High-strength steel pipe and method for manufacturing high-strength steel pipe

Abstract

This invention relates to a high-strength steel pipe. It also relates to a method for manufacturing the high-strength steel pipe. The method is characterized by subjecting a hot-rolled billet pipe to at least two hardening steps and a final tempering step, heating the billet pipe to a quenching temperature of at least Ac3 for hardening, and heating it to a tempering temperature in the range of 400°C to 600°C for tempering.

Description

Technical Field

[0001] This invention relates to a high-strength steel pipe and a method for manufacturing a high-strength steel pipe. Background Technology

[0002] Many applications require tubes, or tube products made from them, to be high in strength while still maintaining sufficient toughness. These requirements are particularly stringent in applications such as inflatable tubes.

[0003] In the manufacture of steel pipes, it is known that the blank pipe, especially the hot-rolled blank pipe (also known as the tube blank), undergoes heat treatment. In particular, it is known that the tube blank undergoes a hardening step followed by a tempering step. A disadvantage of this heat treatment is that the austenite grain size and average martensite bulk size present before quenching are very large. Therefore, the yield strength of the steel pipe is very small, which is detrimental to the corresponding applications. Summary of the Invention

[0004] Therefore, the objective of this invention is to provide a steel pipe that meets the requirements of the corresponding application.

[0005] According to one aspect, the task is solved by a method for manufacturing high-strength steel pipes. The method is characterized by subjecting a hot-rolled billet pipe to at least two hardening steps and a final tempering step, heating the billet pipe to a quenching temperature above the Ac3 temperature for hardening, and heating it to a tempering temperature in the range of 400°C to 600°C for tempering.

[0006] High-strength steel pipe refers to steel pipe with a tensile strength Rm of at least 900 MPa, preferably at least 1050 MPa. The hot-rolled blank used to manufacture steel pipe is also called a tube blank. In order to manufacture steel pipe from the blank, it is preferable to at least heat-treat the blank.

[0007] According to one embodiment, the heat treatment includes at least two hardening steps and a final tempering step. The hardening process, which involves heating or preheating to the quenching temperature, maintaining at the quenching temperature, and quenching, is referred to as the hardening step. Preferably, the quenching is performed at a temperature below the martensite initiation temperature (Ms).

[0008] In one embodiment, the final hardening step is a tempering step. In the tempering step, the quenched blank tube is heated to the tempering temperature, held at the tempering temperature, and cooled from the tempering temperature.

[0009] Preferably, in one embodiment of the method, two hardening steps are performed. The two hardening steps, along with the single tempering step immediately following the second hardening step, are also referred to below as double quenching and tempering or double quenching and tempering (DQ&T).

[0010] Preferably, each hardening step includes heating to the quenching temperature, holding at the quenching temperature, and quenching. The quenching temperature is preferably higher than the Ac3 temperature, thereby austenitizing the steel by heating and holding at the quenching temperature. Quenching is performed after each austenitization.

[0011] Cooling at a high cooling rate is called quenching. Therefore, quenching is distinct from cooling processes such as air cooling. Furthermore, in the quenching according to the invention, the blank tube is preferably quenched such that the entire wall thickness of the blank tube is reduced to a desired temperature. The desired temperature is, in this case, the temperature at which austenite transforms into martensite. This can be, in particular, the so-called martensitic initiation temperature Ms, but is preferably the martensitic termination temperature of the steel alloy, at which most or all of the austenite is transformed into martensite. Here, it is, for example, a temperature below 300°C. However, the blank tube can also be quenched to approximately room temperature or, after quenching, can be slowly further cooled for the martensitic transformation.

[0012] Furthermore, according to the invention, at least two hardening steps are performed after hot rolling. No further deformation of the blank tube occurs during the hardening steps themselves.

[0013] According to one embodiment, in the hardening step, i.e., for hardening, the blank tube is heated or preheated to a quenching temperature greater than Ac3. Ac3 temperature represents a temperature at which the material, i.e., the steel of the blank tube, is austenitized.

[0014] In the tempering step, the blank tube is heated to a tempering temperature, preferably in the range of 400°C to 600°C. More preferably, the tempering temperature is in the range of 400°C to 500°C. Particularly preferably, the tempering temperature is above 400°C within the said range.

