EP3658692B1

EP3658692B1 - Steel strip, sheet or blank for producing a hot formed part, part, and method for hot forming a blank into a part

Info

Publication number: EP3658692B1
Application number: EP18740258.1A
Authority: EP
Inventors: Radhakanta RANA
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2017-07-25
Filing date: 2018-07-23
Publication date: 2021-11-10
Anticipated expiration: 2038-07-23
Also published as: EP3658692A1; JP7326247B2; BR112020000917A2; KR20200035259A; KR20240142610A; ES2899238T3; JP2020528963A; MX2020000928A; WO2019020575A1; US20210156012A1

Description

The present invention relates to a steel strip, sheet or blank for producing a hot formed part; a hot formed part; and a method for producing a hot formed 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 protection of passengers.
In order to meet the automotive industry's requirements in terms of improved mechanical properties, such as improved tensile strength, energy absorption, workability, ductility and toughness, cold-forming and hot-forming processes have been developed to produce steels that meet these requirements.
In cold-forming 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 it reduces crash performance.
In hot-forming 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 forming technique and steel compositions adapted to be used therefor were already described in GB1490535 . WO2015/144318 discloses a method for hot forming a part using a coated steel blank used in the automotive industry e.g. body-in-white of automotive vehicles.
A steel typically used for hot-forming is 22MnB5 steel. This boron steel can be furnace-heated and is usually austenitized between 870-940 °C, transferred from furnace to forming tool, and stamped into the desired part geometry, while the part is at the same time cooled. 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, but at the same time they display a low ductility and bendability which in turn results in a limited toughness and thus a poor impact-energy absorptive crashworthiness.
Fracture toughness measurement is an 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 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 formed into a part that has a combination of an excellent ultimate tensile strength, ductility and bendability, thereby providing an excellent crash energy absorption when compared to conventional cold-formed and hot-formed steels.
It is another subject of the present invention to provide a hot formed part which is produced from such a steel strip, sheet or blank, and the use of such a hot formed part as a structural part of a vehicle.
Yet another object of the present invention is to provide a method for hot-forming 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 carbon, manganese, chromium, titanium and nitrogen, relatively small amounts of niobium and boron.
Accordingly, the present invention relates to a steel strip, sheet or blank for producing hot formed parts as disclosed in appended claim 1.
The hot formed 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 toughness when compared to conventional hot-formed boron steels.
Two automotive components for this steels are in mind, namely the front longitudinal bars and the B-pillar. For the front longitudinal, currently a cold-formed dual phase steel (DP800) is used and for the B-pillar a hot stamped 22MnB5 steel is used. The DP steel has 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 - 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 high ductility. 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 formed parts as described above has the following composition in weight%:

C: 0.05 - 0.17, preferably 0.07 - 0.15, and/or
Mn: 1.0 - 2.1, preferably 1.2 - 1.8, and/or
Cr: 0.5 - 1.7, preferably 0.8 - 1.5, and/or
Ti: 0.015 - 0.07, preferably 0.025 - 0.05, and/or
Nb: 0.02 - 0.08, preferably 0.03 - 0.07, and/or
B: 0.0005 - 0.004, preferably 0.001 - 0.003, and/or
N: 0.001 - 0.008, preferably 0.002 - 0.005
and optionally one or more of the elements selected from:
- Si: ≤ 0.1, preferably ≤ 0.05,
- Mo: ≤ 0.1, preferably ≤ 0.05,
- Al: ≤ 0.1, preferably ≤ 0.05,
- Cu: ≤ 0.1, preferably ≤ 0.05,
- P: :≤ 0.03, preferably ≤ 0.015
- S: ≤ 0.025, preferably ≤ 0.01
- O: ≤ 0.01, preferably ≤ 0.005,
- V: ≤ 0.15, preferably ≤ 0.05,
- Ca: ≤ 0.01
- the remainder being iron and unavoidable impurities.

