High strength steel tube and method of manufacturing a high strength steel tube

Abstract

The present invention relates to a high strength steel tube. In addition, the invention relates to a method of manufacturing a high strength steel tube. The method is characterized in that a hot rolled pre-tube is subjected to at least two hardening steps with a final tempering step, the pre-tube is heated to a quenching temperature of at least Ac3 temperature for hardening and is heated to a tempering temperature in the range of 400 to 600° C. for tempering.

Claims

1. A process for producing a high-strength steel tube, characterized in that a hot-rolled pre-tube made of a material having an Ac3 temperature at which the material of the pre-tube has been austenitized is subjected to at least two hardening steps with a final tempering step, the pre-tube is heated to a quenching temperature of at least the Ac3 temperature for hardening and is heated to a tempering temperature in a range of 400 to 600° C. for tempering, wherein the steel tube has a microstructure with a former average austenite grain size (D.sub.avg) of <5 μm.

2. The process according to claim 1, wherein the steel tube has a martensitic structure and a tensile strength of at least 900 MPa.

3. The process according to claim 1, wherein the steel tube is made of an alloy comprising the following alloying elements in Ma-%, in addition to iron and impurities due to melting: C 0.07-0.50 Si 0.01-0.60 Mn 0.3-1.7 Cr max. 1.2 Mo max. 1.2 Ni max. 0.4 Al 0.01-0.10 V max. 0.15 Nb max. 0.06 Ti max. 0.06.

4. The process according to claim 3, wherein manganese is present in an amount in a range of 0.5-1.7 Ma %.

5. The process according to claim 1, wherein the steel tube has a microstructure with an average martensitic parcel size of d.sub.avg<3 μm.

6. The process according to claim 1, wherein the steel tube has a microstructure of stretched tempered martensite.

7. The process according to claim 1, wherein the steel tube has a wall thickness of less than 4 mm.

8. The process according to claim 1, wherein each hardening step comprises heating to a quenching temperature, holding at the quenching temperature, and quenching.

9. The process according to claim 8, wherein quenching to a temperature below a martensite starting temperature (Ms) occurs in each hardening step.

10. The process according to claim 1, wherein two hardening steps are performed followed by a single tempering step.

11. The process according to claim 1, wherein the pre-tube is heated to a temperature greater than Ac3 for hardening.

12. The process according to claim 1, wherein the pre-tube is drawn after the tempering step.

13. The process according to claim 12, wherein the pre-tube is subjected to stress relieving annealing after drawing.

14. The process according to claim 1, wherein the heating is by induction heating.

15. The process according to claim 1, wherein the quenching is at a t8/5 time of less than 4 s.

16. The process according to claim 1, wherein the pre-tube is maintained at the quenching temperature for a period of 1 to 10 seconds prior to quenching.

17. The process according to claim 1, wherein the pre-tube is maintained at the tempering temperature for a period of time greater than 5 seconds.

18. The process according to claim 1, wherein the pre-tube is heated to a temperature of Ac3+50° C. for hardening.

19. The process according to claim 14, wherein the induction heating is at a heating rate greater than 50K/s.

20. The process according to claim 1, wherein the steel tube has a tensile strength of at least 1,050 MPa.

21. The process according to claim 1, wherein manganese is present in an amount in a range of 0.6-1.7 Ma %.

22. The process according to claim 1, wherein the steel tube has a microstructure with a former average austenite grain size (D.sub.avg) of <4.6 μm.

23. The process according to claim 1, wherein the steel tube has a microstructure with a former average austenite grain size (D.sub.avg) of <4.0 μm.

24. The process according to claim 1, wherein the steel tube has a microstructure with a former average austenite grain size (D.sub.avg) of <3.5 μm.

25. A process for producing a high-strength steel tube, characterized in that a hot-rolled pre-tube made of a material having an Ac3 temperature at which the material of the pre-tube has been austenitized is subjected to at least two hardening steps with a final tempering step, the pre-tube is heated to a quenching temperature of at least the Ac3 temperature for hardening and is heated to a tempering temperature in a the range of 400 to 600° C. for tempering, wherein the quenching is at a t8/5 time of less than 4 s.

26. The process according to claim 25, wherein the steel tube has a microstructure with a former average austenite grain size (D.sub.avg) of <5 μm.