[0015] According to a preferred embodiment, the microstructure of very fine grains in the blank tube is adjusted by subjecting the blank tube to multiple, especially two, quenching and tempering processes (DQ&T). Specifically, a significantly higher grain refinement is achieved, meaning a smaller grain size compared to a blank tube subjected to only one hardening and subsequent tempering. In particular, a significantly smaller average austenite grain size is achieved compared to the austenite grains present before quenching. This austenite grain size is also referred to as the initial or prior average austenite grain size (DQ&T). avgFurthermore, it increases the yield strength and toughness of the steel pipe material. This is primarily due to the increase in tensile strength and the cessation of crack propagation through grain boundaries, especially through a smaller martensite envelope size (d). avg )produce.

[0016] According to one embodiment, the blank tube is heated to a temperature of Ac3+50°C for hardening. This ensures that the material of the blank tube is fully austenitized.

[0017] According to a preferred embodiment, the blank tube is drawn after the tempering step. In particular, the blank tube is subjected to at least one cold drawing step after the tempering step. This embodiment can produce steel tubes with small wall thickness and, despite this, properties tunable by heat treatment, particularly high tensile strength, high yield strength, and simultaneously high toughness, especially cold toughness. In this embodiment, the steel tubes produced by this method can be used, in particular, as airbag tubes.

[0018] According to one embodiment, stress-relief annealing is performed in the blank tube that has been cold-drawn after tempering.

[0019] According to a preferred embodiment, heating is performed by induction heating. Induction heating enables a high heating rate.

[0020] According to a preferred embodiment, the material is heated to the quenching temperature at a heating rate greater than 50 K / s, preferably greater than 70 K / s, for example 200 K / s.

[0021] According to a preferred embodiment, quenching is performed in a t8 / 5 time period of less than 4 seconds. The time period required to cool from 800°C to 500°C is referred to as the t8 / 5 time period.

[0022] According to one implementation, the blank tube is held at the quenching temperature for a period of 1 to 10 seconds, for example 3 to 6 seconds, before quenching.

[0023] According to one implementation, the blank tube is held at the tempering temperature for a period of more than 5 seconds.

[0024] According to another aspect, the task is accomplished by a high-strength airbag tube, characterized in that the steel tube has a prior average austenite grain size (D0) of <5 μm. avg It has a martensitic microstructure and a tensile strength of at least 900 MPa, preferably at least 1050 MPa, and a transformation temperature of at most -60°C. The airbag tube is hereby also referred to as a steel tube.

[0025] Preferably, the previous average austenite grain size (D avg<4.6μm, especially <4.0μm, and particularly preferably <3.5μm.

[0026] The transition temperature, also known as the ductile-brittle transition temperature (DBTT), defines the temperature at which ductile properties transition from a high-energy layer (which can be simply referred to as the high layer) to a low-energy layer (which can be simply referred to as the low layer). When cooled below the transition temperature, notched impact toughness decreases drastically, and brittle fracture occurs. The transition temperature can be determined in a ring Charpy test, in which a ring-shaped notch is cut from a manufactured gas generator tube, and then tested in a pendulum impact apparatus. In particular, the steel tube also exhibits ductile properties up to -60°C. The Charpy impact strength is preferably measured according to JIS Z 2242, the standard of the Japanese Standards Association (JSA), corresponding to ISO 179.

[0027] According to another aspect, the task is solved by a high-strength steel pipe, characterized in that the steel pipe is manufactured according to the method of the invention. In one embodiment, the steel pipe manufactured according to the method of the invention has a previous average austenite grain size (D0) of <5 μm. avg The steel pipe has a microstructure and a tensile strength of at least 900 MPa and a maximum transition temperature of -60°C. The preferred steel pipe is an airbag tube.

[0028] The advantages and features described with respect to this method (where applicable) also apply to the steel pipes according to the invention and vice versa. Furthermore, the advantages and features described with respect to the steel pipes according to the invention (where applicable) also apply to steel pipes manufactured according to the method according to the invention and vice versa.

[0029] According to a preferred embodiment, the steel pipe has a martensitic structure.

[0030] According to one embodiment, the steel pipe is made of an alloy that, apart from iron and impurities caused by melting, contains the following alloying elements in mass percent:

[0031] C 0.07 to 0.50

[0032] Si 0.01 to 0.60

[0033] Mn 0.3 to 1.7

[0034] Cr maximum 1.2

[0035] Momax 1.2

[0036] Ni maximum 0.4

[0037] Al 0.01 to 0.10

[0038] V maximum 0.15

[0039] Nb maximum 0.06

[0040] Ti maximum 0.06.

[0041] Alloys are also referred to below as steel alloys, steel, or materials. The content data of alloying elements are given as a percentage by mass, but only as a percentage when necessary.

