WO2014009004A1

WO2014009004A1 - Coated hot-formable steel strip, sheet or blank and method for making the same

Info

Publication number: WO2014009004A1
Application number: PCT/EP2013/002016
Authority: WO
Inventors: Uazir Orion Bezerra DE OLIVEIRA; Jose Reyes Flores Ramirez; Fouzia Hannour
Original assignee: Tata Steel Nederland Technology B.V.
Priority date: 2012-07-10
Filing date: 2013-07-09
Publication date: 2014-01-16

Abstract

The invention relates to a hot-formable steel for use in hot forming comprising a hot- formable steel strip, sheet or blank, a metal or metal alloy coating formed on the hot- formable steel strip, sheet or blank and a metallurgical slag based coating formed on the metal or metal alloy coating, wherein the metal or metal alloy coating comprises zinc and/or aluminium and the metallurgical slag based coating comprises a metallurgical slag and a polymeric material.

Description

COATED HOT- FORMABLE STEEL STRIP, SHEET OR BLANK AND METHOD

FOR MAKING THE SAME

The invention relates to a coated hot-formable steel strip, sheet or blank suitable for hot-forming and to a method for manufacturing the same. The invention also relates to a method for hot-forming the coated hot-formable steel strip, sheet or blank and to the hot-formed product thus produced.

Hot-forming is a process that is commonly used to produce high-strength, lightweight body parts for motor vehicles. The process generally comprises the steps of cutting a blank from a steel strip or sheet, heating the blank to above the Acl temperature of the steel material being processed, hot-pressing the heated blank to form an automotive body part and hardening the part.

During the heat treatment uncoated steel is susceptible to high temperature oxidation, which is a corrosion process where iron in the steel reacts with atmospheric oxygen at elevated temperatures. The presence of iron oxides at the steel surface reduces the weldability of the hardened part and also adversely affects the aesthetic appearance of the hardened part after paint baking. To prevent or at least minimise the formation of iron oxides at the steel surface, the steel material may be heated in a controlled atmosphere and/or provided with a metallic coating. The use of Al-Si metallic coatings to inhibit high temperature oxidation is known from EP0971044. Although Al-Si coatings protect the steel from high temperature oxidation, they do not provide cathodic corrosion protection and are difficult to paint. Zinc or zinc alloy coatings can also be used to protect steel from oxidation during hot-forming. Such coatings are known from EP1 143029. However, due to the relatively low melting point of zinc, such a method suffers from the disadvantage that zinc is lost during the heat treatment and/or hot-pressing due to evaporation. Consequently the degree of passivation and cathodic corrosion protection afforded to the steel is reduced. A further disadvantage of using zinc based coatings is that a thin layer of zinc oxide forms at elevated temperatures, which reduces the weldability of the zinc coated steel. An additional process step is therefore required to remove the zinc oxide layer before welding can occur.

Accordingly, it is an object of the present invention to provide a hot-formable steel strip, sheet or blank provided with a metallic coating, which enables improved retention of the coating during hot-forming. Another object of the present invention is to provide a hot-formable steel strip, sheet or blank that affords improved passivation corrosion protection and cathodic corrosion protection to the steel. A further object of the invention is to provide a coated steel strip, sheet or blank suitable for hot forming that is economical to produce. It is also an object of the invention to a provide hot- formable steel strip, sheet or blank that exhibits improved weldability after hot-forming.

As a by-product of the steel manufacturing process, vast quantities of steel slag is generated by the steel industry that must be recycled or disposed, Such 'steel making slag' includes blast furnace slag, blast oxygen furnace slag, ladle furnace slag and electric arc furnace slag

Blast furnace (BF) slag is produced as a by-product during the manufacture of pig iron. Although BF slag may be recycled externally, for instance as an aggregate in cements or as a fertiliser in agriculture, BF slag is often not re-used by the steel manufacturer. In this respect BF slag may not be re-introduced into the sinter plant or into the blast furnace without further treatment due to the presence of phosphorous, which forms phosphate precipitates that are detrimental to the steel thus produced.