Carbon is added for securing good mechanical properties. C is added in an amount of 0.03 wt% or more 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 in the range of from 0.03 - 0.17 wt%, preferably in the range of from 0.05 - 0.17 wt%, and more preferably in the range of from 0.07 - 0.15 wt%.
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 minimising 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 Ac3 temperature. A higher Mn content is advantageous in lowering the temperature necessary for hot press forming. When the Mn content exceeds 2.5 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 - 2.5 wt%, preferably in the range of from 1.0 - 2.1 wt%, and more preferably in the range of from 1.2 - 1.8 wt%.
Chromium improves the hardenability of the steel and facilitates avoiding the formation of ferrite and/or pearlite during press quenching. 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 in this invention. The amount of Cr used in the invention is in the range of from 0.2 - 2.0 wt%, preferably in the range of from 0.5 - 1.7 wt%, more preferably in the range of 0.8 - 1.5 wt%.
Preferably, manganese and chromium are used in such an amount that Mn + Cr < 2.7, preferably Mn + Cr is in the range of from 0.5 - 2.5, and more preferably Mn + Cr is in the range of from 2.0 - 2.5.
Titanium is added to form 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, which is also an essential element for this invention, becomes more effective. Stoichiometrically, the ratio of Ti to N (Ti/N) addition should be > 3.42. In accordance with the invention the amount of titanium is in the range of from 0.01 - 0.1 wt%, preferably in the range of from 0.015 - 0.07 wt%, and more preferably in the range of from 0.025 - 0.05 wt%.
Niobium has 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-forming 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 in the range of from 0.01 - 0.1 wt%, preferably in the range of from 0.02 - 0.08 wt%, and more preferably in the range of from 0.03 - 0.07 wt%.
Boron is an important element 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 in an amount in the range of from 0.0005 - 0.005 wt%, preferably in the range of from 0.0005 - 0.004 wt%, more preferably in the range of from 0.001 - 0.003 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 0.01 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%.
In accordance with the present invention Mn, Cr and B are used in such amounts that (B x 1000)/(Mn + Cr) is in the range of from 0.185 - 2.5, preferably in the range of from 0.2 - 2.0, and more preferably in the range of from 0.5 - 1.5. The (B x 1000)/(Mn + Cr) ratio as applied in accordance with the present invention establishes an adequate hardenability of the steel.
The amounts of Si, Mo, Al, Cu, P, S, O, V, Ni and Ca, if present, should all be low.
Silicon is also added to promote hardenability and adequate substitutional solid solution strengthening. The Si amount used in the invention is at most 0.1 wt%, preferably at most 0.5 wt%.
Aluminium is added to deoxidize the steel. The Al amount is at most 0.1 wt%, preferably at most 0.05 wt%.
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 0.1 wt%, preferably at most 0.05 wt%.
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 0.1 wt%, preferably at most 0.05 wt%.
P 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 0.03 wt%, preferably at most 0.015 wt%.
The amount of sulphur needs to be minimised to reduce harmful non-metallic inclusions. S forms a sulfide 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 0.01 wt%.
Steel products need to be deoxidised 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%.
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%.
Nickel may be added in an amount of at most 0.15 wt%. Ni can be added to increase the strength and toughness of the steel.
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.
According to a preferred embodiment, 1000*B divided by the sum of Mn and Cr has to be between 0.185 and 2.5, preferably between 0.5 and 1.5. This limitation improves the hardenability of the steel.
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 forming process.
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 formed part produced from a steel strip, sheet or blank in accordance with the present invention has a microstructure comprising at most 60% bainite, the remainder being martensite. Preferably, the microstructure comprises at most 50 vol. % of bainite, the remainder being martensite. More preferably , the microstructure comprises at most 40 vol. % of bainite, the remainder being martensite. 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 formed part in accordance with the present invention displays excellent mechanical properties. The part has a tensile strength (TS) of at least 750 MPa, preferably of at least 800 MPa, more preferably of at least 900 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 100 °, 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 formed 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 high strength, high ductility and a high bendability. In particular parts in the form of structural parts of vehicles 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-formed boron steels and cold-formed 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-forming a steel blank or a preformed part into an part as disclosed in appended claim 11.
In accordance with the present method it was found that through forming 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-formed boron steels and cold-formed multiphase steel.
After the cooling of the part to a temperature below the Mf 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 preformed steel part may also be used. The preformed 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 T1 for a time period t1. Preferably, in step (a) the temperature T1 is 50-100 °C higher than the Ac3 temperature of the steel, and/or the temperature T2 is above the Ar3 temperature. When T1 is 50 - 100 °C above the Ac3 temperature, the steel is fully or almost fully austenitized within the time period t1, 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 t1 is at least 1 minute and at most 7 minutes. Too long a time period t1 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 preformed part is transferred to a hot-forming tool during a transport time t2 during which the temperature of the heated steel blank or preformed part decreases from temperature T1 to a temperature T2, wherein the transport time t2 is at most 20 seconds. Time t2 is the time needed to transport the heated blank from the heating apparatus to the hot-forming tool (e.g. press) and till the hot-forming apparatus is closed. During the transfer of the blank or preformed part may cool from temperature T1 to temperature T2 by the act of natural air-cooling and/or any other available cooling method. The heated blank or preformed part may be transferred from the heating apparatus to the forming tool by an automated robotic system or any other transfer method. Time t2 may also be chosen in combination with T1, t1 and T2 in order to control the microstructural evolution of steel at the commencement of forming and quenching. Suitably, t2 is equal or less than 12 seconds, preferably t2 is equal or less than 10 s, more preferably t2 is equal or less than 8s, and most preferably equal or less than 6s. In step (b), the blank or preformed 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 preformed 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 preformed 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-forming tool is cooled to a temperature below the Mf 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-forming operation the desired bainitic phase into the steel microstructure. The present method enables the production of hot formed 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 decarburisation of said steel.

