FLAT STEEL PRODUCT HAVING AN AL-COATING, PROCESS FOR PRODUCTION THEREOF, STEEL COMPONENT AND PROCESS FOR PRODUCTION THEREOF

Abstract

The invention relates to a flat steel product for hot forming, consisting of a steel substrate which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating applied to the steel substrate. The iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 2.50% Mg. In addition, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

Claims

1. A flat steel product for hot forming, comprising a steel substrate comprising a steel which, as well as iron and unavoidable impurities, comprises (in wt %): C: 0.04-0.45 wt %, Si: 0.02-1.2 wt %, Mn: 0.5-2.6 wt %, Al: 0.02-1.0 wt %, P: ?0.05 wt %, S: ?0.02 wt %, N: ?0.02 wt %, Sn: ?0.03 wt % As: ?0.01 wt % Ca: ?0.005 wt % and of an Al-based protective coating applied to the steel substrate, where the iron-free mass fraction of the aluminum-based protective coating includes a total of up to 10% of additional alloy constituents and the rest of the iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg, wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

2. The flat steel product as claimed in claim 1, wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.80% Mn.

3. The flat steel product as claimed in claim 2 wherein the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 0.80% Si.

4. The flat steel product as claimed in claim 3 wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to less than 1.80% Mn.

5. A metallurgically bonded steel component, having a thickness of the Fe seam of greater than 2.5 ?m, comprising a steel substrate comprising a steel which, as well as iron and unavoidable impurities, comprises (in wt %): C: 0.04-0.45 wt %, Si: 0.02-1.2 wt %, Mn: 0.5-2.6 wt %, Al: 0.02-1.0 wt %, P: ?0.05 wt %, S: ?0.02 wt %, N: ?0.02 wt %, Sn: ?0.03 wt % As: ?0.01 wt % Ca: ?0.005 wt % and of an Al-based protective coating (15) applied to the steel substrate, where the iron-free mass fraction of the aluminum-based protective coating optionally includes a total of up to 10% of additional alloy constituents and the rest of the iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg, wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

6. The steel component as claimed in claim 5 wherein the steel component is an automobile component, comprising one of a bumper beam, a bumper reinforcement, a door reinforcement, a B pillar reinforcement, an A pillar reinforcement, a roof frame or a sill.

7. A method of producing a flat steel product having the characteristics as claimed in claim 4, comprising the following steps: providing a steel substrate composed of a steel which, as well as iron and unavoidable impurities, consists (in wt %) of C: 0.04-0.45 wt %, Si: 0.02-1.2 wt %, Mn: 0.5-2.6 wt %, Al: 0.02-1.0 wt %, P: ?0.05 wt %, S: ?0.02 wt %, N: ?0.02 wt %, Sn: ?0.03 wt % As: ?0.01 wt % Ca: ?0.005 wt % coating the steel substrate with an Al-based protective coating, where the iron-free mass fraction of the aluminum-based protective coating includes a total of up to 10% of additional alloy constituents and the remaining iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg and where the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

8. The method as claimed in claim 7, wherein the protective coating is applied to the steel substrate by hot dip coating.

9. The method as claimed in claim 8, wherein the flat steel product is prealloyed immediately after coating by keeping it at a prealloying temperature of 500? C.-600? C. for a prealloying time of 15-30 seconds.

10. A method of producing a steel component having the characteristics as claimed in claim 6, comprising the following steps producing a flat steel product by employing a method specified in claim 8; annealing the flat steel product in a furnace preheated to a temperature T for an annealing time t within a polygon formed by the points ABCD for flat steel products having a thickness between 0.7 mm and 1.5 mm, or in a furnace preheated to a temperature T for an annealing time t within a polygon formed by the points EFGH for flat steel products having a thickness between 1.5 mm and 3.0 mm, so as to form an Fe seam having a thickness greater than 2.5 ?m, where the points ABCD/EFGH are as follows: TABLE-US-00005 Point Temperature [? C.] Annealing time t [min] A 930 1.5 B 930 7 C 880 12 D 880 2.5 E 940 2.5 F 940 9 G 900 13 H 900 4 hot forming the flat steel product to give the steel component.

