The invention relates to a method for connecting a hot formed steel article to a steel object, and to a combination of a hot formed steel article and a steel object connected in that way. In this context, hot forming is also known as hot press forming (HPF) or hot stamping or press hardening.

Hot formed articles are used in increasing numbers in the body in white of cars to provide increased strength and to reduce the weight of the cars. General information about hot forming can be found in document EP 0971044 B1.

However, the hot formed products that are used are mainly aluminium alloy coated, because the use of a zinc or zinc alloy coating makes the process more difficult to perform, and usually results in a phenomenon called Liquid Metal Embrittlement (“LME”) or Liquid Metal Assisted Cracking (“LMAC”) during hot forming as well as during assembling the automobile parts by resistance spot welding. LME is caused by melting of zinc during the process and the presence of stress (thermal or mechanical) allows the liquid Zn to penetrate through the substrate-coating interface and along the grain boundaries of the steel substrate causing microcracks to form. The stress can be applied stress or internal stress resulting from restraint, thermal dilatation/contraction or phase transformations. The resulting microcracks reduce the performance of the product, as for example bendability and fatigue performance among other things. Also, corrosion resistance of the coated steel may suffer due to loss of zinc by evaporation and degradation of the quality of the zinc coating by the formation of microcracks.

Furthermore, the use of 22MnB5 steel as substrate for the hot forming process provides a high strength of 1500 MPa or more, but the ductility of the hot formed articles is low.

In document US 2016/0312323 A1 it is already described that an improved ductility can be provided by using a steel substrate that contains higher amounts of manganese, in comparison to the standard boron-containing steels such as 22MnB5. Steels containing higher amounts of manganese are often called medium Mn steels when manganese content is lower than approximately 12 wt.%; document US 2016/0312323 A1 indicates that 2.5 to 15 wt.% manganese can be used, resulting in hot formed articles with a strength of up to 1600 MPa.

In document WO 2019/155014 A1 it is described that liquid metal embrittlement (LME) in the zinc coated hot formed article can be avoided by carefully choosing the annealing temperature at which the strip is annealed from which the blanks are cut to hot form in hot formed articles, in conjunction with the reheating and soaking temperature of the blank during the hot forming process.

For use in service condition the hot formed articles have to be connected to other steel parts, for instance of the body in white of cars. Usually most of the connecting is performed by applying a resistance spot welding (“RSW’) operation. However, the resistance spot welding can result in in LME induced cracks being formed in the heat affected zone (“HAZ”) of the welds, even when the hot formed article itself does not suffer from LME induced microcracking during hot forming. Melting of zinc combined with pressure exerted by the electrodes and the presence of thermal stress can cause LME to occur during the RSW process.

It is an object of the invention to provide a method for connecting a hot formed zinc or zinc alloy coated medium Mn steel article to a steel object whereby LME cracking is reduced and preferably does not take place.

It is a further object of the invention to provide a method for connecting a hot formed zinc or zinc alloy coated medium Mn steel article to a steel object that can be used in the automotive industry without problems relating to LME.

It is another object to provide a combination of a hot formed zinc or zinc alloy coated medium Mn steel article and a steel object in which the welds are substantially free or fully free from LME cracks.

According to a first aspect of the invention a method for connecting a hot formed steel article to a steel object is provided, comprising the following steps:

• Providing a zinc or zinc alloy coated steel blank containing 3.0 - 12.0 wt.% Mn;

• Heating the zinc or zinc alloy coated steel blank to the soaking temperature;

• Soaking the zinc or zinc alloy coated steel blank during 2 - 12 minutes at a soaking temperature between 650 and 750 °C;

• Hot press forming the zinc or zinc alloy coated steel blank into a zinc or zinc alloy coated hot formed steel article;

• Cooling the zinc or zinc alloy coated hot formed steel article ;

• Providing a steel object to which the zinc or zinc alloy coated hot formed steel article has to be connected;

• Bringing the zinc or zinc alloy coated hot formed steel article at least partially into contact with the steel object; Resistance spot welding the zinc or zinc alloy coated hot formed steel article to the steel object.

