Method of Manufacturing High Strength Steel Tubing from a Steel Composition and Components Thereof

Abstract

A method of manufacturing tubing from a well-defined steel composition. in particular fat a suited gas inflator pressure vessel comprises the steps: a) producing a steel tubing from a steel composition including at least one hot rolling or hot forming pass: b) subjecting the steel tubing to a cold-drawing process to obtain desired dimensions. wherein the cold-drawing process comprises at least too pulls and before the first pull of the cold-drawn tug process an intermediate austenizing and quenching step: c) subsequently performing a final recovery heat treatment on the cold-drawn steel tubing at a temperature in the range of 200-600° C.

Claims

1. A method of manufacturing tubing from a steel composition, in particular for a stored gas inflator pressure vessel, comprising the steps: a) producing a steel tubing from a steel composition including at least one hot rolling or hot forming pass; b) subjecting the steel tubing to a cold-drawing process to obtain desired dimensions, wherein the cold-drawing process comprises at least two pulls and before the final pull of the cold-drawing process an intermediate austenizing and quenching step; and c) subsequent to the final pull of the cold-drawing process performing a final recovery heat treatment on the cold-drawn steel tubing at a temperature in the range of 200-600° C. wherein the steel composition comprises, in wt. %, C: 0.04-0.15; Mn: 0.90-1.60; Si: 0.10-0.50; Cr: 0.05-0.80; Al 0.01-0.50; N 0.0035-0.0150; Mo: 0-0.50; Ni: 0-0.50; Cu 0-0.25; V 0-0.40; Nb 0-0.20; Ti 0-0.10; B 0-0.005; Ca 0-0.005. As 0-0.05; Sb 0-0.05; Sn 0-0.05; Pb 0-0.05. Bi 0-0.005; S 0-0.015; P 0-0.025; the remainder being Fe and other inevitable impurities.

2. The method according to claim 1, wherein the total reduction of area of the one or more pulls after the intermediate austenizing and quenching step is at least 10%.

3. The method according to claim 1, wherein the intermediate austenizing and quenching step is carried out between the penultimate and final pull of the cold-drawing process.

4. The method according to claim 1, wherein in the intermediate austenizing and quenching step comprises quenching at a quenching rate of at least 50° C./s.

5. The method according to claim 1, wherein the step a) of producing a steel tubing comprises the substeps of preparing the steel composition, casting the composition into a billet, piercing the billet at elevated temperature, and hot rolling the pierced billet in at least one hot rolling pass.

6. The method according to claim 1, wherein the rolling reduction in each hot rolling pass is at least 3%.

7. The method according to claim 1, wherein in step b) the intermediate austenizing and quenching step comprises heating to a temperature above Ac3.

8. The method according to claim 1, wherein the method further comprises a normalizing heat treatment, which comprises either heat treating the hot rolled tubing at a temperature above Ac3 after hot rolling or normalizing rolling in the final hot rolling pass at a temperature above Ar3.

9. The method according to claim 8, wherein the normalizing heat treatment comprises heat treating the hot rolled tubing at a temperature between Ac3 and 1000° C. after hot rolling.

10. The method according to claim 8, wherein the normalizing heat treatment comprises normalizing rolling in the final hot rolling pass at a temperature between Ar3 and the grain coarsening temperature.

11. The method according to claim 1, further comprising a cold forming step d) of cold forming the tubular product from step c), in particular the ends thereof.

12. The method according to claim 1, wherein [% Sn]+[% Sb]+[% Pb]+[% As]+[% Bi]≤0.10%, wherein [%] is wt. %.

13. The method according to claim 1, wherein
0.3≤Ceq≤0.7, wherein
Ceq=[% C]+[% Mn]/6+([% Cr]+[% Mo]+[% V])/5+([% Ni]+[% Cu])/15, or [% Al]/1.9+[% Ti/3.4]+[% V]/3.6+[% Nb]/6.6≥[% N], wherein [%] is wt. %.

14. The method according to claim 1, wherein in the steel composition, in wt. %, C: 0.06-0.12; Mn: 1.00-1.40; Si: 0.20-0.35; Cr: 0.30-0.60; Al 0.015-0.030; N 0.006-0.010.

15. The method according to claim 1, wherein [% Al]/1.9+[% Ti]/3.4+[% V]/3.6+[% Nb]/6.6≥1.1 [% N], wherein [%] is wt. %.

16. The method according to claim 1, wherein the resulting tubing has one or more of the properties: yield strength (YS): ≥896 MPa (130 ksi); tensile strength (TS): ≥1103 MPa (160 ksi); total elongation (A 5D): ≥9%; wherein YS, TS and A 5D are determined according to ASTM E8 DBTT: Burst: ≤−60° C.; ≥50% ductile at −60° C.

17. The method according to claim 1, wherein the resulting tubing has a mainly martensitic microstructure comprising 80% or more martensite and lower bainite, the remainder being coarse bainite and ferrite.

