EP3122486A1

EP3122486A1 - Method for hot forming a coated steel blank

Info

Publication number: EP3122486A1
Application number: EP15719148.7A
Authority: EP
Inventors: David Neal Hanlon; Stefanus Matheus Cornelis VAN BOHEMEN; Guido Cornelis Hensen
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2014-03-28
Filing date: 2015-03-23
Publication date: 2017-02-01
Also published as: WO2015144318A1; CN106715745A

Abstract

The invention relates to a method for hot forming a part, wherein the hot forming is performed starting with a metal coated steel blank, which is heated to a temperature between the Ac3-temperatur of the steel and 1000° C in a furnace, after which the heated blank is transported to a hot forming press, and the heated blank is pressed in the hot forming press to form a part. According to the invention, the steel has the following composition in weight%: C: 0.10 - 0.25, Mn: 1.6 - 2.6, Si: < 0.4, Cr: < 1.0, Al: < 0.5, P: < 0.02, S: < 0.004, O: < 0.008, N: < 0.03, B: < 0.0004, and optionally: Ti: < 0.1, Mo: < 0.5, Nb: < 0.3, V: < 0.5, Ca: < 0.05, the remainder being iron and unavoidable impurities, and the blank is cooled between the furnace and the hot forming press such that the blank has a temperature of at most 770° C and at least 450° C when placed in the hot forming press, and the hot formed part is at least partially martensitic.

Description

METHOD FOR HOT FORMING A COATED STEEL BLANK

State-of-the-art The invention relates to a method for hot forming a part using a coated steel blank.

Hot forming of coated steel blanks is much used in the automotive industry. The parts formed from these steels get high mechanical properties (such as a high strength) after the blanks are heated to a temperature above the Ac3 temperature, for instance a temperature between 850 °C and 950 °C, pressed in a hot forming press and quenched at a velocity above the critical quenching rate. Before heating, these steels have a good formability and a tensile strength between 300 MPa and 500 MPa, for most grades. After the hot forming process, these steels have a very high tensile strength, which can be above 1500 MPa, and nowadays up to 2000 MPa. The high tensile strength makes the hot formed products especially suitable for use in the body-in-white of automotive vehicles. For automotive purposes usually a boron-alloyed steel is used, in particular steel grade 22MnB5.

Hot forming is generally used for the direct hot forming process, but is also used in the indirect hot forming process. A general picture of hot forming (or hot stamping) is given by A. Naganathan and L. Penter, Chapter 7: Hot Stamping, in Sheet Metal Forming - Processes and Applications, (T. Altan and A. E. Tekkaya, editors), ASM International, 2012.

It has however been found that hot forming of parts using Zn or Zn alloy coated steel is susceptible to the forming of micro-cracks, due to the hot forming process. The micro-cracks are present not only in the coating, but also in the part itself. The micro-cracks in the hot formed part are seen as undesirable. Objective / Abstract

It is an object of the invention to provide a method for manufacturing a coated steel part having high mechanical properties using hot forming techniques that avoid or at least diminish the occurrence of micro-cracks in the hot formed part.

It is another object of the invention to provide a method for hot forming that can be used by the automotive industry without changes to the equipment used at present.

According to the invention at least one of these objects is reached using a method for hot forming a part, wherein the hot forming is performed starting with a metal coated steel blank, which is at least partially heated to a temperature between the Ac3-temperature of the steel and 1000° C in a furnace, after which the heated blank is transported to a hot forming press, and the heated blank is pressed in the hot forming press to form a part, wherein the steel has the following composition in weight%:

C: 0.10 - 0.25,

Mn: 1 .6 - 2.6,

Si < 0.4,

Cr: < 1 .0,

Al < 0.5,

P: < 0.02,

S: < 0.004,

O: < 0.008,

N: < 0.03,

B: < 0.0004

and optionally:

Ti: < 0.1 ,

Mo: < 0.5,

Nb: < 0.3,

V: < 0.5,

Ca: < 0.05,

the remainder being iron and unavoidable impurities, wherein the blank is cooled between the furnace and the hot forming press such that the blank has a temperature of at most 770° C and at least 450° C when placed in the hot forming press, and wherein the hot formed part is at least partially martensitic. Description of the invention

In order to reduce the depth of micro-cracks that occur during the forming of zinc coated blanks, the inventors have found that is of significant importance to control the temperature at which the part is formed. The temperature of forming is preferably chosen such that any liquid components that exist in the coating solidify prior to forming. The inventors have found that it is beneficial to further reduce the forming temperature to at most 770°C and at least 450° C.

