Steel with improved processing characteristics for elevated temperature forming

Abstract

The invention relates to a flat steel product for hot forming, a formed sheet metal part and methods for their production, wherein the flat steel product and the sheet metal part have improved properties, particularly in conjunction with an aluminum-based anti-corrosion coating.

Claims

1. A steel flat product for hot forming, comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in wt. %), consists of C: 0,06-0,5 %, Si: 0,05-0,6 %, Mn: 0,4-3,0 %, Al: 0,06-1,0 %, Note: 0,001-0,2 %, Ti: 0,001-0,10 %, B: 0,0005-0,01 %, P: ≤ 0,03 %, S: ≤ 0,02 %, N: ≤ 0,02 %, Sn: ≤ 0,03 %, Ace: ≤ 0,01 %, and optionally one or more of the elements Cr, Cu, Mo, Ni, V, Ca, W in the following contents Cr: 0,01-1,0 %, Cu: 0,01-0,2 %, Mon: 0,002-0,3 %, No: 0,01-0,5 %, V: 0,001-0,3 %, Approx: 0,0005-0,005 %, W: 0,001-1,0 %, where the ratio Al / Nb from Al content to Nb content is: Al / Nb ≤ 20 .0 wenn Mn ≤ 1,6 Gew . − % and Al / Nb ≤ 30 .0 wenn Mn ≥ 1,7 Gew . − % 2. Flat steel product according to one of the preceding claims characterized in that it has an anti-corrosive coating on at least one side.

3. Flat steel product according to claim 2, characterized in that the anti-corrosive coating is an aluminum-based anti-corrosive coating and has an alloy layer and an Al base layer.

4. Flat steel product according to one of claims 1 to 3 characterized in thatthe steel flat product has a yield strength with a continuous curve (Rp0.2) or a yield strength with a difference (ΔRe) between the upper yield strength value (ReH) and the lower yield strength value (ReL) of at most 45 MPa and / or the steel flat product has a uniform elongation Ag of at least 10% and / or the steel flat product has an elongation at break A80 of at least 15%, preferably at least 20%.

5. A method for producing a flat steel product for hot forming with a corrosion protection coating, comprising the following steps: a) providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in wt. %), consists of C: 0,06-0,5 %, Si: 0,05-0,6 %, Mn: 0,4-3,0 %, Al: 0,06-1,0 %, Note: 0,001-0,2 %, Ti: 0,001-0,10 %, B: 0,0005-0,01 %, P: ≤ 0,03 %, S: ≤ 0,02 %, N: ≤ 0,02 %, Sn: ≤ 0,03 %, Ace: ≤ 0,01 %, and optionally one or more of the elements "Cr, Cu, Mo, Ni, V, Ca, W" in the following contents Cr: 0,01-1,0 %, Cu: 0,01-0,2 %, Mon: 0,002-0,3 %, No: 0,01-0,5 %, V: 0,001-0,3 %, Approx: 0,0005-0,005 %, W: 0,001-1,0 %, where the ratio Al / Nb from Al content to Nb content is: Al / Nb ≤ 20 .0 wenn Mn ≤ 1,6 Gew . − % and Al / Nb ≤ 30 .0 wenn Mn ≥ 1,7 Gew . − % ; b) through-heating the slab or thin slab at a temperature (T1) of 1100-1400°C; c) optionally pre-rolling the through-heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000-1200°C; d) hot rolling to a hot-rolled flat steel product, the final rolling temperature (T3) being 750-1000°C; e) optionally coiling the hot-rolled flat steel product, the coiling temperature (T4) being no more than 700°C; f) descaling the hot-rolled flat steel product; g) optionally cold rolling the flat steel product, the cold rolling degree being at least 30%; h) annealing the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6) which is 650-800°C, preferably 670-800°C;j) coating the flat steel product cooled to the immersion temperature with a corrosion protection coating by hot-dip coating in a molten bath with a melt temperature (T7) of 660-800°C, preferably 680-740°C; k) cooling the coated flat steel product to room temperature, wherein the first cooling time t; mT in the temperature range between 600°C and 450°C is more than 10s, in particular more than 14s and the second cooling time t nT in the temperature range between 400°C and 300°C is more than 8s, in particular more than 12s; l) optional temper rolling of the coated flat steel product.