18. The method according to claim 1, wherein the grain size number (ASTM E112), in the resulting tubing is 9 or higher.

19. An automotive component, in particular an airbag inflator pressure vessel, comprising a length of tubing manufactured according to claim 1.

Description

EXAMPLES

[0102] Micro-alloyed steel compositions as listed in Table 1 were prepared under clean practice and casted into a round billet having a diameter of about 148 mm. This billet was subjected to a process comprising the steps of induction heating to a temperature of 870° C., i.e. above Ac3, piercing, hot-rolling using floating mandrel technology with intermediate reheating and final stretch reducing rolling, cooling and furnace normalizing.

TABLE-US-00002 TABLE 1 Chemical composition Composition A B C D E C 0.11 0.1 0.1 Mn 1.34 1.27 1.27 1.28 1.3 Si 0.26 0.24 0.25 0.29 0.25 P 0.014 0.011 0.014 0.015 0.011 S 0.002 0.0013 0.001 0.001 0.001 Cr 0.61 0.36 0.61 0.43 0.44 Mo 0.18 0.15 0.17 0.14 0.14 Ni 0.11 0.07 0.15 0.14 0.12 Cu 0.15 0.14 0.17 0.17 0.21 V 0.1 0.063 0.1 0.06 0.06 Nb 0.002 0 0.001 0.002 0.002 Al 0.028 0.031 0.036 0.028 0.029 Ti 0.023 0 0.014 0.003 0.002 N 0.0091 0.0058 0.007 0.0088 0.0078 B 0.0004 0.0002 0.0002 0.0002 0.0005 As 0.007 0.004 0.006 0.006 0.008 Sb 0.002 0 0.0004 0.0015 0.0017 Sn 0.01 0.011 0.016 0.016 Pb 0.0006 0.0006 0.0004 0.0001 0.0001 Bi 0.0002 0.0002 0.0002 0.0004 0.0005 Ca 0.0014 0.0011 0.0013 0.0012 0.0011 Al/1.9 + 0.0496 0.0338 0.0510 0.0326 0.0328 Ti/3.4 + V13.6 + Nb/6.6 Ceq 0.52 0.43 0.52 0.46 0.47 Pcm 0.2 0.25 0.22 0.23

Example 1 (Comparative)

[0103] The hot-rolled hollow thus obtained from composition A having an outer diameter (OD) of 42.4 mm and a wall thickness (VVT) of 2.9 mm was cold drawn in two pulls to a size of 30*1.85 mm (OD*WT), heat treated in the range of 900-1030° C. and quenched using a water spray. The tubular product thus obtained was subjected to straining simulated by cold f (mandrel-free cold-drawing) to an OD of 25 mm in order to simulate the effect of finishing forming operations. A recovery treatment was not applied.

Example 2 (Comparative)

[0104] In another example the same composition A was also used for manufacturing a tube according to a similar process under the same conditions, except that a quench and temper heat treatment was performed at 400° C. before the simulation of straining (mandrel-free cold-drawing).

[0105] The below table 2 lists the properties as measured using the respective standards ASTM E8 and ASTM E10, for the products as obtained (“as received”) in these Examples prior to the simulation and for the products after cold-working, that simulates straightening and straining (“strained”).

TABLE-US-00003 TABLE 2 Example 1 Example 2 HT (as CD HT (as CD received) (strained) received) (strained) Propertie 1303 (189) 1441 (209) 1158 (168) 1199 (174) TS in MPa (ksi) YS in MPa 1013 (147) 1172 (170) 1061 (154) 1034 (150) (ksi) A 5D in % 14 8 13 10 Strain 1868 (271) 1516 (220) hardening K in MPa (ksi) n 0.11 0.07 Hardness 429 449 387 379 HV 10 Burst 1813 2469 1732 2146 pressure in (26, 298) (35, 810) (25, 130) (31, 126) bar (psi)

[0106] From comparison of these examples it appears that Example 1 (drawn-quenched-redrawn) outperforms Example 2 (drawn-quenched and tempered-redrawn) in almost every aspect, except for the decrease in elongation (A 5D).

Example 3 (Invention)

[0107] A tubular product was made from steel composition B according to the process outlined for Example 1, however with the incorporation of an intermediate austenizing and quenching treatment prior to the final cold drawing pull and the incorporation of a final recovery heat treatment at 430° C. after the final cold drawing. Austenizing was carried out by induction heating to 950° C. and a soaking time of 5 seconds, followed by quenching to room temperature using an external water spray (cooling rate over 50° C./s). After hot rolling the hollow measured 48.3*3.4 mm (OD*VVT). The final size of the cold-drawn product was 35*2 mm.