At forming temperatures of at most 770°C, the inventors found that it is complicated to obtain a fully martensitic substrate with the commonly used 22MnB5 steel. To overcome this shortcoming, the inventors have come up with multiple alternative substrate compositions. The inventors claim that these compositions, which do not rely on the alloying with boron (B) for hardenability, are much less dependent on forming temperature to attain the required mechanical properties. The mechanical properties after forming and quenching remain high for a larger range of forming temperatures compared to 22MnB5. The invention will therefore provide more freedom to the hot former because he can select the forming temperature that fits best with his requirements regarding formability and microcrack depth without concerns with regard to mechanical properties. The inventors have found that the required formation of martensite will not be possible when a blank has a temperature below 450 °C when placed in the hot forming press.

The inventors have found that it is advantageous when a steel type is used in which less carbon and boron is present. Instead of these elements, Mn and/or Cr can be used to create a steel type that has the same strength as a 22MnB5 steel, but having beneficial other properties. Moreover, the inventors have found that it is beneficial to reduce the amount of other elements, apart from C, Mn and/or Cr, and Si as well.

Non-metallic constituents reduce the homogeneity of the substrate and these inhomogeneities can lead to local stress concentrations and pre-mature failure of a mechanically loaded product, especially products with very high mechanical properties such as hot formed products with yield strengths >900MPa and tensile strengths >1400MPa. Typical non-metallic constituents in steel are TiN, BN, Fe₂6(B,C)₆, MnS, AIN, CaS, AI₂O₃, P, Fe₃C etc. The invented steel composition is aimed to reduce the size and amount of all these non-metallic constituents by reducing the amount of B, Ti, S, Ca, Al, P and other required chemical elements.

The nowadays commonly used 22MnB5 substrate composition contains 20 to 40 ppm boron (B) to improve the hardenability during hot forming operations. To maintain this element in its functional state, the steelmaker adds titanium (Ti) to the cast to prevent B to form boron nitride (BN). The Ti is normally added in an over-stochiometric ratio to the nitrogen (N) to maximize the efficiency of the added amount of B. Boron is also known to form fine Fe₂₆(B,C)₆ complex precipitates that can lead to local stress concentrations in the matrix. Therefore the inventors have removed the B from the steel composition to limit the presence of B based non-metallic constituents. To compensate for the loss of hardenability by reducing the amount of B, the inventors added manganese (Mn) and/or chromium (Cr).

Mn is a favourable metallic component because of its compatibility with the iron matrix. Moreover, the addition of Mn reduces the Aci and Ac₃ temperature of the steel substrate. This means that a lower furnace temperature can be utilized to austenitize the substrate prior to hot forming. The invented compositions demonstrate a reduction in Ac₃ of approximately 25°C compared to the commonly used 22MnB5. Reducing the furnace temperature is economically and environmentally favourable and also opens up new process opportunities for Zn, Zn alloy or Al and Al alloy coatings. For Zn alloy coatings it is commonly known that an increased furnace temperature reduces the corrosion performance of the hot formed product. For Al or Al alloy coatings it is known that high furnace temperatures reduce the weldability of the component. A steel composition that enables the use of lower furnace temperatures is therefore favourable over the commonly used 22MnB5.

In contrast to B, Mn does strengthen the substrate by solid solution strengthening. Furthermore, Mn additions also lower the M_s temperature, which means that less (auto-)tempering will occur and therefore the substrate will have a higher martensite strength at room temperature. The M_s temperature of the invented compositions is approximately 25°C lower compared to the commonly used 22MnB5. Due to both strengthening mechanisms, the inventors claim that they can reduce the amount of carbon (C) in steel substrates for hot forming and obtain a similar strength level as achieved with 22MnB5. Reducing the amount of C is favourable to prevent Fe3C formation during (auto-)tempering during the hot forming process step. Fe3C precipitates can introduce local inhomogeneities and stress concentrations during mechanically loading, leading to premature failure of the product. Furthermore, the spot-weldability of hot-formed products will improve due to the lower C content in the inventive steel substrate.