27. A process for producing a high-strength steel tube, characterized in that a hot-rolled pre-tube made of a material having an Ac3 temperature at which the material of the pre-tube has been austenitized is subjected to at least two hardening steps with a final tempering step, the pre-tube is heated to a quenching temperature of at least the Ac3 temperature for hardening and is heated to a tempering temperature in a the range of 400 to 600° C. for tempering, wherein the pre-tube is maintained at the quenching temperature for a period of 1 to 10 seconds prior to quenching.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail by the following description of the figures. Showing:

(2) FIG. 1: time-temperature curve of an embodiment of a manufacturing process according to the invention;

(3) FIG. 2: time-temperature curve of a further embodiment of a manufacturing process according to the invention;

(4) FIG. 3: a schematic representation of former austenitic grain size distribution of steel tube embodiments of the invention; and

(5) FIG. 4: schematic representation of a martensitic structure with former austenite grain boundaries and with martensitic package boundaries.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) FIG. 1 schematically shows the time-temperature curve of an embodiment of a manufacturing process according to the invention. As can be seen from FIG. 1, in a first hardening step, the hot rolled pre-tube is heated to a quenching temperature greater than 900° C. The heating is carried out at a heating rate greater than 50 Kelvin/second (K/s), for example greater than 70 K/s. The pre-tube is then held at the quenching temperature for a period of 1-10 seconds, for example 4-5 seconds. Thereafter, the pre-tube is quenched to a temperature below the Ms temperature at a cooling rate t8/5 of less than 4 Kelvin/seconds. This hardening step is then performed again. After the second hardening step, the pre-tube is heated to a tempering temperature above 400° C., in particular 400 to 500° C. The heating is carried out in particular at a heating rate of more than 15K/s. After a holding time of more than 5 seconds, the pre-tube is cooled.

(7) FIG. 2 schematically shows the time-temperature curve of a further embodiment of a manufacturing process according to the invention. In this embodiment, following the tempering step, the pre-tube is formed by drawing, in particular cold drawing, and then subjected to stress-relief annealing. The stress-relief annealing is carried out at a temperature of more than 420° C. and the pre-tube is preferably kept at this temperature for longer than 500 s.

(8) FIG. 3 shows a schematic diagram of the former austenitic grain size distribution according to a steel pipe according to embodiments of the invention (DQ&T). As can be seen from this diagram, the grain size according to the embodiments of the invention, which has been subjected to double hardening and tempering, is predominantly around 3.0 μm. The grain sizes were measured from longitudinal sections at a surface section of 27950 μm.sup.2 of the steel tubes.

(9) The measurements were carried out on hollows with an outer diameter of 30 mm and a wall thickness of 2.3 mm (30×2.3).

(10) The process according to the invention is also advantageous compared to processes in which cold-drawn tubes are hardened (Final-QT) and processes in which a simply once quenched and tempered steel tube is cold-drawn and stress-relieved (QT+SR). Compared with the finished tubes produced by means of the final-QT process, the probability of the small former austenite grain size obtained according to the invention is higher. Compared to the finished tubes produced by means of the QT+SR process, the former austenite grain size of the tubes produced according to the invention is substantially smaller.

(11) FIG. 4 schematically shows a martensitic microstructure. In particular, the former austenite grain sizes and the martensitic package boundaries are shown. The average martensite package size is denoted by d.sub.avg and the average former austenite grain size by D.sub.avg.

(12) The present invention thus relates to a method of double quenching and tempering (DQ&T) of high strength hot rolled steel tubes. Preferably, the heating to the quenching temperature is performed by induction heating. The present invention produces very fine-grained microstructures. In particular, the former austenite grain size is smaller compared to conventional steel tubes produced by means of hardening and tempering (Q&T). With the process according to the invention, a significant grain refinement is obtained with an average austenite grain size (D.sub.avg) of 4.6 μm compared to QT tubes with D.sub.avg equal to 7.8 μm.

(13) The grain refinement in quenched and tempered microstructure and the small martensitic package size leads to an increase in the yield strength of the materials according to the Hall-Petch relationship and also in the toughness of the material by increasing the breaking strength and stopping the crack propagation through the grain boundaries. These properties are also obtained in the process DQ&T according to the invention.

(14) With the method according to the invention after cold drawing, d.sub.avg<3 μm and D.sub.avg<5 μm can be achieved.

(15) The present invention has a number of advantages. In particular, high toughness (especially at low temperatures) can be achieved while maintaining high yield strength/strength. Thus, secure high-strength components, for example secure airbag tubes, and the products with quenched and tempered microstructure are created. In addition, fine-grained steels with the best surface qualities are created. The advantages can also be achieved on tubes with larger dimensions, for example AD>30 mm and WD>2 mm, which can be used for example in the airbag sector. The invention is not limited to seamless steel tubes, but may also concern welded steel tubes.