Blast oxygen furnace slag is produced as a by-product of the basic oxygen steel (BOS) making process in which molten pig iron from a blast furnace is charged into a basic oxygen furnace (BOF) together with scrap metal, fluxes, alloys and high purity oxygen. Oxygen is used primarily for the decarburisation and conversion of molten pig iron to liquid steel, while alloys are added to tailor the properties of the steel itself. Fluxes such as burnt lime or dolomite are provided to form slag, the purpose of which is to absorb impurities in the steel and those introduced during the steel making process.

The chemical composition and physical properties of BOF slag prevents or at least limits steel manufacturers from recycling BOF slag themselves. In this respect, the presence of phosphorous in BOF slag prevents it from being re-introduced into the steel making process.

The relatively high CaO content in BOF slag also makes BOF slag less preferred as material for use in the manufacture of concrete. This is primarily due to the increased risk of cracks forming in the concrete caused by the hydration reaction of CaO to calcium hydroxide ((CaOH₂).

A further disadvantage is that BOF slag contains Cr(lll) (Cr₂0₃) compounds, which when heated to above 1400^°C, for instance in a cement kiln, oxidise to form hexavalent chromium compounds that are hazardous to human health,

Thus, it is a further object of the invention to find a new use for steel making slag, particularly BOF slag.

It is another object of the invention to find a new use for steel making slag in the manufacture of steel products.

One or more of the above objects are satisfied by the present invention, which according to a first aspect defines a hot-formable steel for use in hot forming comprising a hot-formable steel strip, sheet or blank, a metal or metal alloy coating formed on the hot-formable steel strip, sheet or blank and a metallurgical slag based coating formed on the metal or metal alloy coating, wherein the metal or metal alloy coating comprises zinc and/or aluminium and the metallurgical slag based coating comprises a metallurgical slag and a polymeric material. The inventors found that the metallurgical slag based coating adheres very well to the steel strip, sheet or blank, which ensures that the coating does not delaminate prior to hot-forming. The inventors also observed that the extent of high temperature oxidation can be reduced by providing a steel strip, sheet or blank with the metallurgical slag based coating prior to subjecting the coated steel to a high temperature heat treatment. This has been attributed to the formation of a layer on the metal or metal alloy coating, that is formed from the metallurgical slag during a heat treatment up to 1200^"C. This layer also acts as physical barrier that reduces zinc evaporation when the metallic coating is zinc or a zinc alloy. Consequently the corrosion protective properties of the zinc or zinc alloy coating after the heat treatment are also improved. After the heat treatment, the hot- formable steel strip, sheet or blank comprising the metal or metal alloy coating and the layer formed from the metallurgical slag exhibited an electrical resistivity below 1 mOhm. The hot-formable steel strip, sheet or blank of the invention is therefore very suitable for welding.

In a preferred embodiment of the invention the polymeric material is water soluble and comprises polyimides, preferably polyamideimide.

Other suitable polymeric materials include polyphenylenesulphides (PPS), liquid crystal polymers (LCP), polyarylether sulphones (PES), polyetherimides (PEI), polysulphones (PSU), polyetherketones (PEEK), fluoropolymers (FP), polyacrylates (PAR) and polyphthalamides (PPA).

In a preferred embodiment of the invention the thermal degradation temperature of the polymeric material is at least 250 C, preferably at least 400^°C. The cured polymeric material provides flexibility to the metallurgical slag based coating and ensures that the coating adheres to the steel strip, sheet or blank prior to the heat treatment. The polymeric material also acts as a physical barrier that prevents or at least reduces abrasion of the underlying metal or metal alloy coating. Below the thermal degradation temperature of the polymeric material, both the polymeric material and the metallurgical slag protect the coated steel from oxidation at lower temperatures. Above the thermal degradation temperature of the polymeric material, the metallurgical slag protects the coated steel from high temperature oxidation.