Similar to Mn, Cr increases the hardenability, and it also lowers the M_s temperature. Furthermore, Cr contributes to the strength of the substrate by solid solution strengthening.

Si also delivers a solid solution strengthening contribution. In addition, Si retards the (auto)tempering because of its weak solubility in carbides.

Sulphur (S) is a common element found in steel substrates. Steelmakers use various desulphurization methods to reduce the amount of S because it could lead to hot-shortness during continuous casting. S can also precipitate with manganese (Mn) to form soft MnS inclusions. During hot rolling and subsequent cold rolling, these inclusions are elongated and form relatively large inhomogeneities that could lead to premature failure, especially when loaded in the tangential direction. Calcium (Ca) can be added to spherodize the S containing inclusions and to minimize the amount of elongated inclusions. However, the presence of CaS inclusions will still lead to inhomogeneities in the matrix. Therefore, it is best to reduce S.

Aluminium (Al) is normally added to steel in an over-stoichiometric ratio to oxygen (O) to prevent carbon monoxide (CO) formation during continuous casting by reducing the available amount of free O through formation of aluminium oxide AI₂O₃. The formed AI₂O₃ normally forms a slag on top of the liquid steel, but can be entrapped in the solidifying steel during casting. During subsequent hot and cold-rolling, this inclusion will become segmented and forms non-metallic inclusions that lead to premature fracture upon mechanically loading the product. The over-stoichometric Al precipitates as aluminium nitrides (AIN) which also leads to local inhomogeneities in the steel matrix.

Preferably the more limited amounts of the elements according to claim 2 or 3 are used. It will be clear that a more limited amount of the elements as specified in claims 2 and 3 provides a steel in which the number of non- metallic constituents in the steel substrate are further reduced. For instance, the over-stochiometric amount of Ti will form titanium nitrides, which are known as hard, non-deformable inclusions. By limiting the amount of Ti and N, the TiN inclusions are limited. Claim 3 shows that it is possible to use a steel for hot forming in which no boron is added, such that the boron in the steel will be only present as an unavoidable impurity. Though the amount of boron that will be present as an impurity will depend on the raw materials used in the ironmaking process and also depends on the steelmaking process, the inventors have found that the impurity level for boron that is nowadays obtained has a maximum of 0.0001 weight% or 1 ppm.

According to a preferred embodiment, the steel contains Mn + Cr > 2.5 weight%, preferably Mn + Cr > 2.6 weight%. The inventors found that Mn and Cr in combination should be high enough to provide the desired strength of the hot formed part.

The reduced forming temperature of the blank in the press reduces the need for advanced cooling techniques in the press. The reduced forming temperature also lowers the thermal load on the forming tools and will therefore improve the tool life.

Preferably, the metal coated steel blank has a metal coating comprising a layer of zinc or a zinc based alloy. Zinc or zinc based alloy coated blanks are preferred in the automotive industry to improve in-service corrosion performance.

According to a preferred embodiment the metal coating is an iron-zinc diffusion coating obtained by heat treating a zinc layer, the zinc layer comprising Al < 0.18 wt% and Fe < 15 wt%, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 μιτι per side, more preferably a thickness between 6 and 13 μιτι per side. This zinc pre-coating provides good corrosion properties.

According to a further preferred embodiment the metal coating comprises 0.5 to 4 wt% Al and 0.5 to 3.2 wt% Mg, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 μιτι per side, more preferably a thickness between 6 and 13 μιτι per side. This pre-coating provides even better corrosion properties.

Preferably the metal coated steel blank is at least partially heated to a temperature between 850 and 950° C, more preferably between 850 and 900° C. At these temperatures the steel blanks are austenitised fast enough.

According to a preferred embodiment the coated steel blank is heated in a furnace during a time period in which at least part of the blank reaches a desired austenitising temperature plus an additional 10 to 600 seconds. This time period is suitable to heat steel blanks having the usual thicknesses for automotive purposes.