11. The method as claimed in claim 10, wherein in that the flat steel product is taken from the furnace after the annealing time t at a heating temperature, where the heating temperature is sufficiently high that the flat steel product at the start of forming has a hot forming temperature at which the microstructure of the steel substrate has been fully or partly converted to austenitic microstructure, and in that the flat steel product is quenched after the forming or in the course of forming, such that hard microstructure is formed in the microstructure of the steel substrate of the flat steel product.

12. The method as claimed in claim 11, wherein the heating temperature is between 880? C. to 950? C.

13. The flat steel product of claim 1 wherein the steel substrate further comprises: one or more of the elements Cr, B, Mo, Ni, Cu, Nb, Ti, V in the following contents: Cr: 0.08-1.0 wt %, B: 0.001-0.005 wt % Mo: ?0.5 wt % Ni: ?0.5 wt % Cu ?0.2 wt % Nb: 0.02-0.08 wt %, Ti: 0.01-0.08 wt % V: ?0.1 wt %.

14. The steel component of claim 5 wherein the steel substrate further comprises: one or more of the elements Cr, B, Mo, Ni, Cu, Nb, Ti, V in the following contents: Cr: 0.08-1.0 wt %, B: 0.001-0.005 wt % Mo: ?0.5 wt % Ni: ?0.5 wt % Cu ?0.2 wt % Nb: 0.02-0.08 wt %, Ti: 0.01-0.08 wt % V: ?0.1 wt %.

Description

[0092] The invention is elucidated in detail with reference to the working examples that follow, in conjunction with the figures. The figures show:

[0093] FIG. 1 a cross section of a steel component with a comparative protective coating;

[0094] FIG. 2 a cross section of a steel component of the invention with a protective coating;

[0095] FIG. 3 a cross section of a steel component with an alternative comparative protective coating;

[0096] FIG. 4 a cross section of a steel component of the invention with an alternative protective coating;

[0097] FIG. 5 the thickness of the Fe seam as a function of the annealing time in three different variants for the iron-free mass fractions of manganese;

[0098] FIG. 6 the thickness of the Fe seam as a function of the annealing time in three different variants for the iron-free mass fractions of manganese in the simultaneous presence of Si;

[0099] FIG. 7 the thickness of the Fe seam with an annealing time of 3 minutes as a function of the iron-free mass fraction of manganese;

[0100] FIG. 8 the thickness of the Fe seam with an annealing time of 3 minutes as a function of the iron-free mass fraction of silicon;

[0101] FIG. 9 the thickness of the Fe seam as a function of the annealing time in three different variants without magnesium;

[0102] FIG. 10 the thickness of the Fe seam as a function of the annealing time in three different variants without magnesium after a prealloying operation;

[0103] FIG. 11 suitable annealing parameters for the method of producing a steel component;

[0104] FIG. 12 a schematic diagram of a flat steel product.

[0105] FIGS. 1-4 show cross sections of steel components that have been produced with the same steel substrate and by the same forming method. All that has been varied is the composition of the protective coating.

[0106] Blanks were cut from a 1.5 mm-thick strip of steel type D according to table 2 with a double-sided 25 ?m-thick aluminum-based protective coating. The cutting method employed was either a punching tool or a laser. The exact chemical composition of the substrate was: C: 0.223 wt %, Si: 0.294 wt %, Mn: 1.275 wt %, P: 0.008 wt %, S: 0.002 wt %, Al: 0.046 wt %, Cr: 0.181 wt %, Cu: 0.054 wt %, Mo: 0.001 wt %, N: 0.001 wt %, Ni: 0.035 wt %, Nb: 0.002 wt %, Ti: 0.033 wt %, V: 0.007 wt %, B: 0.0033 wt %, Sn: 0.002 wt %.

