KR20160014658A - Method for producing a component by hot forming a pre-product made of steel - Google Patents

Abstract

The present invention relates to a method of manufacturing a component by hot forming a semi-finished product made of steel. The semi-finished product is heated to the molding temperature and then re-molded, and the part has a bainite-based microstructure having a minimum tensile strength of 800 MPa after the molding process. In the process, a semi-finished product having a certain alloy composition is heated to a temperature below the Ac ₁ transformation temperature, and the semi-finished product is already made of steel having a microstructure of at least 50% bainite.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a component by hot-

The present invention relates to a method of manufacturing a component by hot-forming a semi-finished product made of steel according to the premise of claim 1. In the following, the term semi-finished product means, for example, a sheet cut from a coil or a cut plate or a seamless tube or a welded tube, optionally this can additionally be cold drawn.

These components are used primarily in the automotive and utility vehicle industries, but can also be found in mechanical or building structures.

Because of the fiercely competitive marketplace, automakers must constantly seek solutions to reduce the expense and maintain the best possible comfort and passenger protection. An important factor in this context is, on the one hand, to reduce the weight of all vehicle components, but on the other hand, the favorable behavior of the individual components when exposed to high static or dynamic stresses during operation or in the event of a collision.

Pre-material manufacturers are seeking to meet these requirements by providing high strength and ultra high strength steels that can reduce wall thickness and at the same time have improved part properties during manufacture and operation.

These steels therefore have to meet strength, toughness, toughness, energy absorption and corrosion resistance, and relatively high demands on their machinability, for example during cold forming and welding.

In view of the above-described aspects, manufacturing parts with hot-moldable steel is of increasing importance as it ideally meets the requirements raised at low material costs.

The manufacture of parts by semi-finished products made of press-hardenable steel by hot forming with a forming tool is known from DE 601 19 826 T2. In this case, the sheet metal blank is preheated to a temperature in excess of the austenitizing temperature to 800 - 1200 ° C, optionally coated with zinc or provided as a zinc main component, molded into parts in a tool optionally cooled by hot forming Where the sheet or part is quickly cured (press-cured) in the forming tool during molding by quickly removing the heat, resulting in the required microstructure and strength.

The metal coating is applied on the hot rolled steel sheet or the cold rolled steel sheet, or on the semi-finished product produced therefrom, as corrosion inhibition, for example by hot-dip galvanizing or hot-dip aluminum plating, by a continuous melt coating method.

The plate is then cut to size for the forming tool used for hot forming. It is also possible to provide a workpiece or blank that is formed including a molten coating.

The application of the metal coating on the semi-finished product to be formed before hot forming is advantageous in this process because it effectively prevents scaling of the base material and prevents excessive tool wear due to additional lubrication effects.

Known hot-moldable steels for this application are, for example, manganese-boron steel "22MnB5 ", and also air hardenable steels according to the recent DE 10 2010 024 664 A1.

In order to obtain a component having a very high strength exceeding 980 MPa while having sufficient toughness, a steel having a microstructure that is ferrite in an initial state is formed by die-curing from EP 2 546 375 A1, It is known to form microstructures of bainite, tempered martensite and retained austenite in the finished part. The sheet metal to be formed here is first heated to a temperature of 750 to 1000 占 폚, held at this temperature for 5 to 1000 seconds, then molded at 350 to 900 占 폚, and cooled to 50 to 300 占 폚. Finally, the part is reheated to a temperature of 350 to 490 캜, which is maintained for a time of 5 to 1000 seconds. In this case, the microstructure of the finished part consists of 10 to 85% of martensite, 5 to 40% of retained austenite and at least 5% of bainite.

However, the production of parts by hot-pressing by press-curing has several disadvantages.

In one respect, this method requires a step of heating the semi-finished product to the austenitizing temperature and also requires a large amount of energy during the ferrite to austenite transformation, which makes this process expensive, Of CO ₂ .

Further, in order to prevent excessive scale formation of the sheet metal surface, a considerable post-treatment of a further metal protective layer or a lacquer-based protective layer as described above or a scale-formed surface due to heating and molding is required.

Molding at temperatures in excess of the Ac ₃ temperature is typically carried out at a temperature well in excess of 800 [deg.] C, thus giving very high demands on the temperature stability of these layers.

A further disadvantage is that, to obtain the corresponding part strength after press-hardening, only the transformation-capable steel with sufficient transformation-inertness can be used, Additives should be included.

In summary, the known methods of making steel components by hot forming at temperatures above the austenitization temperature require large furnaces in conjunction with long term heat treatment times, resulting in high manufacturing and energy costs.

To improve the forming ability of high strength steels, DE 10 2004 028 236 B3 describes the further processing of workpieces as parts by hot forming at temperatures of 400 to 700 DEG C instead of cold forming (warm forming) have. The disadvantage here is that the molded part is softened by heating below the transformation temperature, i.e., the strength is reduced compared to the starting state.

