METHOD FOR PRODUCING METALLIC COMPONENTS HAVING ADAPTED COMPONENT PROPERTIES

Abstract

The invention relates to a method for producing a sheet steel component by means of a press hardening or form hardening process, the sheet steel component being produced by virtue of the fact that a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one dual-phase steel is cold-formed, then heated, and then quenched in a cooling press or a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one region made of a dual-phase steel is heated to a temperature above the austenitization temperature of the highly hardenable steel material and is then formed into the sheet steel component in a single stroke or in a plurality of strokes in a forming and cooling press, wherein as a softer material and as a partner for the highly hardenable carbon/manganese/boron steel, a dual-phase steel is used, whose Ac3 value is increased until at the required annealing temperatures, with the austenitization of the carbon/manganese/boron steel, only a partial austenitization of the dual-phase steel takes place so that when loaded into the cooling press, the dual-phase steel has a ferritic matrix, and in addition to this, austenite is present.

Claims

1. A method for producing a sheet steel component by means of a press hardening or form hardening process, the sheet steel component being produced by virtue of the fact that a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one dual-phase steel is cold-formed, then heated, and then quenched in a cooling press or a sheet bar composed of at least one region made of a highly hardenable carbon/manganese/boron steel and at least one region made of a dual-phase steel is heated to a temperature above the austenitization temperature of the highly hardenable steel material and is then formed into the sheet steel component in a single stroke or in a plurality of strokes in a forming and cooling press, characterized in that as a softer material and as a partner for the highly hardenable carbon/manganese/boron steel, a dual-phase steel is used, whose Ac3 value is increased until at the required annealing temperatures, with the austenitization of the carbon/manganese/boron steel, only a partial austenitization of the dual-phase steel takes place so that when loaded into the cooling press, the dual-phase steel has a ferritic matrix, and in addition to this, austenite is present.

2. The method according to claim 1, characterized in that the annealing temperatures are >800° C., preferably 840° C., in particular >870° C., but less than the Ac3 of the dual-phase steel.

3. The method according to claim 1 or 2, characterized in that the dwell time in the furnace is between 0 and 600 seconds, preferably between 5 and 300 seconds.

4. The method according to one of the preceding claims, characterized in that the Ac3 value of the dual-phase steel is high enough that the degree of austenitization occurring with the dwell time and the temperature is between 50 volume % and 90 volume %.

5. The method according to one of the preceding claims, characterized in that in the transfer of the formed component or the sheet bar from the furnace into the cooling and/or forming press, the cooling rate is between 5 Kelvin/sec and 500 Kelvin/sec, in particular between 5 Kelvin/sec and 100 Kelvin/sec, and especially preferably, between 10 Kelvin/sec and 70 Kelvin/sec.

6. The method according to one of the preceding claims, characterized in that the loading temperature in the press is between 450 and 850° C., preferably between 450 and 750° C.

7. The method according to claim 6, characterized in that the loading temperature in the form hardening process is set to 700 to 850° C.

8. The method according to claim 6, characterized in that the loading temperature in the press hardening process is set to 400 to 650° C., preferably 440 to 600° C., and especially preferably, 450 to 520° C.

9. The method according to one of the preceding claims, characterized in that the cooling rate in the press is 10 Kelvin/sec.

10. The method according to one of the preceding claims, characterized in that as the dual-phase steel, a steel is used, which contains 0.5 to 1.5%, preferably 0.6 to 1.3%, aluminum.

11. The method according to one of the preceding claims, characterized in that the annealing temperature is set so that the dual-phase steel is intercritically annealed, i.e. is annealed at a temperature between its Ac1 and Ac3 temperature.

12. A dual-phase steel material for use in the method according to one of the preceding claims, characterized in that the material has the following composition in mass %: C 0.02-0.12%, Si 0.01-2.0%, Mn 0.5-2.0%, Cr 0.3-1.0%, Al 0.5-1.5%, Nb <0.10%, Ti <0.10% Residual quantities of iron and inevitable smelting-related impurities.

