PRODUCT The invention relates to a hot formable strip, sheet or blank which is formable at a temperature of 600° C or above, comprising a substrate of hot formable steel coated with a corrosion protective coating. The invention also relates to a process for producing such a strip, sheet or blank, a method for hot forming a product and a hot formed product made therefrom.

Such a coated strip, sheet or blank is known from EP 0971044A1 , relating to an

Al-Si coated boron steel; the process of hot forming a zinc coated boron steel is known from for instance EP 1143029A1.

Uncoated boron steels are known to form Fe oxides during the heat treatment preceding the hot forming step in a die, as a consequence whereof loose and thick oxide layers are formed on the surface, which can pollute and damage the surface of the die. Moreover, such oxide layers interfere with the welding process of the formed product during the subsequent use of the formed product, and also contaminate subsequent painting processes. Therefore, the oxide layers have to be removed after the hot forming process of the uncoated steel products, which is inefficient and costly.

To overcome the above problems and to improve the corrosion performance of the steel, coated boron steels have been developed, and the boron steel substrate has been covered with a metallic coating such as an Al-Si coating or a Zn based coating. However, it has been found that during the hot forming process the coatings are covered by an oxide layer due to the high temperature needed for the hot forming process, which oxide layer is formed from the coating material. A zinc-oxide layer makes that it is difficult to spot weld the hot formed products. Good spot weldability is needed for the automotive industry, where hot formed parts are used.

It is an object of the invention to provide a coated strip, sheet or blank suitable for hot forming that has a good spot weldability after hot forming.

It is a further object of the invention to provide a coated strip, sheet or blank suitable for hot forming that is economical to produce.

It is also an object of the invention to provide a process for the production of a strip, sheet or blank that meets one or more of the objects hereinabove.

It is furthermore an object of the invention to provide a method for hot forming a product and a hot formed product that has a good spot weldability. According to the invention one or more of the above objects are reached with a steel strip, sheet or blank for hot forming at a temperature of Ac1 or above, comprising a substrate of hot formable steel coated with a corrosion protective coating, wherein the coated steel substrate is covered by a siloxane layer.

The inventors have found that such a siloxane layer is suitable to reduce the extent of oxidation during the hot forming of a steel strip, sheet and blank coated with a corrosion protective coating. The siloxane layer also reduces zinc losses due to evaporation. When hot formed, the siloxane coated material will be oxidised to leave a thin adhering silicon oxide layer, which has a low contact resistance and consequently good weldability. Because of the reduced oxidation of the corrosion protective coating, the siloxane layer also improves the effectiveness of the coating for corrosion protection of the steel. The steel substrate is for hot forming at Ac1 -temperature or above. Such steel substrates can be used for hot forming and quenching of the hot formed product, which usually provides a tensile strength of 1500 MPa or more.

Siloxane layers are normally used as chromate replacing passivation treatment or as phosphate or chromate replacing conversion treatment to improve adhesion with adhesives or paints. They are formed after curing of a water-based silane film eventually containing alcohol as well, applied by roll coating, dip/squeezing or spray/squeezing and contain silicon atoms that are bound with oxygen atoms and/or hydroxyl groups as well as with the carbon atoms of hydrocarbon and/or hydrocarbyl groups. The carbon atoms of the hydrocarbon and/or hydrocarbyl groups can be bound with each other or with intermediate amine or carbonyl groups and/or sulphur atoms.

According to a preferred embodiment the siloxane layer originates from a bis- silane containing an amount of carbon such that C/Si≤ 1. The siloxane layer thus adds optimal to the protection of the steel due to the little formation of carbon dioxide resulting in an almost porous-free, homogeneous silicon oxide layer, which has shown to provide best welding possibilities for the hot formed product.

According to a preferred embodiment, the thickness of the siloxane layer is such that at most 200 mg Si/m ² remains in the silicon oxide layer after hot forming, preferably between 20 and 100 mg Si/m ² , and more preferably between 40 and 100 mg Si/m ² . The inventors have found that the thickness of the metal-phosphate layer should be at most 200 mg Si/m ² since with higher thickness the silicon oxide layer will itself hamper the contact resistance of the spot welded material. Moreover, also the costs of the layer become too high. Therefore, thinner layers are preferred. According to a preferred embodiment the corrosion protective layer is a zinc or zinc alloy layer. A coating of zinc or a zinc alloy gives a very good, active corrosion protection of the steel substrate.