Preferably the blank has a temperature of at least 480 °C when placed in the hot forming press, preferably a temperature of at least 500 or 520 or 550 or 580 or 600 or 620 °C. The required formation of martensite will not be possible when a blank has a temperature below 450 °C when placed in the hot forming press. It is possible to use temperatures that are higher then 450 °C, such as 480 °C, but the inventors have found that the higher the temperature is at which a zinc coated steel blank is placed in the hot forming press, the bigger the depth of the micro-cracks will be in the deformed portions of the hot formed parts.

Preferably the blank has a temperature of at most 750 °C when placed in the hot forming press, preferably a temperature of at most 725 or 700 or 680 or 650 °C. A maximum temperature for the steel blank that is placed in the hot forming press provides a guarantee that the micro-cracks in the hot formed part will not be too deep.

According to a preferred embodiment the blank is transported from the furnace to the hot forming press in a time period of at most 20 seconds, preferably in a time period of at most 15 seconds, more preferably in a time period of at most 12 seconds, even more preferably in a time period of at most 10 seconds, most preferably in a time period of at most 8 seconds. Transportation times that are that short provide a zinc coated steel blank that is oxidised only to a limited extent.

It is envisaged the blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 20 °C/s during the forced cooling, more preferably with an average cooling velocity of at least 30 °C/s during the forced cooling, even more preferably with an average cooling velocity of at least 40 °C/s during the forced cooling, still more preferably with an average cooling velocity of at least 50 °C/s during the forced cooling, most preferably with an average cooling velocity of at least 60 °C/s during the forced cooling. Such cooling rates make it possible to cool the blank in a few seconds, so the transportation time between the furnace and the hot forming press remains short. Forced cooling is usually performed using forced air or metal cooling plates, but other ways of forced cooling are also possible.

The invention also encompasses a product produced using the method as described above. This product has the mechanical properties provided by the hot forming method, as needed for automotive or other purposes.

Preferably a product as described above is used in a motor vehicle. For this purpose also other properties besides mechanical properties are have to be taken into account, such as the weldability of the product. Examples

In invention will be elucidated with reference to the accompanying drawings.

Figure 1 shows the depth of micro-cracks as a function of time and temperature of the blanks in the hot forming press.

Figure 2 shows the depth of micro-cracks for different circumstances. Figure 3 shows the dimensions of pre-cooled area and location of cross- section (dashed line).

Figure 4 shows the cracks in the outer vertical wall of a B-pillar. Figure 5 shows that the inventive steel ID0083 does not show ferrite formation just below the coating in contrast to comparative material ID0322.

Figure 6 shows the depletion of boron just underneath the coating in regular 22MnB5 material

Figure 7 shows the hardness of the hot formed parts using different substrates and temperatures.

The inventors have demonstrated the effect of forming temperature on microcrack depth by performing various hot forming trials with zinc coated 22MnB5 and forced cooling of the heated blank prior to forming. A 1 .65mm gauge blank with 130 gr/m2 ZnFe coating was heated in a furnace to 900°C for a number of minutes, taken from the furnace and transported to a cooling station that cooled the blank to the required temperature with compressed air. After the forced cooling the blank was transported to the press and formed into a top hat geometry with vertical walls, a draw depth of 50mm and a die radius of 2.1 mm. The draw gap and spacers used were equal to the blank thickness + 0.15mm. The forced cooling took place in 1 -3 seconds depending on the required forming temperature. The total transport time from furnace to press was 10 seconds. The process was monitored with pyrometers during the entire transportation. The maximum microcrack depth found in cross-section for each tested sample has been recorded and is plotted in Figure 1 .

The results in Figure 1 show that for short total furnace dwell times the crack depth is very large when forming starts at 800°C. This is because the solidification temperature of the phases present in the coating after such a dwell time is ~782°C and forming starting at 800°C will therefore result in liquid metal assisted cracking. When the blanks are cooled to 770°C, just below the solidification temperature of the coating, the crack depth becomes nearly independent of furnace time, but is still large. Further reduction of the forming temperature leads to a further reduction of the crack depth. The invention has been demonstrated to work for multiple Zn coatings and geometries as can be seen in Figure 2.

