WO2020239891A1

WO2020239891A1 - Steel strip, sheet or blank for producing a hot-stamped part, part, and method for hot-stamping a blank into a part

Info

Publication number: WO2020239891A1
Application number: PCT/EP2020/064803
Authority: WO
Inventors: Radhakanta RANA
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2019-05-28
Filing date: 2020-05-28
Publication date: 2020-12-03
Also published as: CN113891952A; EP3976846A1; JP2022535352A; KR20220013393A

Abstract

The present invention relates to a steel strip, sheet or blank for producing hot-stamped parts having the following composition in wt%: C: < 0.20, Mn: 0.65 - 3.0,5 W: 0.10 - 0.60, and optionally one or more of the elements selected from: Si: < 0.10, Mo: ≤ 0.10, Al: ≤ 0.10,10 Cr: ≤ 0.10, Cu: ≤ 0.10, N: ≤ 0.010, P: ≤ 0.030, S: ≤ 0.025,15 O: ≤ 0.01, Ti: ≤ 0.02, V: ≤ 0.15, Nb: ≤ 0.01, B: ≤ 0.0005,20 the remainder being iron and unavoidable impurities.

Description

STEEL STRIP, SHEET OR BLANK FOR PRODUCING A HOT-STAMPED PART, PART, AND METHOD FOR HOT-STAMPING A BLANK INTO A PART

The present invention relates to a steel strip, sheet or blank for producing a hot- stamped part; a hot-stamped part; and a method for producing a hot-stamped part. There is an increasing demand for steel alloys that allow for weight reduction of automobile parts in order to reduce fuel consumption, whilst they provide at the same time improved safety to passengers.

In order to meet the requirements of the automotive industry in terms of improved mechanical properties, such as improved tensile strength, crash energy absorption, workability, ductility and toughness, cold-stamping and hot-stamping processes have been developed to manufacture steel components that meet these requirements.

In cold-stamping processes, the steel is shaped into a product at near-room temperature. Steel products produced in this way are for instance dual phase (DP) steels which have a ferritic-martensitic microstructure. Although these DP steels display a high ultimate tensile strength, their bendability and yield strength are low which is undesirable since these reduce crash performance in service.

In hot-stamping processes, steels are heated beyond their recrystallization temperature, and quenched to obtain desired material properties, usually by a martensitic transformation. The basics of the hot-stamping technique and steel compositions adapted to be used therefor were already described in GB1490535.

A steel typically used for hot-stamping is 22MnB5 steel. This boron steel can be reheated in a furnace to austenitize usually between 870 and 940 °C, transferred from furnace to the hot-stamping press, and stamped into the desired part geometry, while the part is cooled at the same time. The advantage of such boron steel parts produced this way is that they display a high ultimate tensile strength for anti-intrusive crashworthiness due to their fully martensitic microstructure achieved by press-quenching, but at the same time they display a low bendability and ductility which in turn results in a limited toughness and bending fracture resistance and thus a poor impact-energy absorptive crashworthiness.

Fracture toughness measurement is a useful tool to indicate the crash energy absorption of steels. When the fracture toughness parameters are high, generally a good crash behavior is obtained.

In view of the above, it will be clear that there is a need for steel parts that display an excellent ultimate tensile strength, and at the same time an excellent ductility, yield strength and bendability, and in turn excellent crash energy absorption. It is therefore an object of the present invention to provide a steel strip, sheet or blank that can be hot-stamped into a part that has a combination of an excellent ultimate tensile strength, yield strength, bendability and ductility, thereby providing an excellent crash energy absorption when compared to conventional cold-stamped and hot- stamped steels.

It is another subject of the present invention to provide a hot-stamped part which is produced from such a steel strip, sheet or blank, and the use of such a hot-stamped part as a structural part of a vehicle.

Yet another object of the present invention is to provide a method for hot-stamping a steel blank into a part.

It has now been found that these objects can be established when use is made of a low alloy steel that contains in addition to manganese a relatively high amount of tungsten. Accordingly, the present invention relates to a steel strip, sheet or blank for producing hot-stamped parts having the following composition in wt%:

C: < 0.20,

Mn: 0.65 - 3.0,

W: 0.10 - 0.60,

and optionally one or more of the elements selected from:

Si: < 0.10,

Mo: £ 0.10,

Al: £ 0.10,

Cr: £ 0.10,

Cu: < 0.10,

N: £ 0.010,

P: £ 0.030,

S: £ 0.025,

O: £ 0.01 ,

Ti: £ 0.02,

V: £ 0.15,

Nb: £ 0.01 ,

B: £ 0.0005,

the remainder being iron and unavoidable impurities.