The inventors have found that at soaking temperatures below 650 °C LME cracks in the HAZ of the resistance spot welds are likely to be formed, due to a fully intact inhibition layer between the fully intact steel substrate and the zinc coating which contains less than 5 wt.% Fe, as shown in scanning electron microscope (“SEM”) images of cross sections at the welds of samples. In contrast, the SEM image of cross sections at the welds of samples with an article of which the soaking temperatures were between 650 and 750 °C for 2 or more minutes show that in the coating the inhibition layer has disappeared and that part of the steel substrate has dissolved in the zinc layer, including Mn and Si internal oxides. This results in a Fe-zinc intermetallic layer of 2 - 5 pm thickness containing 18 or more wt.% Fe (the r phase) near the substrate, whereas the remainder of the zinc coating contains approximately 10 wt.% zinc (the 8 phase). Due to the higher Fe content of the zinc layer, less liquid Zn (or no liquid Zn) will be present in the intermetallic layer during resistance spot welding and that results in a lower LME sensitivity, or no LME sensitivity at all. Hence, the inventors found that the soaking time and temperature is of prime importance to avoid the appearance of LME cracks in the HAZ of the resistance spot welds to connect a zinc or zinc alloy coated hot formed article to a steel object.

When the soaking temperature is lower than 650 °C, not enough Fe dissolves into the Zn layer and as a result the melting point of the Zn-Fe phase remains low and zinc-rich liquid is present during resistance spot welding. On the other hand, at temperatures higher than 750 °C, LME occurs during hot forming of the zinc or zinc alloy coated steel substrate itself due to presence of too much liquid zinc or zinc alloy. Preferably, the Fe content in the zinc coating is at least 16 wt.% to get the r phase.

It can be expected that LME during welding is also reduced for higher soak temperatures; however, higher soak temperatures are specifically unfavourable for hot forming, and also have a negative effect on weldability due to excessive oxidation of the zinc coating, and may lead to reduced corrosion protection.

It is preferred when the zinc or zinc alloy coated steel blank is soaked during 2 - 10 minutes, and it is even more preferred when the soaking time is 2 - 8 minutes or even 2 - 5 minutes. Shorter soaking times are preferred in the industry for reasons of mass production. Also, long soaking times are not favourable for the blank in view of grain coarsening and the like. The thickness of the zinc or zinc alloy coating on the steel blank is as usual in the art. This can be a minimum of 5 micron and a maximum of 30 micron. However, thicker coatings will increase the LME sensitivity, as is known in the art. The coating thickness is therefor preferably 5 - 15 micron.

The heating of the zinc or zinc alloy coated steel blank is usually performed at a heating rate of 5 to 100 °C/s. These are the heating rates used in the furnaces for hot press forming. The cooling of the zinc or zinc alloy coated steel article is usually performed at an average cooling rate of 1 °C/s or more, to a temperature below 200 °C. Below the temperature of 200 °C the zinc or zinc alloy coated hot formed steel article is usually cooled in air.

Preferably, the zinc or zinc alloy coated steel blank contains 5.0 - 9.0 wt.% Mn, preferably 6.0 - 8.0 wt.% Mn. For these more limited ranges, the soaking temperature and time can result in a better performance of the resistance spot welds with regard to LME cracking.

According to a preferred embodiment of the method, the zinc or zinc alloy coated steel blank consists of, in wt.%:

C: 0.05 - 0.3, preferably 0.08 - 0.2, more preferably 0.08 - 0.15

Mn: 3.0 - 12.0, preferably 5.0 - 9.0, more preferably 6.0 - 8.0

Al: 0.5 - 2.1 , preferably 0.7 - 2.1 , more preferably 1.0 - 1.8

Si: 0.1 - 1.0, preferably 0.1 - 0.7, more preferably 0.15 - 0.4 optionally one or more further alloying elements selected from the group consisting of:

Cr: less than 1.0;

V: less than 0.1 ;

Nb: less than 0.1 ;

Ti: less than 0.15;

Mo: less than 0.3 inevitable impurities; the remainder being Fe.

Using a steel blank having this composition results in a hot formed article having the advantages of a high tensile strength of at least 950 MPa up to 1600 MPa and a good bendability, as is for instance advantageous in many parts of the body in white of a car. It is preferred when the hot formed article has a tensile strength of at least 980 MPa. The inevitable impurities result from the ironmaking and steelmaking process. In this respect, S, P and N at least are considered as residual or inevitable impurities. Therefore, their contents are usually less than 0.01 wt.% for S, 0.04 wt.% for P and 0.008 wt.% for N.

The composition above is to be understood as the nominal composition, i.e. the composition of the steel which is present in the zone centred on the mid-thickness of the sheet.

Preferably the soaking temperature is between 670 and 750 °C, preferably between 670 and 730 °C. By using these soaking temperatures one can be sure that the melting point of the Zn-Fe phase is high enough, and that the substrate has not dissolved into the zinc layer to a too large extent and/or that not too much liquid Zn is present and/or that Zn evaporation and oxidation remain low.