To demonstrate the advantages of the steel according to the invention, samples from 2 types of industrially zinc coated steel (see table 1 and 2) have been hot fornned into a B-pillar shaped die with drawing depth of 55 mm.

Table 1 : Details of used steel material

Table 2: Composition of used steel material Due to size limitations of the pre-cooling installation, only part of the B- pillar was pre-cooled as indicated in figure 3. This part is the most sensitive to micro-crack formation due to the high deformations.

The samples were heated in a furnace at 900°C for a total of 3 minutes. After this, the samples were transported to the press within 10 seconds and formed into shape. Dependent on the desired pressing temperature, the part was pre-cooled in a pre-cooling station for 2-3 seconds during this transport.

In the area with high deformations, cross-sections were made to observe the crack depth and substrate microstructure (see Figure 3). The results of this analysis are given in Figure 4 and Figure 5.

It can be observed that in both steels the crack depth is reduced significantly. The improvement brought about by the inventive steel over the comparative steel can be seen in Figure 5. Here it can be seen that the comparative steel suffers from a martensite + ferrite band just below the coating. This inhomogeneous substrate band makes the substrate more prone to failure and is caused by depletion of Boron at the surface due to the oxygen rich atmosphere that draws the Boron to the surface. This Boron enrichment at the surface and depletion below the FeZn coating is illustrated by a GDOES profile in Figure 6 of the comparative 22MnB5 material. It can be clearly seen that there is a sharp increase in Boron content at the surface while below the coating, the Boron content is much less than the bulk content of the substrate. The inventive steel that does not rely on Boron for its hardenability and hence does not suffer from this issue.

To further demonstrate the relevance of the invented substrate compositions, the inventors have casted multiple compositions into 25kg ingots. These ingots were subsequently hot rolled with a finish temperature of 900°C, a coiling temperature of 630°C and a hot rolled gauge of 4mm. Subsequently the strips were pickled and cold rolled to 1 .5mm gauge. Using dilatometry the Ac₃ temperature, the M_s temperature and critical cooling rate of the compositions have been determined. For these tests, samples were heated in a Bahr 805A Dilatometer to a temperature of 900°C with an average heating rate of 15°C/s from room temperature up to 650°C and with an average heating rate of 3°C/s from 650-900°C. After 3 minutes of soaking at 900°C the samples were quenched with various cooling rates to determine the critical cooling rate at which the substrate transforms completely to martensite. The obtained data is given in table 1 for various chemical compositions. The data illustrates that it is possible to obtain similar critical cooling rate compared to 22MnB5 without the use of the element B. It also illustrates that the M_s and Ac₃ temperature are decreased. A reduction in these temperatures is beneficial to reduce auto- tempering and furnace temperature respectively.

Table 3: compositions with Ms and Ac1 temperatures and CCR.

ID Composition C Mn Si Cr Al S P N Ti B O M_s Ac₃ CCR wt% wt% wt% wt% wt% wt% wt% ppm wt% ppm wt% °C °C °C/s A 2.0Mn 0.190 1.97 0.2 0.00 0.019 0.0027 0.008 43 <0.005 <3 <0.005 n.a. 810 >130 B 2.2Mn 0.190 2.16 0.2 0.00 0.015 0.0027 0.008 43 <0.005 <3 <0.005 n.a. 805 60 A 2.0Mn-0.3Cr 0.190 1.99 0.2 0.30 0.016 0.0027 0.009 47 <0.005 <3 <0.005 n.a. 805 55 B 2.0Mn-0.6Cr 0.190 1.97 0.2 0.59 0.014 0.0026 0.009 35 <0.005 <3 <0.005 375 805 35 A 1.8Mn-0.6Cr 0.190 1.79 0.2 0.60 0.021 0.0030 0.009 40 <0.005 <3 <0.005 n.a. 805 60 B 2.2Mn-0.6Cr 0.185 2.17 0.2 0.60 0.019 0.0025 0.009 34 <0.005 <3 <0.005 370 800 20 A 1.6Mn-0.8Cr 0.190 1.59 0.2 0.81 0.021 0.0023 0.009 55 <0.005 <3 <0.005 n.a. 810 65 B 1.8Mn-0.8Cr 0.190 1.79 0.2 0.77 0.019 0.0021 0.009 52 <0.005 <3 <0.005 375 805 35

1A 22MnB5 0.230 1.17 0.238 0.20 0.01 1 0.0040 0.009 29 0.021 26 <0.005 400 830 30 With the usual measuring equipment the amount of Ti and B could not be measured more accurately then indicated in Table 3. The table shows that the amount of Ti is low enough. The amount of O has not been measured but it is known that for such steel types the amount is less then 50 ppm in laboratory samples. Steel produced during commercial full-scale production of these steel types has shown to contain less then 30 ppm O.