Figure 1 shows a schematic representation of an embodiment of the method according to the invention.
Figure 2 shows a cross-section through a drop tower for axial crash tests.

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 preformed part is (re)heated up to the austenitizing temperature above Ac1 at a particular (re)heating rate. Once the Ac1 has been exceeded the (re)heating rate is lowered until the blank or preformed part has reached a temperature higher than the Ac3. 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 forming tool, during which cooling of the blank by air occurs to some extent. The blank or preformed 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 Mf temperature of the steel, the hot-forming 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.

Ac1:Temperature at which, during heating, austenite starts to form.
Ac3: Temperature at which, during heating, transformation of the ferrite into austenite ends.
Ar3: The temperature at which transformation of austenite to ferrite starts during cooling.
Ms: Temperature at which, during cooling, transformation of the austenite into martensite starts.
Mf: 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 forming thermal cycles in a hot dip annealing simulator (HDAS) and an SMG press. The HDAS was used for slower cooling rates (30-80°C/s) whereas the SMG press was used for fastest cooling rate (200 °C/s). The steel blanks were reheated to a T1 of respectively 900°C (36°C above Ac3) and 940°C (76°C above Ac3), 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 (A50 specimen geometry) were prepared and tested with 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 mm thickness (= original bending angle x 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 conditions 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))]^0.5 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). 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 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-formed CR590Y980T-DP (steel composition B and commonly known as DP1000 steel) was also tested since it has a similar strength level as the steel blank in accordance with the invention. In addition, and also for comparative reasons, a standard hot-formed 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 after a variety of cooling rates V3. In addition, Table 3 shows the microstructure in terms of martensite (M) and bainite (B). It will be clear from Table 3 that an ultimate tensile strength of greater than 800 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 horforming 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-formed 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 formed 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 need 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-formed and hot-formed steel products.