According to a preferred embodiment of the method, the steel object is a forming steel sheet, such as a DX grade sheet, preferably a DX54 grade steel, that is optionally formed, or wherein the steel object is a zinc or zinc alloy coated hot formed steel article as described above. DX grade steel is standardised in EN 10346:2015 (E). A DX54 grade sheet is often used in the automotive industry, and has to be connected to hot formed articles in an automotive vehicle. However, other (forming) grades steel are also used in the body in white of automotive vehicles. Also hot formed articles according to the invention have to be connected to each other.

Often the resistance spot welding operation is performed to connect one zinc or zinc alloy coated hot formed steel article and one steel object as a 2-stack combination. In assembling the parts of a car a 2-stack combination has to be welded many times.

On the other hand, there are situations wherein the resistance spot welding operation is performed to connect one zinc or zinc alloy coated hot formed steel article and two steel objects as a 3-stack combination. In assembling the parts of a car a 3- stack combination also has to be welded on certain occasions.

In rare occasions, even a 4-stack combination can be resistance spot welded.

In an embodiment, the zinc or zinc alloy coating consists of up to 1.0. wt.% aluminium and optionally at most 0.3 wt.% of one or more additional alloying elements each, the remainder being zinc and unavoidable impurities, or the zinc or zinc alloy coating consists of 0.2 - 5.0 wt.% Al and 0.2 - 5.0 wt.% Mg and optionally at most 0.3 wt.% of one or more additional alloying elements each, the remainder being zinc and unavoidable impurities. These coatings are very suitable for the hot formed steel articles, since they provide a good galvanic protection when used in for instance a car. The additional alloying elements are known to the person skilled in the art and are Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Si, Ni, Zr and Bi. At most 0.3 wt.% can be added of each additional alloying element, preferably at most 0.1 wt.% of each additional element, more preferably at most 0.03 wt.% of each additional element. These additional elements and their role are for instance known from document WO2014/135999. Such amounts of alloying elements do not influence the galvanising a steel strip. Unavoidable impurities are for instance Fe, Pb, Cu, Cd, Mg, Sn, Ni, Bi, Sb, but need not be described further.

The invention is not limited by the coating process that is used. Thus, the zinc or zinc alloy coating can be applied on the steel surface by hot dip galvanising (Gl), hot dip galvannealing (GA), electrogalvanising (EZ), physical vapour deposition (PVD) or chemical vapour deposition (CVD). The presence of LME cracks in the welds will be (substantially) avoided regardless of the method of zinc or zinc alloy coating process that is used to apply the coating on the steel that is used in this invention.

According to a preferred embodiment the zinc or zinc alloy coated steel blank and the steel object each have a thickness of 0.5 - 2.5 mm, preferably a thickness of 0.8 - 2.0 mm. Such thicknesses are suitable for automotive vehicles and can be well resistance spot welded.

Preferably the resistance spot welding is performed using welding electrodes, with a hold time of 100 - 500 milliseconds and an electrode force of 2 - 6.5 kN. Using such hold times and forces results in good spot welds as required in assembling automotive vehicles.

For the resistance spot welding described above, the welding electrodes used preferably have a tip diameter of 5 - 20 mm. Such diameters result in good spot welds.

When the resistance spot welding is performed as described herein, the total weld time is preferably 100 - 1500 milliseconds, either as single pulse weld time or as multi pulse weld time. Usually two, three or four pulses are used within the multi pulse weld time.

A weld current I between I min and 1.10 I max should be used, where I min is the minimum current above which pullout failure is observed when the resistance spot weld is submitted to shear tensile test, and Imax is the current at which expulsion of liquid metal from the interface between the welded objects starts to be observed in resistance spot welding. The weld current depends inter alia on the thickness of the objects to be welded and the type steel that has to be welded.

According to a second aspect of the invention there is provided a resistance spot welded combination of a zinc or zinc alloy coated hot formed steel article and a steel object, produced in accordance with the method according to the first aspect of the invention as described above, wherein the resistant spot welds are free or substantially free from liquid metal embrittlement cracks in cross section of the heat affected zone of the resistance spot welds.

