METHOD FOR PRODUCING A STEEL STRIP WITH AN ALUMINIUM ALLOY COATING LAYER

Abstract

A method for producing a steel strip with an aluminium alloy coating layer in a continuous coating process. Also, a steel strip coated with an aluminium alloy coating layer that can be produced in accordance with the method, the use of such a coated steel strip and the product made by using the coated steel strip.

Claims

1. A method for producing a steel strip coated on one or both sides with an aluminium alloy coating layer in a continuous hot-dip coating and a subsequent pre-diffusion annealing process, said process comprising a hot-dip coating stage in which the steel strip is passed with a velocity v through a bath of a molten aluminium alloy to apply an aluminium alloy coating layer to one or both sides of the steel strip, and a pre-diffusion annealing stage, wherein the thickness of the applied aluminium alloy coating layer on the one or both sides of the steel strip is between 5 and 40 m and wherein the aluminium alloy coating layer comprises 0.4 to 4.0 weight % silicon, and wherein the aluminium alloy coated steel strip enters the pre-diffusion annealing stage while at least the outer layer of the aluminium alloy coating layer or layers is above its liquidus temperature, and the strip is annealed at an annealing temperature of at least 600 and at most 800 C. for at most 40 seconds to promote the diffusion of iron from the steel strip into the aluminium alloy coating layer or layers to form a substantially fully-alloyed aluminium-iron-silicon coating layer or layers, substantially entirely consisting of iron-aluminides; followed by cooling the pre-diffusion annealed coated steel strip to ambient temperatures.

2. The method according to claim 1, wherein the composition of the fully-alloyed aluminium-iron-silicon coating layer or layers is 50-55 wt. % Al, 43-48 wt. % Fe, 0.4-4 wt. % Si and inevitable elements and impurities consistent with the hot dip coating process.

3. The method according to claim 1, wherein the molten aluminium alloy in the bath contains between 0.4 and 4.0 wt. % silicon, and wherein the molten aluminium alloy has a temperature of between 630 and 750 C.

4. The method according to claim 3, wherein the temperature of the steel strip entering the molten aluminium alloy bath is between 550 and 750 C., and/or wherein the velocity v is between 0.6 m/s and 4.2 m/s.

5. The method according to claim 1, wherein the fully-alloyed aluminium-iron-silicon coating layer contains at least 0.5 wt. % Si and/or at most 3.5 wt. % Si.

6. The method according to claim 1, wherein the thickness of the fully-alloyed aluminium-iron-silicon coating layer is at least 8 and/or at most 40 m.

7. The method according to claim 1, wherein the thickness d (in m) of the fully-alloyed aluminium-iron-silicon coating layer in dependence of the silicon content (in wt. %) of the fully-alloyed aluminium-iron-silicon coating layer is enclosed in the Si-d space by the equations (1), (2) and (3):
d1.39.Si+12.6 and(1)
d9.17.Si+43.7 and(2)
Si0.4%.(3)

8. The method according to claim 1, wherein the annealing time in the pre-diffusion annealing stage is at most 30 seconds.

9. The method according to claim 1, wherein the immersion time of the steel strip in the molten aluminium alloy bath in the hot-dip coating stage is between 2 and 10 seconds.

10. A steel strip having a composition comprising (in wt. %): TABLE-US-00008 C: 0.01-0.5 P: 0.1 Nb: 0.3 Mn: 0.4-4.0 S: 0.05 V: 0.5 N: 0.001-0.030 B: 0.08 Ca: 0.05 Si: 3.0 O: 0.008 Ni 2.0 Cr: 4.0 Ti: 0.3 Cu 2.0 Al: 3.0 Mo: 1.0 W 0.5 the remainder being iron and unavoidable impurities, coated on one or both sides with a fully alloyed aluminium-iron-silicon coated steel strip, obtainable by the process according to claim 1, and wherein the composition of the fully-alloyed aluminium-iron-silicon coating layer or layers is 50-55 wt. % Al, 43-48 wt. % Fe, 0.4-4 wt. % Si and inevitable elements and impurities consistent with the said process.

11. The coated steel strip according to claim 10, the steel strip having a composition comprising (in wt. %): TABLE-US-00009 C: 0.10-0.25 P: 0.02 Nb: 0.3 Mn: 1.0-2.4 S: 0.005 V: 0.5 N: 0.03 B: 0.005 Ca: 0.05 Si: 0.4 O: 0.008 Ni 0.05 Cr: 1.0 Ti: 0.3 Cu 0.05 Al: 1.5 Mo: 0.5 W 0.02 the remainder being iron and unavoidable impurities.

12. The coated steel strip according to claim 10, wherein the fully-alloyed aluminium-iron-silicon coating layer or layers contain between 0 and 10 area. % of -phase, and wherein the -phase, if present, is dispersed in the coating layer.