13. The material according to claim 12, wherein C=0.04-0.10 mass %, in particular 0.05-0.08 mass %.

14. The material according to claim 12 or 13, wherein Si=0.05-1.50 mass %, in particular 0.10-1.25 mass %, and especially preferably 0.50-1.25 mass %.

15. The material according to one of claims 12 to 14, wherein Mn=0.60-1.50 mass %, in particular 0.70-1.25 mass %.

16. The material according to one of claims 12 to 15, wherein Cr=0.45-0.80 mass %, in particular 0.55-0.75 mass %.

17. The material according to one of claims 12 to 16, wherein Al=0.40-1.20 mass %, in particular 0.50-1.00 mass %, and especially preferably 0.50-0.80 mass %.

18. The material according to one of claims 12 to 17, characterized in that with an annealing temperature of 800 to 950° C. and a furnace dwell time of up to 300 seconds, the material has a degree of austenitization of 50 to 90% and after quench hardening, has an existing dual-phase structure with ferritic matrix and 5 to 20% martensite and possibly some bainite.

19. A welded sheet bar including at least one dual-phase steel material and a highly hardenable steel, in particular a carbon/manganese/boron steel, characterized in that the dual-phase material has the following composition in mass %: C 0.02-0.12%, Si 0.01-2.0%, Mn 0.5-2.0%, Cr 0.3-1.0%, Al 0.5-1.5%, Nb <0.10% Ti <0.10% Residual quantities of iron and inevitable smelting-related impurities.

20. The welded sheet bar according to claim 19, wherein the dual-phase material contains C=0.04-0.10 mass %, in particular 0.05-0.08 mass %.

21. The welded sheet bar according to claim 19 or 20, wherein the dual-phase material contains Si=0.05-1.50 mass %, in particular 0.10-1.25 mass %, and especially preferably 0.50-1.25 mass %.

22. The welded sheet bar according to one of claims 19 to 21, wherein the dual-phase material contains Mn=0.60-1.50 mass %, in particular 0.70 to 1.25 mass %.

23. The welded sheet bar according to one of claims 19 to 22, wherein the dual-phase material contains Cr=0.45-0.80 mass %, in particular 0.55-0.75 mass %.

24. The welded sheet bar according to one of claims 19 to 23, wherein the dual-phase material contains Al=0.40-1.20 mass %, in particular 0.50-1.00 mass %, and especially preferably 0.50-0.80 mass %.

Description

[0043] The invention will be explained by way of example based on the drawings. In the drawings:

[0044] FIG. 1: shows the elongation and strength of dual-phase structures and ferritic-perlitic structures according to the prior art;

[0045] FIG. 2: shows the behavior of fully austenitically annealed dual-phase steels with high cooling rates in the press, first showing the strength as a function of the loading temperature and then showing the elongation as a function of the loading temperature as well as the achievable structure;

[0046] FIG. 3: shows the behavior of fully austenitically annealed dual-phase steels at high and low cooling rates in the press;

[0047] FIG. 4: shows the influence of carbon on the mechanical characteristic values as a function of the loading temperature;

[0048] FIG. 5: shows structure images of dual-phase steels with different carbon contents;

[0049] FIG. 6: shows the influence of manganese on the mechanical characteristic values;

[0050] FIG. 7: shows the structure images with different manganese contents;

[0051] FIG. 8: shows the influence of aluminum on the mechanical characteristic values;

[0052] FIG. 9: shows the structure images with different aluminum contents;

[0053] FIG. 10: shows the influence of the intercritically annealed aluminum-alloyed dual-phase steel concept according to the invention in comparison to fully austenitically annealed carbon/manganese alloys.