According to a preferred embodiment the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 8 to 12 weight %. It has been found that a siloxane layer as mentioned above on this type of coating will give good spot weldability. Using this galvannealed zinc or zinc alloy layer makes it easier to hot form the coated steel as well.

According to an embodiment the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 1 to 5 weight %.

It is possible that the zinc alloy layer consists of 0.3 to 2.3 weight % magnesium and 0.6 to 2.3 weight % aluminium, optional less than 0.2 weight % of one or more additional elements, the remainder being zinc and unavoidable impurities. An optional element that could be added in a small amount, less than 0.2 weight %, could be Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form spangles. This type of coating has an improved corrosion resistance as compared to usual zinc layers.

Preferably the corrosion protective layer has a thickness of 2 to 25 m. These thicknesses are suitable to produce hot formed parts for automotive purposes.

According to a preferred embodiment the hot formable steel substrate is a steel substrate, having a composition in weight percent:

C between 0.04 and 0.5 %

Mn between 0.5 and 3.5 %

Si less than 2.0 %

Cr less than 1.0 %

Ti less than 0.2 %

Al less than 2.0 %

Mo less than 0.5 %

P less than 0.1 %

N less than 0.015 %

S less than 0.05 %

B less than 0.015 %

the remainder being Fe and unavoidable impurities.

These steel types are suitable for hot forming, wherein the alloying elements can still be chosen within limits. Usually the amount of boron is between 0.001 and 0.005 weight %. Preferably, the hot formable steel substrate is a steel substrate, having a composition in weight percent:

C between 0.04 and 0.5 %

Mn between 0.5 and 3.5 %

Si less than 2.0 %

Cr less than 1.0 %

Ti less than 0.2 %

Al less than 2.0 %

Mo less than 0.5 %

P less than 0.1 %

Nb less than 0.1 %

N less than 0.015 %

S less than 0.05 %

B less than 0.015 %

the remainder being Fe and unavoidable impurities.

These steel types have better mechanical properties when Nb is added. Also in this embodiment the amount of boron is usually between 0.001 and 0.005 weight %.

Preferably, the hot formable steel substrate is a steel substrate, having a composition in weight percent:

C between 0.04 and 0.5 %

Mn between 0.5 and 3.5 %

Si less than 2.0 %

Cr less than 1.0 %

Ti less than 0.2 %

Al less than 2.0 %

Mo less than 0.5 %

P less than 0.1 %

N less than 0.015 %

S less than 0.05 %

B less than 0.015 %

W less than 0.3 %

the remainder being Fe and unavoidable impurities.

These steel types have better mechanical properties when W is added. Also in this embodiment the amount of boron is usually between 0.001 and 0.005 weight %.

Preferably the steel substrate has a thickness between 0.5 and 3.0 mm. Such thicknesses are used in the automotive industry. The invention also relates to a process for producing a hot formable strip, sheet or blank as described above, wherein a substrate of hot formable steel coated with a corrosion protective coating is covered by a solution of silane, whereafter the strip, sheet or blank is heated to form the siloxane layer. In this way it is possible to provide the steel substrate with the siloxane layer as exemplified above.

According to a preferred embodiment the silane solution is applied on the coated steel strip, sheet or blank as a wet film having a thickness of 1 to 10 ml/m ² by spraying or dipping and subsequently squeezing or by roll coating, preferably at the end of a coating line, and the wet film is dried and cured at a temperature of between 70 °C and 200 °C, preferably between 70 °C and 100 °C. This process of applying the wet film is easy to perform and gives an even wet layer on the coated substrate, which is dried and cured in the usual manner, for instance in an oven.

Preferably, the siloxane covered corrosion protection coated substrate embodies one or more of the features of the strip, sheet or blank as exemplified above.

