METHOD FOR PRODUCING A HOT-FORMED COATED STEEL PRODUCT

Abstract

An AlSi-alloy coated steel strip for hot press forming and to a method for producing the AlSi-alloy coated steel strip in a continuous coating process.

Claims

1. A process for producing a hot-formed steel product, wherein the hot-formed product comprises a steel substrate and an aluminium alloy coating layer, the aluminium alloy coating layer comprising a surface layer and a diffusion layer between the surface layer and the steel substrate, and wherein the surface layer contains between 0 and 10 area % of -phase, and wherein the -phase, if present, is dispersed in the surface layer, and wherein the process at least comprises the subsequent steps of: providing a steel strip or sheet provided with an aluminium alloy coating layer by means of immersing the steel substrate in a molten aluminium alloy bath comprising at least 0.4 wt. % and at most 4.0 wt. % of Si; cutting the coated steel strip or sheet to obtain a blank; hot-forming the blank into a product by means of a direct or indirect hot-forming process wherein the hot-forming process involves heating the blank, or the hot-formed steel product in case of the indirect hot-forming process, to a temperature above the Ac1-temperature, preferably above the Ac3-temperature of the steel; cooling the product to form the desired final microstructure to obtain the hot-formed steel product.

2. The process according to claim 1, wherein the surface layer is free from -phase.

3. The process according to claim 1, wherein the outermost surface layer is free from -phase.

4. The process according to claim 1, wherein the molten aluminium alloy bath comprises 0.6 to 4.0 wt. % of silicon.

5. The process according to claim 1, wherein the molten aluminium alloy bath comprises 0.6 to 1.4 wt. % of silicon.

6. The process according to claim 1, wherein the molten aluminium alloy bath comprises at least 1.6 wt. % to 4.0 wt. % of silicon.

7. The process according to claim 1, wherein the coated steel strip or sheet with the aluminium alloy coating layer is subjected to a pre-diffusion annealing step before the hot-forming step.

8. The process according to claim 1, wherein the coated strip or sheet with the aluminium alloy coating layer is subjected to a pre-diffusion annealing step: as a strip in a hot-dip coating line immediately following the hot-dip coating, as a strip, sheet or blank in an induction furnace optionally in combination with a radiation and/or convection heating oven.

9. The process according to claim 1, wherein the alloy layer on the coated steel strip or sheet prior to heating and hot-forming, and prior to the optional pre-diffusion annealing step, comprises at least three distinct layers, from the steel strip surface outwards: intermetallic layer 1, consisting of Fe.sub.2Al.sub.5 with silicon in solid solution intermetallic layer 2, consisting of FeAl.sub.3 with silicon in solid solution outer layer having the composition of the melt.

10. The process according to claim 1, wherein the thickness of the aluminium alloy coating layer prior to heating and hot-forming, and prior to the optional pre-diffusion annealing, is between 10 and 40 m.

11. The process according to claim 1, wherein the composition of the steel strip comprises (in weight. %): 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.

12. A hot-formed steel product, said hot-formed product comprising a steel substrate and an aluminium alloy coating layer comprising at least 0.4 wt. % of Si and at most 4.0 wt. %, the aluminium alloy coating layer comprising a surface layer and a diffusion layer between the surface layer and substrate, and wherein the surface layer contains between 0 and 10 area % of -phase, and wherein the -phase is dispersed in the surface layer.

13. The hot formed product according to claim 12 having at least one feature selected from the group consisting of: wherein the aluminium alloy coating layer comprises 0.6 to 4.0 wt. % of silicon, wherein the surface layer is free from -phase, wherein the outermost surface layer is free from -phase, and wherein the contiguity of the -phase C.sub. is 0.4.

14. The hot formed product according to claim 12, wherein the composition of the steel substrate comprises (in weight. %): TABLE-US-00009 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.

15. A vehicle part made from the hot-formed product obtainable by the method of claim 1.

16. The process according to claim 1, wherein the molten aluminium alloy bath comprises at least 1.8 wt. % to 4.0 wt. % of silicon.