The hot-stamped part produced from the steel strip, sheet or blank in accordance with the present invention displays an improved combination of tensile strength, ductility and bendability, and thereby impact-energy absorptive crashworthiness when compared to conventional hot-stamped boron steels.

The automotive components which are in mind to be made from these steels are the front and back longitudinal bars and the B-pillar. For the front longitudinal, currently a cold-stamped dual phase steel (e.g. DP800) is used and for the B-pillar a hot-stamped 22MnB5 steel is used. The DP800 steel exhibits a lower energy absorption, and using a higher strength steel (Ultimate Tensile Strength > 800 MPa) will enable more weight saving through downgauging and enhanced passenger safety by higher crash energy absorption. On the other hand, for the B-pillar one currently used solution is using two types of steels, an ultra high strength (-1500 MPa) 22MnB5 for the upper part and a lower strength (-500 MPa) steel for the lower part. The two steel blanks are joined by laser welding before hot-stamping and then the hybrid blank is stamped into the B-pillar. By using this solution, during crash the upper part resists intrusion whereas the lower part absorbs energy due to its higher bendability and ductility combination. The current invention offers better performance and weight saving potential: the invented higher strength steel can replace the lower strength steel of the lower part with a higher energy absorption capability.

Preferably, the steel strip, sheet or blank for producing hot-stamped parts as described above has the following composition in wt%:

C: 0.05 - 0.18, preferably 0.07 - 0.16, and/or

Mn: 1.00 - 2.50, preferably 1.20 - 2.20, and/or

W: 0.10 - 0.50, preferably 0.13 - 0.30, and/or

Si: < 0.009, preferably £ 0.005, and/or

Al: £ 0.05, preferably £ 0.04

N: 0.001 - 0.008, preferably 0.002 - 0.005. Carbon is added to the steel for securing good mechanical properties. C is added in an amount of less than 0.20 wt% to achieve high strength and to increase the hardenability of the steel. When too much carbon is added there is the possibility that the toughness and weldability of the steel sheet will deteriorate. The C amount used in accordance with the invention is therefore < 0.20 wt%, preferably in the range of from 0.05 - 0.18 wt%, and more preferably in the range of from 0.07 - 0.16 wt%. For some applications it is advantageous if the C amount is from 0.07 - 0.15 wt%. This may be advantageous for higher ductility parameters, such as bendability and/or elongation.

Manganese is used because it promotes hardenability and gives solid solution strengthening. The Mn content is at least 0.65 wt% to provide adequate substitutional solid solution strengthening and adequate quench hardenability, while minimizing segregation of Mn during casting and while maintaining sufficiently low carbon equivalent for automotive resistance spot-welding techniques. Further, Mn is an element that is useful in lowering the A_C3 temperature. A higher Mn content is advantageous in lowering the temperature necessary for hot-stamping. When the Mn content exceeds 3.0 wt%, the steel sheet may suffer from poor weldability and poor hot- and cold-rolling characteristics that affect the steel processability. The Mn amount used in accordance with the invention is in the range of from 0.65 - 3.0 wt%, preferably in the range of from 1.00 - 2.50 wt%, and more preferably in the range of from 1.20 - 2.20 wt%. Lower Mn contents should be used with higher W and C combinations and vice versa to ensure the adequate hardenability of the steel.

Tungsten is very effective in delaying the diffusion-controlled transformations at high temperatures in steel. It postpones the formation of ferrite and pearlite by lengthening the incubation times of ferrite and pearlite transformations. In other words, W increases the hardenability of the steel. It is important that for this effect of hardenability improvement, W is in solid solution of iron. This is ensured by austenitising the steel sufficiently above the A_C3 temperature and for a suitable duration. In this respect it is observed that the presence of ferrite and/or pearlite in the microstructure is detrimental to mechanical properties for the targeted microstructure according to the present invention. The amount of W used in the invention is more than 0.10 wt% and at most 0.60 wt%, preferably in the range of from 0.10 - 0.50 wt%, more preferably in the range of from 0.13 - 0.50 wt%, still more preferably in the range of from 0.13 - 0.30 wt%. The amount of W should not be too high because it will increase the alloying cost too much compared to the advantages obtained and it should not be too low as it will not be effective to give the metallurgical effect as explained above.

