempered after hot working, so that a molded component is formed which, compared to the cured component state, at least in some areas has a higher yield strength and an elastic yield strength.

Description

The The invention relates to methods for producing a molded component from a semifinished product of hardenable and hot-forming steel sheet, by hot forming with cold Forming tools, wherein the hot forming a hardened component which is a martensitic and / or bainitic structure having. In particular, the invention relates to methods for press-hardening Steel sheets for the manufacture of body parts with high toughness.

For the weight reduction in bodywork and bodywork components, the approach is pursued Reduce sheet thickness. It is necessary to use the conventional steels through high and highest strength To replace steel materials. The saving of material must have a increased Strength of the material can be made up for. With regard to the Requirements for tensile strength, for example, the steel 22MnB5, which belongs to the class of ultra-high-strength steels a strength of 1200 MPa and more.

For series production of molded parts, press hardening is a particularly economical process. This is especially true for sheet-like semi-finished products, since a hardening of the entire sheet is possible here due to the small thickness. The press hardening is for example from the DE 198 15 022 A1 in which a semi-finished product is first cold formed by a drawing process, then heated and then press-cured in a hot-forming tool.

From the DE 197 43 802 C1 is a method for press hardening of sheet metal blanks known. In this case, a metallic mold component for motor vehicle components, which has areas with a higher ductility, made of a boron-manganese steel alloy. The composition of the steel is expressed in weight percent: carbon (C) 0.18 to 0.3%, silicon (Si) 0.1% to 0.7%, manganese (Mn) 1.0% to 2.50%, Phosphorus (P) maximum 0.025, chromium (Cr) 0.1% to 0.8%, molybdenum (Mo) 0.1% to 0.5%, sulfur (S) maximum 0.01, titanium (Ti) 0, 02 to 0.05, boron (B) 0.002 to 0.005, aluminum (Al) 0.01% to 0.06. The metal sheet is heated to temperatures around 900 ° C and formed in the cold pressing tool, with a compensation takes place. Subsequently, individual regions of the molded component are subjected to a partial heat treatment, which is at a temperature between 600 ° C and 900 ° C.

From the DE 103 32 722 B3 is the use of high-strength steel alloys as a material for the production of spring plates for motor vehicle struts, methods for their production and such spring plate known. Steel alloys are used with carbon contents of 0.18% to 0.28 and manganese contents of 1.20% to 4.0%. Other alloy components are silicon, chrome and boron. Depending on the steel alloy, blanks are cold or hot formed into a spring plate and hardened in liquid or in air.

Press-hardened components are generally very brittle, so that their use in motor vehicle bodies, in particular in crash structures, in This condition is very limited. In particular, a material behavior with a pronounced Yield strength and high yield ratio Re / Rm meets the requirement close to the body shop. Also is for component design for a passenger compartment in addition to the component geometry and the component material yield strength prevail and not the maximum component-material tensile strength.

It is therefore an object of the invention an economical manufacturing process for high strength steel moldings to provide, in which the material both a high strength as well as a high ductility having.

• a) Erwärmung des Halbzeugs auf Austenitisierungstemperatur und
• b) Presshärten in einem kalten Werkzeug wobei sich durch Abschreckung zumindest teilweise ein martensitisches und/oder bainitisches Gefüge einstellt und
• c) Anlassen des umgeformten Halbzeugs bei Temperaturen unterhalb 400°C unter Bildung eines Formbauteils mit gegenüber dem gehärteten Bauteil erhöhter Streckgrenze und/oder Bruchdehnung, mit den Merkmalen des Anspruchs 1.

The object is achieved by a method for producing a molded component from a semifinished product of curable and heat-formable steel sheet, comprising the steps

• a) heating of the semifinished product to Austenitisierungstemperatur and
• b) press-hardening in a cold mold, wherein at least partially a martensitic and / or bainitic microstructure sets by means of quenching and
• c) tempering of the formed semi-finished product at temperatures below 400 ° C to form a molded component with respect to the cured component increased yield strength and / or elongation at break, with the features of claim 1.

Further advantageous embodiments are the subject of the dependent claims.