[0042] Carbon (C) is preferably present in an amount ranging from 0.07 to 0.50% by mass. Carbon increases the strength of steel alloys.

[0043] Silicon (Si) is preferably present in an amount ranging from 0.01 to 0.60% by mass, more preferably from 0.01 to 0.50% by mass. Silicon increases tensile strength and yield strength.

[0044] Manganese (Mn) is preferably present in an amount ranging from 0.3 to 1.7% by mass. Manganese improves the yield strength and overall strength of steel alloys. Additionally, manganese improves weldability as a carbon substitute. According to a preferred embodiment, manganese is present in an amount ranging from 0.5 to 1.7% by mass, particularly preferably from 0.6 to 1.7% by mass.

[0045] Chromium (Cr) is preferably present in a maximum amount of 1.2% by mass. Chromium improves the toughness and tensile strength of steel alloys.

[0046] Molybdenum (Mo) is preferably present in a maximum amount of 1.2% by mass. Molybdenum, in particular, improves the tensile strength and weldability of steel alloys.

[0047] Nickel (Ni) is preferably present in a maximum amount of 0.4% by mass. Nickel improves tensile strength and yield strength.

[0048] Aluminum (Al) is preferably present in an amount ranging from 0.01 to 0.10% by mass.

[0049] Vanadium (V) is preferably present in a maximum amount of 0.15% by mass. Vanadium increases the tensile strength of the alloy.

[0050] Niobium (Nb) is preferably present in a maximum amount of 0.06% by mass.

[0051] Titanium (Ti) is preferably present in a maximum amount of 0.06% by mass.

[0052] Impurities that are caused by melting or smelting, especially during steel manufacturing, particularly through materials added during the manufacture and processing of the melt, are impurities that reach the steel alloy.

[0053] In a variant of the implementation, the steel pipe according to the invention is made of a steel alloy, which is particularly suitable for use in the method according to the invention, the steel alloy being composed of the following alloying elements, which, except for iron and impurities caused by melting, are present in the steel alloy in a mass percentage:

[0054] C 0.08 to 0.15

[0055] Si 0.01 to 0.60

[0056] Mn 1.0 to 1.7

[0057] Cr maximum 1.0; preferably 0.2 to 0.9.

[0058] Momax 0.2

[0059] Ni maximum 0.4; preferably 0.15 to 0.4.

[0060] Al 0.01 to 0.10

[0061] V maximum 0.15

[0062] Nb maximum 0.06

[0063] Ti maximum 0.06.

[0064] The steel pipe is particularly the airbag tube. According to a preferred embodiment, the steel pipe has an average initial austenite grain size (D0) of <5 micrometers (μm), preferably 4.6 μm, especially <4.0 μm, and particularly preferably <3.5 μm. avg ) organizational structure.

[0065] According to a preferred embodiment, the steel pipe, especially the airbag tube, has a d avg Microstructure with an average martensite packing size of <3 μm.

[0066] According to one embodiment, the steel pipe, especially the airbag tube, has a microstructure composed of extended, tempered martensite. This microstructure is preferably achieved, particularly through heat treatment and subsequent cold drawing.

[0067] Preferably, the airbag tube has a transition temperature of less than -60°C. The transition temperature is preferably determined by the aforementioned annular Charpy test. For example, the transition temperature is determined here by sampling from a corresponding longitudinal section of a narrow tube section that has a target notch introduced for testing purposes, acting as an annular notch impact sample. The axial impact sample is subjected to cooling to the low temperature conditions of the test. The low temperature at which the sample transitions from ductile fracture characteristics to brittle fracture characteristics is called the transition temperature. It is understood that, for its determination, multiple samples characterizing the longitudinal section must be tested.

[0068] According to one embodiment, the steel pipe, especially the airbag tube, has a wall thickness of less than 4 millimeters (mm). Steel pipes with this wall thickness are particularly suitable for use as airbag tubes.

[0069] Preferably, the steel pipe has a tolerance zone for the outer diameter (AD) and / or inner diameter (ID) that is reduced relative to the final quenched and tempered steel pipe, in particular half of the “value” of standard EN10305-1.

[0070] Preferably, the steel pipe is free of oxide scale (zunderfrei) compared to the final tempered and untreated steel pipe.