6. Method according to claim 5 characterized in thatin hot-dip coating, a molten bath is used which contains the corrosion protection to be applied to the flat steel product in liquid form, which consists of up to 15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5% by weight of alkali or alkaline earth metals and optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0% by weight, and the remainder being aluminium.

7. Sheet metal part formed from a flat steel product, comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0,06-0,5 %, Si: 0,05-0,6 %, Mn: 0,4-3,0 %, Al: 0,06-1,0 %, Note: 0,01-0,2 %, Ti: 0,001-0,10 %, B: 0,0005-0,01 %, P: ≤ 0,03 %, S: ≤ 0,02 %, N: ≤ 0,02 %, Sn: ≤ 0,03 %, Ace: ≤ 0,01 %, and optionally one or more of the elements Cr, Cu, Mo, Ni, V, Ca, W in the following contents Cr: 0,01-1,0 %, Cu: 0,01-0,2 %, Mon: 0,002-0,3 %, No: 0,01-0,5 %, V: 0,001-0,3 %, Approx: 0,0005-0,005 %, W: 0,001-1,0 %, where the ratio Al / Nb from Al content to Nb content is: Al / Nb ≤ 20 .0 wenn Mn ≤ 1,6 Gew . − % and Al / Nb ≤ 30 .0 wenn Mn ≥ 1,7 Gew . − % , and an anti-corrosion coating.

8. Sheet metal part according to claim 7 characterized in thatthe steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite and / or lower bainite, preferably more, at least partially more than 90% martensite and / or lower bainite, and wherein preferably the former austenite grains of the martensite have an average grain diameter which is less than 14 µm, in particular less than 12 µm, preferably less than 10 µm.

9. Sheet metal part according to one of claims 7 to 8 characterized in thatthe sheet metal part at least partially has a yield strength of at least 950 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1500 MPa and / or the sheet metal part at least partially has a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1800 MPa and / or the sheet metal part at least partially has an elongation at break A80 of at least 4%, preferably at least 5%, particularly preferably at least 6% and / or the sheet metal part at least partially has a bending angle of at least 30°, in particular at least 40°, preferably at least 50°.

10. Sheet metal part according to one of claims 7 to 9, characterized in that the sheet metal part has fine precipitates in the structure, in particular in the form of niobium carbonitrides and / or titanium carbonitrides.

11. Sheet metal part according to one of claims 7 to 10, characterized in thatthe electrochemical potential of the surface of the sheet metal part in a corrosive medium is at least -0.50 V.

12. Sheet metal part according to one of claims 7 to 11, characterized in that the corrosion protection coating is an aluminum-based corrosion protection coating and comprises an alloy layer and an Al base layer, and wherein in the cross-section of the alloy layer over a measuring length of 500 µm, the porous area in the alloy layer is less than 250 µm 2 and in particular the proportion of the area occupied by pores with a diameter greater than or equal to 0.1 µm is less than 10%.

13. Sheet metal part according to one of claims 7 to 12, characterized in that the welding range is at least 0.9 kA.

14. Sheet metal part according to one of claims 7 to 13, characterized in that the Nb content in the alloy layer is greater than 0.010 wt.%, preferably greater than 0.015 wt.%, in particular greater than 0.018 wt.%.

15. A method for producing a sheet metal part, comprising the following steps: a) providing a sheet metal blank from a flat steel product according to one of claims 1-4; b) heating the sheet metal blank such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg the blank when placed in a forming tool intended for hot press forming (working step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c) placing the heated sheet metal blank in a forming tool, wherein the transfer time required for removing the blank from the heating device and placing it in the forming tool is t Trans at most 20s, preferably at most 15s; d) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is hot-press formed over a period t WZof more than 1s with a cooling rate r of at least partially more than 30 K / s WZ to the target temperature T Ziel cooled and optionally kept there; e) removing the sample cooled to the target temperature T Ziel cooled sheet metal part from the tool.

Images