The amounts of Si, Mo, Al, Cr, Cu, N, P, S, O, Ti, Nb, B and V, if present, should all be low.

Silicon is not added and not needed to exert the desired metallurgical effects in this invention. The Si amount used in the invention is less than 0.10 wt%, preferably less than 0.009 wt% and preferably at most 0.005 wt%.

Chromium may improve the hardenability of the steel and facilitate avoiding the formation of ferrite and/or pearlite during press quenching. The amount of Cr used in the invention is at most 0.10 wt%, preferably at most 0.05 wt% and more preferably at most 0.009 wt%, the latter because Cr in a greater amount could cause formation of Cr containing carbides which may deteriorate the mechanical properties. Molybdenum is added to improve the hardenability of the steel and facilitate the formation of bainite. The Mo amount used in accordance with the invention is at most 0.10 wt%, preferably at most 0.05 wt% and more preferably at most 0.009 wt%, the lower amounts being preferred because higher amounts of Mo will considerably increase the alloying cost.

Aluminium is added to deoxidize the steel. The Al amount is at most 0.10 wt%, preferably at most 0.05 wt%, more preferably at most 0.04 wt%. If more aluminium is added some ferrite may form during press quenching causing deterioration of the mechanical properties.

Copper is added to improve hardenability and increase strength of the steel. If present, Cu is used in accordance with the invention in an amount of at most 0.10 wt%, preferably at most 0.05 wt%, more preferably at most 0.04 wt% and even more preferably at most 0.009 wt%, the latter because the presence of Cu can cause hot shortness during high temperature processing

Phosphorus is known to widen the intercritical temperature range of a steel. P is also an element useful for maintaining desired retained austenite. However, P may deteriorate the workability of the steel. In accordance with the invention P should be present in an amount of at most 0.030 wt%, preferably at most 0.015 wt%.

Sulphur needs to be minimized to reduce harmful non-metallic inclusions. S forms sulphide based inclusions such as MnS, which initiates crack, and deteriorates processability. Therefore, it is desirable to reduce the S amount as much as possible. In accordance with the present invention the amount of S is at most 0.025 wt%, preferably an amount of at most 0.010 wt%.

Titanium, when present, forms TiN precipitates to scavenge out N at high temperatures while the steel melt cools. Formation of TiN prohibits formation of B3N4 at lower temperatures so that B, if present, becomes more effective. Stoichiometrically, when B is added, the ratio of Ti to N (Ti/N) addition should be > 3.42. In accordance with the invention the amount of titanium is £ 0.02 wt.%.

Niobium may have the effect of forming strengthening precipitates and refining microstructure. Nb increases the strength by means of grain refinement and precipitation hardening. Grain refinement results in a more homogeneous microstructure improving the hot-stamping behavior, in particular when high localized strains are being introduced. A fine, homogeneous microstructure also improves the bending behavior. The amount of Nb used in the invention is £ 0.01 wt.%. Vanadium may be added to form V(C, N) precipitates to strengthen the steel product. The amount of vanadium, if any, is at most 0.15 wt%, preferably at most 0.05 wt% and more preferably at most 0.009 wt%, the lower amounts being preferred for cost reasons and for the reason that V can cause the formation of complex carbides together with micro-alloying elements which formation could reduce the ductility properties of the product.

Boron is for increasing the hardenability of steel sheets and for further increasing the effect of stably guaranteeing strength after quenching. In accordance with the invention B is present at £ 0.0005 wt.%.

Nitrogen has an effect similar to C. N is suitably combined with titanium to form

TiN precipitates. The amount of N according to the invention is at most 0.010 wt%. Preferably the amount of N is in the range of 0.001 - 0.008 wt%. Suitably, N is present in an amount in the range of from 0.002 - 0.005 wt%.

Oxygen: Steel products need to be deoxidized because oxygen reduces various properties such as tensile strength, ductility, toughness, and/or weldability. Hence, the presence of oxygen should be avoided. In accordance with the present invention, the amount of O is at most 0.01 wt%, preferably at most 0.005 wt%.