[0054] The method according to the invention provides producing a tailored welded blank (TWB) by combining at least one usually flat sheet part, which is composed of a highly hardenable steel material such as a boron/manganese steel and in particular a steel from the family of 22MnB5 or 20MnB8 and steels of the like, with at least one usually flat sheet part composed of a dual-phase steel.

[0055] Such a combined tailored welded blank can then be sufficiently heated in the direct or indirect method and then formed or else formed and then heated and quenched.

[0056] According to the invention, a dual-phase steel with a relatively high aluminum content is used. According to the invention, it has been discovered that aluminum decreases the sensitivity of the mechanical characteristic values to the loading temperature and sharply decreases their sensitivity to the cooling rate in the press.

[0057] With high cooling rates in the press, simple carbon/manganese alloys, which are fully austenitically annealed in the furnace, are highly dependent on the loading temperature.

[0058] The composition of the dual-phase steel according to the invention is as follows, with all percentages being indicated in mass %:

C 0.02-0.12%, preferably 0.04-0.10%
Si 0.05-2.0%, preferably 0.20-1.60%, and especially preferably, 0.50-1.50%
Mn 0.5-2.0%, preferably 0.6-1.50%
Cr 0.3-1.0%, preferably 0.45-0.80%
Al 0.4-1.5%, preferably 0.50-1.30%, and especially preferably, 0.60-1.20%
Nb <0.20%, preferably 0.01-0.10%
Ti <0.20%, preferably 0.01-0.10% Residual quantities of iron and inevitable smelting-related impurities.

[0059] With a dwell time in the furnace of up to 600 seconds, in particular up to 300 seconds, at the annealing temperatures of about 840° C. that are typical for the austenitization of the highly hardenable partner material, only a partial austenitization is achieved with regard to the dual-phase steel.

[0060] The degree of austenitization that occurs in the dual-phase steel is between 50 and 90% by volume, with the desired structure being a fine dual-phase steel with ferritic matrix and 5 to 20% by volume martensite and possibly some bainite.

[0061] The desired structure occurs if the following cooling sequence is maintained and thus if—during the manipulation of the component or sheet bar in the cooling press, i.e. during handling—a cooling rate of 5 to 500 Kelvin/sec is maintained and the loading temperature in the cooling press is 400 to 850° C., preferably 450 to 750° C., the loading temperature being adjusted to 700 to 800° C. in the cooling press during the form hardening process (indirect method).

[0062] In the press hardening process (direct method), the loading temperature is set to 400 to 650° C., preferably 440 to 600° C., and especially preferably, 450 to 520° C.

[0063] The special effect—primarily in the direct process, i.e. press hardening—that is achieved with a loading temperature of 450 to 520° C. is that this permits the structure to be established in an optimal way, yielding a system that is particularly robust with regard to cooling rates.

[0064] Furthermore with TWB sheet bars or components, there is a need on the one hand for the loading temperature based on the desired structure for the dual-phase part to not be excessively high and there is a need on the other hand for the loading temperature to not be excessively low since otherwise, the carbon/manganese/boron steel falls below the Ms temperature.

[0065] The cooling rate in the press should be 10 Kelvin/sec.

[0066] To achieve this, an air cooling (for example a cooling rate of 5 Kelvin/sec to 70 Kelvin/sec) or for example a plate cooling can be carried out (cooling rates of more than 80 Kelvin/sec are easily achievable).

[0067] The resulting mechanical properties according to the invention are as follows:

R.SUB.p0.2 .250 to 500 MPa

R.SUB.m .400 to 900 MPa

A≥10%.

[0068] FIG. 1 shows the differences with regard to the ratio of the elongation to the tensile strength R.sub.m with a ferritic-perlitic structure (gray) and a dual-phase structure (black). It is clear that a dual-phase structure is very well-suited for the purposes according to the invention.

