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, comprising the steps of: providing a sheet bar having at least one region that includes a hardenable carbon-manganese-boron steel, and at least one other region that includes a dual-phase steel; and forming the sheet bar into the steel sheet component; wherein the forming of the sheet bar includes the steps of a) cold forming, then heating to an annealing temperature, then quenching the sheet bar in a cooling press, or b) heating the sheet bar to an annealing temperature above an austenization temperature of the hardenable steel and forming and quenching the sheet bar using one or more strokes in a forming and cooling press; and wherein the dual phase steel is softer than the hardenable steel and has an Ac1 temperature and an Ac3 temperature, and the annealing temperature is between the Ac1 temperature and the Ac3 temperature so that only partial austenization of the dual phase steel occurs at the annealing temperature, yielding a matrix that includes ferritic and austenitic components when the dual phase steel enters the cooling press or the forming and cooling press.

2. The method according to claim 1, wherein the annealing temperature is greater than about 800° C. and lower than the Ac3 temperature of the dual phase steel.

3. The method according to claim 1, wherein the heating step is performed in a furnace using a dwell time of between about zero and about 600 seconds.

4. The method according to claim 3, wherein the Ac3 temperature of the dual-phase steel is high enough that a degree of austenitization occurring with the dwell time and the annealing temperature is between 50 volume % and 90 volume %.

5. The method according to claim 1, wherein the quenching in a) or b) is performed at a cooling rate between 5 Kelvin/sec and 500 Kelvin/sec.

6. The method according to claim 1, wherein the sheet bar is formed using a press having a loading temperature between 450 and 850° C.

7. The method according to claim 6, wherein the loading temperature is 700 to 850° C.

8. The method according to claim 6, wherein the loading temperature 400 to 650° C.

9. The method according to claim 5, wherein the cooling rate is between 10 Kelvin/sec and 500 Kelvin/sec.

10. The method according to claim 1, wherein the dual-phase steel contains, in mass %, 0.5 to 1.5% aluminum.

11. The method according to claim 1, wherein the annealing temperature is set so that the dual-phase steel is intercritically annealed at a temperature between its Ac1 temperature and its Ac3 temperature.

12. A welded sheet bar including at least one dual-phase steel material in a first region and a hardenable carbon-manganese-boron steel in a second region, wherein 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 and a balance of residual quantities of iron and smelting-related impurities.

13. The welded sheet bar according to claim 12, wherein the dual-phase material contains 0.04-0.10 mass % C.

14. The welded sheet bar according to claim 12, wherein the dual-phase material contains 0.05-1.50 mass % Si.

15. The welded sheet bar according to claim 12, wherein the dual-phase material contains 0.60-1.50 mass % Mn.

16. The welded sheet bar according to claim 12, wherein the dual-phase material contains 0.45-0.80 mass % Cr.

17. The welded sheet bar according to claim 12, wherein the dual-phase material contains 0.40-1.20 mass % Al.

Description

(1) The invention will be explained by way of example based on the drawings. In the drawings:

(2) FIG. 1: shows the elongation and strength of dual-phase structures and ferritic-perlitic structures according to the prior art;

(3) 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;

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

(5) FIG. 4: shows the influence of carbon on the mechanical characteristic values as a function of the loading temperature;

(6) FIG. 5: shows structure images of dual-phase steels with different carbon contents;

(7) FIG. 6: shows the influence of manganese on the mechanical characteristic values;

(8) FIG. 7: shows the structure images with different manganese contents;

(9) FIG. 8: shows the influence of aluminum on the mechanical characteristic values;

(10) FIG. 9: shows the structure images with different aluminum contents;

(11) 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.

(12) FIG. 11: corresponds to Table 1 and describes specific alloys that are within and not within the scope of the present invention.

(13) 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.

(14) 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.

(15) 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.

(16) 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.

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

(18) C 0.02-0.12%, preferably 0.04-0.10%

(19) Si 0.05-2.0%, preferably 0.20-1.60%, and especially preferably, 0.50-1.50%

(20) Mn 0.5-2.0%, preferably 0.6-1.50%

(21) Cr 0.3-1.0%, preferably 0.45-0.80%

(22) Al 0.4-1.5%, preferably 0.50-1.30%, and especially preferably, 0.60-1.20%

(23) Nb <0.20%, preferably 0.01-0.10%

(24) Ti <0.20%, preferably 0.01-0.10% Residual quantities of iron and inevitable smelting-related impurities.

(25) 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.

(26) 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.

(27) 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).

(28) 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.

(29) 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.

(30) 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.

(31) The cooling rate in the press should be 10 Kelvin/sec.

(32) 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).

(33) The resulting mechanical properties according to the invention are as follows:

(34) R.sub.p0.2 250 to 500 MPa

(35) R.sub.m 400 to 900 MPa

(36) A≥10%.

(37) 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.

(38) The following problems, however, occur when adjusting the alloy according to the prior art:

(39) 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.

(40) 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.

(41) 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.

(42) 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.

(43) 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.

(44) 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.

(45) 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.

(46) 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.

(47) 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.

(48) 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.

(49) 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.

(50) Specific alloys within and not within the present invention are shown in the Table 1 below.

(51) TABLE-US-00001 TABLE 1 C, Si, Mn, Al, Cr, Nb + Ti, Ae1, Ae3, according to alloy wt. % wt. % wt. % wt. % wt. % wt. % ° C. ° C. the invention alloy A 0.06 0.2 1.5 1.0 0.5 0.03 719 1000 yes alloy B 0.08 0.2 1.5 1.0 0.5 0.03 718 981 yes alloy C 0.10 0.2 1.5 1.0 0.5 0.03 718 968 yes alloy D 0.08 0.2 1.2 1.0 0.5 0.03 729 1001 no alloy E 0.08 0.2 1.7 1.0 0.5 0.03 710 975 no alloy F 0.08 0.2 1.5 0.5 0.5 0.03 704 904 no alloy G 0.08 0.2 1.5 1.4 0.5 0.03 730 1074 yes alloy H 0.30 0.3 2.2 <0.05 <0.05 <0.05 669 767 no alloy I 0.26 0.3 1.8 0.3 <0.05 <0.05 658 818 no alloy J 0.05 0.6 0.7 0.7 0.35 <0.05 739 1028 yes alloy K 0.08 0.8 1.3 0.9 0.5 <0.05 734 1020 yes alloy L 0.10 1.3 1.8 1.3 0.7 <0.05 741 1087 yes alloy M 0.11 1.8 1.9 1.1 0.6 <0.05 738 1063 yes