In a preferred embodiment of the invention the dry film thickness of the metallurgical slag based coating is up to 30 μηι, preferably between 2 and 10 μιη. The inventors found that a metallurgical slag based coating having a dry film thickness below 2 Mm did not offer sufficient barrier protection prior to the heat treatment. Moreover, such a thickness was not effective at retaining zinc during the heat treatment when the metal or metal alloy coating was a zinc or zinc alloy. A dry film thickness of up to 30 μιη is also preferred since above this thickness the metallurgical slag based coating may delaminate. A dry film thickness between 2 and 10 m is most preferred since such coatings are robust and zinc evaporation is kept to a minimum.

In a preferred embodiment of the invention the metallurgical slag comprises basic oxygen furnace (BOF) slag, blast furnace (BF) slag or a mixture thereof. The inventors found that layers formed from BOF and/or BF slag are capable of retarding oxidation and retaining the metal or metal alloy at the steel surface. CaO is particularly effective as a high temperature oxidation protective barrier. After the heat treatment, the hot-formable steel strip, sheet or blank is afforded passive corrosion protection from the layer formed from the metallurgical slag and cathodic corrosion protection from the metal or metal alloy coating when the metal or metal alloy comprises zinc. Electric arc furnace slag (EAF) may also be used in accordance with the invention. The composition and properties of EAF slag are similar to BOF slag, although EAF slag generally has a lower content of free magnesium and calcium oxides. In addition to BOF, BF and EAF slags, it is also possible to use ladle slag obtained from the ladle furnace. Ladle slag contains more aluminium oxide and less iron oxide compared to BOF slag. Such steel making slags comprise free oxides and mixed oxide compounds formed from oxide anions and two or more elemental cations. As an example, BOF slag comprises both CaO and calcium silicate, with calcium silicate being formed from the reaction between CaO and silica (Si0₂). Such compounds are only formed at high temperatures such as those used in blast oxygen, blast, electric arc and ladle furnaces.

In a preferred embodiment of the invention the metallurgical slag comprises one or more oxide particles of Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S. Preferably the metallurgical slag comprises one or more of CaO, Al₂0₃, Si0₂, FeO, Fe₂0₃, MgO, MnO, Mn₃0₄, Ti0₂, P₂0₅, V₂0₅, Cr0₂, Na₂0, K₂0 and S0₃.

It has been found that metallurgical slag based coatings comprising one or more of the above oxides are very suitable for retarding oxidation and preserving the metal or metal alloy coating during hot-forming. The inventors found that oxides of at least Al, Si and Mn preferentially react with the metal or metal coating during the heat treatment to form a layer on the metal or metal alloy coating capable of providing passive corrosion protection and protection against high temperature oxidation. Preferably the metallurgical slag comprises a mixed oxide compound formed from oxide anions and at least two elemental cations selected from Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S. Such mixed oxide compounds contribute to retarding oxidation during hot-forming and to providing passivation corrosion protection to the underlying hot-formable steel substrate. Examples of such mixed oxide compounds include (Fei._xMg_x)0, Ca₂Fe₂0_5i Ca₂Si0₄ as well as those disclosed in Table 1.

In a preferred embodiment of the invention the metallurgical slag comprises 20 to 75 wt% of calcium oxides. When the content of calcium oxides is within the aforementioned range the extent of high temperature oxidation at the steel surface is greatly reduced. As well as reducing the extent of high temperature oxidation, it was found that the layer formed from such a metallurgical slag composition afforded very good passive corrosion protection to the underlying substrate.

Preferably the metallurgical slag comprises:

- 40 to 65 wt% of calcium oxides

- up to 45 wt%, preferably between 20 and 45 wt% of aluminium oxides

up to 35 wt%, preferably between 1 and 15 wt% of silicon oxides

- up to 15 %, preferably between 1 and 10 wt% of iron oxides

In another embodiment of the invention, the metallurgical slag comprises:

30 to 50 wt% of calcium oxides

- up to 10 wt%, preferably between 1 and 5 wt% of aluminium oxides

up to 35 wt%, preferably between 10 and 20 wt% of silicon oxides

up to 35 %, preferably between 15 and 35 wt% of iron oxides

In a preferred embodiment of the invention the metallurgical slag additionally comprises:

- up to 10 wt % of magnesium oxides

up to 6 wt% of manganese oxides

- up to 2 wt% of titanium oxides

up to 3 wt% of phosphorous oxides

- up to 2 wt% of vanadium oxides

- up to 2 wt% of chromium oxides

up to 2 wt% of sodium oxides

The above oxides can all contribute to improving passivating properties of the layer formed from the metallurgical slag after hot-forming.

