METHOD FOR PRODUCING A WELDED COMPONENT MADE OF A DEFORMED HIGH-STRENGTH STEEL, AND COMPONENT PRODUCED IN THIS MANNER

Abstract

A welded component having mechanical properties in a welding seam region comparable or better to those in the non-influenced base material via a method including producing a hot-rolled steel product made of a high-strength air-hardenable steel with a material thickness of at least 1.5 mm having a chemical composition by mass in one embodiment of: C: 0.03 to 0.4; Mn: 1.0 to 4.0; Si: 0.09 to 2.0; Al: 0.02 to 2.0; P<=0.1; S<=0.1; N: 0.001 to 0.5; Ti: 0.01 to 0.2; Cr: 0.05 to 2.0; B: 0.001 to 0.1; Mo: 0.01 to 1.0; V: 0.01 to 0.2; optionally: Ni: 0.02 to 1.0; Nb: 0.01 to 0.1; and residual iron including conventional steel-accompanying elements, subsequently air hardening the produced hot-rolled steel product, then deforming the hot-rolled steel product in the air-hardened state to form a component, and producing welding connections using a fusion welding process on the component.

Claims

1.-15. (canceled)

16. A method for producing a welded component, comprising: producing a hot-rolled steel product from a high-strength air-hardenable steel having a material thickness of at least 1.5 mm, wherein the hot-rolled steel product comprises the following chemical composition in mass %: C: 0.03 to 0.4, Mn: 1.0 to 4.0, Si: 0.09 to 2.0, Al: 0.02 to 2.0, P: <=0.1, S: <=0.1, N: 0.001 to 0.5, Ti: 0.01 to 0.2, Cr: 0.05 to 2.0, B: 0.001 to 0.1, Mo: 0.01 to 1.0, V: 0.01 to 0.2, with the remainder being iron including typical steel-associated elements, subsequently air-hardening solely by cooling in air the hot-rolled steel product produced, after which the hot-rolled, air-hardened steel product has a minimum yield strength Rp0.2 of 450 MPa, a minimum tensile strength Rm of 700 MPa and/or an elongation at fracture A5 of at least 6%; forming the hot-rolled steel product in the air-hardened state into a component; and producing welded connections by fusion welding on the component.

17. The method of claim 16, wherein the hot-rolled steel product has the following chemical composition in mass % in said producing a hot-rolled steel product: C: 0.06 to 0.12, Mn: 1.80 to 2.20, Si: 0.22 to 0.34, Al: 0.02 to 0.06, P: <=0.1, S: <=0.1, N: 0.0030 to 0.0125, Ti: 0.010 to 0.050, Cr: 0.60 to 1.0, B: 0.0015 to 0.0060, Mo: 0.10 to ≤0.40, V: 0.05 to ≤0.09, with the remainder being iron including typical steel-associated elements.

18. The method of claim 17, wherein the hot-rolled steel product has the following chemical composition in mass % in said producing a hot-rolled steel product: C: 0.08 to 0.10, Mn: 1.80 to 2.00, Si: 0.25 to 0.30, Al: 0.02 to 0.05, P: ≤0.020 S: ≤0.010 N: 0.0030 to 0.0080 Ti: 0.020 to 0.030 Cr: 0.70 to 0.80 B: 0.0025 to 0.0035 Mo: 0.15 to 0.30 V: 0.05 to 0.08 with the remainder being iron including typical steel-associated elements.

19. The method of claim 16, wherein the hot-rolled steel product further comprises in mass % in said producing a hot-rolled steel product: Ni: 0.02 to 1.0, Nb: 0.01 to 0.1, with the remainder being iron including typical steel-associated elements.

20. The method of claim 16, wherein said method consists exclusively of the following steps: producing a hot-rolled steel product from a high-strength air-hardenable steel having a material thickness of at least 1.5 mm, wherein the hot-rolled steel product comprises the following chemical composition in mass %: C: 0.03 to 0.4, Mn: 1.0 to 4.0, Si: 0.09 to 2.0, Al: 0.02 to 2.0, P: <=0.1, S: <=0.1, N: 0.001 to 0.5, Ti: 0.01 to 0.2, Cr: 0.05 to 2.0, B: 0.001 to 0.1, Mo: 0.01 to 1.0, V: 0.01 to 0.2, with the remainder being iron including typical steel-associated elements, subsequently air-hardening solely by cooling in air the hot-rolled steel product produced, after which the hot-rolled, air-hardened steel product has a minimum yield strength Rp0.2 of 450 MPa, a minimum tensile strength Rm of 700 MPa and/or an elongation at fracture A5 of at least 6%; forming the hot-rolled steel product in the air-hardened state into a component; and producing welded connections by fusion welding on the component.

