DE102019115165A1 - Method for producing a steel composite material - Google Patents

Abstract

The invention relates to a method for producing a steel composite, wherein at least two steel sheets, which consist of at least two different steel grades, in particular a first steel grade made of a hardenable manganese-containing steel and a second steel grade made of a microalloyed steel, are placed on top of one another and plated by hot rolling and thereby The composite material obtained is then optionally cold-rolled, with the composite material consisting of at least two layers with different steel compositions being form or press hardened after the rolling process, the composite material being subjected to a subsequent temperature treatment, the composite material being subjected to a subsequent temperature treatment to increase the bending angle by at least 5 ° between 150 ° C and 300 ° C for 15 minutes to 60 minutes.

Description

The present invention relates to a method for producing a steel composite material and a steel composite material component which is produced according to this method.

Hardened steel components have the advantage, especially in the body shop of motor vehicles, that their outstanding mechanical properties make it possible to create a particularly stable passenger cell without having to use components that would be much more massive and therefore heavier with normal strengths.

To produce such hardened steel components, types of steel which can be hardened by quench hardening are used. Such types of steel are, for example, boron-alloyed manganese carbon steels, the most widely used being the 22MnB5 or 20MnB8. However, other boron-alloyed manganese carbon steels are also used for this purpose.

In order to produce components hardened from these types of steel, the steel material must be heated to the austenitizing temperature (> Ac ₃ ) and wait until the steel material is austenitized. Depending on the required degree of hardness, partial or full austenitization can be achieved here.

If such a steel material is cooled after austenitizing at a rate above the critical hardening rate, the austenitic structure is transformed into a martensitic, very hard structure. In this way, tensile strengths R _{m of} over 1500 MPa can be achieved.

There are currently two ways of producing the steel components.

In the so-called form hardening, a sheet steel blank is separated from a steel strip, e.g. cut or punched and then deep-drawn into the finished component in a conventional, for example five-step deep-drawing process. This finished component is dimensioned slightly smaller in order to compensate for a subsequent thermal expansion during austenitizing.

The component produced in this way is then austenitized and then placed in a form hardening tool, in which it is pressed, but not or only slightly deformed, and the heat flows out of the component into the pressing tool as a result of the compression, at a rate above the critical hardening speed Speed.

The second, alternative method is the so-called press hardening, in which a blank is separated from a sheet steel strip, e.g. cut or punched, then the blank is austenitized and the hot blank is formed in a single-stage or multi-stage forming step and at the same time with a hardening speed above the critical hardening speed Speed is cooled.

In both cases, metallic corrosion protection layers can be used, e.g. plates provided with zinc or an alloy based on zinc can be used. Press hardening is also known as an indirect process and press hardening as a direct process. The advantage of the indirect process is that more complex workpiece geometries can be implemented.

The advantage of the direct process is that a higher degree of material utilization can be achieved. However, the component complexity that can be achieved is lower, especially in the one-step forming process.

In this way, ready-formed and usually ready-perforated components are passed through an oven and heated to austenitizing temperature during press hardening. These components are placed on furnace supports for transport.

During press hardening, the blanks must be conveyed through the furnace using chain conveyors or walking beams.

Furthermore, it is known to produce such form-hardened or press-hardened components with zones of different properties, in particular in areas with different mechanical characteristics such as tensile strength and elongation at break.

Here it is common, for example, not to heat certain areas of the blank or the component up to the austenitizing temperature, so that these areas are not hardened during the subsequent quenching.

This allows zones with a lower hardness and higher ductility to be created. It is known to achieve such softer zones by applying absorption masses, shielding these areas from thermal radiation or not exposing these areas to additional thermal radiation.

These are so-called Tailored Property Parts (TPP).

Furthermore, it is known to produce components with different areas by using different steel grades, i. For example, to combine steel grades that can be hardened with a form or press hardening process with steel grades that, for example, are not or not so highly hardenable.

Such components, which are also referred to as Tailor Welded Parts (TWP), consist, for example, of a boron-manganese steel that can be hardened in the form or press hardening process, such as a 22MnB5 and, in addition, a microalloyed steel and other steels that are hardened show different behavior compared to hardenable steels.

