DEFORMATION-HARDENED COMPONENT MADE OF GALVANIZED STEEL, PRODUCTION METHOD THEREFOR AND METHOD FOR PRODUCING A STEEL STRIP SUITABLE FOR THE DEFORMATION-HARDENING OF COMPONENTS

Abstract

A deformation-hardened component is made of galvanized steel by cutting a plate from a steel strip or steel sheet coated with zinc or with a zinc-based alloy and subsequently heating the plate to a deformation temperature above Ac3 for deformation and hardening. The galvanized steel has an at least partially martensitic transformation structure and includes as a chemical composition in wt. % C: 0.10-0.50, Si: 0.01-0.50, Mn: 0.50-2.50, P<0.02, S<0.01, N<0.01, Al: 0.015-0.100, B<0.004, remainder iron, including unavoidable smelting-induced, steel-accompanying elements. The chemical composition further includes at least one element selected from the group consisting of Nb, V, Ti, with a sum of the contents Nb+V+Ti being in a range of 0.01 to 0.20 wt. %. The structure of the steel after deformation-hardening has an average grain size of the former austenite grains of <15 ?m.

Claims

1.-12. (canceled)

13. A deformation-hardened component made of galvanized steel by cutting a plate from a steel strip or steel sheet coated with zinc or with a zinc-based alloy and subsequently heating the plate to a deformation temperature above Ac3 for deformation and hardening, said galvanized steel having an at least partially martensitic transformation structure and comprising as a chemical composition in wt. % C: 0.10-0.50 Si: 0.01-0.50 Mn: 0.50-2.50 P: <0.02 S: <0.01 N: <0.01 Al: 0.015-0.100 B: <0.004 remainder iron, including unavoidable smelting-induced, steel-accompanying elements, wherein the chemical composition further comprises at least one element selected from the group consisting of Nb, V, Ti, with a sum of the contents Nb+V+Ti being in a range of 0.01 to 0.20 wt. %, wherein the structure of the steel after deformation-hardening comprises an average grain size of the former austenite grains of <15 ?m.

14. The deformation-hardened component of claim 13, wherein the steel has a C content of 0.20 to 0.40 wt. %, an Si content of 0.15 to 0.25 wt. %, an Al content of 0.015 to 0.04 wt. %, wherein the total of the contents of Nb+V+Ti is in a range of 0.03 to 0.15 wt. %.

15. The deformation-hardened component of claim 13, wherein the steel has an Nb content of greater than 0.03 to less than or equal to 0.08 wt. % and/or a V content of 0.03 to 0.08 wt. % and/or a Ti content of greater than 0.09 to less than or equal to 0.2 wt. %.

16. The deformation-hardened component of claim 13, wherein the structure of the steel after deformation-hardening has an average grain size of former austenite grains of <12 ?m.

17. The deformation-hardened component of claim 13, wherein the structure of the steel after deformation-hardening has an average grain size of former austenite grains of <9 ?m.

18. The deformation-hardened component of claim 13, wherein the deformation-hardened component has a bending angle of at least 60?.

19. A method for producing a steel strip suitable for deformation-hardening of a component, said method comprising: smelting a steel with a following chemical composition in wt. % C: 0.10-0.50 Si: 0.01-0.50 Mn: 0.50-2.50 P: <0.02 S: <0.01 N: <0.01 Al: 0.015-0.100 B: <0.004 remainder iron, including unavoidable smelting-induced, steel-accompanying elements, wherein the chemical composition further comprises at least one element selected from the group consisting of Nb, V, Ti, with a sum of the contents of Nb+V+Ti being in a range of 0.01 to 0.20 wt. %; casting the steel by a continuous casting process to form individual slabs with subsequent cooling in static air; reheating the slabs to a temperature in a range of 1200? C. to 1280? C.; hot-rolling the reheated slabs at a final rolling temperature in a range of 780? C. to 920? C. to form a hot strip; winding the hot strip at a temperature in the range of 630? C. to 750? C.; optional cold-rolling the hot strip with subsequent optional recrystallisation annealing; coating the hot-rolled or cold-rolled strip with zinc or a zinc-based alloy; and optional heat treatment to transfer the zinc coating or zinc alloy coating into a zinc-iron alloy layer.

20. The method of claim 19, wherein the steel has a C content of 0.20 to 0.40 wt. %, an Si content of 0.15 to 0.25 wt. %, an Al content of 0.015 to 0.04 wt. %, wherein the sum of the contents of Nb+V+Ti is in a range of 0.03 to 0.15 wt. %.