Calcium may be present in an amount of up to 0.05 wt%, preferably up to 0.01 wt%. Ca is added to spheroidize the sulphur containing inclusions and to minimize the amount of elongated inclusions. However, the presence of CaS inclusions will still lead to inhomogeneities in the matrix; it is thus best to reduce the amount of S.

Preferably, the steel strip, sheet or blank, is provided with a zinc based coating, an aluminium based coating or an organic based coating. Such coatings reduce oxidation and/or decarburization during a hot-stamping process, and provides corrosion protection in service.

It is preferred when the zinc based coating is a coating containing 0.2 - 5.0 wt% Al, 0.2 - 5.0 wt% Mg, optionally at most 0.3 wt% of one or more additional elements, the balance being zinc and unavoidable impurities. The additional elements can be selected from the group comprising 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.

Preferably, the total amount of additional elements in the zinc alloy is at most 0.3 wt.%. These small amounts of an additional element do not alter the properties of the coating nor the bath to any significant extent for the usual applications.

When one or more additional elements are present in the zinc alloy coating, each is preferably present in an amount of at most 0.03 wt%, preferably each is present in an amount of at most 0.01 wt%. Additional elements are usually only added to prevent dross forming in the bath with molten zinc alloy for the hot-dip-galvanizing, or to form spangles in the coating layer.

The hot-stamped part produced from a steel strip, sheet or blank in accordance with the present invention has a microstructure comprising at most 50 vol.% bainite, the remainder being martensite. Preferably, the microstructure comprises at most 40 vol. % of bainite, the remainder being martensite. More preferably, the microstructure comprises at most 30 vol. % of bainite, the remainder being martensite. The presence of bainite is applicable only for the slow cooling rates encountered during press quenching. During press quenching, the typical cooling rate of the blanks is greater than about 30 °C/s. Above a cooling rate of 60 °C/s a fully martensitic microstructure is formed. In such situations, the martensite provides a high strength, whereas the softer bainite improves the ductility. The small strength difference between martensite and bainite helps in maintaining a high bendability due to lack of weak phase interfaces.

The hot-stamped part in accordance with the present invention displays excellent mechanical properties. The part has a tensile strength (TS) of at least 745 MPa, preferably of at least 1070 MPa, more preferably of at least 1300 MPa, and further has a tensile strength of at most 1400 MPa.

The part suitably has a total elongation (TE) of at least 5 %, preferably 5.5 %, more preferably at least 6 % and most preferably at least 7 %, and/or a bending angle (BA) at

1.0 mm thickness of at least 78°, preferably at least 100°, more preferably at least 115°, more preferably at least 130° and most preferably at least 140°.

It will be clear that the steel products in accordance with the present invention exhibit excellent crash energy absorption.

The present invention also relates to the use of hot-stamped parts as described above, as structural part in the body-in-white of a vehicle. Such parts are made of the present steel strip, sheet or blank. These parts have a combination of high strength, high ductility and a high bendability. In particular, for parts in the form of structural parts of vehicles, the steels of the present invention are very attractive since they exhibit excellent crash energy absorption and in turn, down-gauging and lightweighting opportunities based on crashworthiness compared to the use of conventional hot-stamped boron steels and cold-stamped multiphase steels.

The present invention also relates to a method for producing a part in accordance with the present invention. Accordingly, the present invention also relates to a method for hot-stamping a steel blank or a pre-formed part into a part comprising the steps of:

(a) heating the blank, or a pre-formed part produced from the blank, according to any one of claims 1 - 3 to a temperature Ti and holding the heated blank at Ti during a time period ti , wherein Ti is higher than the A_C3 temperature of the steel, and wherein ti is at most 10 minutes;

(b) transferring the heated blank or pre-formed part to a hot-stamping tool during a transport time t2 during which the temperature of the heated blank or pre formed part decreases from temperature Ti to a temperature T2, wherein the transport time t2 is at most 20 seconds;

(c) hot-stamping the heated blank or pre-formed part into a part; and

(d) cooling the part in the hot-stamping tool to a temperature below the M_f temperature of the steel with a cooling rate of at least 30 °C/s.