In addition, it is known to realize different properties also through different sheet metal thicknesses, so that a press or form hardened component has zones of different sheet metal thicknesses and thus also different properties. Areas of different sheet metal thickness can also be made of different steel grades, so that a thinner area consists of a first steel grade and a thicker area consists of a second steel grade, and both areas can also consist of one and the same steel grade.

As stated above, there are many ways to influence the ductility over the width or extent of a component.

In the past, however, considerations were also made as to how different properties could be set in the thickness of a component.

In particular, it is desirable to influence the outer edge zones of steel sheets while the core retains its usual properties. In conventional steel processing methods, this is achieved, for example, by carrying out an edge carburization, whereby the carbon diffuses into a workpiece from the outside so that, depending on the carbon content, the edges can be hardened to a greater extent or an edge decarburization that has taken place through machining, in particular thermal treatment, is restored is balanced.

In order to make such edge zones not harder for subsequent forming processes, but rather softer, i.e. more ductile, edge decarburization can be provided. In these carburizing and decarburizing processes, however, the corresponding temperature control and the corresponding gas control must be ensured in a relatively complex manner.

In the case of steel sheets or steel strips, this is of course also possible, but the surfaces of the steel strip would then have to be treated accordingly in a continuous process.

In addition, it is known to form a steel sheet as a composite, the outer surfaces consisting of a different steel quality than the core. For this purpose, three sheets, for example a sequence A-B-A, are rolled onto one another, the A standing for steel grades that are located on the outer surface of the finished steel strip and B for a steel quality that is embedded between the two outer steel grades.

The company ThyssenKrupp has published corresponding sandwich structures under the title BP3-Metall-Metallverbunde, whereby the outer sheets are high-carbon steels, while the inner steel sheet is a so-called low carbon steel. This product is also known in reverse composition under the brand name Tribond®.

It could be observed here that the carbon diffuses from the higher carbon material into the lower carbon-containing material.

Such a diffusion behavior is of course unfavorable, because it would lead to the carbon content and thus the hardness equalizing each other during a heat treatment and a subsequent rolling treatment.

In addition, the high carbon content in one of the layers makes processing more difficult in production, especially during cold rolling.

From the WO 2017/054862 A1 a multilayer composite with an edge area and a core area is known, the edge area being designed to be softer and the total of carbon, silicon, manganese, chromium and nickel to be greater than 1.45% by weight.

From the EP 2 286 332 B1 a sandwich sheet for press hardening is known, which should have a solid core made of a steel with 0.26% carbon and soft cover layers, which were produced by roller plating, at most 0.13% carbon.

From the EP 2 271 541 B1 it is known to use a composite material in a vehicle structure.

From the DE 10 2016 115 036 A1 chassis components with high durability are known, which consist of a multilayer composite and an optional remuneration consisting of hardening and tempering can be carried out.

From the DE 10 2016 108836 A1 a motor vehicle component is known in which what is known as tailor-heated rapid heating is carried out, so that only certain areas are heated up quickly, the tensile strength R _m preferably being 2000 MPa and thus 20 W> 6%. It can also be a multi-layer sheet, with an increased bending angle and optional partial annealing being disclosed here.

The object of the invention is to create a method for producing a steel composite material with which a steel composite material is obtained which has improved properties with regard to crash properties and formability.

The object is achieved with a method having the features of claim 1.

Advantageous further developments are characterized in the subclaims.

A further object is to create a steel composite material which has improved crash properties and improved deformation properties.

The object is achieved with a material having the features of claim 13.

Advantageous further developments are characterized in the dependent claims.

Low temperatures and glow durations e.g. B. 180 ° C for 25 minutes with boron manganese steels, such. B. the 22 MnB5, have a slight positive effect on the yield point, but reduce the strength and increase the bending angle slightly. Composite materials with a martensitic core and ferritic outer layers are manufactured in the same way. The ductile outer layer increases the bending angle.

In addition, edge decarburization for setting a strength / ductility profile is known, with the bending angle also being significantly increased here.