According to the invention it is thus provided that the hot-formed and hardened in a cold tool hot and tempered component is annealed, so that a molding member is formed, compared to the cured construction Partial state at least partially has a higher and a pronounced yield strength. By annealing the component, an improvement of the mechanical properties takes place. It is essential that the yield strength is increased and a pronounced yield strength is achieved. As a result, the component is both high strength, and ductile enough for use in body construction.

A pronounced Yield point is given when the stress-strain curves of a Tensile test according to the linear range (Hook's range) clearly kink and a more or less pronounced short almost horizontal Have history. The yield strength (Rm) is determined by the location of the Deflection point defined. For materials without pronounced yield strength becomes common as a basis for the structural design is based on the Rp 0.2 limit.

in the another is provided according to the invention, that the press-hardened Component undergoes a temperature treatment below 400 ° C, resulting in a Molded parts with opposite the hardened Component increased Yield point and / or elongation at break results. This temperature treatment is to be understood as tempering at a low temperature level. The selected temperatures according to the invention are well below the for the corresponding high and highest strength Steels usual Temperature level for starting. By the temperature treatment according to the invention a component of high strength is produced at the same time as high ductility, which meets the requirements for body components and in particular also meets crash structures in automobiles.

Prefers the temperature and duration of annealing or tempering is chosen so that the yield strength is increased by at least 20. It can be also increase the elongation at break of the mold component or at least only slightly lose weight.

at The crash structures in case of deformation is a high energy absorption desired by the deformation of the component. This is done by the with the method according to the invention achieved achievable combination of strength and ductility.

In Preferred embodiment of the invention, the semi-finished product of a Chrome-molybdenum steel is formed. The content of Cr and Mo is preferably for Cr at 0.8 to 1.3% and for Mo at 0.13 to 0.4%, with the content of carbon at 0.2 to 0.5%.

suitable Representative of these steels are 25CrMo4, 34CrMo4 or 42CrMo4, as well as 25CrMoS4, 34CrMoS4 or 42CrMoS4. For these steels are hardened by manufacturers in the range of 840 to 880 ° C and tempering temperatures at 540 to 680 ° C are recommended. The specified Cause tempering temperatures Although to improve the ductility, but at the same time to a intolerable decrease in strength or yield strength. For the inventively provided Temperatures of tempering below 400 ° C will no more embrittlement observed, but rather an increase in the Brucheinschnürung, the Yield strength and elongation at break. Preference is given in the temperature range of 250 ° C tempered up to 400 ° C. Particularly preferred in these steels is the tempering temperature in Range from 250 to 350 ° C.

In preferred development of the invention, the semi-finished product of a boron-alloyed tempered and tempered steel is formed. Suitable representatives of these steels are for example 17MnB3, 22MnB5 or 27MnCr5-2. Particularly preferred in these steels is the Tempering temperature in the range of 300 to 330 ° C.

The needed for starting Time depends in particular on the material thickness of the component. For a material thickness For example, 2 to 10 minutes are suitable for 1 mm. One too long elected Holding time can be dependent the steel alloy lead to a negative influence on the component or material properties.

For the production process according to the invention Sheet-like semi-finished products are particularly suitable. Preferably exists the semi-finished product from a flat board, from an already cold-formed Semi-finished steel or from a trimmed component blank. The material thickness is typically in the range of 0.8 to 3 mm.

By the forming process according to the invention in particular profile carrier or hollow beams for bodywork and chassis made.

It is expedient that the cured semifinished product relate only immediately before or after tempering Annealing is cut to final contour. For this purpose, the surface of the cured molded component is preferably cleaned before tempering. Contamination of the surface during tempering should be avoided as much as possible, so that post-processing of the surface after tempering can be minimized. A preferred cleaning method is powder blasting and dry blasting by blasting.

In Another preferred embodiment of the invention is in Tempering (in step c)) or annealing at the same time a surface treatment for the formation of a defined corrosion protection and / or wear protection for the Molded component performed. The temperature of the annealing at about 250 to 400 ° C, for example for firing a coating, in particular anti-corrosion coating, be used.

Especially is preferred during tempering or tempering as a surface treatment galvanizing or thin-layer galvanizing carried out. As a result, a substantial process simplification is possible; the Processes of tempering and galvanizing fall together.

The Semifinished can in a further embodiment of the invention be composed of several different sheet metal parts. in this connection If the semifinished product preferably consists of several hot-formable steel sheets, at least one of which consists of a hardenable steel.