For the avoidance of doubt the wt% of the oxides (Ca, Al, Si, Fe, Mg, Mn, Ti, P. V, Cr and Na) mentioned hereinabove takes into account both the wt% of the free oxide and the wt% of the oxides comprised in a mixed oxide compound. The above weight percentages were determined using wavelength dispersive X-ray fluorescence (WDXRF).

In a preferred embodiment of the invention the oxide particles have a size dimension between 0: 1 and 30 μηα, preferably between 0.1 and 10 μηη and more preferably between 0.1 and 1 μηη. It was found that the surface quality of the metallurgical slag based coating and the coated hot-formable steel strip, sheet or blank after the heat treatment can be improved by providing a metallurgical slag containing oxide nanoparticles.

In a preferred embodiment of the invention the metal or metal alloy coating is a galvannealed coating containing zinc and Fe up to 70 weight %, preferably containing Fe up to 40 weight %, more preferably containing Fe up to 20 weight %, still more preferably containing Fe up to 10 weight %. The inventors found that both zinc and iron were homogeneously distributed on the surface of the steel and that the metallurgical slag based coating exhibited good adherence to the galvannealed coating. After the heat treatment, the surface of the galvannealed coating was covered with a layer formed from the metallurgical slag. The inventors found that the surface coverage of the layer could be increased by reducing the iron content in the galvannealed coating.

In a preferred embodiment of the invention the hot formable steel is a boron steel, preferably having the composition in weight percent:

C between 0.04 and 0.5%

Mn between 0.5 and 3.5%

Si less than 1.0%

Cr O.01 and 1.0%

Ti less than 0.2%

Al less than 2.0%

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.

Such steel types are generally known and used for hot forming purposes.

The second aspect of the invention relates to a method for manufacturing a hot-formable steel according to the first aspect of the invention, which comprises the steps of:

(i) providing a hot-formable steel strip, sheet or blank; (ii) hot-dip galvanising or hot-dip aluminising the hot-formable steel strip, sheet or blank;

(iii) providing a coating composition, which composition comprises water, a polymeric material and a metallurgical slag;

(iv) applying the coating composition on the hot-dip galvanised or hot-dip aluminised steel strip, sheet or blank, and

(v) heating the coated steel strip, sheet or blank to cure the coating composition.

In this way it is possible to provide the hot-formable steel as exemplified above. In a preferred embodiment of the invention the coating composition comprises

1-30 wt% metallurgical slag, 0.1 -10 wt% polymeric material and water. Such a composition can be produced at low cost. The inventors found that if the metallurgical slag content is greater than 30 wt% then the composition is difficult to process and the cured coating is brittle once formed. The inventors also found that a composition comprising more than 10 wt% of the polymeric material is less suitable for retaining the metal or metal alloy coating during hot-forming. Preferably the coating composition comprises 5-30 wt% metallurgical slag.

In a preferred embodiment of the invention the step of providing the coating composition comprises the consecutive steps of providing the metallurgical slag in water to form a metallurgical slag solution and then adding the polymeric material to the metallurgical slag solution. In this way metallurgical slag solutions having a high solids content may be obtained. Coatings obtained from such solutions exhibit increased 'compactness' relative to coatings obtained from solutions having a lower solids content. The inventors found that the more compact the coating, the more effective the coating is at retaining the metal or metal alloy coating and reducing high temperature oxidation. The compactness of the coating may be increased by providing a metallurgical slag comprising nanometer sized particles.

In a preferred embodiment of the invention the coated steel strip, sheet or blank is heated using electromagnetic radiation, preferably near infrared radiation. The use of infrared radiation means that the coating composition may be cured in 5-10 seconds. This is much faster than curing by conventional curing means.