The invention moreover relates to a method for hot forming a product using a blank as described above or using a blank formed from a strip or sheet as described above, wherein the blank or a pre-form formed out of the blank is heated to the Ac1- temperature or above, the blank or pre-form is hot formed in a hot forming die, and the hot formed product is quenched. This is the usual hot forming process, which is now performed using a blank according to the invention or a blank produced from the strip or sheet according to the invention. Using such a blank with a siloxane layer according to the invention, the product formed has a good weldability.

The invention furthermore relates to a hot formed product using the strip, sheet or blank as described above or produced according to the method above, wherein the coated hot formed substrate has a contact resistance below 1 mOhm. The contact resistance was measured according to a method described in DVS2929:2007 with the exceptions that a 7.5kN force and 300mm electrode radius were used. A contact resistance of below 1 mOhm stands for good weldability.

The invention will be elucidated referring to some background information and a number of experiments hereinafter, shown in the accompanying figures.

Fig. 1 shows the contact resistance of heated zinc coated steel for two different siloxane layer thickness as well as the expected siloxane layer thickness needed to achieve good weldability (< 1 mOhm).

Fig. 2A shows a zinc coated substrate without post treatment after heating. Fig.2B. shows the same substrate with a siloxane layer after heating.

Due to the high temperatures during hot forming, zinc coated material oxidises rapidly. Due to the nature of the formed oxide, the contact resistance of the material will increase. In general, a hot formed material with a contact resistance larger than 1 mOhm is regarded as unweldable. After a commonly used heat treatment of 6 minutes in a furnace of 900 °C , the contact resistance is usually larger than 5 mOhm. The amount of oxide can be reduced by a lower furnace temperature and/or shorter dwell times in the furnace but this will seriously affect the robustness of the hot forming process. Low furnace temperatures make the material susceptible to ferrite formation which will decrease the final material strength. A short dwell time minimizes the flexibility of the process, since sometimes the samples have to remain in the furnace for a longer time period due to technical issues at the pressing station behind the furnace.

It is the opinion of the inventors that the amount of oxide should be lowered (without lowering the furnace temperature or shortening the maximum furnace dwell time) in order to maintain a very robust hot forming process. Therefore they investigated several post-treatments that would minimize the total amount of oxidation at comparable heating cycles.

One type of silane namely bis-(trimethoxysilyl)ethane (BTSE) (which is commercially available) was chosen for the investigation. This silane was applied as a water-based solution for two different concentrations namely 0.5 % weight % and 3 weight % by roll-coating, resulting in a wet film thickness of ca. 10 ml/m ² to give a siloxane coating weight of respective 2 and 12 mg Si/m ² after drying in an oven. The substrates were 22MnB5 steel sheets coated with a zinc coating that had an additional heat treatment to have 12 % Fe in the coating.

Table 1 below shows the results the inventors obtained for different post-treatments after a heat cycle of 6 min in a chamber furnace heated to 880 °C . It will be clear that the applied siloxane layer increases the amount of zinc remaining in the sheets, contributing to a better corrosion protection, and reduces the amount of zinc-oxide, which reduces contact resistance. However, the difference between both concentrations is too small to find a significant effect of concentration. Nevertheless, it is clear that by using higher silane concentrations we will achieve the needed contact resistance lower than 1 mOhm that is required for hot formed parts in the automotive industry to provide good welding.

Figure 1 tells us that when we extrapolate the results (dotted line), we at least have to apply siloxane layers containing > 20 mg Si/m ² and preferably > 40 mg Si/m ² , taking deviation into account, to achieve a contact resistance lower than 1 mOhm. Amount Amount of Zn in

Post treatment Contact resistance

of ZnO sheet

(mOhm) (g/m ² ) (g/m ² ) mean Standard deviation

oil 12 5.9 68 48

BTSE (2 mg Si/m ² 8.3 2.1 52 77

BTSE (12 mg Si/m ² ) 5.2 2.8 44 67

Table 1

Figure 2 shows the difference between a zinc coated substrate without post treatment (Fig. 2A) and with the siloxane layer (Fig. 2B). It is clear from the figures that the use of the siloxane layer minimizes the oxide layer formed in the zinc coating due to the heating of the silane treated zinc coated steel.