[0071] In the airbag tube, the stretching process, especially the cold drawing step, is preferably performed during manufacturing after heat treatment. Particularly preferred is stress-relief annealing after stretching. Attached Figure Description

[0072] The invention will be described in detail below with reference to the accompanying drawings. In the drawings:

[0073] Figure 1 A time-temperature variation curve is shown for one embodiment of the manufacturing method according to the present invention;

[0074] Figure 2 A time-temperature variation curve is shown for another embodiment of the manufacturing method according to the present invention;

[0075] Figure 3 A schematic diagram illustrating the prior austenite grain size distribution of a steel pipe according to an embodiment of the present invention; and

[0076] Figure 4 A schematic diagram of a martensitic microstructure with prior austenitic grain boundaries and martensitic enclosure boundaries is shown. Detailed Implementation

[0077] exist Figure 1 The diagram schematically illustrates a time-temperature variation curve according to one embodiment of the manufacturing method of the present invention. (As shown by...) Figure 1It is found that the hot-rolled billet tube is heated to a quenching temperature greater than 900°C in the first hardening step. This heating is carried out at a heating rate greater than 50 Kelvin / second (K / s), for example, greater than 70 K / s. The billet tube is then held at the quenching temperature for 1 to 10 seconds, for example, 4 to 5 seconds. Afterward, the billet tube is quenched to a temperature below the martensitic initiation temperature at a cooling rate of less than 4 Kelvin / second (t8 / 5). The hardening step is then performed again. Following the second hardening step, the billet tube is heated to a tempering temperature greater than 400°C, particularly 400°C to 500°C. This heating is particularly carried out at a heating rate greater than 15 K / s. The billet tube is cooled after a holding time of more than 5 seconds.

[0078] exist Figure 2 The diagram schematically illustrates a time-temperature curve according to another embodiment of the manufacturing method of the invention. In this embodiment, immediately following the tempering step, the blank tube is formed by stretching, particularly cold drawing, and subsequently subjected to stress-relief annealing. Stress-relief annealing is performed at a temperature greater than 420°C, and preferably the blank tube is held at this temperature for more than 500 seconds.

[0079] exist Figure 3 The diagram schematically illustrates the prior austenite grain size distribution of a steel tube (DQT) according to an embodiment of the invention. As can be seen from the diagram, according to the embodiment of the invention, the grain size after double hardening and tempering is primarily about 3.0 μm. The grain size was measured by longitudinal grinding (Längschliffen) at a surface cross-section of 27950 μm² for the steel tube.

[0080] Measurements were performed on a tube blank with an outer diameter of 30 mm and a wall thickness of 2.3 mm (30 × 2.3).

[0081] The method according to the invention is also advantageous compared to methods that harden cold-drawn tubes (Final-QT) and methods that involve cold-drawing and stress-relieving annealing of steel tubes after simple quenching and tempering (QT+SR). The possibility of achieving a smaller prior austenite grain size is higher in the tubes manufactured according to the invention compared to finished tubes produced using the Final-QT method. The prior austenite grain size in the tubes manufactured according to the invention is significantly smaller compared to finished tubes produced using the QT+SR method.

[0082] Figure 4 The martensitic microstructure is schematically illustrated. In particular, the preceding austenite grain size and martensite inclusion boundaries are shown. The average martensite inclusion size is represented by d. avg The average previous austenite grain size is represented by D. avg express.

[0083] Therefore, this invention relates to a method for double quenching and tempering (DQ&T) of high-strength hot-rolled steel pipes. Preferably, heating to the quenching temperature is performed using induction heating. This invention produces a very fine-grained microstructure. In particular, the austenite grain size is smaller compared to steel pipes conventionally manufactured using quenching and tempering (Q&T). Compared to having D... avg For a 7.8 μm QT transistor, an average austenite grain size of 4.6 μm was achieved using the method according to the present invention. avg Significant grain refinement.

[0084] The grain refinement and small martensite size in the tempered microstructure lead to an increase in the yield strength and toughness of the material according to the Hall-Petch-Beziehung relation, by increasing tensile strength and preventing crack propagation through grain boundaries. These properties are also achieved in the DQ&T method according to the invention.

[0085] The method according to the present invention can achieve d after cold drawing. avg <3μm and D avg <5μm.

[0086] This invention offers a number of advantages. In particular, it achieves high toughness (especially at low temperatures) while simultaneously possessing high yield strength / strength. Therefore, it provides safe, high-strength components (e.g., safe airbag tubes) and products with a tempered microstructure. Furthermore, it provides steel with fine grains and optimal surface quality. These advantages can also be achieved with tubes having larger dimensions (e.g., AD > 30 mm and WD > 2 mm), which can be used, for example, in airbag regions. This invention is not limited to seamless steel tubes but can also relate to welded steel tubes.