21. The method as claimed in claim 16, wherein said forming is a conventional cold sheet forming method.

22. The method as claimed in claim 21, wherein the conventional cold sheet forming comprises deep drawing, folding, roll-profiling, bending or flanging.

23. The method as claimed in claim 16, wherein the forming is carried out at a temperature in the range of −5° C. to 40° C.

24. The method as claimed in claim 23, wherein the forming is carried out at room temperature in the range of 15° C. to 25° C.

25. The method as claimed in claim 16, wherein the fusion welding comprises metal arc welding or beam welding.

26. The method as claimed in claim 25, wherein the metal arc welding comprises protective gas welding, and wherein the beam welding comprises laser beam welding.

27. A welded component produced by forming from a hot-rolled and air-hardened steel product consisting of an air-hardenable high-strength steel with a material thickness of at least 1.5 mm, wherein the steel product has the following chemical composition in mass %: C: 0.03 to 0.4, Mn: 1.0 to 4.0, Si: 0.09 to 2.0, Al: 0.02 to 2.0, P: <=0.1, S: <=0.1, N: 0.001 to 0.5, Ti: 0.01 to 0.2, Cr: 0.05 to 2.0, B: 0.001 to 0.1, Mo: 0.01 to 1.0, V: 0.01 to 0.2, wherein prior to being formed into a component the hot-rolled and air-hardened steel product in the air-hardened state has a minimum yield strength Rp0.2 of 450 MPa, a minimum tensile strength Rm of 700 MPa and/or an elongation at fracture A5 of at least 6%, and wherein the hot-rolled steel product which is air-hardened by cooling solely in air has a complex phase microstructure with a bainite content of more than 50%, and wherein the hot-rolled, air-hardened and formed steel product in the region of a heat-affected zone of a welded connection has a complex phase microstructure with a bainite content of more than 50%.

28. The welded component as claimed in claim 27, wherein the steel product has the following chemical composition in mass %: C: 0.06 to 0.12, Mn: 1.80 to 2.20, Si: 0.22 to 0.34, Al: 0.02 to 0.06, P: <=0.1, S: <=0.1, N: 0.0030 to 0.0125, Ti: 0.010 to 0.050, Cr: 0.60 to 1.0, B: 0.0015 to 0.0060, Mo: 0.10 to 0.40, V: 0.05 to 0.09.

29. The welded component as claimed in claim 28, wherein the steel product has the following chemical composition in mass %: C: 0.08 to 0.10, Mn: 1.80 to 2.00, Si: 0.25 to 0.30, Al: 0.02 to 0.05, P: ≤0.020, S: ≤0.010, N: 0.0030 to 0.0080, Ti: 0.020 to 0.030, Cr: 0.70 to 0.80, B: 0.0025 to 0.0035, Mo: 0.15 to 0.30, V: 0.05 to 0.08.

30. The welded component as claimed in claim 27, wherein the steel product further comprises in mass %: Ni: 0.02 to 1.0, Nb: 0.01 to 0.1.

31. The welded component as claimed in claim 27, wherein prior to being formed into a component the hot-rolled and air-hardened steel product in the air-hardened state has a minimum yield strength Rp0.2 of more than 600 MPa, a minimum tensile strength Rm of more than 800 MPa and/or an elongation at fracture A5 of at least 13% and wherein the hot-rolled steel product which is air-hardened by cooling solely in air has a complex phase microstructure with a bainite content of more than 80% and wherein the hot-rolled, air-hardened and formed steel product in the region of a heat-affected zone of a welded connection has a complex phase microstructure with a bainite content of more than 80%.