17. The hot formed product according to claim 12, wherein the composition of the steel substrate comprises (in wt. %) C: 0.10-0.25 Mn: 1.0-2.4 N: 0.03 Si: 0.4 Cr: 1.0 Al: 1.5 P: 0.02 S: 0.005 B: 0.005 O: 0.008 Ti: 0.3 Mo: 0.5 Nb: 0.3 V: 0.5 Ca: 0.05 Ni0.05 Cu0.05 W0.02 the remainder being iron and unavoidable impurities.

18. The hot-formed product according to claim 13 as a part in a vehicle, e.g. as a body part.

19. The hot formed product according to claim 12: wherein the aluminium alloy coating layer comprises 0.6 to 4.0 wt. % of silicon, wherein the surface layer is free from -phase, wherein the outermost surface layer is free from -phase, and wherein the contiguity of the -phase C.sub. is 0.4.

20. The vehicle part of claim 15, wherein the vehicle part is a vehicle body part.

21. A vehicle part made from the hot-formed product made according to claim 12.

22. The vehicle part of claim 21, wherein the vehicle part is a vehicle body part.

Description

EXAMPLES

[0061] Hot-formed coated steel products were produced from a steel substrate having 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

[0062] Aluminium alloy coating layers were provided onto the steel substrate by immersing the substrate in a molten aluminium alloy bath (a.k.a. hot-dipping or hot-dip coating), and the silicon content of the bath, and thus the aluminium alloy coating layers was 1.1 and 9.6 wt. % respectively. The bath temperature was 700 C., the immersion time was 3 seconds, and the thickness of the aluminium alloy coating layers was 30 m.

[0063] After applying the coating, sheets of steel were heated for 6 minutes in a radiation furnace at a temperature of 925 C. At the end of heating the blanks were transferred in less than 10 seconds to a press and subsequently stamped and quenched. After hot stamping the steels were covered with an aluminium alloy coating layer of 40-50 m thickness. The increase of the thickness of the aluminium alloy coating layer is caused by the diffusion and alloying processes taking place in the surface layer and by the formation of the diffusion layer between the surface layer and the steel substrate. This diffusion layer is formed by diffusion of aluminium into the steel substrate, thereby enriching the steel substrate with aluminium to a level that the steel substrate locally does not transform to austenite any longer, and stays ferritic during the hot stamping and this ductile layer stops any surface cracks from reaching the steel substrate. The coating of the steel coated with a 1.1% Si layer (Sample A) consists of three layers while in the coating of the steel coated with the 9.6% Si (Sample B) four layers can be distinguished, as illustrated in FIG. 4. In sample B the presence of a continuous layer of -phase in the aluminium alloy coating layer (indicated with 3 in FIG. 4) as well as significant amounts of the same phase on the surface can be identified.

[0064] Energy-dispersive X-ray spectroscopy (EDX or EDS), is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on an interaction of some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing a unique set of peaks on its electromagnetic emission spectrum[2] (which is the main principle of spectroscopy). To stimulate the emission of characteristic X-rays from a specimen or a beam of X-rays, is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive spectrometer. As the energies of the X-rays are characteristic of the difference in energy between the two shells and of the atomic structure of the emitting element, EDS allows the elemental composition of the specimen to be measured (https://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy).

[0065] Energy Dispersive X-ray analysis (EDX or EDS) of the sub layers revealed the following structure for sample A: [0066] layer 1: Diffusion layer [0067] layer 2: FeAl.sub.2 (46-52 wt. % Fe, 44-50 wt. % Al and <3 wt. % Si) [0068] layer 3: Fe.sub.2Al.sub.5 (40-47 wt. % Fe, 51-58 wt. % Al and <3 wt. % Si)
In the four layered structure of sample B the identified phases were: [0069] layer 1: Diffusion layer [0070] layer 2: Fe.sub.2Al.sub.5 [0071] layer 3: -phase (Fe.sub.2SiAl.sub.2) [0072] layer 4: Fe.sub.2Al.sub.5
Note that these layer structures are dependent on the annealing time. After prolonged annealing the composition of layer 2 of sample B will likely become FeAl.

[0073] In addition both layers contain low concentrations Cr and Mn. EPMA line scans on cross sections of the steel coated with Al-1.1 wt. % Si revealed Cr and Mn diffused from the substrate into the layers. Concentrations found in the coating are about 50% of the concentration in the substrate. An example is given in FIG. 7 for heat treatment of 6 minutes at 900 C. It is noted that intermetallic layer 1 can be very thin, even almost absent for short and/or low annealing temperatures (see FIG. 8).