13. A method of using the fully-alloyed aluminium-iron-silicon coated steel strip obtainable by the method according to claim 1, to produce a hot-formed product in a hot-forming process comprising the steps of: cutting the coated steel strip to obtain a blank; heating the blanks to above the Ac.sub.1 temperature of the steel, optionally to above the Ac.sub.3 temperature of the steel; hot-forming the blank into a product; cooling the hot-formed product,

14. The method of using the fully-alloyed aluminium-iron-silicon coating coated steel strip in a hot-forming process according to claim 13, wherein the heating of the blanks from ambient temperature to above the Ac.sub.1 temperature of the steel, optionally to above the Ac.sub.3 temperature of the steel, is performed by means of induction heating, contact heating or resistance heating.

15. A vehicle part made according to claim 13.

16. The method according to claim 1, wherein the molten aluminium alloy in the bath contains between 0.4 and 4.0 wt. % silicon, and wherein the molten aluminium alloy has a temperature of at least 660 C. and/or at most 700 C.

17. The method according to claim 3, wherein the temperature of the steel strip entering the molten aluminium alloy bath is at least 660 C. and/or at most 700 C., and/or wherein the velocity v is at most 3.0 m/s.

18. The method according to claim 3, wherein the temperature of the steel strip entering the molten aluminium alloy bath is at least 660 C. and/or at most 700 C., and/or wherein the velocity v is at least 1.0 and/or at most 2.0 m/s.

19. The method according to any one of the preceding claims claim 1, wherein the thickness of the fully-alloyed aluminium-iron-silicon coating layer is at least 10 and/or at most 30 m.

20. The method according to any one of the preceding claims claim 1, wherein the thickness of the fully-alloyed aluminium-iron-silicon coating layer is at least 12 and/or at most 25 m.

21. The method according to any one of the preceding claims claim 1, wherein the thickness of the fully-alloyed aluminium-iron-silicon coating layer is at most 20 m.

22. The method according to claim 1, wherein the immersion time of the steel strip in the molten aluminium alloy bath in the hot-dip coating stage is at least 3 and/or at most 6 seconds.

Description

EXAMPLES

[0039] The invention will now be further explained by means of the following, non-limitative examples. The steel substrate for the experiments had the composition as given in Table 1.

TABLE-US-00004 TABLE 1 Composition of steel substrate, balance Fe and inevitable impurities. 1.5 mm, cold-rolled, full-hard condition. C Mn Cr Si P S Al B Ca wt. % wt. % wt. % wt. % wt. % wt. % wt. % ppm ppm 0.20 2.18 0.64 0.055 0.010 0.001 0.036 0 17

Example 1

[0040] Two aluminium-alloy coated steels were produced. Sample A was produced by hot-dipping a steel strip in a molten aluminium alloy bath comprising 0.9 wt. % Si. Sample B was produced by hot-dipping in a prior art aluminium alloy bath comprising 9.6 wt. % Si. Both baths were saturated with Fe (about 2.8 wt. %). The steel grade used is a 1.5 mm cold rolled steel, in full hard condition and having a composition suitable for hot forming applications. Prior to hot-dipping the steels were recrystallisation annealed. Immediately following the recrystallisation annealing the steels were immersed in the respective aluminium alloy bath fora period of 3 seconds, which is consistent with a line speed of about 120 m/min. The strip entry temperature in the bath was 680 C., and the bath temperature was 700 C. After hot dipping the layer thickness of the coating was adjusted by wiping with nitrogen gas at 20 m. The steels were annealed in the pre-diffusion annealing stage for 20 s at 700 C. to obtain pre-alloying and then cooled down by forced nitrogen gas.

[0041] FIG. 2 shows the annealed aluminium-alloy coating layers. The coating on sample A is a fully-alloyed aluminium-iron-silicon coating layer while the coating on sample B consists of an alloyed layer of less than 10 m thick (with a different composition than the fully-alloyed aluminium-iron-silicon coating layer on sample A!) with a non-alloyed layer with the coating bath composition on top. Additional experiments with sample B with varying annealing times in the pre-diffusion annealing stage at 700 C. show that the growth rate of the alloyed layer is very slow (see table 1). The remainder of the coating layer is still liquid.

TABLE-US-00005 TABLE 1 thickness measurements of alloy layer on Sample B annealed at 700 C. Sample ID i ii iii iv Heat treatment time [s] 0 10 20 60 Alloy layer thickness [m] 5 7 9 11

[0042] So a prior art coating with 9.6wt. % Si is not suitable for inline pre-alloying according to the invention, because the pre-diffusion annealing stage does not produce a fully-alloyed aluminium-iron-silicon coating layer. The coating with 0.9% Si on the other hand shows a fully alloyed layer of 20 m thickness already after 20 seconds.