[0107] These blanks were annealed in a roller hearth furnace at 920? C. for an annealing time t. This heating temperature is above the Ac3 temperature, which is about 860? C. for this type of steel. Thus, at least a partly austenitic microstructure was formed in the steel substrate. Subsequently, the blanks were formed and quenched in a forming tool.

[0108] Table 1 shows the thickness of the Fe seam for various variants of the iron-free mass fractions of the elements Mg, Mn and Si, and for various annealing times t.

[0109] FIGS. 1-4 show illustrative cross sections of the steel component thus produced with different compositions. FIGS. 5-8 show diagrams that are used to elucidate the various effects.

[0110] FIG. 1 shows a steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The hot dip coating melt consisted of aluminum with the following added to the alloy: 0.4 wt % of Mg, 0.8 wt % of Mn and 2.0 wt % of Si. The flat steel product 11 (see FIG. 12) accordingly had, after the coating, a protective coating 15 having a thickness of 25 ?m, and the protective coating 15 had an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si. The hot forming elucidated resulted in the steel component 21 shown in cross section with the aluminum-based protective coating 15, and the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si.

[0111] FIG. 2 shows a steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The hot dip coating melt consisted of aluminum with the following added to the alloy: 0.4 wt % of Mg, 1.6 wt % of Mn and 2.0 wt % of Si. The flat steel product 11 (see FIG. 12) after the coating operation accordingly had a protective coating 15 having a thickness of 25 ?m, and the protective coating 15 had an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si. The hot forming elucidated resulted in the steel component 21 shown in cross section with the aluminum-based protective coating 15, and the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 1.6% Mn and 2.0% Si.

[0112] The two steel components 21 in FIG. 1 and FIG. 2 show merely hints of an Fe seam. The thickness of the Fe seam is below 1 ?m. The protective coating 15 is thus insufficiently metallurgically bonded within the 3 minutes at 920? C.

[0113] FIG. 3 shows a steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The hot dip coating melt consisted of aluminum with the following added to the alloy: 0.4 wt % of Mg, 0.8 wt % of Mn. Apart from impurities in the region of 0.2 wt %, the melt did not contain any silicon. The flat steel product 11 (see FIG. 12) after the coating operation accordingly had a protective coating 15 having a thickness of 25 ?m, and the protective coating 15 had an iron-free mass fraction of 0.4% Mg, 0.8% Mn and less than 0.50% Si. The hot forming elucidated resulted in the steel component 21 shown in cross section with the aluminum-based protective coating, and the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn and less than 0.50% Si.

[0114] It is clearly apparent in FIG. 3 that an Fe seam 17 has formed in the protective coating. This has a thickness of 3 ?m. The metallurgical bonding of the protective coating 15 is thus complete after the 3 minutes at 920? C. Even though the steel components 21 shown in FIG. 1 and FIG. 3 have undergone an identical production method apart from the silicon content of the melt, the result in FIG. 3 is a pronounced Fe seam. The effect of reducing the silicon content is thus that the process duration can be shortened significantly.

[0115] FIG. 4 shows a steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The hot dip coating melt consisted of aluminum with the following added to the alloy: 0.4 wt % of Mg, 1.6 wt % of Mn. Apart from impurities in the region of 0.2 wt %, the melt did not contain any silicon. The flat steel product after the coating operation accordingly had a protective coating 15 having a thickness of 25 ?m, and the protective coating 15 had an iron-free mass fraction of 0.4% Mg, 1.6% Mn and less than 0.50% Si. The hot forming elucidated resulted in the steel component 21 shown in cross section with the aluminum-based protective coating, and the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 1.6% Mn and less than 0.50% Si.

[0116] In FIG. 4, a distinct Fe seam 17 with a thickness of 7 ?m is apparent. The metallurgical bonding of the protective coating 15 is thus complete after the 3 minutes at 920? C., and is additionally further advanced than in FIG. 3 owing to the higher manganese content. The increase in the manganese content thus further shortens the process duration. At the same time, comparison of FIG. 4 with FIG. 2 shows that this effect of manganese occurs only in the case of a low silicon content. Even though the manganese content is elevated in the variant according to FIG. 2 by comparison with the variant according to FIG. 1, it is not the case that a more intense Fe seam is observed in FIG. 2 by comparison with FIG. 1 because of the higher silicon content.