32. The welded component as claimed in claim 31, wherein the hot-rolled steel product which is air-hardened by cooling solely in air has a complex phase microstructure with a bainite content of more than 90% and wherein the hot-rolled, air-hardened and formed steel product in the region of a heat-affected zone of a welded connection has a complex phase microstructure with a bainite content of more than 90%.

33. The welded component as claimed in claim 27, wherein the hot-rolled and air-hardened steel product has a material thickness of between 1.5 mm to 25 mm.

34. The welded component as claimed in claim 27, wherein the hot-rolled and air-hardened steel product has a material thickness of up to 15.

35. The welded component as claimed in claim 27, wherein the welded component (i) is used in the automotive industry as a chassis component, a bumper member, or a cross member, or (ii) is used in the construction industry, or (iii) is used in the household appliance industry, or (iv) is used in chemical apparatus engineering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] FIG. 1 shows a section of a welded component 1 in accordance with the invention in a lateral sectional view;

[0072] FIG. 2 shows the qualitative results of a grid-type hardness test over the section shown in FIG. 1;

[0073] FIG. 3 shows a microstructure-micrograph of the region of the base material of the welded component 1;

[0074] FIG. 4 shows a microstructure-micrograph of the region of the heat-affected zone of the welded component 1;

[0075] FIG. 5 shows a microstructure-micrograph of the region of the fusion line of the welded component 1; and

[0076] FIG. 6 shows a microstructure-micrograph of the region of the weld seam of the welded component 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0077] FIG. 1 shows a section of a welded component 1 in accordance with the invention in a lateral sectional view of a typical overlapping connection. The welded component 1 in the form of a chassis component consists essentially of a first lower steel product 2, in particular flat steel product, on which a further second upper steel product 3, in particular flat steel product, is placed partially covering it. The steel products 2, 3 each have a material thickness of 3 mm. The upper steel product 3 is connected to a surface 20 of the lower steel product 2 in the region of a front edge 3v via a welded connection 4 in the manner of an overlapping joint.

[0078] FIG. 2 shows qualitatively the result of a grid-like hardness test over the entire illustrated section of the welded component 1. Starting from the hardness of the lower steel product 2 in the basic state, which is shown as the first hardness region 5a with vertical hatching, a heat-affected zone adjoins in the direction of the welding material 4a or the melt in the usual manner in the steel products 2, 3 and is shown as the second hardness region 5b without hatching or pattern. Based upon the air-hardening properties of the steel, the second hardness region 5b is hardened compared to the first hardness region 5a. The welding material 4a of the welded connection 4 has a third hardness region 5c which is characterised by a dot pattern. This third hardness region 5c is adjustable in relation to the hardness depending on the choice of welding wire and will typically be in a hardness region of the steel product 2, 3 in the basic state.

[0079] The determined measurement values for the first hardness region 5a are in the range of 280 to 320 HV 0.1, for the second hardness region 5b in the range of 430 to 470 HV 0.1 and for the third hardness region 5c in the range of 230 to 270 HV 0.1.

[0080] FIGS. 3 to 6 each show a microstructure-micrograph from different regions of the welded component 1. All microstructure-micrographs have been etched with nital in connection with sample preparation and are illustrated at a magnification of 1:500.

[0081] FIG. 3 shows a microstructure-micrograph of the region of the base material of the welded component 1. At this location, a complex phase microstructure is present having 5% ferrite, 3% pearlite, 90% bainite and 2% martensite. The average ferrite grain size is 13.5 μm.

[0082] FIG. 4 shows a microstructure-micrograph of the region of the heat-affected zone of the welded component 1. A microstructure consisting of bainite with some ferrite can be seen. The average ferrite grain size is 13.5 μm.

[0083] FIG. 5 shows a microstructure-micrograph of the region of the fusion line of the welded component 1. At this location, the microstructure is 100% bainite.

[0084] FIG. 6 shows a microstructure-micrograph of the region of the weld seam 4a of the welded component 1. The microstructure which can be seen is bainite interspersed with a light network of ferrite and bainite.

[0085] As already explained above, the welded component 1 in accordance with the invention achieves a high fatigue strength or operating strength by reason of the predominantly fine-grained bainitic microstructure, which occurs in the region of the welded connection 4, for the heat-affected zone as well as for the weld seam 4a. The base material of the welded component is already present as a complex phase microstructure with a predominant proportion of bainite.