Table 1: chemistry (wt%)

Steel	C	Mn	Si	Nb	B	Cr	Ti	N	Remainder
A	0.075	1.48	-	0.05	0.0025	1.01	0.03	0.0045	Fe + impurities
B	0.15	2.3	0.1	0.01	-	-	0.015	0.0035	Fe + impurities
C	0.23	1.25	0.2	-	0.003	-	-	0.004	Fe + impurities

Table 2: Transformation temperatures steel composition A

A_c1 (°C)	A_c3 (°C)	M_s (°C)	M_f (°C)
770	864	486	287

Table 3: Mechanical properties and microstructures for steel composition A

T1 (°C)	V3 (°C/s)	YS (MPa)	UTS (MPa)	UE (%)	TE (%)	Microstructure (vol. %)
900	30	696	893	2.8	5.6	55M + 45B
900	40	699	911	2.8	5.6	73M +27B
900	50	741	955	3.2	6.2	79M + 21B
900	60	772	998	3.2	5.8	91M + 9B
900	80	784	1003	3.7	6.4	94M + 6B
900	200	879	1090	3.2	6.1	100M
940	30	757	962	3.6	6.9	60M + 40B
940	40	763	975	3.7	6.8	70M + 30B
940	50	741	985	4.4	8.1	82M + 18B
940	60	782	1006	4.4	8.3	93M + 7B
940	80	777	1021	4.4	8.1	960M + 4B
940	200	892	1089	3.2	6.3	100M

Table 4: Bending angles for steel composition A

T1 (°C)	V3 (°C/s)	BA (1.5 mm)	BA (1.5 mm)	BA (1 mm)	BA (1 mm)
		L sample (°)	T sample (°)	L sample (°)	T sample (°)
900	30	126.8	123	155.3	150.7
900	40	123.5	123.5	151.2	151.2
900	50	126.2	126.4	154.5	154.8
900	60	123	124.1	150.7	152
900	80	119.2	115.3	146	141.3
900	200	111.7	113	136.8	138.5
940	30	120.7	122.4	147.8	149.9
940	40	127.8	121	156.5	148.1
940	50	121.2	125.9	148.5	154.2
940	60	122.6	120.5	150.2	147.6
940	80	118.6	132.5	145.3	162.3
940	200	122.1	117.9	149.5	144.4

Table 5. Mechanical properties Steel composition A after baking

YS (MPa)	UTS (MPa)	UE (%)	TE (%)	BA (1.5 mm)	BA (1.5 mm)	BA (1 mm)	BA (1 mm)
				L sample (°)	T sample (°)	L sample (°)	T sample (°)
937	1072	2.5	5.7	113.6	116	139.1	142.1

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

Steel	YS (MPa)	UTS (MPa)	UE (%)	TE (%)	BA (1.5 mm)	BA (1.5 mm)	BA (1 mm)	BA (1 mm)
					L sample (°)	T sample (°)	L sample (°)	T sample (°)
DP1000	747	1022	7.3	14.4	65.4	74.4	71.1	81.5
22MnB5	912	1374	4.1	6.6	83.5	69.2	102.3	84.7

Table 7: Fracture toughness parameters for steel compositions A and B (DP1000)

Steel	Orientation	CTOD (mm)	J (J/mm²)	KJ (MPa.m^0.5)	K_Q (MPa.m^0.5)

Composition A	L	0.361	0.638	381	90
Composition A	T	0.245	0.434	314	104.7
DP1000	L	0.139	0.231	229	86
DP1000	T	0.146	0.243	235	79.2

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

Steel	Condition	Mean force at 1.5 mm (NM)	Visual observation
Steel composition A	heated and pressed	107	good folding; no cracking
Steel composition A	heated, pressed and baked	98	good folding; no cracking
DP1000	as annealed	82	severe cracking in folds