21. A method for producing a deformation-hardened component from a steel strip, comprising: producing the steel strip by smelting a steel with a following chemical composition in wt. % C: 0.10-0.50, Si: 0.01-0.50, Mn: 0.50-2.50, P<0.02, S<0.01, N<0.01, Al: 0.015-0.100, B<0.004, remainder iron, including unavoidable smelting-induced, steel-accompanying elements, wherein the chemical composition further comprises at least one element selected from the group consisting of Nb, V, Ti, with a sum of the contents of Nb+V+Ti being in a range of 0.01 to 0.20 wt. %, casting the steel by a continuous casting process to form individual slabs with subsequent cooling in static air, reheating the slabs to a temperature in a range of 1200? C. to 1280? C., hot-rolling the reheated slabs at a final rolling temperature in a range of 780? C. to 920? C. to form a hot strip, winding the hot strip at a temperature in the range of 630? C. to 750? C., optional cold-rolling the hot strip with subsequent optional recrystallisation annealing; coating the hot-rolled or cold-rolled strip with zinc or a zinc-based alloy; and optional heat treatment to transfer the zinc coating or zinc alloy coating into a zinc-iron alloy layer; deforming the steel strip to form a slit pipe; welding the slit pipe along its strip edges; and deformation-hardening the welded slit pipe to form a component.

22. The method of claim 21, wherein the slit pipe is welded by high-frequency induction welding (HFI) or laser welding.

23. The method of claim 21, wherein the deformation-hardening includes hot-deforming the welded slit pipe and thereby hardening the component.

24. The method of claim 23, wherein the hot-deforming is a bending or internal high pressure deformation.

Description

[0021] The object of the invention is to provide a deformation-hardened component of galvanized steel which is inexpensive to produce and in which micro-cracks of >10 ?m after deformation-hardening are avoided to the greatest possible extent. Furthermore, a method for the production of a steel strip suitable for the deformation-hardening of components and a method for the production of a deformation-hardened component from this steel strip is to be provided.

[0022] According to the teaching of the invention this object is achieved by a deformation-hardened component of galvanized steel in which firstly a plate is cut from a steel strip or steel sheet coated with zinc or with a zinc-based alloy, then the plate is heated to a deformation temperature above Ac3 and deformed and thus hardened, comprising an at least partially martensitic transformation structure after forming, wherein the steel has the following chemical composition in wt. % [0023] C: 0.10-0.50 [0024] Si: 0.01-0.50 [0025] Mn: 0.50-2.50 [0026] P<0.02 [0027] S<0.01 [0028] N<0.01 [0029] Al: 0.015-0.100 [0030] B<0.004 remainder iron, including unavoidable smelting-induced, steel-accompanying elements, having at least one element from the group Nb, V, Ti, wherein the total of the contents of Nb+V+Ti is in a range of 0.01 to 0.20 wt. % and wherein the structure of the steel after deformation-hardening comprises an average grain size of the former austenite grains of less than 15 ?m.

[0031] Surprisingly it has been discovered by trials that by using plates with the stated alloy composition in combination with the establishment of an extremely fine-grained structure, micro-cracks could be drastically reduced or even prevented during deformation-hardening. In relation to this, the addition of micro-alloy elements from the group of niobium, titanium and vanadium in the stated amounts and the resulting controlled establishment of a very fine-grained structure during production of the steel strip has a decisive role. If a structure with a grain size of the former austenite grains of less than 15 ?m is established, the inclination to form micro-cracks is drastically reduced. The result is even clearer when grain sizes of less than 12 ?m or less than 9 ?m are established.

[0032] The establishment of a very fine-grained structure is assumed to prevent or markedly reduce the introduction of cracks and the progression of cracks. Furthermore, the addition of niobium, vanadium or titanium increases the grain boundary cohesion of the austenite grains, which is likewise assumed to have a positive effect on the avoidance of crack formation during deformation-hardening.

[0033] In a preferred alloy composition, the steel has a C content of 0.20 to 0.40 wt. %, an Si content of 0.15 to 0.25 wt. %, an Al content of 0.015 to 0.04 wt. %, wherein the total of the contents of Nb+V+Ti is in a range of 0.03 to 0.15 wt. %.

[0034] In order to achieve the desired effects with respect to the most fine-grained structure possible the steel has an Nb content of greater than 0.03 to less than or equal to 0.08 wt. % and/or a V content of 0.03% to 0.08 wt. % and/or a Ti content of greater than 0.09 to less than or equal to 0.2 wt. %.

