STEEL MATERIAL FOR A TORSIONALLY STRESSED COMPONENT, METHOD FOR PRODUCING A TORSIONALLY STRESSED COMPONENT FROM SAID STEEL MATERIAL, AND COMPONENT MADE THEREOF

Abstract

A steel material for a torsionally stressed component, such as a driveshaft, having a minimum tensile strength of 800 MPs, and the microstructure consists of more than 50 vol. % of bainite, having an alloy with the following composition in wt. %: C: 0.02 to 0.3; Si: up to 0.7; Mn: 1.0 to 3.0; P: max. 0.02; S: max. 0.01; N: max. 0.01; Al: up to 0.1; Cu: up to 0.2; Cr: up to 3.0; Ni: up to 0.3; Mo: up to 0.5; Ti: up to 0.2; V: up to 0.2; Nb: up to 0.1; B: up to 0.01; where 0.02≤Nb+V+Ti≤0.25, residual iron, and smelting impurities. The steel material is inexpensive and has good torsional fatigue strength when used for a torsionally stressed component. The invention also relates to a method for producing a component made of the material and to such a component.

Claims

1. A steel material for a torsionally stressed component, in which the steel material has a minimum tensile strength of 800 MPa and the microstructure consists of more than 50 vol. % bainite, having an alloy with the following composition in wt. %: C: 0.02 to 0.3; Si: up to 0.7; Mn: 1.0 to 3.0; P: max. 0.02; S: max. 0.01; N: max. 0.01; Al: up to 0.1; Cu: up to 0.2; Cr: up to 1.0; Ni: up to 0.3; Mo: up to 0.5; Ti: up to 0.2; V: up to 0.2; Nb: up to 0.1; and B: up to 0.01; wherein 0.02≤Nb+V+Ti≤0.25 is met, with the remainder being iron and melting-induced impurities.

2. The steel material as claimed in claim 1, wherein the microstructure consists of at least 90 vol. % bainite and the proportions of residual austenite and martensite and ferrite are <10 vol. %.

3. The steel material as claimed in claim 2, wherein with respect to one or more of the following elements the composition is in wt. %: C: 0.02 to 0.11; and/or Si: 0.01 to 0.5; and/or Mn: 1.4 to 2.2; and/or Al: 0.015 to 0.1; and/or Cr: up to 0.3; and/or Ni: up to 0.2; and/or Mo: 0.05 to 0.5; and/or B: max. 0.004; and/or wherein 0.05≤Nb+V+Ti≤0.2.

4. The steel material as claimed in claim 3, wherein with respect to the following elements the composition is in wt. %, as follows: C: 0.05 to 0.11; and/or Si: 0.1 to 0.5; and/or Mn: 1.5 to 2.0; and/or N: 0.003 to 0.01; and/or Al: 0.03 to 0.1; and/or Ni: up to 0.15; and/or Mo: 0.1 to 0.3; and/or Ti: 0.04 to 0.2.

5. A method for producing a tubular component that is configured to be torsionally stressed, wherein the tubular component is produced from a seamless or welded pre-tube consisting of a steel material as claimed in claim 1, and wherein the pre-tube has an enlarged diameter and greater wall thickness in comparison with the required final dimension of the component, said method comprising: annealing the pre-tube in a furnace in a temperature range of 650 to 850° C. with a furnace dwell time of 5 to 30 min followed by cooling to room temperature; drawing the pre-tube with at least one drawing procedure to the required final dimension in which the wall thickness of the pre-tube is reduced by a greater percentage than the outer diameter of the pre-tube; and optionally adjusting the tubular component to the required length of the component.

6. The method as claimed in claim 5, wherein the pre-tube is annealed to a temperature in the range of 700 to 800° C.

7. The method as claimed in claim 6, wherein the pre-tube is annealed to a temperature in the range of 720 to 780° C.

8. The method as claimed in claim 5, wherein during said drawing the pre-tube the percentage ratio of the decrease in the thickness of the pre-tube to the reduction in the outer diameter of the pre-tube is greater than 2:1.

9. The method as claimed in claim 5, wherein the pre-tube produced without seams is hot-rolled and the welded tube is produced from hot or cold strip.