DE 10 2011 108 162 A1 discloses a method of producing parts by warm-forming a semi-finished product made of steel at a temperature below the Ac ₁ transformation temperature, wherein the increase in the strength of the part causes the semi-finished article to become cold . Optionally, additional strength increases can be achieved by using higher strength materials such as bainitic steels, martensitic steels, micro-alloyed steels and dual phase or multi phase steels. The disadvantage here is the additional cost due to the required cold forming before heating to the shaping temperature. The dual phase steel during hot forming also has the disadvantage that it is sensitive to fracture caused by edge braking during molding.

There is no disclosure on the specification or adherent concrete alloy composition regarding the microstructure of semi-finished products for setting the mechanical properties of the parts after warm-forming in a targeted manner in the case of using higher-strength steels.

It is an object of the present invention to provide a semi-finished article made of steel at a temperature less than the Ac ₁ transformation point, which is cost-effective and which can achieve the properties of a molded part which is comparable to or better than that of hot- A method of manufacturing a component by hot forming. Particularly, it is an object of the present invention to achieve a strength of more than 800 MPa, a yield strength of more than 700 MPa, a elongation at break (A ₈₀ ) of more than 8% and a plastic failure behavior of the finished part.

According to the present invention, this object is solved by a method of manufacturing a component by hot-forming a semi-finished product made of steel, wherein the semi-finished product is molded after being heated to a molding temperature and the molded component has a minimum tensile strength of 800 MPa The semi-finished product is heated to a temperature below the Ac ₁ transformation temperature and the semi-finished product is already made of steel having a microstructure of at least 50% bainite, and the semi-finished product has the following weight percent alloy composition .

C: 0.02 to 0.3

Si: 0.01 to 0.5

Mn: 1.0 to 3.0

P: 0.02 max

S: Up to 0.01

N: Up to 0.01

Al: 0.1 max

Cu: Up to 0.2

Cr: Up to 3.0

Ni: Up to 0.2

Mo: Up to 0.2

Ti: Up to 0.2

V: Up to 0.2

Nb: 0.1 max

B: Up to 0.01

Compared to the process known from DE 601 19 826 T2 or EP 2 546 375 A1 for the production of parts by press-curing, the process according to the invention can be carried out by using a steel already having bainite in its initial state, Has the advantage of providing components with mechanical properties equal to or better than their mechanical properties at considerably lower heating energy requirements. This saves energy costs.

Compared to DE 10 2011 108 162 A1, using an intermediate product according to the invention and a semi-finished product already having at least 50% of the bainite microstructure in the initial state, an additional cold forming step of the semi-finished product for increasing the strength is unnecessary And the required mechanical properties of the part can be adjusted in a targeted manner after warm-forming.

The use of semifinished steels with already mentioned alloy composition and already imbenite is very advantageous since the starting material already has a high tensile strength and electrical conductivity, which is maintained or even higher after non-transformation.

The bainite steels used for the process according to the invention have their own microstructure through the corresponding temperature profile already during the production of the semi-finished product. In the case of a hot-rolled steel sheet, this fine structure is formed, for example, through thermomechanical rolling, and in the case of a cold-rolled steel sheet, for example, by an annealing process after cold rolling or during hot- dip galvanizing.

"Softening" observed after molding in other high strength steels is not observed in these bainitic steels. "Softening" is often associated with microstructure transformation, and thus is time and temperature critical. On the other hand, the use of semi-finished products according to the invention made of metal bainite steels is non-reactive, for example intentional and unintentional time and temperature changes during heating and molding do not lead to defects in mechanical properties. As a result of this advantageous material behavior, complex multi-step process steps can be performed reproducibly.

A particular advantage of using this alloy concept and bainite microstructure is also an extremely fine and homogeneous microstructure with at least 50% bainite, small amounts of ferrite, retained austenite and martensite.

When the microstructure has at least 70% bainite, the ratio of retained austenite + martensite is less than 10%, and the remainder is iron, it is very advantageous to achieve the required mechanical properties.

Very homogeneous and homogeneous material properties can be achieved when the bainite based semi-finished steel has the following alloy composition in weight percent.

C: 0.02 to 0.11%

Si: 0.01 to 0.5%

Mn: 1.0 to 2.0%

P: Up to 0.02%

S: Up to 0.01%

N: Up to 0.01%

Al _min : 0.015 to 0.1%

B: Up to 0.004%

Nb + V + Ti: Up to 0.2%

In a further improved embodiment of the present invention, the semi-finished steel has the following alloy composition in weight percent.