Example 2

[0043] Sample A from Ex. 1 (recrystallised cold-rolled 1.5 mm thick strip) was hot-dip coated in aluminium-alloy baths with different Si concentrations according to the invention, varying between 0.5, 0.9, 1.1 and 1.6 wt. % and pre-diffusion annealing times ranged from 0 to 30 seconds. The pre-diffusion annealing temperature was 700 C. The coating layer thickness was adjusted at 30 to 40 m by nitrogen jets after exiting the coating bath. Producing relatively thick layers was a deliberate choice as the purpose of these examples was to determine the maximum achievable pre-alloying thickness without a limiting effect of the applied coating thickness. The steels were treated the same as in Ex. 1, except for the varying annealing time. In FIG. 3 cross sections (SEM) of the produced coatings are shown. The images clearly reveal an increased alloy layer thickness at lower Si levels and longer heat treatment times. Alloy layer thickness are presented in FIG. 4. Measurements demonstrate that depending on Si concentration and heat treatment time the alloy layer thickness ranges from 10 to 35 m. Based on the measurements and extrapolation of the measurements a triangle is drawn in FIG. 4 that displays the thickness of fully alloyed coatings that can be produced with dipping times of 3 s in combination with heat times between 0 and 30 s.

Example 3

[0044] Hot-forming steel (1.5 mm) coated with an aluminium alloy coating layer with 0.9 wt. % Si and 2.3 wt. % Fe with immersion times in the molten aluminium alloy bath of 3, 5 and 10 seconds. After exiting the coating bath the layers thickness was controlled at 25 m by wiping with nitrogen. Next the steels were cooled down with forced nitrogen. Bath and strip entry temperature were as before. The thickness of the alloy layer thicknesses are given in table 2. The increase of alloy layer thickness at longer dipping times, i.e. lower line speeds, is clearly illustrated.

TABLE-US-00006 TABLE 2 thickness measurements (0.9 wt. % Si) Sample ID v vi vii Dipping time[s] 3 5 10 Alloy layer thickness [m] 13 15 18
By changing the dipping time the fabrication window of Ex. 3 (FIG. 4) can be enlarged. Combining data of both examples resulted in a production window of fully alloyed coatings as shown in FIG. 5.

Example 4

[0045] Hot-forming steel (1.5 mm) coated with an aluminium alloy coating layer with 1.9 wt. % Si and 2.3 wt. % Fe with immersion times in the molten aluminium alloy bath of 3, 5 and 10 seconds. After exiting the coating bath the layers thickness was controlled at 25 m by wiping with nitrogen. Next the steels were cooled down with forced nitrogen. Bath and strip entry temperature were as before. The thickness of the alloy layer thicknesses are given in table 3. The increase of alloy layer thickness at longer dipping times, i.e. lower line speeds, is clearly illustrated.

TABLE-US-00007 TABLE 3 thickness measurements in m (1.9 wt. % Si) pre-diffusion annealing Dipping Dipping Dipping time (s) time 3 s time 5 s time 10 s 0 9 10 12 10 14 16 18 20 20 21 23

Example 5

[0046] The layer structure of sample A after pre-diffusion annealing (for 20 s at 700 C., according to the invention) and B as hot-dipped (so no pre-diffusion annealing, which is the prior art situation) are compared in FIG. 6 (SEM cross section images). Sample A shows a fully-alloyed aluminium-iron-silicon coating layer, whereas the coating on sample B is a thin alloy layer at the steel interface, while the top part of the coating is not alloyed and has an average composition equal to the coating bath composition. As a consequence the top layer starts to melt at a temperature of about 575 C. The steels in this condition were heat treated in a radiation furnace set at 900 C. with a thermocouple welded to the strips to record the heat-up rates. The heating curves of both steels (see FIG. 7) clearly illustrate the faster heat up rate of the pre-alloyed sample A compared to comparative sample B. Especially at lower temperatures the heating rate is improved by pre-alloying as during this stage the reflection of radiation is markedly reduced by the dull appearance of the pre-alloyed coating. Faster heating rate enables higher throughput with the same furnace. Alternatively shorter furnaces can be used requiring a smaller foot print and lower investment. Samples taken at temperatures of 700, 800, 850 C. during the heating of sample B revealed that only at after reaching a temperature of 850 C. a fully alloyed layer is obtained. This means that the outer part of the coating layer remained liquid over the entire temperature range of 575 to 850 C. During the time the coating is molten roll build up during contact with the furnace rolls occur. Roll build up not only leads to increased maintenance and furnace down time but is also a source of product damage. Sample A with the non-melting pre-alloyed coating is not causing any roll build up at any temperature.

Example 6

[0047] Sample A (1.1 wt. % Si) and sample B sheets (9.6 wt. % Si) were heated in a radiation furnace set at 900 C. At various time intervals samples were taken out of the furnace for examination in cross section to determine the growth rate of the diffusion layer. A thickness of the diffusion layer of 10 m is considered to be a proper diffusion zone with good crack propagation resistance. The investigation showed that a thickness of this thickness was achieved for sample A after 170 seconds at 900 C. and for sample B after 400 s. With sample A (according to the invention) a furnace time saving of more than 50% is achieved compared to sample B (prior art). The relevant images are shown as FIGS. 8A and B.