[0035] In terms of process technology, the invention for the production of a steel strip suitable for the deformation-hardening of components is carried out by the following steps:

[0036] smelting of a steel with the following chemical composition in wt. % [0037] C: 0.10-0.50 [0038] Si: 0.01-0.50 [0039] Mn: 0.50-2.50 [0040] P<0.02 [0041] S<0.01 [0042] N<0.01 [0043] Al: 0.015-0.100 [0044] B<0.004
remainder iron, including unavoidable smelting-induced, steel-accompanying elements, having at least one element from the group Nb, V, Ti, wherein the sum of the contents of Nb+V+Ti is in a range of 0.01 to 0.20 wt. %, [0045] casting of the steel using a continuous casting method to form individual slabs with subsequent cooling in static air, [0046] reheating of the slabs to a temperature in the range of 1200? C. to 1280? C.the dwell time at over 1200? C. must be a minimum of 30 minutes, [0047] hot-rolling of the reheated slabs at a final rolling temperature in the range of 780? C. to 920? C., [0048] winding of the hot strip at a winder temperature in the range of 630? C. to 750? C., [0049] optional cold-rolling of the hot strip with subsequent optional recrystallisation annealing, [0050] coating of the hot-rolled or cold-rolled strip with zinc or a zinc-based alloy, [0051] optional heat treatment for the transfer of the zinc coating or zinc alloy coating into a zinc-iron alloy layer.

[0052] It has been recognised in accordance with the invention that the carbide-forming micro-alloy elements, such as niobium carbide, must undergo sufficient dissolution from the preceding continuous casting process to form fine deposits at the austenite grain boundaries during hot-rolling, which deposits are then definitive for nucleation during the phase transformation and prevention of grain coarsening at high temperatures and therefore for grain fineness and cracking resistance in the subsequent deformation-hardened component.

[0053] Thus, in accordance with the invention, reheating of the slabs to a temperature in the range of 1200? C. to 1280? C. takes place. The dwell time at over 1200? C. must be at least 30 minutes.

[0054] In addition, the final rolling temperature in accordance with the invention is lowered with respect to conventional temperatures to values in a range of 780? C. to 920? C. in order to achieve a high dislocation density at the end of the hot-rolling process. During the subsequent cooling of the hot strip this leads to a high nucleation density for the phase transformation and therefore to the desired extremely fine grain size.

[0055] In accordance with the invention, the hot strip is then wound to form a coil at a winder temperature in the range of 630? C. to 750? C. This temperature range has been established in accordance with the invention since it has been recognised that, in this temperature range, the precipitation pressure for the precipitations is at its greatest.

[0056] The hot strip thus produced can then be galvanized and directly processed further to form a component or a cold-rolling step is provided upstream of the galvanization in order to produce correspondingly thin strips of e.g. less than 1.5 mm in thickness. If the hot strip has been subjected to a cold-rolling step, the cold-rolled strip can then optionally be subjected to recrystallisation annealing. This can take place in a batch-type annealing process or in a continuous annealing installation, wherein the continuous annealing can also take place during hot-dipping galvanization.

[0057] Both hot-dipping and also electrolytic galvanization are also considered as coating methods. The coating is based on zinc as the main component, wherein, however, e.g. aluminium, magnesium, nickel and iron individually or in combination can also be contained therein. Combined coatings from electrolytic deposition of e.g. nickel, iron or zinc and subsequent annealing and hot-dipping refinement are also possible. Furthermore, it is possible to produce a thin coating by deposition from the gas phase and then to refine the strip electrolytically or by hot-dipping with a coating of zinc or a zinc alloy. It is also possible to transfer the produced layers into zinc-iron alloy layers by suitable annealing treatment in order e.g. to permit shorter furnace times or inductive rapid heating during deformation-hardening. This can take place either directly after the hot-dipping process (galvannealing) or in a separate process step in the form of a batch-type or continuous annealing process.

[0058] The deformation-hardened component thus produced has an extraordinarily good level of deformability, wherein bending angles, detected in a bending test, of over 60? and even over 80? are possible, in particular when the roll-hardened cold strip has been subjected, prior to galvanization, to a recrystallizing batch-type annealing process in a temperature range of 650? C. to 700? C. and a dwell time of 24 to 72 hours.

[0059] According to requirements for corrosion protection, the thickness of the coating can amount to between 5 ?m and 25 ?m, wherein greater thicknesses are also possible.

[0060] Welded pipes can also be produced from the steel strip produced by the above-described method, these pipes then each being deformation-hardened to form a component. The deformation-hardening can take place e.g. during a bending process or by internal high pressure deformation.

[0061] The pipes can be in the form of welded pipes or the steel strip is deformed to form a slit pipe which is then welded along its strip edges, wherein e.g. high-frequency induction welding (HFI) or laser welding can be considered as welding processes for the production of welded pipes.