[0108] The obtained product had the following metallurgical and mechanical properties: [0109] UTS: 1248 MPa (182 ksi); [0110] YS: 1228 MPa (178 ksi); [0111] Total elongation: 10%; [0112] Grain size number (ASTM E112): 13; [0113] Hardness HVio: 394; [0114] Burst at ambient temperature: 1731-1738 bar (25.1-25.2 ksi); [0115] Burst fracture appearance at −69° C.: >50% shear area.

Example 4 (Invention)

[0116] A tubular product was made from steel composition C according to the process outlined in Example 1, however again with the incorporation of an intermediate austenizing and quenching treatment prior to the final cold drawing pull and the incorporation of a final recovery heat treatment at 400° C. after the final cold drawing. Austenizing was carried out by induction heating to 900-1030° C., followed by quenching to room temperature using an external water spray (cooling rate over 50° C./s). After hot rolling the hollow measured 38.0*2.9 mm. At a reduction of 29% in the first cold drawing pull the hollow measured 34.5*2.25 mm. After the second cold drawing pull at a reduction of 26% the final size of the cold-drawn product was 30*1.92 mm.

[0117] The product thus obtained had the following metallurgical and mechanical properties: [0118] UTS: 1262 MPa (183 ksi); [0119] YS: 1172 MPa (170 ksi); [0120] Total elongation: 16.8%; [0121] Grain size number (ASTM E112): 11-12; [0122] Hardness HVio: 428; [0123] Burst at ambient temperature: average 1972 bar (28.6 ksi); [0124] Burst fracture appearance at −60° C.: >50% shear area.

Example 5 (Comparative)

[0125] Example 1 was repeated using steel composition D, except that the cold drawing involved a single pull, after which the quenching step was performed. After hot rolling the hollow measured 38.1*2.7 mm. The hollow after the single cold drawing step at reduction of 32% had dimensions of 33.2*2.08 mm.

The product had the following metallurgical and mechanical properties: [0126] UTS: 1277 MPa (183 ksi); [0127] YS: 992 MPa (170 ksi); [0128] Total elongation: 15%; [0129] Grain size number (ASTM E112): 11-12; Hardness HVio: 413;

Example 6 (Comparative)

[0130] Example 2 was repeated using steel composition E, except that cold drawing involved a single pull, after which quenching and tempering at 380° C. was performed. After hot rolling the hollow measured 38.1*2.7 mm. The hollow after the single cold drawing step at reduction of 33% had dimensions of 32*2.15 mm.

[0131] The product had the following metallurgical and mechanical properties: [0132] UTS: 1084 MPa (183 ksi); [0133] YS: 911 MPa (170 ksi); [0134] Total elongation: 13%; [0135] Grain size number (ASTM E112): 11-12; [0136] Hardness HVio: N.A.

[0137] The tubular products from Examples 4-6 were subjected to straining simulated by cold forming (mandrel-free cold drawing) at an area reduction of 17%. The below Table 3 summarizes the results, wherein “as-received” indicates the tubular products manufactured according to these Examples and “strained” the tubular products after the simulated straining.

TABLE-US-00004 TABLE 3 EXPERIMENTAL DATA EXAMPLES 4-6 Ex. 4 Ex. 5 Ex 6 As As As Property received Strained received Strained received Strained Rm in 1262 1310 1277 1358 1084 1110 MPa (ksi) (183) (190) (185) (197) (157) (161) A5 D in % 16.8 6.3 15 4.3 13 5

[0138] From this table it appears that upon straining the tensile strength of the Example 4 according to the invention is higher than that of Example 6. This also applies to the elongation. Although the strength of Example 5 is higher than that of Example 4, the elongation value of Example 4 according to the invention, for both the as-received tubular product and the strained product, is higher. Thus the favourable combination of strength and ductility properties of the product manufactured according to the invention remains upon cold working allowing to finish the product properly.

[0139] Furthermore it has been found that the dislocation density in Example 4 according to the invention is significantly lower than that of Example 5 as is apparent from FIG. 1, that shows the average microstrain £ (note that dislocation density p is proportional to £.sup.2 (p=A*(0) with A being a material's constant)). As can be seen in the invention the dislocation density is much lower than in the embodiment of Example 5. Moreover, upon cold working (=straining) the dislocation density in the invention remains almost the same, while the material of Example 5 showed a significant increase in microstraining and thus dislocation density. An increase in dislocation density increases the hardness and strength, but decreases the ductility and toughness properties, it can be assumed that straining affects the steel tubing according to the invention regarding strength and elongation and thus formability to a lesser extent than the material of Example 5.

Airbag Inflator Pressure Vessel

[0140] A seamless tube manufactured according to the invention is cut to length and then cold formed using known techniques, e.g. crimping, swaging and the like, into a desired shape. As an alternatively, a welded tube processed according to the invention, could be used. To each end of cold formed tube an end cap and a diffuser are welded using known techniques, e.g. friction welding, arc welding and laser welding, thereby producing the airbag inflator pressure vessel.