10. A tubular stressed component produced from a steel material as claimed in claim 1.

11. Use of a steel material as claimed in claim 1 for producing a tube, wherein the tube comprises a torsionally stressed component comprising a drive shaft.

12. The tubular component as claimed in claim 10, wherein the tubular component comprises a tubular drive shaft.

13. The method as claimed in claim 8, wherein during said drawing the pre-tube the percentage ratio of the decrease in the thickness of the pre-tube to the reduction in the outer diameter of the pre-tube is greater than 5:1.

14. The steel material as claimed in claim 1, wherein the microstructure consists of at least 70 vol. % bainite and the proportions of residual austenite and martensite and ferrite are <30 vol. %.

15. The steel material as claimed in claim 14, wherein with respect to one or more of the following elements the composition is in wt. %: C: 0.02 to 0.11; and/or Si: 0.01 to 0.5; and/or Mn: 1.4 to 2.2; and/or Al: 0.015 to 0.1; and/or Cr: up to 0.3; and/or Ni: up to 0.2; and/or Mo: 0.05 to 0.5; and/or B: max. 0.004; and/or wherein 0.05≤Nb+V+Ti≤0.2.

16. The steel material as claimed in claim 15, wherein with respect to the following elements the composition is in wt. %, as follows: C: 0.05 to 0.11; and/or Si: 0.1 to 0.5; and/or Mn: 1.5 to 2.0; and/or N: 0.003 to 0.01; and/or Al: 0.03 to 0.1; and/or Ni: up to 0.15; and/or Mo: 0.1 to 0.3; and/or Ti: 0.04 to 0.2.

17. The steel material as claimed in claim 1, wherein with respect to one or more of the following elements the composition is in wt. %: C: 0.02 to 0.11; and/or Si: 0.01 to 0.5; and/or Mn: 1.4 to 2.2; and/or Al: 0.015 to 0.1; and/or Cr: up to 0.3; and/or Ni: up to 0.2; and/or Mo: 0.05 to 0.5; and/or B: max. 0.004; and/or wherein 0.05≤Nb+V+Ti≤0.2.

18. The steel material as claimed in claim 17, wherein with respect to the following elements the composition is in wt. %, as follows: C: 0.05 to 0.11; and/or Si: 0.1 to 0.5; and/or Mn: 1.5 to 2.0; and/or N: 0.003 to 0.01; and/or Al: 0.03 to 0.1; and/or Ni: up to 0.15; and/or Mo: 0.1 to 0.3; and/or Ti: 0.04 to 0.2.

19. The steel material as claimed in claim 17, wherein the microstructure consists of at least 70 vol. % bainite and the proportions of residual austenite and martensite and ferrite are <30 vol. %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a graph of tensile tests on samples according to Table 2 of the specification; and

[0043] FIG. 2 is a graph of the results of torsion tests.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Comparative tests on the mechanical properties were carried out on steels with the alloy compositions specified in the following Table 1, divided for reasons of space, in wt. %. A first alloy composition in accordance with the invention, called KSG 1000, is indicated with a conventional comparative alloy KSG 800. The remainder of iron and smelting-induced impurities are present as a matter of course but are not specifically listed.

TABLE-US-00001 Material C Si Mn P S N Al B Invention 0.08 0.46 1.9 0.009 0.0010 0.0080 0.061 — (KSG 1000) KSG 800 0.22-0.29 0.15-0.3 1.2-1.5 max. max. 0.02-0.08 max. 0.035 0.035 0.004 Nb + Material Cu Cr Mo Ni Nb Ti V V + Ti Invention 0.020 0.03 0.14 0.04 0.04 0.14 0.01 0.19 (KSG 1000) KSG 800 max. 0.4 max. 0.05

[0045] For the investigations into the influence of the various possible final machining states of the component, in particular the drive shaft, hot strip with the alloy composition in accordance with the invention as shown in Table 1 was investigated as the primary material for the tubes. Pre-tubes (hollows) with the dimension (outer diameter×wall thickness in mm) 60×2 in the states “CR1” and “A” according to DIN 10305 part 2 and part 3 were manufactured from this primary material. In addition to the pre-tube produced in this way, the hollows were annealed in accordance with the invention and then cold-drawn with different diameter and wall thickness decreases. In addition, the influence of the bend-straightening process during adjustment was examined.