C: 0.05 to 0.11%

Si: 0.01 to 0.5%

Mn: 1.0 to 2.0%

P: Up to 0.02%

S: Up to 0.01%

N: 0.003 to 0.01%

Al _min : 0.03 to 0.1%

B: Up to 0.004%

Mo: 0.04 to 0.2

Si: 0.04 to 0.2

Nb + V + Ti: 0.1 to 0.2%

Addition of at least 0.03 to 0.01% by weight of nitrogen in combination with carbon and a minimum content of titanium of 0.04 to 0.2% by weight ensures fine grain microstructure by the formation of titanium carbonitride with high strength and toughness properties. When molybdenum is added in an amount of 0.04 to 0.2% by weight, the precipitate formed is also advantageously kept very small.

Comparative tests were carried out on steels having the alloy compositions listed in Table 12. The results of the mechanical properties before and after the warm-forming are shown in Table 2.

The sheet metal tested had a thickness of 1.8 to 2.25 mm, which was heated in a furnace to a temperature of 600 DEG C for 3 minutes and then cooled between two flat tool parts in a forming press.

The materials tested are indicated by the letters b, c, d, e and f in Tables 1 and 2. The alloy composition of the material is consistent with the alloy composition according to the present invention, but the microstructure is set to be different from the starting state. Therefore, in the initial state, the steel a has a ferrite-bainite-based basic microstructure ("FB") before being heated to the forming temperature, the steel b has a bainite microstructure ("B &("MBF") of bismuth, site, bainite and ferrite, where the content of martensite is dominant. The steel d and the steel e have a ferrite-based microstructure ("F") and the steel f has a martensitic microstructure ("M"). In the steel a and the steel c, the bainite content in the microstructure is less than 50%, and the bainite content in the steel b exceeds 50%. Table 2 shows that only steel b with the dominant bainite-based microstructure of the semi-finished product meets the requirements imposed on the mechanical properties of a minimum tensile strength of 800 MPa and a minimum fracture elongation (A ₈₀ ) exceeding 8% after warm-forming Lt; / RTI >

Typical applications to take advantage of the weight savings and high strength possibilities of components are movable derrick structures, longitudinal members and transverse members of trucks and trailers, safety components and chassis components of vehicle structures in cars and trains.

The steel according to the invention and the parts produced therefrom are characterized by a very high yield strength and tensile strength in excess of 800 MPa at sufficient electrical strength. In addition, its chemical composition also leads to good weldability.

As is known, the above-described steel may further comprise a scale resistant layer or a corrosion resistant layer or metal coating on the lacquer base. The metal coating may contain zinc and / or magnesium and / or aluminum and / or silicon.

In contrast to the conventional manufacturing process, a hot-rolled steel sheet or a cold-rolled steel sheet which has been subjected to surface treatment for forming after heating can be used because it can withstand warm-forming accompanied by a low degree of adhesion and low deformation. The metal coating is resistant to short time reheating of a combination of substrate / coating (steel sheet / coating) at temperatures below Ac ₁ temperature of the substrate to withstand reheating prior to warm forming and actual warming.

Due to the relatively low calorie, a large reheat assembly such as a pusher type furnace or a pumped furnace can be omitted for quick and direct acting systems (inductive, conductive and especially radiation type systems).

The method described above also requires significantly less heat energy. That is, energy efficiency is higher than press-hardening. As a result, the process cost is lower and the CO ₂ emissions are reduced.

Preferably, the reheating is effected by radiation prior to warming, which is considerably more efficient than heating in the furnace or the case of conductive heating in the case of radiation, and the introduction of energy into the material is faster and more effective Because.

This material is very suitable for partial heating. For example, by using a radiator, the individual regions of the semi-finished product to be formed can be heated in a targeted manner so as to obtain an optimal area regarding the molding ability. Advantageously, this allows the use of conventional cold forming dies and thus does not require a complicated hot forming system required in press-curing.

For transporting between the heat source and the forming tool, especially for very thin sheet metals (e.g., less than 0.8 mm), it is more useful to provide a sheet metal with profiling to increase local stiffness . This is not possible with conventional press curing, since the achieved strength requires quenching through the inner surface of the tool that is excluded due to profiling.

In the process according to the invention, the semi-finished product is heated to a temperature of less than 720 DEG C, advantageously in the temperature range of 400 to 700 DEG C, and then molded into parts. The optimum forming temperature depends on the strength required of the part, and is preferably 500 ° C to 700 ° C. In order to obtain a bainitic microstructure as described in EP 2 546 375 A1, a long retention time is not required and the process time for the manufacture of the part is considerably shortened.