The Method is preferred for producing a shell part for in Passenger compartment of a vehicle body integrated hollow beam or for in the chassis of a vehicle integrated hollow beams applied. Especially preferred are the hollow beams in the crash structure or in deformation structures of automobiles arranged.

Example:

To For comparison purposes, several semi-finished products were made of a 22MnB5 steel on the one hand hot-formed according to the invention or press-hardened and then fishing (components A) and on the other hand only heat-formed or press-hardened (Components B); The material thickness of the sheet metal blanks was about 1.0 mm.

For starting (Component A) was a temperature of 320 ° C and a holding time of approx. 4-6 minutes dialed.

For the components A, the following averaged measured values result: Stretch limit 1226 MPa tensile strenght 1335 MPa breaking stress 1000 MPa ges.Dehn.Höchstlast 2.4 ges.Dehn.Bruch 4.1 Fracture, Z 29.8

For the components B, the following averaged measured values result: Stretch limit 923 MPa tensile strenght 1659 MPa breaking stress 1351 MPa ges.Dehn.Höchstlast 3.2% ges.Dehn.Bruch 5.0% Fracture, Z 18.3%

It shows that the inventive process control a significant increase the yield strength can be achieved. For use as a body component is the measured decrease in tensile strength without meaning, because this plays for the design of the construction only a minor role.

The Elongation at break decreases only insignificantly by the process control according to the invention 5 to 4.1%. On the other hand, there is a reduction in the crack sensitivity in comparison of the fractional necking values. Again, the state shows with subsequent tempering or annealing with 29.8 instead of 18.3% a significant increase in performance compared to only the hot-formed and hardened Status.

The stress characteristics of the tensile specimens of components A or B are in the form of stress / strain diagrams in FIG 1 for the components A and in 2 for the components B reproduced.

For the components A are pronounced Yield limits detectable. The curves bend according to the linear (Hookschen) Range clearly, in a short almost horizontal course. The ensuing long range of plastic deformation is also comparatively horizontal.

In contrast, the curves show in 2 For the components b no kink and no apparent yield strength.

Claims (12)

A process for producing a molded component from a semifinished product of curable and hot-formable steel sheet, comprising the steps of a) heating the semifinished product to austenitizing temperature and b) press-hardening in a cold mold, at least partially establishing a martensitic and / or bainitic structure by means of quenching and c) a Tempering of the formed semi-finished product at temperatures below 400 ° C to form a molded component with respect to the cured component increased yield strength and / or elongation at break, characterized in that for the semi-finished a chromium-molybdenum steel compositions 25CrMo4, 34CrMo4, 42CrMo4, 25CrMoS4, 34CrMoS4 or 42CrMoS4 is selected. Method according to claim 1, characterized in that that the temperature and duration of tempering are chosen so that the yield strength increased by at least 20%. Method according to one of the preceding claims, characterized in that the semifinished product consists of a flat board, made of a cold-formed steel semi-finished or trimmed component blank consists. Method according to one of the preceding claims, characterized characterized in that the tempering at a temperature in the range from 250 to 400 ° C he follows. Method according to one of the preceding claims, characterized characterized in that the tempering at a temperature of 300 to 330 ° C takes place. Method according to one of the preceding claims, characterized characterized in that the cured semi-finished product before or after step c) is trimmed to final contour. Method according to one of the preceding claims, characterized characterized in that the surface of the cured molded component before the Tempering is cleaned, in particular by a Pulverstrahlverfahren. Method according to one of the preceding claims, characterized characterized in that when starting in step c) at the same time a surface treatment for the formation of a defined corrosion protection and / or wear protection for the Molded part takes place. Method according to claim 12, characterized in that that as a surface treatment galvanizing or thin-layer galvanizing carried out becomes. Method according to one of the preceding claims, characterized characterized in that the semifinished product is made of several thermoformable Composed of steel sheets, of which at least one of a curable Steel exists. Method according to one of the preceding claims, characterized characterized in that from the mold component shell parts for in the Passenger compartment of a vehicle body integrated hollow beams are produced. Method according to one of the preceding claims, characterized characterized in that from the mold component shell parts for in the Suspension of a vehicle integrated hollow beams are manufactured.