[0046] The results for the mechanical properties are shown in Table 2 below. All values correspond to the specifications for the mechanical properties to be achieved.

TABLE-US-00002 Tensile Yield strength strength Elongation at Testing state R.sub.m [MPa] R.sub.p0.2 [MPa] fracture A [%] Strip, transverse 824 764 18.5 Strip, longitudinal 800 701 21.5 Hollow + CR1 837 770 14.5 Hollow + A 871 608 19.5 Tube 50 × 1.7 1066 1034 9.5 Tube 57 × 1.5 1080 1033 7.8

[0047] FIG. 1 shows the results of the tensile tests on the samples according to Table 2. As expected, particularly high strengths can be achieved with cold-drawn tubes, wherein the adjusting bend-straightening process does not exert any significant influence.

[0048] The results for the torsion tests are illustrated in FIG. 2. Excellent results for fatigue strength are achieved, in particular on 57 mm×1.5 mm cold-drawn tubes. In the case of these tubes, the percentage decrease in the wall thickness of the pre-tube of 25% is higher than the percentage decrease in the diameter of the pre-tube (5%). In contrast thereto, in the case of the 50 mm×1.7 mm tube, the percentage decrease in the wall thickness of the pre-tube is ca. 12% and the percentage decrease in the diameter of the pre-tube is ca. 17%.

[0049] A comparison of the results for cold-drawn components, in particular drive shafts, produced from a standard grade KSG 800 and the steel KSG 1000 in accordance with the invention is shown in Table 3 below.

TABLE-US-00003 KSG800 KSG1000 Yield strength min. 700 MPa min. 900 MPa Tensile strength min. 800 MPa min. 1000 MPa Elongation (axial) min. 8% min. 10% Microstructure Ferrite-pearlite Bainite Application example: Dimension OD 60 MM × WT 1.6 mm OD 57 × 1.5 mm Torque/200,000 LC 1400 Nm 1400 Nm Torque/cross-section 477 × 10.sup.3 N/m 540 × 10.sup.3 N/m Weight/metres 2.30 kg/m 2.03 kg/m Mass moment of 1964 kgm 1585 kgm inertia/m

[0050] The required 200,000 load cycles at a torque of 1400 Nm are achieved with a tube having an outer diameter of 60 mm and a wall thickness of 1.6 mm in standard quality and with a 57 mm×1.5 mm tube consisting of the bainitic steel in accordance with the invention. The resulting advantages for the use of the steel in accordance with the invention, particularly with regard to the weight reduction per metre of tube length and the mass moment of inertia per metre of tube length, are significant.

[0051] In addition to the purely mechanical-technological properties, notches have the impact of reducing the service life of components subjected to fatigue stress, in particular torsionally stressed components, such as drive shafts. A distinction must be made between external notches (scratches, grooves on the surface) and internal notches (defects, inclusions, phase boundaries between the same and different phases); the inherent stress condition present in the component also has an influence on the load cycles of the component to be achieved, in addition to the external operating loads of the component.

[0052] The outer notches can be reduced by the manufacturing process of the component. The density and size of the internal notches are influenced by the production process of the steel material. The reduction in density and size of defects and inclusions to improve the quality of steel materials is continuously pursued in the steel works. However, the density and type of phase boundaries are dependent upon the set microstructure. In this case, the bainitic microstructure proves to be advantageous compared to classical multiphase microstructures. The reason is that microstructure components of bainite are generally comparatively small and the differences in hardness between the components are comparatively small. As a result, for a given density of phase boundaries, the stress concentration at the phase transitions is lower compared to a classical multiphase microstructure (e.g. dual phase microstructure with ferrite and martensite). A lower stress concentration is to be equated to a lower notch effect. Ideally, a completely bainitic microstructure is formed, which is also retained during further tube production. Purely bainitic microstructures contain less inherent stress than materials with martensitic microstructures; they allow the achievement of very high strength combined with high elongation and toughness. High toughness prevents, in turn, rapid crack growth under recurring loads. In addition to the chemical composition of the steel, a high adjusted bainite proportion is therefore of great importance for achieving the previously described properties of the components, such as e.g. the drive shaft.