The inventors have found that by using the method according to the invention, the resulting resistance spot welds are (substantially) free from LME cracks in the heat affected zone (“HAZ”) of the welds. This is shown by visual examination of the cracks after dezincing the welds and also by making cross sections of the resistance spot welds. Thus, for instance the automotive industry can use the zinc or zinc alloy coated hot formed articles according to the invention and use these articles as parts in a car, and resistance spot weld these parts to other parts, such that these connections are (substantially) free from LME cracks in the resistance spot welds. With substantially free is meant that the cracks are (much) smaller than the LME cracks observed when the invention is not used, and/or the amount of cracks is (far) less that the amount of cracks observed when the invention is not used. To do so, a reasonable amount of welds should be observed, for instance at least 5 welds, preferably more.

The invention will also work to avoid the LME cracks in the welds of the hot formed article to other steels, when welding is done by laser welding or arc welding, where the presence of thermal stresses can cause LME cracks in the zinc or zinc alloy coated hot formed article.

Preferably the zinc or zinc alloy coated hot formed steel article has a tensile strength of 950 MPa or more outside the resistance spot welds and/or a total elongation of 15 % or more (tested according to ISO 6892-1 norm with 50 mm gauge length and 12.5 mm width of the specimens) outside the resistance spot welds and/or at 1.0 mm thickness a minimum bending angle of 90° or more (tested according to VDA 238-100 norm) outside the resistance spot welds. Such hot formed articles are especially suitable for use in a body in white of an automotive vehicle.

Preferably the zinc or zinc alloy coated hot formed steel article has a microstructure comprising, in vol.%: ferrite 30% or more austenite 20% or more martensite 50% or less including 0%.

Such a microstructure results in very good mechanical properties of the hot formed article.

It is preferred when the zinc or zinc alloy coated steel blank is obtained from an intercritically annealed cold rolled or hot rolled steel strip that has been subjected to intercritical annealing at an annealing temperature in the same range as the soaking temperature according to the invention, as mentioned as a preferred method in document WO 2019/155014. The blank is preferably heated at a heating rate of 30 °C/s to the soaking temperature, also as mentioned as a preferred method in document WO 2019/155014.

The invention also relates to the use of a resistance spot welded combination as described above for the fabrication of parts of automotive vehicles.

The invention will be elucidated with reference to the below example, which is not limitative.

Figure 1 shows: (a) a portion of a hot formed -shaped galvanised medium-Mn grade steel sheet according to the invention (i.e. a HPF1000 Gl grade steel), and positions of the samples used for the RSW tests; (b) resistance spot weld configuration and LME HAZ crack classification from dezinced weld (top view); (c) dimensions (in mm) of the high temperature tensile test sample.

Figure 2 shows: (a) the average HAZ crack length from visual analyses for the various soaking conditions, and (b) the relative ductility ratio determined from the high temperature tensile tests of the non-HPF and 675 °C hot formed samples

Figure 3 shows cross sections of the resistance spot welds made on material from the top side of -profiles, showing deep HAZ cracks for the lower temperature HPF cycles, and no cracks for the 5 min. @675 °C cycle.

Figure 4 shows Glow discharge optical emission spectroscope (GDOES) profiles for the Zn coating layers of various samples.

Figure 5 shows SEM images of the coating-substrate interface for (a) a non-HPF sample (which is similar to those observed for the 530 and 620 °C variants), and (b) a sample hot press formed at 675 °C.

A HPF1000 steel (HPF is the abbreviation of “hot press formed”) with a nominal composition of 0.1C-7Mn-1.5AI-0.2Si (wt.%), the remainder being Fe and unavoidable impurities, was cast and processed in the laboratory to a final thickness of 1.5 mm employing hot and cold rolling, followed by continuous annealing at 675 °C and hot dip coating with a Zn-0.4 wt.% Al alloy in a Rhesca annealing simulator. The coating used had a thickness of approximately 20 micron. These hot dip galvanised sheets were pressed to 100 mm-long -shaped profiles (see a section of the profile in Fig. 1a) in a Schuler SMG hot forming press by direct hot press forming using three soaking temperatures of 530 °C, 620 °C and 675 °C, all for 5 minutes. The hot press formed (HPF) samples were analysed with scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES) to characterise the coating and coating-substrate interface.

The composition of the HPF1000 steel used for the experiments is: C = 0.13; Mn = 6.91 ; Al = 2.019; Si = 0.242; P = 0.0037, S = 0.007, Ni = 0.0024, Cr = 0.0006, Sn = 0.002, Mo = 0.0137, Nb = 0.0026, V = 0.008, Ti = 0.0028, Pb = 0.0368; Zr = 0.0009; As = 0.0044; Ca = 0.00045; N = 0.00228, all in wt.%. Only C, Mn, AL and Si are alloying elements. The other elements are inevitable impurities result from the ironmaking and steelmaking process Boron has not been added as an alloying element, but can be present as an impurity, up to 5ppm.