According to the invention, it was recognized that the optimum from bending angle to strength to yield point comes up against a limit despite cladding layers with a higher strength core. According to the invention, it was recognized that the maximum or optimal performance can only be achieved in the resulting gradient material through an optimal structure of the composite material and ideal thermal conditioning of the individual layers. Accordingly, if the yield point or the strength properties are retained, the bending angle should be increased in order to improve the deformation properties. Here, a higher yield point and a higher strength should be achieved, with each layer being loaded to the maximum, with a higher bending angle being available at the same time and folding being made possible. Surprisingly, it has been found that with a temperature treatment of the finished composite in one Temperature range from 150 ° C to 300 ° C, in particular from 160 ° C to 250 ° C and in particular at 175 ° to 225 ° C and an annealing time from 15 minutes to one hour and in particular from 20 to 40 minutes with an increase in the yield point A significant improvement in the bending angle of up to 20 ° is achieved, while the tensile strength is essentially retained. The maximum change in the tensile strength value can be less than 5%.

The effect with composite materials is significantly more pronounced than with pure boron manganese steel grades, so that it is assumed that the influence on the ferrite of the softer partner, e.g. B. a 340LA, only becomes effective in combination or an intermediate surface effect (interface effect) is activated. According to the invention, a suitable selection of the composite material is necessary for this, it has been found that a special quality combination z. B. is a boron-manganese steel of the type 34MnB5 with a ferritic outer layer, e.g. a 340LA.

Accordingly, it has been found according to the invention that the possible edge materials should have a relatively low carbon content in order to improve the processability, i.e. the ductility in the edge area. These should contain little manganese and chromium and the loss of carbon should have little effect on the strength of the material. It also makes sense if the edge active substance contains few alloying elements and is therefore a so-called micro-alloyed steel. Steels of the following grades have proven to be suitable materials: 340LA; 420LA and conditionally 500LA. These are common material designations that can be determined in the steel key, for example. The 340LA can also be found in ÖNORM EN 10346, for example. The designation can also be HX340LA or H340LA, the abbreviation in front of it denotes the various processing states and does not refer to the chemical composition itself.

IF and ULC steels, austenitic steels and stainless steels are not suitable.

Hardenable steels of the general alloy composition are also particularly suitable for the invention (all data in% by mass): Carbon (C) 0.08-0.6 Manganese (Mn) 0.5-3.0 Aluminum (AI) 0.01-0.07 Silicon (Si) 0.01-0.7 Chromium (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process.

Hardenable steels of the preferred alloy composition are also particularly suitable for the invention (all data in% by mass): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminum (AI) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process.

Hardenable, manganese-containing steels of this alloy composition are particularly suitable for the invention (all data in% by mass): C. Si Mn P S. Al Cr Ti B. N % % % % % % % % % % 0.36 0.23 1.85 0.0057 0.001 0.05 0.11 0.03 0.0030 0.0044 Remainder iron and impurities caused by the melting process, The alloying elements boron, manganese, carbon and optionally chromium and molybdenum are used as conversion retarders in such steels.

In order to improve the corrosion protection, at least one side of the composite material can be provided with a metallic coating, preferably before the form hardening process or press hardening process.
In principle, the coating metal can consist of a wide variety of alloys, nickel, copper, chromium, aluminum, magnesium, zinc or their alloy combination are preferred.

Aluminum-silicon alloys (also known as Usibor) and zinc alloys based on zinc can particularly stand out.
The application can be carried out by means of conventional coating devices such as a hot-dip galvanizing plant or hot-dip aluminizing plant or an electrolytic coating plant.