Other test samples produced under laboratory condition show to contain 1 to 3 ppm B when no boron has been added to the steel. This variation in the amount of boron can be accounted for by a small contamination of the steelmaking equipment with previously produced boron containing steels. Commercial full-scale production of such types of steel to which no boron has been added contain an amount of less then 2 ppm boron; usually an amount of less then 1 ppm boron is measured.

To illustrate the difference between the invented substrates and the commonly used 22MnB5, the inventors conducted high temperature deformation trials in a Gleeble^® 3800 to assess the influence of forming temperature on mechanical properties. These trials simulate the deformation of the steel that can take place during the hot forming process. 1 .5mm thick samples of different chemical composition were heated to 900°C with an average heating rate of 7.5°C/s and held at that temperature for 3 minutes. Subsequently the sample was cooled to the required forming temperature and strained to 20% elongation with a strain rate of 2s^"1. After the deformation was finished, the samples were quenched with a cooling rate of 40-70°C/s. The resulting hardness of the samples was measured and these results are plotted in Figure 7.

The results demonstrate, when using 22MnB5, it is not possible for these test conditions to ensure hardness over 450HV over the whole temperature range. At a forming temperature of 650°C and 700°C, the hardness of 22MnB5 is nearly 25HV lower compared to 750°C and nearly 40Hv lower compared to what is expected based on it's carbon level. This is estimated to be equal to a strength reduction of 75-125MPa (1 MPa ~ 3.2HV). In examination by light optical microscopy, increased amounts of ferrite were observed for these 22MnB5 samples. The invented substrates based on a boron free alloying concept demonstrate a much more constant hardness as a function of forming temperature and are therefore much more suited to be used in a process intended to reduce microcrack depth.

To demonstrate the effect of the beneficial influence of the absence of non-metallic constituents on the mechanical properties, the inventors performed hot forming trials. 1 .5mm gauge steel blanks were heated to 900°C with a total furnace time of 5 minutes. The blanks were taken out of the furnace, transported to the press within 10 seconds and pressed in between flat tools at a temperature of approximately 780°C. The resulting mechanical properties demonstrate that even though the carbon levels of the invented substrates are lower, the yield strength (R_P)_and tensile strength (R_m) are similar to the commonly used 22MnB5. Due to the reduced number of non- metallic constituents, the invented substrates all have higher total elongation compared to the commonly used 22MnB5. This is shown in Table 4.

Table 4: Mechanical properties of various substrates.

Cast: Furnace T Transport time Rp Rm Ag A

[No.] [°C] [s] [MPa] [MPa] [%] [%]

6A 2.0Mn 900 8 1 126 1536 4.0 6.9

6B 2.2Mn 900 8 1 109 1551 3.9 7.1

7A 2.0Mn-0.3Cr 900 8 1 1 1 1 1519 3.7 6.3

7B 2.0Mn-0.6Cr 900 8 1 1 19 1544 4.1 7.3

8A 1.8Mn-0.6Cr 900 8 1 133 1525 3.8 6.6

8B 2.2Mn-0.6Cr 900 8 1 137 1550 4.1 7.0

9A 1.6Mn-0.8Cr 900 8 1 158 1554 3.8 6.5

9B 1.8Mn-0.8Cr 900 8 1 147 1566 3.7 6.4

1A 22MnB5 900 8 1 137 1555 3.7 6.0

Claims

1 . Method for hot forming a part, wherein the hot forming is performed starting with a metal coated steel blank, which is at least partially heated to a temperature between the Ac1 -temperatur of the steel and 1000° C in a furnace, after which the heated blank is transported to a hot forming press, and the heated blank is pressed in the hot forming press to form a part,

characterised in that

the steel has the following composition in weight%:

C: 0.10 - 0.25,

Mn: 1 .6 - 2.6,

Si: < 0.4,

Cr: < 1 .0,

Al: < 0.5,

P: < 0.02,

S: < 0.004,

N: < 0.03,

B: < 0.0004,

O: < 0.008

and optionally:

Ti: < 0.1 ,

Mo: < 0.5,

Nb: < 0.3,

V: < 0.5,

Ca: < 0.05,

the remainder being iron and unavoidable impurities,

and in that the blank is cooled between the furnace and the hot forming press such that the blank has a temperature of at most 770° C and at least 450° C when placed in the hot forming press,

and in that the hot formed part is at least partially martensitic.

2. Method according to claim 1 , wherein:

C: 0.12 - 0.23 and/or

Mn: 1 .6 - 2.5 and/or

Si: < 0.3 and/or

Cr: < 0.8 and/or

Al: < 0.1

O: < 0.005 and/or

N:≤ 0.01 and/or

B: < 0.0003 and preferably B < 0.0002 and/or

Mo: < 0.2 and/or

Nb: < 0.1 and/or

V: < 0.2 and/or

Ca: < 0.01 .

3. Method according to claim 1 or 2, wherein:

C: 0.15 - 0.21 and/or

Mn: 1 .8 - 2.4, preferably Mn 1 .85 - 2.4 and/or

Si: < 0.2 and/or

Cr: < 0.7, preferably Cr 0.2 - 0.7 and/or

Al: < 0.05 and/or

O: < 0.002 and/or

N:≤ 0.006 and/or

Ti: < 0.02, preferably Ti < 0.015 and/or

Mo: < 0.08 and/or

Nb: < 0.02 and/or

V: < 0.02 and/or

Ca: < 0.005

B: < 0.0001 and preferably B < 0.00009.

4. Method according to claim 1 , 2 or 3, wherein Mn + Cr > 2.5 weight%, preferably Mn + Cr > 2.6 weight%.

5. Method according to any one of the preceding claims, wherein the metal coated steel blank has a metal coating comprising a layer of zinc or a zinc based alloy.

6. Method according to claim 5, wherein the metal coating is an iron-zinc diffusion coating obtained by heat treating a zinc layer, the zinc layer comprising Al < 0.18 wt% and Fe < 15 wt%, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 μιτι per side, more preferably a thickness between 6 and 13 μιτι per side.

7. Method according to claim 5, wherein the metal coating comprises 0.5 to 4 wt% Al and 0.5 to 3.2 wt% Mg, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 μιτι per side, more preferably a thickness between 6 and 13 μιτι per side.

8. Method according to any one of the preceding claims, wherein the metal coated steel blank is at least partially heated to a temperature between 850 and 950° C, most preferably between 850 and 900° C.

9. Method according to any one of the preceding claims, wherein the coated steel blank is heated in a furnace during a time period in which at least part of the blank reaches a desired austenitising temperature plus an additional 10 to 600 seconds.

10. Method according to any one of the preceding claims, wherein the blank has a temperature of at least 480 °C when placed in the hot forming press, preferably a temperature of at least 500 or 520 or 550 or 580 or 600 or 620 °C.

1 1 . Method according to any one of the preceding claims, wherein the blank has a temperature of at most 750 °C when placed in the hot forming press, preferably a temperature of at most 725 or 700 or 680 or 650 °C.

12. Method according to any one of the preceding claims, wherein the blank is transported from the furnace to the hot forming press in a time period of at most 20 seconds, preferably in a time period of at most 15 seconds, more preferably in a time period of at most 12 seconds, even more preferably in a time period of at most 10 seconds, most preferably in a time period of at most 8 seconds.

13. Method according to any one of the preceding claims, wherein the blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 20 °C/s during the forced cooling, more preferably with an average cooling velocity of at least 30 °C/s during the forced cooling, even more preferably with an average cooling velocity of at least 40 °C/s during the forced cooling, still more preferably with an average cooling velocity of at least 50 °C/s during the forced cooling, most preferably with an average cooling velocity of at least 60 °C/s during the forced cooling.