In accordance with the present method it was found that through stamping the heated blank into a part as described above, complex shaped parts with enhanced mechanical properties can be obtained. In particular, the parts exhibit excellent crash energy absorption and thus allow for down-gauging and lightweighting opportunities based on crashworthiness compared to the use of conventional hot-stamped boron steels and cold-stamped multiphase steels.

After the cooling of the part to a temperature below the M_f temperature, the part can for instance be further cooled to room temperature in air or can be forcibly cooled to room temperature.

In the method according to the present invention, the blank to be heated in step (a) is provided as an intermediate for the subsequent steps. The steel strip or sheet from which the blank is produced can be obtained by standard casting processes. In a preferred embodiment, the steel strip or sheet is cold-rolled. The steel strip or sheet can suitably be cut to a steel blank. A pre-formed steel part may also be used. The pre-formed part may be partially or entirely formed into the desired geometry, preferably at ambient temperature.

The steel blank is heated in step (a) to a temperature Ti for a time period ti . Preferably, in step (a) the temperature Ti is 40 - 100 °C higher than the A_C3 temperature of the steel, and/or the temperature T2 is above the A_r3 temperature. When Ti is 40 - 100 °C above the A_C3 temperature, the steel is fully or almost fully austenitised within the time period ti , and the cooling during step (b) is easily possible. When the microstructure is a homogenous austenitic microstructure the formability is enhanced. Preferably, the time period ti is at least 1 minute and at most 7 minutes. Too long a time period ti may result in coarse austenitic grains, which will deteriorate the final mechanical properties.

The heating apparatus to be used in step (a) may for instance be an electric or gas powered furnace, electrical resistance heating device, infra-red induction heating device.

In step (b), the heated steel blank or pre-formed part is transferred to a hot- stamping tool during a transport time t2 during which the temperature of the heated steel blank or pre-formed part decreases from temperature Ti to a temperature T2, wherein the transport time t_å is at most 20 seconds. Time t_å is the time needed to transport the heated blank from the heating apparatus to the hot-stamping tool (e.g. press) and till the hot-stamping apparatus is closed. During the transfer of the blank or pre-formed part may cool from temperature Ti to temperature T2 by the act of natural air-cooling and/or any other available cooling method. The heated blank or pre-formed part may be transferred from the heating apparatus to the hot-stamping tool by an automated robotic system or any other transfer method. Time t_å may also be chosen in combination with T 1 , ti and T2 in order to control the microstructural evolution of steel at the commencement of hot-stamping and quenching. Suitably, t_å is equal or less than 12 seconds, preferably t_å is equal or less than 10 seconds, more preferably t_å is equal or less than 8 seconds, and most preferably equal or less than 6 seconds. In step (b), the blank or pre-formed part can be cooled from temperature T1 to a temperature at a cooling rate V2 of at least 10 °C/s. V2 is preferably in the range of from 10 - 15 °C/s. When the blank or pre-formed part should be precooled, the cooling rate should be higher, for instance at least 20 °C/s, up to 50 °C/s or more.

In step (c) a heated blank or pre-formed part is formed into a part having the desired geometry. The formed part is preferably a structural part of a vehicle.

In step (d) the formed part in the hot-stamping tool is cooled to a temperature below the M_f temperature of the steel with a cooling rate V3 of at least 30 °C/s. Preferably, the cooling rate V3 in step (d) is in the range of from 30 - 150 °C/s, more preferably in the range of from 30 - 100 °C/s.

The present invention provides an improved method of introducing during hot- stamping operation the desired bainitic phase into the steel microstructure. The present method enables the production of hot-stamped steel parts displaying an excellent combination of high strength, high ductility and high bendability.

One or more steps of the method according to the present invention may be conducted in a controlled inert atmosphere of hydrogen, nitrogen, argon or any other inert gas in order to prevent oxidation and/or decarburization of said steel.

Figure 1 shows a schematic representation of an embodiment of the method according to the invention.

In Figure 1 , the horizontal axis represents the time t, and the vertical axis represents the temperature T. The time t and temperature T are indicated diagrammatically in Figure 1. No values can be derived from Figure 1.