[0086] The inventive, high-strength, air-hardening steel for lightweight vehicle construction is likewise characterised by the fact that this alloy concept achieves excellent weldability in the case of the typical welding methods, such as e.g. protective gas welding, protective gas soldering or laser welding, without the disadvantages of known air-hardenable steels. High-frequency induction welding (HFI welding) is also unproblematic without the undesirable chromium carbide precipitations in the weld seam.

[0087] The content of C and Mn which is likewise reduced in comparison with the known air-hardening steel for seamless pipes ensures excellent general weldability with simultaneously excellent forming properties. At the same time, the lowered Si content ensures the galvanising capability of the steel and the addition of V ensures the tempering resistance.

[0088] The investigations have shown that the Cr content which is decisive for the air-hardening effect can be reduced to a value which is not critical for the avoidance of chromium carbide precipitations during HFI welding, if the air-hardenability of the steel is then improved at the same time by means of a complex alloy concept based on Cr—Mo—Ti—B.

[0089] In accordance with the invention, the alloy concept for the steel product is based on the knowledge that, in contrast to the known steel for seamless pipes, in which nitrogen must be completely bound by titanium in order to avoid boron nitride precipitations and thus ensure the effectiveness of the added boron, the nitrogen is also bound by other alloy elements such as Cr or Mo. It is therefore no longer absolutely necessary to determine an overstoichiometric titanium addition in relation to nitrogen. The addition of vanadium triggers precipitations of vanadium carbonitrides of the V(C,N) type at higher tempering temperatures, which counteract a drop in strength via secondary hardening.

[0090] However, the disadvantage of these alloy concepts based on Mn—Si—Ti—B is the excessively high Si content which is necessary to achieve high strength values but makes piece-galvanising more difficult. Moreover, from temperatures of ca. 550° C., the strength of the material drops significantly below the required values and so the tempering resistance is also not guaranteed.

[0091] On the basis of these findings, the inventive alloy concept already described above was determined, wherein the following analysis range in mass % turned out to be advantageous. The respective specifications for the analysis ranges can be fulfilled individually or in total:

[0092] C 0.06 to 0.12

[0093] Mn 1.8 to 2.20

[0094] Si 0.22 to 0.34

[0095] Al 0.02 to 0.06

[0096] P≤0.020

[0097] S≤0.010

[0098] N 0.0030 to 0.0125

[0099] Ti 0.010 to 0.050

[0100] Cr 0.60 to 1.0

[0101] B 0.0015 to 0.0060

[0102] Mo 0.10 to 0.40

[0103] V 0.05 to 0.09

[0104] Further advantageous processing and component properties are achieved when the following analysis range is observed in mass %:

[0105] C 0.08 to 0.10

[0106] Al 0.02 to 0.05

[0107] Si 0.25 to 0.30

[0108] Mn 1.80 to 2.00

[0109] P≤0.020

[0110] S≤0.010

[0111] N 0.0030 to 0.0080

[0112] Ti 0.020 to 0.030

[0113] Cr 0.70 to 0.80

[0114] B 0.0025 to 0.0035

[0115] Mo 0.15 to 0.30

[0116] V 0.05 to 0.08

[0117] The results show the high tempering resistance of the steel up to temperatures of 700° C.

[0118] As investigations on welded components consisting of the hot-rolled and air-hardened steel product in accordance with the invention have shown, this steel can be used advantageously not only in the automotive sector, but also in all fields of application in which good cold-formability in combination with high steel strengths or fatigue strengths and operating strengths under dynamic stress of welded components is required. Accordingly, the field of application for such components can be e.g. the automotive industry, in particular for chassis components, the construction equipment industry, the household appliance industry or chemical apparatus engineering. In the automotive industry, a use as a chassis component, bumper or cross member is conceivable.

[0119] The advantages of this air-hardening steel in accordance with the invention are listed once again hereinafter: very good cold-formability in the air-hardened state, very good weldability in the soft and air-hardened state, very good HFI weldability, can be coated effectively with the typical coating methods, such as cathodic dip coating (CDC), hot-dip galvanising and high-temperature galvanising, use for welded components subject to high static and dynamic loads, especially in the chassis of vehicles, cost-effective alloy concept.