In a preferred embodiment of the invention the coated steel strip, sheet or blank is heated, at least in part, using residual heat from the hot-dip galvanised or hot-dip aluminised steel strip, sheet or blank. Making use of the residual heat from the hot-dip galvanising or hot-dip aluminising process allows cost savings to be made since the amount of energy that is required to cure the coating composition, either by conventional or electromagnetic means, is reduced. The third aspect of the invention relates to a method for manufacturing a hot- formed product, which comprises the steps of providing the hot-formable steel strip, sheet or blank according to the first aspect of the invention, heating the hot-formable steel strip, sheet or blank to the Ac1 -temperature or above, hot-forming the hot- formable steel strip, sheet or blank to form the hot-formed product and quenching the hot-formed product. This is the usual hot forming process, which is now performed using the hot-formable steel strip, sheet or blank of the invention. In this way the metallurgical slag based coating protects the metal or metal alloy coated steel strip, sheet or blank from abrasion and corrosion if the steel, strip, sheet or blank is stored prior to hot-forming. As the coated steel is heated towards the Ac1 temperature, the polymeric material thermally degrades leaving behind a layer formed from the metallurgical slag. This layer prevents or at least reduces high temperature oxidation at the steel surface and prevents or at least reduces evaporation of the metal or metal alloy coating.

In a preferred embodiment of the invention the hot-formable steel strip, sheet or blank according to the first aspect of the invention is pre-formed. The inventors found that the metallurgical slag based coating provides very good abrasion protection during pre-forming. Moreover, the metallurgical slag provides active corrosion protection in case the metal or metal alloy coating is damaged during pre-forming or during handling of the preformed strip, sheet or blank.

The fourth aspect of the invention relates to a hot-formed product produced according to the method of the third aspect of the invention wherein the hot-formed product comprises a layer formed from a metallurgical slag on the metal or metal alloy coating. This layer covers the surface of the metal or metal alloy and comprises loosely adherent oxides from the metallurgical slag and reaction products obtained from the reaction between a metal from the metal or metal alloy coating and an oxide from the metallurgical slag. Such reaction products are stable and provide passive corrosion protection, whereas the loosely adherent oxides provide temporary active corrosion protection e.g. during storage of the hot-formed product. Advantageously, the loosely adherent oxides may be easily removed by brushing, rinsing or by air jet cleaning. Sand blasting may also be used but is not necessary and is less preferred since damage to the metal or metal alloy coating may occur.

In a preferred embodiment of the invention the layer formed from the metallurgical slag comprises oxides of aluminium, silicon and manganese. These oxides, particularly Al₂0₃, form an inert barrier on the metal or metal alloy coating. Preferably Al₂0₃ covers at least 50% of the metal or metal alloy coating surface. The invention will be now be elucidated by referring to the non-limitative examples below.

Figure 1 shows the results of potentiodynamic experiments conducted to determine the corrosion protective properties of boron coated steels after a heat treatment at 900^°C for 5 minutes. The results relate to a galvannealed boron steel strip (that has not undergone the heat treatment) (A), a galvannealed boron steel strip provided with the metallurgical slag based coating of the present invention (B), a commercially available Al-Si (Alusi^®) coated boron steel strip (C) and a galvanised boron steel strip provided with the metallurgical slag based coating of the present invention (D).

Example 1 : Preparation of a metallurgical slag based coating solution

A Metallurgical slag (4 g) obtained from a ladle furnace is added to a vessel containing water (100 mL) to form a metallurgical slag solution. The metallurgical slag contained in weight %: CaO (54.03), Al₂0₃ (33.20), Si0₂ (4.78), MgO (3.72) FeO

(2.23) , MnO (1 .05), Na₂0 (0.49), Ti0₂ (0.34), V₂0₅ (0.15), P₂O₅ (0.06) and Cr₂O₃ (0.03) as determined by wavelength dispersive X-ray fluorescence (WDXRF). A solution of polyamide imide (15.5 mL at 7% dilution in water) is then slowly added to the metallurgical slag solution. The polyamide imide used is the commercially available TORLON ^® AI-50 from Solvay Advanced Polymers.