[0117] FIG. 5 shows the thickness of the Fe seam as a function of the annealing time in three different variants for the iron-free mass fractions: [0118] aluminum with 0.4% magnesium, 0.2% silicon [0119] aluminum with 0.4% magnesium, 0.8% manganese, 0.2% silicon [0120] aluminum with 0.4% magnesium, 1.6% manganese, 0.2% silicon

[0121] The effect of the manganese is clearly apparent here. While there is as good as no difference in the case of longer annealing times, the addition of manganese to the alloy has the effect that even the short annealing time of 3 minutes results in an Fe seam of 7 ?m, which confirms sufficient metallurgical bonding.

[0122] FIG. 6 shows the same diagram as FIG. 5. In the variants in FIG. 6, however, the silicon content was additionally raised to an iron-free mass fraction of 2%. Thus, the following variants are shown (the last two curves are identical and therefore indistinguishable in the diagram): [0123] aluminum with 0.4% magnesium, 0% manganese, 2% silicon [0124] aluminum with 0.4% magnesium, 0.8% manganese, 2% silicon [0125] aluminum with 0.4% magnesium, 1.6% manganese, 2% silicon

[0126] It is clearly apparent that the effect of the manganese no longer occurs. After an annealing time of 3 minutes, no significant Fe seam is apparent in any of the three variants.

[0127] FIG. 7 shows the thickness of the Fe seam at an annealing time of 3 minutes as a function of the iron-free mass fraction of manganese. The iron-free mass fraction of silicon is about 0.2%. Over and above an iron-free mass fraction of manganese of about 0.3%, a thicker Fe seam arises, and the thickness of the Fe seam rises with the manganese content.

[0128] FIG. 8 shows the effects of silicon. The plot is of the thickness of the Fe seam at an annealing time of 3 minutes as a function of the iron-free mass fraction of silicon. In all cases, the iron-free mass fraction of manganese was 1.6%. Below 1.8% silicon, the manganese effect is still significant; the smaller the iron-free mass fraction of silicon, the greater the effect.

[0129] FIG. 9 shows the described effects once again for variants without magnesium. What is shown is the thickness of the Fe seam as a function of the annealing time in three different variants for the iron-free mass fractions: [0130] aluminum without addition [0131] aluminum with 1.6% manganese, 0.0% silicon [0132] aluminum with 1.6% manganese, 0.5% silicon

[0133] It is clearly apparent that, in the variants with 1.6% manganese, a significant Fe seam is formed at a much earlier stage. In addition, it likewise becomes clear that the same effect occurs both at 0.5% silicon and at 0% silicon. In the experiments without magnesium addition, it was possible to lower the silicon contamination to below 0.05%. 0% silicon in these experiments should therefore be regarded as up to 0.05% Si.

[0134] FIG. 10 shows the thickness of the Fe seam as a function of the annealing time for the same layer compositions as in FIG. 9. In these variants, however, the annealing was preceded by a prealloying operation in which the blanks were kept at a prealloying temperature of 680? C. for a prealloying time of 13 seconds. By comparison with FIG. 9, in all cases, the Fe seam is formed much earlier and is thicker than without prealloying at the same annealing time. Moreover, the manganese effect is apparent, in that a significant Fe seam is formed with addition of manganese to the alloy. This is the case both at 0% Si and at 0.5% silicon.

[0135] The last three rows of table 1 are not shown in the graph. This test series examined once again whether there are significant effects of the magnesium content. It was found that the magnesium content has no effects on the effect.

[0136] FIG. 12 shows, in a schematic diagram, a flat steel product 11 for hot forming, consisting of a steel substrate 13 which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating 15 applied to the steel substrate 13.