Small (~25 x 25 mm) samples were cut from several positions of the hot formed Q-profiles, as indicated in Fig. 1a and resistance spot welded to a single layer of 2 x 30 x 30 mm forming steel DX54-GI (Fig. 1b), using extreme heat input weld settings that are known to provoke LME cracks (F1-16 x 6 mm electrodes, 4.5 kN electrode force, 1140 ms weld time, 300 ms hold time). In order to reveal the presence of LME cracks, the welded samples were dezinced using a diluted HCI-solution with inhibitor, as known in the art. Visual analyses were performed for the crack location and their size, where the heat affected zone (HAZ) crack length is reported as a proportion of the weld circumference. In addition, four welds for each variant, taken from samples from the top side of the -profile were cross sectioned in the direction perpendicular to the largest observed LME crack and investigated with light optical microscopy to quantify the depth of the LME cracks. Non-HPF samples were also included in RSW for comparison.

An additional series of flat sheets were produced using the same HPF temperature cycles as described earlier. Tensile samples (Fig. 1c) were machined from these sheets, which were subjected to high temperature tensile testing in a Gleeble 3800 thermomechanical simulator, using a 1000 °C/s heating rate to the target test temperature, followed by a 1s soak time at that temperature, and a tensile test with a stroke rate of 3 mm/s. Samples were tested in both single side coated and uncoated (after fully removing the coating by dezincing) conditions. The LME sensitivity was quantified as the relative ductility (stroke at 50% maximum force, F _ma x) ratio of the coated sample to its uncoated equivalent, tested at the same temperature.

Figure 2a shows the average HAZ crack length from the visual observations of the resistance spot welded samples. These results match well with the cracks observed in the weld cross sections in Fig. 3, where large HAZ cracks were found for the non-HPF samples, and for the samples reheated at 530 °C and 620 °C. In contrast, the material produced in accordance with the invention that was reheated at 675 °C showed some very small cracks in the visual observation, and no cracks in the cross sections.

Figure 2b shows the relative ductility ratio of the coated samples as measured from the hot tensile tests for the non-HPF material and the material hot formed at 675 °C. . The relative ductility ratio is the ratio of strokes at 50% of maximum force (F _ma x) of coated to uncoated specimens tested at high temperatures in tension. The heat affected zone (HAZ) crack length is reported as a proportion of the weld circumference.

The LME sensitivity of the hot formed material shows less severe LME, indicated by the higher test temperatures at which a certain amount of ductility drop occurs, as well as a lower loss of ductility at all temperatures, including at 800 °C.

The differences in LME sensitivity of the various samples found in the RSW and high temperature tensile tests can be understood by looking at the substrate coating interactions that take place during hot press forming. Figure 4 shows the elemental concentration profiles (Fe, Zn, Mn and Si) for the coatings of the various samples before and after hot forming at various temperatures. The profiles show that there has been little coating-substate interaction during hot forming reheating at 530 °C and 620 °C. In contrast, in the sample that was reheated at 675 °C significant amount of Fe (~10 wt.% Fe) and also some Mn and Si diffused into the Zn layer from the steel substrate.

Results of the GDOES analyses are in line with SEM observations, that show similar coating-substrate structures for the non-HPF material, shown in Fig. 5a (scale: 300 nm), as for the 530 °C and 620 °C hot formed samples. All these samples show a fully intact steel substrate with some internal and subsurface selective oxidation, a fully intact inhibition layer, and a Zn coating with <5 wt.% Fe. In contrast, the SEM image of the samples hot formed at 675 °C (Fig. 5b; scale: 1 pm) shows that the inhibition layer has fully disappeared, and part of the substrate, including the Mn an Si internal oxides, have dissolved in the Zn layer, leaving a Zn layer with a ~2-5 ,m thick layer of r phase (Zn-25 wt.% Fe) while the rest contains predominantly 5 phase (~12 wt.% Zn). The LME sensitivity is to a large extent driven by the availability of liquid zinc at the temperatures that prevail during welding (and high temperature tensile testing). The higher Fe content in the coating of the samples hot press formed at 675 °C leads to a reduction of liquid zinc available for LME cracking.

It will be apparent to one of ordinary skill in the art that many changes and modification can be made without departing from the scope of the invention as herein described.