In Figure 1 , a steel blank or pre-formed part is (re-)heated up to the austenitising temperature above A_d at a particular (re-)heating rate. Once the A_d has been exceeded the (re-)heating rate is lowered until the blank or pre-formed part has reached a temperature higher than the A_C3. Then the strip, sheet or blank is held at this particular temperature for a period of time. Subsequently, the heated blank is transferred from the furnace to the hot-stamping tool, during which cooling of the blank by air occurs to some extent. The blank or pre-formed part is then hot-formed into a part and cooled down (or quenched) at a cooling rate of at least 30 °C/s. After reaching a temperature below the M_f temperature of the steel, the hot-stamping tool is opened, and the formed article is cooled down to room temperature.

The different temperatures as used throughout the patent application are explained below:

- A_d: Temperature at which, during heating, austenite starts to form.

A_C3: Temperature at which, during heating, transformation of the ferrite into austenite ends.

A_r3: The temperature at which transformation of austenite to ferrite starts during cooling.

- M_s: Temperature at which, during cooling, transformation of the austenite into martensite starts.

M_f: Temperature at which, during cooling, transformation of the austenite into martensite ends.

The invention will be elucidated by means of the following, non-limiting Examples.

Examples

Steel composition A (according to the invention)

Steel blanks with dimensions of 220 mm x 110 mm x 1.5 mm were prepared from a cold- rolled steel sheet having the composition as shown in Table 1. These steel blanks were subjected to hot-stamping thermal cycles in a hot-dip annealing simulator (HDAS) and a hot-stamping press supplied by Schuler SMG GmbH & Co. KG (herein after named as SMG press). The HDAS was used for slower cooling rates (30 - 80°C/s) whereas the SMG press was used for the fastest cooling rate (200 °C/s). The steel blanks were reheated to a Ti of respectively 900°C (50°C above A_C3) and 940°C (90°C above A_c3), soaked for 5 min. in nitrogen atmosphere to minimize surface degradation. The blanks were then subjected to transfer cooling for a drop in temperature of 120°C in 10s, so at a cooling rate V2 of about 12°C/s and then subjected to cooling to 160°C at the following cooling rates V₃: 30, 40, 50, 60, 80, 200°C/s. From the heat treated samples, longitudinal tensile specimens with 50 mm gauge length and 12.5 mm width (Euronorm A50 specimen geometry) were prepared and tested with a quasistatic strain rate. Microstructures were characterized from the RD - ND planes. Bending specimens (40 mm x 30 mm x 1.5 mm) from parallel and transverse to rolling directions were prepared from each of the conditions and tested till fracture by three-point bending test as described in the VDA 238 - 100 standard. The samples with bending axis parallel to the rolling direction were identified as longitudinal (L) bending specimens whereas those with bending axis perpendicular to the rolling direction were denoted as perpendicular (T) bending specimens. The measured bending angles at 1.5 mm thickness were also converted to the angles for 1.0 mm thickness (= original bending angle ^c square root of original thickness). For each type of test, three measurements were done and the average values from three tests are presented for each condition.

For selected conditions (SMG press samples with reheating at 940°C), J-integral fracture toughness and drop tower axial crash tests were conducted. Compact tension specimens according to NFMT76J standard were prepared from both longitudinal and transverse directions for fracture toughness tests. For the transverse specimen, the crack runs along the rolling direction and the loading is transverse to the rolling direction, whereas the opposite applies for the longitudinal specimens. The specimens were tested according to ASTM E1820-09 standard at room temperature. The pre-cracks were introduced by fatigue loading. The final tests were done with tensile loading with anti buckle plates to keep the stress in plane for sheet material. Three tests for each condition were done and following the guidelines in BS7910 standard the minimum values of three equivalents (MOTE values) for different fracture toughness parameters are presented.

A brief description of the fracture toughness parameters is given below. CTOD is the Crack Tip Opening Displacement and is a measure of how much the crack opens at either failure (if brittle) or maximum load. J is the J-integral and is a measure of toughness that takes account of the energy, so it is calculated from the area under the curve up to failure or maximum load. KJ is the stress intensity factor determined from the J integral using an established expression, given as KJ= [J(E/(1-v²))]⁰⁵ where E is the Young’s modulus (= 207 GPa) and v is the Poisson’s ratio (= 0.03). K_q is the value of stress intensity factor measured at load P_q, where P_q is determined by taking the elastic slope of the loading line, then taking a line with 5% less slope and defining P_q as the load where this straight line intersects the loading line.