[0069] The following problems, however, occur when adjusting the alloy according to the prior art:

[0070] With high cooling rates in the cooling press, fully austenitically annealed dual-phase steels have unfavorable properties. FIG. 2 shows that with two different steels, namely one being a steel with 0.06% carbon and 1.2% manganese and another being a dual-phase steel with 0.08% carbon and 1.6% manganese, depending on the loading temperature, there is a very large spread with regard to the tensile strength R.sub.m of approx. 550 MPa to 880 MPa that is achieved in the steel with less carbon and less manganese.

[0071] Even in the steel with the higher carbon content and higher manganese content, the achievable tensile strength is from about 660 MPa to about 920 MPa. But this also means that with the variable loading temperatures and with the fluctuations in the loading temperature that are customary in the process, it is difficult to achieve reproducible strength values within the desired tolerances with the known dual-phase steels. The same is the case with the R.sub.p0.2 value, which fluctuates in a comparable way so that keeping these two important characteristic values within a manageable range is far from possible.

[0072] When it comes to the elongation, the same is true of the two steels, i.e. the elongation values fluctuate so significantly as a function of the loading temperature that conventional dual-phase steels are absolutely not an option for use as partners for a highly hardenable steel with the known process windows and the known loading temperature fluctuations. The structure of the lower-alloyed steel from the two graphic depictions is shown at a 750° loading temperature and a cooling rate that was achieved by means of water cooling.

[0073] FIG. 3 also shows that the depicted characteristic values, particularly when cooling with water, are highly dependent on the loading temperature and the cooling rate in the press, with the structure also differing significantly from the structure according to FIG. 2 since in FIG. 2, there is a much higher cooling rate.

[0074] FIG. 4 shows the influence of carbon on the above-mentioned characteristic values as a function of the loading temperature with the same manganese contents and the same aluminum contents. It is clear that with increasing carbon content, the strength and yield strength are increased. FIG. 5 shows that the ferrite quantity in the given steel decreases as a function of the carbon content with increasing carbon content.

[0075] FIG. 6 and FIG. 8 show the influence of manganese with the same carbon contents and the same aluminum contents. As the manganese content increases, the strength and yield strength also increase whereas, as is clearly shown in FIG. 7, the martensite quantity in the structure increases and the ferrite quantity decreases.

[0076] The decisive factor for the invention is that an increasing aluminum content (FIGS. 8, 9) makes it possible to reduce the sensitivity to the loading temperature in the press. It is very clear in FIG. 8 that the tensile strength is less dependent on the loading temperature with a higher aluminum content than it is with 0.5% aluminum. This effect is even clearer in the R.sub.p0.2 value.

[0077] Also, a homogenization can be achieved with regard to the elongation. In the enlarged detail depicting the strength as a function of the loading temperature, it is once again very clear that the increasing aluminum content results in a significant homogenization.

[0078] FIG. 9 shows that the increasing aluminum content significantly increases the ferrite quantity. FIG. 10 shows that with fully austenitically annealed carbon/manganese alloys, at high loading temperatures, the strength depends to a massive degree on the cooling rate in the press; with intercritically annealed aluminum-alloyed dual-phase concepts, the dependence of the mechanical properties on both the loading temperature and the cooling rate of the press is significantly reduced, as is clear in the two diagrams in FIG. 10; on the left, a non-aluminum-alloyed steel is used and on the right, an aluminum-alloyed steel dual-phase steel is used.

[0079] According to the invention, in order to ensure the presence of a sufficient quantity of ferrite and thus a ferritic matrix in the dual-phase structure, it is sufficient to perform an intercritical annealing in the furnace so that in addition to austenite, ferrite is also present. For the soft partner material, i.e. the dual-phase steel, the Ac3 temperature must be kept high so that the intercritical annealing is even possible. According to the invention, this Ac3 value is increased by means of aluminum.

[0080] With the invention, it is thus advantageous that the favorable properties of dual-phase steel can be transferred to a method for press hardening or form hardening, particularly for producing a tailored welded blank.