TABLE-US-00003 TABLE 1 Annealing Fe Mg Mn Si time seam [%] [%] [%] [minutes] [?m] Prealloyed? 0.4 0 0.2 3 1.0 no 0.4 0 0.2 5 11.0 no 0.4 0 0.2 10 20.0 no 0.4 0.8 0.2 3 3.0 no 0.4 0.8 0.2 5 12.0 no 0.4 0.8 0.2 10 21.0 no 0.4 1.6 0.2 3 7.0 no 0.4 1.6 0.2 5 11.0 no 0.4 1.6 0.2 10 21.0 no 0.4 0.8 2.0 3 1.0 no 0.4 0.8 2.0 5 9.0 no 0.4 0.8 2.0 10 17.0 no 0.4 1.6 2.0 3 1.0 no 0.4 1.6 2.0 5 9.0 no 0.4 1.6 2.0 10 17.0 no 0.4 0 2.0 3 1.0 no 0.4 0 2.0 5 8.0 no 0.4 0 2.0 10 19.0 no 0.4 0.4 2.0 3 2.0 no 0.4 0.4 2.0 5 11.0 no 0.4 0.4 2.0 10 20.0 no 0.4 1.2 2.0 3 5.0 no 0.4 1.2 2.0 5 11.0 no 0.4 1.2 2.0 10 20.0 no 0.4 1.6 0.8 3 5.0 no 0.4 1.6 0.8 5 11.0 no 0.4 1.6 0.8 10 19.0 no 0.4 1.6 1.6 3 3.0 no 0.4 1.6 1.6 5 10.0 no 0.4 1.6 1.6 10 18.0 no 0 0 0 1 0.0 no 0 0 0 1.5 0.0 no 0 0 0 2 0.0 no 0 0 0 2.5 0.0 no 0 0 0 3 1.0 no 0 0 0 5 10.2 no 0 1.6 0 1 0.0 no 0 1.6 0 1.5 0.0 no 0 1.6 0 2 3.0 no 0 1.6 0 2.5 6.1 no 0 1.6 0 3 7.0 no 0 1.6 0 5 12.5 no 0 1.6 0.5 1 0.0 no 0 1.6 0.5 1.5 0.0 no 0 1.6 0.5 2 3.2 no 0 1.6 0.5 2.5 5.9 no 0 1.6 0.5 3 7.2 no 0 1.6 0.5 5 12.2 no 0 0 0 1 0.0 yes 0 0 0 1.5 0.0 yes 0 0 0 2 5.2 yes 0 0 0 2.5 7.8 yes 0 0 0 3 9.2 yes 0 0 0 5 13.0 yes 0 1.6 0 1 0.0 yes 0 1.6 0 1.5 5.2 yes 0 1.6 0 2 6.1 yes 0 1.6 0 2.5 8.6 yes 0 1.6 0 3 9.2 yes 0 1.6 0 5 14.0 yes 0 1.6 0.5 1 0.0 yes 0 1.6 0.5 1.5 4.5 yes 0 1.6 0.5 2 6.0 yes 0 1.6 0.5 2.5 8.0 yes 0 1.6 0.5 3 8.8 yes 0 1.6 0.5 5 13.2 yes 0.4 1.6 0.8 3 5.0 no 1 1.6 0.8 3 6.0 no 1.5 1.6 0.8 3 5.0 no

TABLE-US-00004 TABLE 2 Steel min/ type max C Si Mn P S Al Nb Ti Cr + Mo B A min 0.05 0.05 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 max 0.10 0.35 1.00 0.030 0.025 0.075 0.100 0.150 0.0050 B min 0.05 0.03 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 max 0.10 0.50 2.00 0.030 0.025 0.075 0.100 0.150 0.0050 C min 0.05 0.05 1.00 0.000 0.000 0.015 0.005 0.000 0.00 0.0010 max 0.16 0.40 1.40 0.025 0.010 0.150 0.050 0.050 0.50 0.0050 D min 0.10 0.05 1.00 0.000 0.000 0.005 0.000 0.00 0.0010 max 0.30 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0050 E min 0.250 0.10 1.00 0.000 0.000 0.015 0.000 0.00 0.0010 max 0.380 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0500