Claims

1. A high-strength airbag tube, characterized by, The airbag tube: It has a wall thickness of less than 4 mm. With an initial average austenite grain size D avg Martensitic microstructure with a microstructure of <5 μm, wherein the martensitic microstructure is elongated tempered martensite. It possesses a tensile strength of at least 900 MPa and a maximum ductile-brittle transition temperature of -60°C, and Made of an alloy, which, in mass percent, contains the following alloying elements, excluding iron and impurities caused by melting: C 0.07 to 0.50 Si 0.01 to 0.60 Mn 0.5 to 1.7 Cr maximum 1.2 Momax 1.2 Ni maximum 0.4 Al 0.01 to 0.10 V maximum 0.15 Nb maximum 0.06 Ti maximum 0.

06.

2. The high-strength air bag tube according to claim 1, wherein, Manganese is present in amounts ranging from 0.6 to 1.7% by mass.

3. The high-strength air bag tube according to claim 1 or 2, wherein, the initial average austenite grain size D avg <4.6 μm.

4. The high-strength air bag tube according to claim 1 or 2, wherein, the initial average austenite grain size D avg < 4.0 μm.

5. The high-strength air bag tube according to claim 1 or 2, wherein, the initial average austenite grain size D avg < 3.5 μm.

6. The high-strength air bag tube according to claim 1 or 2, wherein the average size d of the martensite packets of the tempered martensite avg < 3 μm.

7. The high-strength air bag tube according to claim 1 or 2, wherein, The airbag tube has a reduced tolerance zone for the outer diameter AD and / or inner diameter ID, which is half the value of standard EN10305-1.

8. A method for producing the high-strength airbag tube according to any one of claims 1 to 7, characterized by, The hot-rolled blank tube is subjected to at least two hardening steps and a final tempering step. The hot-rolled blank tube is heated to a quenching temperature of at least Ac3 for hardening, and then heated to a tempering temperature in the range of 400°C to 600°C for tempering. The blank tube is held at the tempering temperature for a period of more than 5 seconds and then stretched after the tempering step.

9. The method of claim 8, wherein, Each hardening step includes heating to a quenching temperature of at least Ac3, holding at the quenching temperature, and quenching.

10. The method of claim 9, wherein, In each hardening step, the material is quenched to a temperature below Ms, the temperature at which martensite begins to transform.

11. The method of any one of claims 8-10, wherein, The tempering step is performed following the two hardening steps.

12. The method of any one of claims 8-10, wherein, The blank tube is heated to a temperature greater than Ac3 for hardening.

13. The method of any one of claims 8-10, wherein, The blank tube is heated to a temperature of Ac3+50℃ for hardening.

14. The method of any one of claims 8-10, wherein, The blank tube is subjected to stress-relief annealing after stretching.

15. The method of any one of claims 8 to 10, wherein, Heating is achieved through induction heating.

16. The method of any one of claims 8 to 10, wherein, Heating is performed by induction heating at a heating rate greater than 50 K / s.

17. The method of any one of claims 8 to 10, wherein, Quenching is performed in a time t8 / 5 of less than 4 seconds, where t8 / 5 is the time required to cool from 800°C to 500°C.

18. The method of any one of claims 8 to 10, wherein, The blank tube is held at the quenching temperature for 1 to 10 seconds before quenching.

19. A high-strength air bag tube characterized by The airbag tube is made by the method according to any one of claims 8 to 18.

20. The high-strength balloon tube of claim 19, wherein, The airbag tube has a martensitic structure and a tensile strength of at least 1050 MPa.

21. The high-strength balloon tube of claim 19, wherein, The high-strength airbag tube has an initial average austenite grain size D avg a microstructure of < 5 μm.

22. The high-strength air bag tube of claim 21 wherein, the initial average austenite grain size D avg <4.6 μm.

23. The high-strength balloon tube of claim 21, wherein, the initial average austenite grain size D avg < 4.0 μm.

24. The high-strength balloon tube of claim 21, wherein, the initial average austenite grain size D avg < 3.5 μm.

25. The high-strength air bag tube according to any one of claims 19 to 24, wherein, the average size d of the martensite packets of the tempered martensite avg < 3 μm.

26. The high-strength airbag tube according to any one of claims 19 to 24, wherein, The airbag tube has a reduced tolerance zone for the outer diameter AD and / or inner diameter ID, which is half the value of standard EN10305-1.

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