With regard to the thickness ratios and thickness ranges, the two outer sheets should each have a maximum of 25% of the total composite thickness, preferably less than 15%, particularly preferably less than 5%. The entire composite has a thickness between 0.5 and 5 mm, preferably 0.5 to 3 mm. The usual structure is thus ABA, with A being the outer layers made of a ferritic steel material and B the inner layer made of a hardenable boron-manganese steel material.
Of course, the ABA structure described can also be reversed into BAB. This means that under certain conditions the outer layers are harder than the middle layers. With the BAB combination, a wear layer can be created or the composite can achieve a higher flexural strength, which can enable higher power transmission

It can advantageously also be provided that the steel composite is formed from more than two different steel materials. With a corresponding arrangement of suitable materials, this can serve to ensure that, for example in the case of a five-layer steel composite, the hardness decreases from the inside out and the elongation at break of each individual layer increases accordingly. With the same ductility / same bending angle, a higher strength of the bond can be achieved.

• 1: Der erzielbare Biegewinkel nach VDA283-100 in Abhängigkeit von der Wärmebehandlung bei einem erfindungsgemäßen Verbundwerkstoff im Vergleich zu einem nicht wärmebehandelten Referenzwerkstoff;
• 2: Die Verschiebung des Biegewinkels bei einem Stahlver-bundwerkstoff gegenüber einem reinen presshärtendem Bor-Mangan-Stahl ohne einererfindungsgemäße Wärmebehandlung, dh. Biegewinkel nach dem Stand der Technik;
• 3: Der Kraftverlauf bei einem Biegewinkeltest bei dem erfindungsgemäßen Werkstoff, an einem Vergleichswerkstoff aus einem monolithischem martensitischen presshärtenden Stahl und dem erfindungsgemäßen Werkstoff bei unterschiedlichen Wärmebehandlungstemperaturen bzw. -dauern.

The invention is explained by way of example with reference to drawings. It shows:

• 1 : The achievable bending angle according to VDA283-100 as a function of the heat treatment of a composite material according to the invention compared to a reference material that has not been heat-treated;
• 2 : The shift in the bending angle in a steel composite material compared to a pure press-hardening boron-manganese steel without heat treatment according to the invention, ie. State of the art bending angle;
• 3 : The force profile in a bending angle test with the material according to the invention, on a comparison material made of a monolithic martensitic press-hardening steel and the material according to the invention at different heat treatment temperatures or durations.

According to the invention, an increase in the bending angle by at least 5% is to be achieved, with heat treatment being carried out at temperatures between 150 ° C. and 300 ° C. At 150 ° C the effect is rather low, while at 300 ° C a loss in strength can be observed. Therefore, ranges between 160 ° C and 250 ° C and more preferably between 175 ° C and 225 ° C are preferred. The sensible annealing times range from 15 minutes (rather weak effect) to one hour (strong effect, however, the strength drops) so that 20 to 40 minutes are particularly preferred.

With a temperature treatment at approx. 200 ° C. for approx. 20 to 25 minutes, the test results show bending angle improvements of up to 40 ° with an increase in the yield point and retention of the tensile strength. Especially in the 1 this effect can be observed.

If you look at 1 you can see there that at 200 ° C and 20 minutes the bending angle increases to over 80 °. At 150 ° C and 25 minutes, this effect is much less pronounced, so that a bending angle of around 72 ° is achieved. When the temperature is increased to 180 ° C, a bending angle of 80 ° is also achieved, while when the temperature is increased to 250 ° C and 300 ° C, bending angles of almost 95 ° are achieved.

2 shows the shift in the curve of the monolithic material (in this example a phs-ultraform 1500) with a higher-strength composite material (1700s) to slightly improved bending angles, but this improvement is bought at the expense of strength. Furthermore, the total area under the curve remains essentially constant. This shows that when selecting the materials for the composite material alone, ie. Without heat treatment according to the invention, no significant improvements in the mechanical characteristics can be achieved.

In 3 you can see how compared to the composite material 2 , the composite materials are shifted significantly to improved values after a temperature treatment with regard to the bending angle, without there being any loss of strength or, at most, slight loss of strength. The significant improvement in the mechanical properties is particularly evident from the significant increase in the area under the curve.