A second metallurgical slag based coating solution was prepared by adding a BOF slag (4 g) to a vessel containing water (100 mL). The BOF slag contained in weight % CaO (42.61 ), Al₂0₃ (1.34), Si0₂ (13.59), MgO (7.70) FeO (26.88), MnO

(4.24) , Na₂0 (0.20), Ti0₂ (0.92), V₂O_s (0.71 ), P₂O_s (1.56) and Cr₂0₃ (0.24) as determined by WDXRF. A solution of polyamide imide (15.5 mL at 7% dilution in water) is then slowly added to the metallurgical slag solution. The polyamide imide used is the commercially available TORLON ^® AI-50 from Solvay Advanced Polymers.

Unlike WDXRF, which was used to determine the content of certain elemental oxides in the metallurgical slag, X-ray diffraction (XRD) was used to determine the presence and structure of crystalline compounds in the metallurgical slag. XRD was also used to determine the presence and amount of amorphous phases in the metallurgical slag.

The results are summarised in Table 1.

XRD patterns were recorded in the range of 10 to 130 ⁰ (2 Θ) in reflection mode using a fully automated Panalytical Xpert MPD diffractometer (CoK_a-radiation) equipped with a position sensitive detector. The step size was 0.02 °, time per step was 1 s. Quantitative determination of phase proportions was performed by Rietveld analysis. The refinement was done on the assumption of pure phases. Unit cell parameters, background coefficients, preferred orientations, profile parameters and phase proportions were refined using the TOPAS software package for Rietveld refinement.

Table 1

Example 2: Coating

The metallurgical slag based coating solution is then applied on a hot-dip galvanised boron steel substrate by dipping or by spraying; thereafter the applied coating solution is cured using near infrared radiation at 260^°C for 5-10 seconds. After curing, the dry film thickness of the coating was 6 μηι. The boron steel substrate contains in weight % C (0.21), Si (0.192), Mn (1.189), Ni (0.022), Cr (0.25), Al (0.044), P (0.013), Ti (0.035), N (62 ppm), S (0.006 ppm) and B (31 ppm). The metallurgical slag used in this example was a BOF slag containing in weight % CaO (42.61 ), Al₂0₃ (1.34), Si0₂ (13.59), MgO (7.70), FeO (26.88), MnO (4.24), Na₂0 (0.20), Ti0₂ (0.92), V₂0₅ (0.71), P₂0₅ (1.56) and Cr₂O₃ (0.24), as determined by WDXRF.

Experiment 1 : Scanning electron microscopy

A field emission gun scanning electron microscope (FEG-SEM) Ultra 55 (Zeiss, Germany) (15 kV) was used to elucidate the surface of the coated boron steel after a heat treatment at 900^°C for 5 minutes.

The results show that the galvanised layer fully covers the surface of the steel substrate and that a large proportion of zinc has been retained. This is not the case when galvanised boron steel substrates, i.e. without the metallurgical slag coating, are subjected to the same heat treatment since evaporation of zinc occurs.

The SEM images also show the presence of a thin layer on the galvanised coating after the heat treatment. This layer covers a large proportion of the galvanised surface and contains a high content of aluminium. It is understood that this layer mainly consists of a compound formed between zinc and/or other alloying elements in the galvanised coating and Al₂0₃ from the metallurgical slag. The inventors believe that this layer acts as an isolating layer which prevents corrosive electrolytes from reaching the galvanised coating and the steel substrate surface. This layer does not dissolve in water. The SEM images also show the presence of silicon and manganese in the layer, which suggests that during the heat treatment a reaction involving Si0₂ and MnO with zinc and/or other alloying elements in the galvanised takes place. Si0₂ and MnO also provide active corrosion protection because these oxides can dissociate, which increases the pH and further reduces the rate of corrosion.

Experiment 2: Corrosion protection after hot-forming

Potentiodynamic experiments were performed on coated boron steel substrates to assess the corrosion protective properties of the coatings following a heat treatment at 900^°C for 5 minutes. The experiments were carried out using a Solartron Model 1280C Potentiostat/Galvanostat. The potentiodynamic scan was performed using 0.6M sodium chloride (NaCI) solution as a conductive medium and the polarization was applied at a scanning rate of 0.5mV/sec delineating the anodic and cathodic branches.