Drop tower axial crash tests were done in SMG-pressed condition with a load of 200 kg and a loading speed of 50 km/hour for the load to hit the crash boxes having a closed top hat geometry (Figure 2) with 500 mm height (transverse to the rolling direction) prepared from larger sheets. The dimensions of the cross-section of the drop tower are given in figure 2 in millimetres (t = 1.5 mm, Ro = 3 mm). The back plates of 100 mm width were spot-welded to the profiles to prepare the crash boxes.

For some selected conditions, a paint bake thermal cycle (180 °C for 20 minutes) was also given to the samples, and the tests were done as will be reflected from the results directly.

Steel compositions B and C (not according to the invention)

For comparison reasons a commercially available cold-formable CR590Y980T-DP (steel composition B and commonly known as DP1000 steel) was also tested since it has a strength level in the range of the steel blank in accordance with the invention. In addition, and also for comparative reasons, a standard hot-stamped 22MnB5 steel product (steel composition C) was tested.

In Table 1 , the chemical compositions in wt% of steel compositions A - C are specified.

In Table 2, the transformation temperatures of steel composition A are shown.

The results of the various tests are presented in Tables 3 to 8.

In Table 3, the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE), and total elongation (TE) are shown for steel composition A for a variety of cooling rates V3. In addition, Table 3 shows the microstructure consisting of martensite (M) and bainite (B). It will be clear from Table 3 that an ultimate tensile strength of greater than 740 MPa was achieved at the different cooling rates V3.

In Table 4, bending angles (BA) at 1.0 mm thickness are shown for steel composition A as obtained after different cooling rates V3. It is clear from Table 4 that high bending angles of greater than at least 130° were achieved for both the longitudinal (L) and transverse (T) orientations. In Table 5, the various mechanical properties have been shown for steel composition A after said composition has been subjected to a hot-stamping and baking treatment simulating the paint baking treatment used during automobile manufacturing. Steel composition A was heated to 900 °C, soaked for 5 min. and then cooled at a V₃ of 200 °C/s, following the transfer cooling. The baking treatment was carried out at 180°C for 20 minutes. From Table 5, it will be clear that approximately the same minimum levels of yield strength YS), ultimate tensile strength (UTS), ultimate elongation (UE), total elongation (TE) and bending angels (BA) are also achieved after steel composition A has been subjected to a baking treatment. This means that in automotive manufacturing after paint baking, the properties claimed will be ensured in service condition.

In Table 6, the various mechanical properties of steel compositions B (DP1000) and C (22MnB5) are shown. These steel compositions B and C were tested under the same test conditions as steel composition A. When the contents of Tables 4 and 6 are compared it will become immediately evident that the steel part in accordance with the present invention (steel composition A) constitutes a major improvement in terms of bendability when compared with conventional cold-formed steel products DP1000 (steel composition B) and conventional hot-stamped steel product 22MnB5 (steel composition C).

From Table 7, it is also clear that the fracture toughness parameters of the steel part in accordance with the present invention (steel composition A) is also higher than that of blanks made of DP1000 (steel composition B).

In Table 8, the crash behavior of the steel compositions A and B is shown. From Table 8 it is clear that the crash behavior of steel composition A is better than that of DP1000 (steel composition B) in both hot-pressed as well as hot-pressed and baked conditions. The baking conditions are the same as described here above. The crash boxes of steel composition A did not show any indication of cracking after the tests, whereas the crash boxes of DP1000 (steel composition B) showed severe cracking in the folds. Moreover, steel composition A shows a higher energy absorption capability.

The high and improved crash behavior of hot-stamped steel composition A in accordance with the present invention when compared to the conventional steel products of similar strength is due to the higher bending angle and higher fracture toughness properties. In this respect it is observed that during a crash, the steel component needs to fold which is determined by its bendability, whereas on the other hand the energy absorption capability before failure is determined by its fracture toughness parameters. In view of the above, it will be clear to the skilled person that the steel products in accordance with the present invention constitute a considerable improvement over conventionally known cold-stamped and hot-stamped steel products. Table 1 : chemistry steels A, B and C (wt%)

Table 2: Transformation temperatures steel composition A

Table 3: Mechanical properties and microstructures for steel composition A

Table 4: Bending angles for steel composition A

Table 5. Mechanical properties of steel composition A after bake hardening

Table 6. Mechanical properties Steel compositions B (DP1000), and C (22MnB5)