Claims

Steel strip, sheet or blank for producing hot formed parts having the following composition in weight%:
C: 0.03 - 0.17,

Mn: 0.65 - 2.50,

Cr: 0.2 -2.0,

Ti: 0.01 ― 0.10,

Nb : 0.01-0.10,

B : 0.0005-0.005,

N: ≤ 0.01,

wherein Ti/N ≥ 3.42,

and optionally one or more of the elements selected from :
Si: ≤ 0.1,

Mo: ≤ 0.1 ,

Al: ≤ 0.1,

Cu: ≤ 0.1,

P: :≤ 0.03,

S: ≤ 0.025,

O: ≤ 0.01,

V: ≤ 0.15,

Ni: ≤ 0.15

Ca: ≤ 0.15

the remainder being iron and unavoidable impurities.
Steel strip, sheet or blank according to claim 1,wherein:
C: 0.05 - 0.17, preferably 0.07 - 0.15, and/or

Mn: 1.00 ― 2.10, preferably 1.20 - 1.80, and/or

Cr: 0.5 - 1.7, preferably 0.8 - 1.5, and/or

Ti: 0.015 - 0.07, preferably 0.025 - 0.05, and/or

Nb: 0.02 - 0.08, preferably 0.03 - 0.07, and/or

B: 0.0005 - 0.004, preferably 0.001 - 0.003, and/or

N: 0.001 - 0.008, preferably 0.002 - 0.005

Ca: ≤ 0.01.
Steel strip, sheet or blank according to claim 1 or 2, wherein the sum of the amount of Mn and Cr is less than 2.7, preferably between 0.5 and 2.5 and more preferably between 2.0 and 2.5.
Steel strip, sheet or blank according to claim 1, 2 or 3, wherein Mn, Cr and B are used in such amounts that (B x 1000)/(Mn + Cr) is in the range of from 0.185 - 2.5, preferably in the range of from 0.2 - 2.0, and more preferably in the range of from 0.5 - 1.5.
Steel strip, sheet or blank according to any one of claims 1 - 4, provided with a zinc based coating or an aluminium based coating or an organic based coating.
Steel strip, sheet or blank according to claim 5, 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.
Hot formed 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 750 MPa, preferably at least 800 MPa, more preferably at least 900 MPa, and further having a tensile strength of at most 1400 MPa.
Hot formed part according to claim 7 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 100°, preferably at least 115°, more preferably at least 130 ° and most preferably at least 140°.
Hot formed part according to claim 7 or 8, the part having a microstructure comprising at most 60% bainite, the remainder being martensite, the microstructure preferably comprising at most 50% bainite, more preferably the microstructure comprising at most 40% bainite.
Use of a hot formed part according to claim 7, 8 or 9 as structural part in the body-in-white of a vehicle.
A method for hot-forming 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 T1 and holding the heated blank at T1 during a time period t1, wherein T1 is higher than the Ac3 temperature of the steel, and wherein t1 is at most 10 minutes;

b. transferring the heated blank or pre-formed part to a hot-forming tool during a transport time t2 during which the temperature of the heated blank or preformed part decreases from temperature T1 to a temperature T2, wherein the transport time t2 is at most 20 seconds;

c. hot forming the heated blank or preformed part into a part; and

d. cooling the part in the hot-forming tool to a temperature below the Mf temperature of the steel with a cooling rate of at least 30 °C/s.
Method according to claim 11, wherein the temperature T1 in step (a) is 50-100 °C higher than the Ac3 and/or the temperature T2 is above Ar3.
Method according to claim 11 or 12, wherein the time period t1 in step (a) is at least 1 minute and at most 7 minutes and/or the time period t2 in step (b) is at most 12 seconds, preferably the time period t2 is between 2 and 10 seconds.
Method according to any one of claims 11-13, 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.
Vehicle comprising at least one part according to any one of claims 7 ― 9 and/or produced according to the method of any one of claims 11 - 14.