The coated boron steel substrates under investigation include a galvannealed boron steel substrate having an average coating thickness of 10 μιτι, the coating comprising zinc and 10% iron (A), a galvannealed boron steel substrate provided with the metallurgical slag based coating of the invention (B), a commercially available Al-Si coated boron steel from Arcelor Mittal (USIBOR®), the Al-Si coating having an average thickness of 40 pm and comprising 90% aluminium and 10% silicon (Alusi®) (C) and a galvanised boron steel provided with the metallurgical slag based coating of the invention, the galvanised coating having an average coating thickness of 20 μιη (D). After the heat treatment, the layer formed from the metallurgical slag was brushed to remove any loosely adherent oxides from the galvanised surface.

The potentiodynamic results (Figure 1 ) show that the coated boron steel substrate of the invention (D) exhibits superior corrosion protection properties relative to the galvannealed boron steel substrate and the Al-Si coated boron steel substrate. This has been attributed to the isolating layer that is formed from the metallurgical slag providing passive corrosion protection and the retained galvanised coating providing cathodic corrosion protection. In contrast, the Al-Si coating only affords passive corrosion protection to the steel substrate, whereas the galvannealed coating only affords cathodic corrosion protection. By providing the metallurgical slag based coating on the galvannealed boron steel strip (B), superior corrosion protection properties were obtained relative to the galvannealed boron steel strip (A). Similar results were obtained for galvannealed and galvanised boron steel substrates that were provided with ladle furnace slag based coatings,

Experiment 3: Electrical resistance

In order to determine the weldability of the coated boron substrates after the heat treatment at 900°C for 5 minutes the coated substrates were subjected to an electrical resistance test. The experimental setup for measuring the electrical resistance consists of two copper electrodes (diameter = 12.5mm), a low ohm meter (Rhopoint Instrumnet M210), a pressure gauge and a pneumatic press (capable of 15 ton pressure). The low ohm meter has a resolution of 1 milli-ohm and its copper wires were soldered directly into the copper electrodes to avoid any potential resistance contribution from the setup. The copper electrode surfaces in contact with the testing samples were ground on 4000 grit silicone carbide paper before use, while the reverse sides were covered with insulating tape.

The measured electrical resistivity of the coated boron steel of the invention was

0.6 mOhm. Such a coated steel is therefore very suitable for subsequent welding.

Claims

Hot-formable steel for use in hot forming comprising a hot-formable steel strip, sheet or blank, a metal or metal alloy coating formed on the hot-formable steel strip, sheet or blank and a metallurgical slag based coating formed on the metal or metal alloy coating, wherein the metal or metal alloy coating comprises zinc and/or aluminium and the metallurgical slag based coating comprises a metallurgical slag and a polymeric material.

Hot-formable steel according to claim 1 , wherein the polymeric material comprises a polyimide, preferably a polyamide-imide.

Hot-formable steel according to claim 1 or claim 2, wherein the thermal degradation temperature of the polymeric material is at least 25CTC, preferably at least 400^°C.

Hot-formable steel according to any one of the preceding claims, wherein the dry film thickness of the metallurgical slag based coating is between 1 and 30 μητ, preferably between 2 and 10 μηι.

Hot-formable steel according to any one of the preceding claims, wherein the metallurgical slag is a steel making slag.

Hot-formable steel according to any one of the preceding claims, wherein the metallurgical slag comprises basic oxygen furnace slag, blast furnace slag or a mixture thereof.

Hot-formable steel according to any one of the preceding claims, wherein the metallurgical slag comprises one or more oxide particles of Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S, preferably the metallurgical slag comprises one or more of CaO, Al₂0₃, Si0₂, FeO, Fe₂0₃, MgO, MnO, Mn₃0₄, Ti0₂, P₂0₅, V₂O_s, Cr0₂, Na₂0, K₂0 and S0₃.

Hot-formable steel according to any one of the preceding claims, wherein the metallurgical slag comprises a mixed oxide compound formed from oxide anions and at least two elemental cations selected from Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S.