Table 7: Fracture toughness parameters for steel compositions A - C

Table 8: Crash test results for steel compositions A and B (DP1000)

Claims

1. Steel strip, sheet or blank for producing hot-stamped parts having the

following composition inwt%:

C: < 0.20,

Mn: 0.65 - 3.0,

W: 0.10 - 0.60,

and optionally one or more of the elements selected from:

Si: < 0.10,

Mo: £ 0.10,

Al: £ 0.10,

Cr: £ 0.10,

Cu: < 0.10,

N: £ 0.010,

P: £ 0.030,

S: £ 0.025,

O: £ 0.01 ,

Ti: £ 0.02,

V: £ 0.15,

Nb: £ 0.01 ,

B: £ 0.0005,

the remainder being iron and unavoidable impurities.

2. Steel strip, sheet or blank according to claim 1 , wherein in wt%:

C: 0.05 - 0.18, preferably 0.07 - 0.16, and/or

Mn: 1.00 - 2.50, preferably 1.20 - 2.20, and/or

W: 0.10 - 0.50, preferably 0.13 - 0.30 and/or

Si: < 0.009, preferably £ 0.005, and/or

Al: £ 0.05, preferably £ 0.04

N: 0.001 - 0.008, preferably 0.002 - 0.005.

3. Steel strip, sheet or blank according to any one of claims 1 - 2, wherein the steel strip, sheet or blank is provided with a zinc based coating or an aluminium based coating or an organic based coating.

4. Steel strip, sheet or blank according to claim 3, wherein the zinc based coating is a coating containing 0.2 - 5.0 wt% Al, 0.2 - 5.0 wt% Mg, optionally at most 0.3 wt% of one or more additional elements, the balance being zinc and unavoidable impurities.

5. Hot-stamped part produced from a steel strip, sheet or blank according to any one of the preceding claims, the part having a tensile strength of at least 745 MPa, preferably at least 1070 MPa, more preferably at least 1300 MPa, and more preferably at least 1400 MPa.

6. Hot-stamped part according to claim 5 having a total elongation (TE) of at least 5 %, preferably at least 5.5 %, more preferably at least 6 % and most preferably at least 7 % and/or a bending angle (BA) at 1.0 mm thickness of at least 78°, preferably at least 100°, more preferably at least 115°, more preferably at least

130 ° and most preferably at least 140°.

7. Hot-stamped part according to claim 5 or 6, the part having a microstructure comprising at most 50% bainite, the remainder being martensite, the microstructure preferably comprising at most 40% bainite, more preferably the microstructure comprising at most 30% bainite.

8. Use of a hot-stamped part according to any one of claims 5 - 7 as structural part in the body-in-white of a vehicle.

9. A method for hot-stamping a steel blank or a pre-formed part into a part comprising the steps of:

(e) heating the blank, or a pre-formed part produced from the blank, according to any one of claims 1 - 3 to a temperature Ti and holding the heated blank at Ti during a time period ti, wherein Ti is higher than the A_C3 temperature of the steel, and wherein ti is at most 10 minutes;

(f) transferring the heated blank or pre-formed part to a hot-stamping tool during a transport time t2 during which the temperature of the heated blank or pre-formed part decreases from temperature Ti to a temperature T2, wherein the transport time t2 is at most 20 seconds; (g) hot-stamping the heated blank or pre-formed part into a part; and

(h) cooling the part in the hot-stamping tool to a temperature below the M_f temperature of the steel with a cooling rate of at least 30 °C/s.

10. Method according to claim 9, wherein the temperature Ti in step (a) is 40 -100

°C higher than the A_C3 and/or the temperature T2 is above A_r3.

11. Method according to claim 9 or 10, wherein the time period h in step (a) is at least 1 minute and at most 7 minutes and/or the time period in step (b) is at most 12 seconds, preferably the time period is between 2 and 10 seconds.

12. Method according to any one of claims 9 - 11 , wherein the part is cooled in step

(d) with a cooling rate in the range of 30 - 150 °C/s, preferably with a cooling rate of 30 - 100 °C/s.

13. Vehicle comprising at least one hot-stamped part according to any one of claims 5 - 7 and/or a part manufactured according to the method of any one of claims 9 12