In an advantageous embodiment of the present invention, in order to locally change (e.g., local cure) the properties that are adjusted in accordance with the later requirements of the part in combination with the increased strength of the remaining material in a targeted manner, Is performed during heating of the semi-finished product to the forming temperature.

material C Si Mn P S N Al Cu Cr Ni V Ti Nb Mo B FB a 0.07 0.08 1.4 0.01 0.002 0.005 0.041 0.03 0.04 0.04 0.05 - 0.04 - - B b 0.08 0.47 1.9 0.01 0.001 0.006 0.066 0.03 0.03 0.04 0.01 0.12 0.05 0.14 - MBF c 0.23 0.25 1.2 0.01 0.002 0.005 0.038 0.04 0.16 0.04 0.01 0.03 - - 0.003 F d 0.10 0.28 2.0 0.01 0.001 0.006 0.041 0.02 0.33 0.04 0.01 0.04 0.04 - 0.003 M f 0.15 0.12 1.7 0.01 0.001 0.005 0.045 0.02 0.33 0.04 0.01 0.02 - - - F e 0.09 0.25 1.8 0.01 0.001 0.005 0.041 0.03 0.33 0.04 0.01 - 0.01 - -

material Tensile strength R _m [MPa] Yield strength [MPa] Elongation at break A ₈₀ [%] HWU I After HWU HWU I After HWU HWU I After HWU FB a 591 632 552 589 19 16 B b 788 854 678 833 14 12 MBF c 982 979 915 922 7 7 F d 855 778 644 767 13 12 M f 1343 1246 1047 1173 6 One F e 676 650 407 498 22 18

Claims (15)

The semi-finished product made of steel as a method for producing a component by hot forming, wherein the semi-finished product is formed after the heating to the forming temperature, the part after the molding has a bainite micro-structure having a minimum tensile strength of 800 Mpa, Ac ₁ Wherein the heating is carried out at a temperature below the transformation temperature and the semi-finished product is already made of steel having a microstructure of at least 50% bainite, and the semi-finished product has the following weight percent alloy composition.
C: 0.02 to 0.3
Si: 0.01 to 0.5
Mn: 1.0 to 3.0
P: 0.02 max
S: Max 0.01
N: 0.01 max
Al: 0.1 max
Cu: 0.2 max
Cr: up to 3.0
Ni: 0.2 max
Mo: 0.2 max
Ti: 0.2 max
V: 0.2 max
Nb: max 0.1
B: 0.01 max The method according to claim 1,
Wherein the microstructure is composed of at least 70% of bainite, the content of residual austenite + martensite is less than 10%, and the remainder is composed of ferrite. 3. The method according to claim 1 or 2,
Wherein the steel has an alloy of the following weight percent composition:
C: 0.02 to 0.11%
Si: 0.01 to 0.5%
Mn: 1.0 to 2.0%
P: 0.02% max.
S: Up to 0.01%
N: Up to 0.01%
Al _min : 0.015 to 0.1%
B: Up to 0.004%
Nb + V + Ti: Up to 0.2% The method according to claim 1,
Wherein the steel has an alloy of the following weight percent composition:
C: 0.05 to 0.11%
Si: 0.01 to 0.5%
Mn: 1.0 to 2.0%
P: 0.02% max.
S: Up to 0.01%
N: 0.003 to 0.01%
Al _min : 0.03 to 0.1%
B: Up to 0.004%
Mo: 0.04 to 0.2
Ti: 0.04 to 0.2
Nb + V + Ti: 0.1 to 0.2% 5. The method according to any one of claims 1 to 4,
Wherein the heating of the semi-finished product to the hot forming temperature is carried out only partially and the partial heating is optionally carried out at a temperature exceeding the Ac ₁ transformation temperature. 6. The method according to at least one of claims 1 to 5,
Wherein the semi-finished product is heated to a temperature of less than 720 占 폚. The method according to claim 6,
Wherein the semi-finished product is heated to a temperature in the range of 400 to 720 占 폚. 8. The method of claim 7,
Wherein the semi-finished product is heated to a temperature in the range of 500 to 700 占 폚. 9. The method according to any one of claims 1 to 8,
Wherein the semi-finished product is provided with a metallic coating or a lacquered coating prior to the heating. 10. The method of claim 9,
Wherein the metal coating contains Zn and / or Mn and / or Al and / or Si. 11. The method according to at least one of claims 1 to 10,
Wherein the heating to the shaping temperature is achieved by induction, conduction or radiation. 12. The method according to at least one of claims 1 to 11,
A metal plate or tube is used as a semi-finished product. 13. The method of claim 12,
Wherein the metal plate is made of a hot-rolled steel sheet or a cold-rolled steel sheet. 13. The method of claim 12,
Wherein the tube is a seamless hot-rolled tube or welded tube made of hot-rolled steel or cold-rolled steel. 15. The method of claim 14,
The tube is a seamless hot-rolled or welded tube made of hot-rolled or cold-rolled steel, which is one or more additional drawing and / or annealing processes.