[0074] On the hot formed panels an E-coat was applied by the following process steps:

TABLE-US-00005 Time Temperature Process step Agent [s] [ C.] Alkaline degreasing Gardoclean S5176 90 55 Spray rinsing Tap water 60 room Activating Gardolene V6513 60 24 Phosphating Gardobond 24 TA 180 51 Dip rinsing Deionised water 60 room E-coating Guard 900 BASF 300 32 Dip rinsing Deionised water 60 room Drying n.a. 30 room Curing n.a. 1380 160

[0075] E-coat adhesion of four sheets of sample A and B was tested by immersion of the panels in deionised water of 50 C. during 10 days. After removing the panels from the warm water bath a cross hatch pattern per sheet was made according NEN-EN-ISO 2409 (June 2007). Paint adhesion was tested on the cross-cut area by a tape peel off test as described in aforementioned standard. Test results were ranked according table 1 of this standard.

[0076] The four sheets of sample A exhibit excellent paint adhesion. The edges of the cuts are completely intact and none of the squares of the lattice is detached (FIG. 5). Therefore the adhesion performance is rated as 0. The four sheets of sample B show a poor paint adhesion. The rating varies between 2 and 4, meaning cross-cut areas of 15 to 65% have flaked off.

[0077] A typical test to determine whether a coated product meets the automotive manufacturer's requirements is the scribe undercreep test. In this test loss of E-coat adhesion due to corrosive creepback at a deliberately made scribe is determined. These test results are considered to be an indicator for cosmetic corrosion in service. E-coated sheets used for this test were produced according the route described above. Scribes were made on the sheets through the E-coat and metallic coating just into the substrate. Two types of scribes per panel were made, one with a Sikkens tool and one with a van Laar knife. Sheets were tested in a corrosion cabinet using the VDA233-102 accelerated corrosion test. Corrosive creepback from the scribe lines was evaluated after 10 weeks of testing. Average creepback width was determined over a scribe length of 70 mm. As a measurement tool rectangular transparent templates with a length of 70 mm and a varying width in steps of 0.5 mm from 1 to 15 mm were used. The width of the template with an area matching best with the delaminated area was taken as average creepback width. Four sheets of sample A and of B were scribed and tested. The results showed a significant improvement of undercreep resistance of A compared to B. Measured undercreep on A range from 3 to 4 mm while on B values between 7 and 10.5 mm were found.

[0078] In another example aluminium coating layers were provided onto the 1.5 mm cold-rolled full hard steel substrate by hot dipping, and the silicon content of the coating bath was 1.9 wt. % and 9.8 wt % respectively. The coating bath temperature was 690 C., the immersion time was 5 seconds, and the resulting layer thickness was adjusted from 15 to 25 m, as indicated in the following table.

TABLE-US-00006 TABLE 2 Si bath concentration, layer thickness and furnace conditions. Bath Si Layer thickness Furnace T Furnace t Series Sheet id [wt %] [m] [ C.] [minutes] 1 617024 1.9 15 925 3.5 617025 1.9 15 925 3.5 617026 1.9 15 925 3.5 2 633037 9.8 15 925 4.5 633038 9.8 15 925 4.5 633039 9.8 15 925 4.5 3 633022 9.8 25 925 6.0 633023 9.8 25 925 6.0 633025 9.8 25 925 6.0

TABLE-US-00007 TABLE 3 Paint adhesion rating Paint Bath Si Area adhesion Series Sheet id [wt %] [%] C rating 1 617024 1.9 0 0 1 617025 1.9 0 0 1 617026 1.9 0 0 1 2 633037 9.8 >10 1 3 633038 9.8 >10 1 2-3 633039 9.8 >10 1 2 3 633022 9.8 >10 1 2 633023 9.8 >10 1 3 633025 9.8 >10 1 3

[0079] After coating application the sheets of steel were heated for 3.5 to 6 minutes, depending on coating thickness and Si level, in a radiation furnace at a temperature of 925 C. At the end of heating the blanks were transferred in less than 10 seconds to a press and subsequently stamped and quenched. After hot stamping the metallic coating layer was measured and was between 20-50 m.