9. Hot-formable steel according to any one of the preceding claims, wherein the metallurgical slag comprises 20 to 75 wt% of calcium oxides. 10. Hot-formable steel according to any one of the preceding claims, the metallurgical slag comprises:

- 30 to 50 wt% of calcium oxides

- up to 10 wt%. preferably between 1 and 5 wt% of aluminium oxides

- up to 35 wt%, preferably between 10 and 20 wt% of silicon oxides

- up to 35 %, preferably between 15 and 35 wt% of iron oxides

1 1. Hot-formable steel according to any one of claims 1-9, wherein the metallurgical slag comprises:

- 40 to 65 wt% of calcium oxides

- up to 45 wt%, preferably between 20 and 45 wt% of aluminium oxides

- up to 35 wt%, preferably between 1 and 15 wt% of silicon oxides

- up to 15 %, preferably between 1 and 10 wt% of iron oxides

12. Hot-formable steel according to any one the preceding claims, wherein the metallurgical slag additionally comprises:

- up to 10 wt% of magnesium oxides

- up to 6 wt% of manganese oxides

- up to 2 wt% of titanium oxides

- up to 3 wt% of phosphorous oxides

- up to 2 wt% of vanadium oxides

- up to 2 wt% of chromium oxides

- up to 2 wt% of sodium oxides

13. Hot-formable steel according to any one of claims 7-12, wherein the oxide particles have a size dimension between 0.1 and 30 μηι, preferably between 0.1 and 10 μιη and more preferably between 0.1 and 1 μηι.

14. Hot-formable steel according to any one of the preceding claims, wherein the metal or metal alloy coating is a galvannealed coating containing zinc and Fe up to 70 weight %, preferably containing Fe up to 40 weight %, more preferably containing Fe up to 20 weight %, still more preferably containing Fe up to 10 weight %. Hot-formable steel according to any one of the preceding claims, wherein the hot formable steel is a boron steel, preferably having the composition in weight percent:

C between 0.04 and 0.5%

Mn between 0.5 and 3.5%

Si less than 1.0%

Cr O.01 and 1.0%

Ti less than 0.2%

Al less than 2.0%

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

Method for manufacturing a hot-formable steel according to any one of the preceding claims, which comprises the steps of:

(i) providing a hot-formable steel strip, sheet or blank;

(ii) hot-dip galvanising or hot-dip aluminising the hot-formable steel strip, sheet or blank;

Method according to claim 16, wherein the coating composition comprises 1 -30 wt% metallurgical slag, at least 0.1 wt% polymeric material and water.

Method according to claim 16 or claim 17, wherein the step of providing the coating composition comprises the consecutive steps of providing the metallurgical slag in water to form a metallurgical slag solution and then adding the polymeric material to the metallurgical slag solution. Method according to any one of claims, 16-18 wherein the coated steel strip, sheet or blank is heated using electromagnetic radiation, preferably near infrared radiation.

Method according to any one of claims, 16-19 wherein the coated steel strip, sheet or blank is heated, at least in part, using residual heat from the hot-dip galvanised or hot-dip aluminised steel strip, sheet or blank.

Method for manufacturing a hot-formed product, which comprises the steps of providing the hot-formable steel strip, sheet or blank according to any one of claims 1 -15, heating the hot-formable steel strip, sheet or blank to the Ac1 - temperature or above, hot-forming the hot-formable steel strip, sheet or blank to form the hot-formed product and quenching the hot-formed product.

Method according to claim 21 , wherein the hot-formable steel strip, sheet or blank according to any one of claims 1 -15 is pre-formed.

Hot-formed product produced according to the method of claim 20 or claim 21 , wherein the hot-formed product comprises a layer formed from a metallurgical slag on the metal or metal alloy coating, which layer comprises a reaction product obtained from the reaction between a metal from the metal or metal alloy coating and an oxide from the metallurgical slag.

24. Hot-formed product according to claim 20 or 21 , wherein the layer formed from the metallurgical slag comprises oxides of aluminium, silicon, preferably Al₂0₃ covers at least 50% of the metal or metal alloy coating surface.