The advantage of the invention is that a heat treatment at low temperatures improves the bending angle and thus the crash behavior and deformation behavior without the strength properties of the steel composite material being impaired.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• WO 2017/054862 A1 [0031]
• EP 2286332 B1 [0032]
• EP 2271541 B1 [0033]
• DE 102016115036 A1 [0034]
• DE 102016108836 A1 [0035]

Claims (16)

A method for producing a steel composite, wherein at least two steel sheets, which consist of at least two different steel grades, in particular a first steel grade made of a hardenable manganese-containing steel and a second steel grade made of a micro-alloyed steel, are placed on top of one another and plated by hot rolling and the composite material thus obtained is subsequently optionally is cold-rolled, the composite material of at least two layers with different steel compositions being form-hardened or press-hardened after the rolling, characterized in that the composite material subsequently increases the bending angle by at least 5 ° between 150 ° C and 300 ° C for 15 minutes to Heat-treated for 60 minutes. Procedure according to Claim 1 , characterized in that the hardenable manganese-containing steel has the following composition: Carbon (C) 0.08-0.6 Manganese (Mn) 0.5-3.0 Aluminum (AI) 0.01-0.07 Silicon (Si) 0.01-0.7 Chromium (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process. Method according to one of the preceding claims, characterized in that the hardenable steel has a steel composition as follows (all data in% by mass): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminum (AI) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process. Method according to one of the preceding claims, characterized in that a microalloyed steel is used as the ferritic partner material for the hardenable, in particular martensitically hardenable, boron-manganese steel, with the following composition (all data in% by mass): Carbon (C) 0.02-0.12 Manganese (Mn) 0.2-2.0 Aluminum (Al) 0.01-0.07 Silicon (Si) <0.5 Chromium (Cr) <0.3 Titanium (Ti) + niobium (Nb) 0.01-0.15 Nitrogen (N) <0.02 Boron (B) <0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process. Method according to one of the preceding claims, characterized in that one steel grade consists of a martensitically hardenable boron-manganese steel and the other steel grade consists of a steel of the grades 340LA, 420LA or 500LA. Method according to one of the preceding claims, characterized in that three steel sheets are placed on top of one another and hot-rolled together, the middle steel sheet being made of a martensitically hardenable boron-manganese steel and the two outer sheets being made of a 340LA, 420LA or 500LA. Method according to one of the preceding claims, characterized in that the hardenable manganese-containing steel which is martensitically hardenable is a steel of the alloy composition (all data in mass%): C. Si Mn P S. Al Cr Ti B. N % % % % % % % % % % 0.36 0.23 1.85 0.0057 0.001 0.054 0.11 0.03 0.0030 0.0044 Method according to one of the preceding claims, characterized in that the outer regions make up a total thickness of 25%, preferably <15%, more preferably 5% of the thickness of the entire steel sheet after rolling. Method according to one of the preceding claims, characterized in that the composite material is heat-treated between 160 ° C and 250 ° C and in particular between 175 ° C and 225 ° C. Method according to one of the preceding claims, characterized in that the composite material is heat-treated for 20 to 40 minutes. Method according to one of the preceding claims, characterized in that at least one side of the composite material before the form hardening process or press hardening process a metallic coating with aluminum or an alloy containing essentially aluminum or an alloy of aluminum and zinc and / or another zinc alloy containing essentially zinc learns. Method according to one of the preceding claims, characterized in that the steel composite is rolled from three or four or more different steel materials, the steel composite being formed with three, four, five or more layers. Sheet steel composite material in particular produced by one of the methods according to one of the Claims 1 to 12th , characterized in that the composite is press-hardened or press-hardened, the softer sheet metal layers developing a tensile strength R _m of 0.3 to 0.9 GPa after the press or press hardening of the composite and the harder sheet-metal layers a tensile strength R _m between 1 GPa and develop 2.8 GPa. Sheet steel composite according to Claim 13 , characterized in that the sheet steel composite has softer types of steel on the outside and a harder type of steel in between or vice versa. Use of a sheet steel composite material according to Claim 13 or 14th for the manufacture of press-hardened or form-hardened components, in which the component is cold-formed, austenitized and then quench-hardened or austenitized, reshaped and quench-hardened Hardened component made from a sheet steel composite material Claim 13 or 14th , produced by a method according to one of the Claims 1 to 12th .

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