[0080] After stamping the coating of the steel coated with a 1.9% Si layer is completely free of Fe.sub.2SiAl.sub.2 (-phase) while the area fraction of Fe.sub.2SiAl.sub.2 (-phase) in the surface layer of the steels coated with 9.8% Si is >10%. Furthermore the contiguity of -phase (CT) in the 1.9% Si coating is 0 and CT of the 9.8% Si coatings is 1 which is far above the preferred value of at most 0.4. Cross section images illustrating the microstructural differences of the coatings are shown in FIG. 9a to c.

[0081] On the hot formed panels an E-coat was applied by going through the same process steps and tested in the same way as explained above. The three sheets of series 1 exhibit very good paint adhesion. The edges of the cuts are to a large extent intact and only very minor flaking off is observed (FIG. 10a). Therefore the adhesion performance is rated as 1. The sheets of series 2 show a poor paint adhesion. The rating varies between 2 and 3, meaning cross-cut areas of 15 to 35% have flaked off (FIG. 10b). The sheets of series 3 show similar performance and are also rated between 2 and 3 (FIG. 10c).

[0082] The invention is further explained by means of the following, non-limiting figures.

[0083] In FIG. 1A the process according to the invention is summarised and has been described in detail above as well as FIG. 1B in which the build-up and development of the coating layer is described.

[0084] FIG. 2 shows the development of the different layers of intermetallic compounds during heat treatment of an steel substrate provided with an aluminium alloy coating comprising 1.6 wt. % Si. Figure A shows the as-coated layer, with the layers that are formed immediately after the immersion, and the top layer having the composition of the bath, B shows the development during reheating once the sample has reached 700 C. and C is the situation after annealing at 900 C. for 5 minutes. In sample C the diffusion zone is now clearly visible, and the top layer having the composition of the bath has completely vanished (EDS: acceleration voltage (EHT) 15 keV, working distance (wd) 6.0, 6.2 and 5.9 mm)

[0085] FIG. 3 shows the development of the different layers of intermetallic compounds during heat treatment of an steel substrate provided with an aluminium alloy coating comprising 3.0 wt. % Si (EHT 15 keV, wd 6.6, 6.5, 6.2 mm respectively). Figure A shows the as-coated layer, with the layers that are formed immediately after the immersion, and the top layer having the composition of the bath, B shows the development during reheating once the sample has reached 850 C. and C is the situation after annealing at 900 C. for 7 minutes. In sample C the diffusion zone is now clearly visible, and the top layer having the composition of the bath has completely vanished. Also visible is a degree of -phase (Fe.sub.2SiAl.sub.2) which is dispersed in the Fe.sub.2Al.sub.5 layer, and does not form a continuous layer. C.sub.0.4.

[0086] FIG. 4 shows the development of the different layers of intermetallic compounds during heat treatment of an steel substrate provided with an aluminium alloy coating comprising 1.1 wt. % Si (Sample A) and 9.6 wt. % (Sample B) on a hot-formed product which was heated for 6 minutes at 925 C. (EHT 15 keV, wd 7.3 and 6.1 mm). The continuous -phase (Fe.sub.2SiAl.sub.2) layer in sample B is clearly visible, as well as the notable absence thereof in sample A.

[0087] FIG. 5 shows the results of the paint adhesion tests of samples A and B which have been discussed herein above. FIG. 6 shows the average undercreep values of samples A and B.

[0088] FIG. 7 shows the diffusion profile of sample A after annealing for 6 minutes at 900 C.

[0089] FIG. 8 (EHT 15 keV, wd 7.4 and 7.3 mm). shows the emergence of the FeAl.sub.2 layer for different heat treatment times of sample A. After 3.5 minutes at 925 C. the FeAl.sub.2-layer starts to appear, whereas after 6 minutes there is a layer of this compound present. Also notable is the crack-stopping ability of the diffusion layer in the 6 minute sample.

[0090] FIG. 9 shows the cross sections of an hot-formed specimen having 1.9 wt. % Si (FIG. 9a) in the aluminium coating layer or 9.8 wt. % Si (FIGS. 9b and 9c). FIGS. 10a to 10 c show the paint adhesion performance of these samples.