CN113891952A

CN113891952A - Steel strip, sheet or blank for producing hot-stamped parts, part and method for hot stamping a blank into a part

Info

Publication number: CN113891952A
Application number: CN202080039175.7A
Authority: CN
Inventors: R·拉那
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2019-05-28
Filing date: 2020-05-28
Publication date: 2022-01-04
Also published as: JP2022535352A; EP3976846A1; KR20220013393A; WO2020239891A1

Abstract

The invention relates to a steel strip, sheet or blank for producing a hot-stamped part having a composition, in% by weight, of C <0.20, Mn from 0.65 to 3.0, 5W from 0.10 to 0.60, and optionally comprising one or more of the elements selected from the group consisting of: si is less than 0.10 percent, Mo is less than or equal to 0.10 percent, Al is less than or equal to 0.10 percent, 10Cr is less than or equal to 0.10 percent, Cu is less than or equal to 0.10 percent, N is less than or equal to 0.010 percent, P is less than or equal to 0.030 percent, S is less than or equal to 0.025 percent, O is less than or equal to 0.01 percent, Ti is less than or equal to 0.02 percent, V is less than or equal to 0.15 percent, Nb is less than or equal to 0.01 percent, B is less than or equal to 0.0005 percent, and the balance of 20 is iron and inevitable impurities.

Description

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

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

To meet the requirements of the automotive industry in terms of improved mechanical properties, such as improved tensile strength, impact energy absorption, workability, ductility and toughness, cold and hot stamping processes have been developed to produce steel parts meeting these requirements.

In the cold stamping process, steel is formed into a product at near room temperature. The steel product produced in this way is, for example, a Dual Phase (DP) steel with a ferritic-martensitic microstructure. Although these DP steels exhibit high ultimate tensile strength, their bendability and yield strength are low, which is undesirable because these reduce crash performance in service.

In the hot stamping process, steels are heated above their recrystallization temperature and quenched to obtain the desired material properties, typically by martensitic transformation. The basis for hot stamping techniques and steel compositions suitable for use therein has been described in GB 1490535.

A typical steel used for hot stamping is 22MnB5 steel. This boron steel can be reheated in a furnace to austenitize typically between 870 ℃ and 940 ℃, transferred from the furnace to a hot stamping press, and stamped into the desired part geometry, with the part being simultaneously cooled. The advantages of such boron steel parts produced in this way are: due to their fully martensitic microstructure achieved by pressure quenching, they exhibit high ultimate tensile strength, but at the same time they exhibit low bendability and ductility, which in turn leads to limited toughness and resistance to bending fracture, resulting in poor impact energy absorption crashworthiness.

Fracture toughness measurements are useful tools to indicate impact energy absorption of steel. Good crash behavior is generally obtained when the fracture toughness parameter is high.

In view of the above, it will be clear that there is a need for the following steel parts: the steel part exhibits excellent ultimate tensile strength and, at the same time, excellent ductility, yield strength and bendability, and, in turn, excellent impact 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 having a combination of excellent ultimate tensile strength, yield strength, bendability and ductility, thereby providing excellent impact energy absorption when compared to conventional cold stamped and hot stamped steels.

Another subject of the invention is to provide a hot-stamped part 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 (vehicle).

It is a further object of the present invention to provide a method for hot stamping a steel blank into a part.

It has now been found that these objectives can be achieved when using low alloy steels containing relatively high amounts of tungsten in addition to manganese. The invention therefore relates to a steel strip, sheet or blank for producing hot-stamped parts, having the following composition in% by weight:

C:<0.20,

Mn:0.65-3.0,

W:0.10-0.60,

and optionally comprising 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 balance being iron and unavoidable impurities.

The hot stamped parts produced from the steel strip, sheet or blank according to the invention show an improved combination of tensile strength, ductility and bendability, when compared to conventional hot stamped boron steel, thereby showing impact energy absorbing crashworthiness.

Consider automotive parts made of these steels to be the front longitudinal bar, the rear longitudinal bar and the B-pillar. For the front side rail, cold stamped dual phase steel (e.g., DP800) is currently used, and for the B-pillar, hot stamped 22MnB5 steel is used. DP800 steel exhibits lower energy absorption and the use of higher strength steel (ultimate tensile strength > 800MPa) will achieve more weight savings through size reduction (downsizing) and enhanced passenger safety through higher crash energy absorption. On the other hand, for B-pillars, one currently used solution is to use two types of steel, ultra-high strength (-1500 MPa)22MnB5 for the upper portion and lower strength (-500 MPa) steel for the lower portion. The two steel blanks are joined by laser welding prior to hot stamping and the hybrid blank is then stamped into a B-pillar. By using this solution, during a collision, the upper part resists intrusion while the lower part absorbs energy due to its higher combination of flexibility and ductility. The invention provides better performance and weight reduction possibilities: the higher strength steel of the present invention may replace lower strength steel having a higher energy absorption capacity.

Preferably, the steel strip, sheet or blank used for producing the hot stamped part described above has the following composition in weight%:

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 ≦ 0.009, preferably ≦ 0.005, and/or

Al: 0.05 or less, preferably 0.04 or less

N: 0.001-0.008, preferably 0.002-0.005.

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

Manganese is used because it promotes hardenability and provides solid solution strengthening. The Mn content is at least 0.65 wt% to provide sufficient substitutional solid solution strengthening and sufficient quench hardenability while minimizing Mn segregation during casting and while maintaining a sufficiently low carbon equivalent for automotive resistance spot welding techniques. In addition, Mn is useful for lowering A_c3The element of temperature. Higher Mn content is beneficial to lower the temperature required 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, which affect the workability of the steel. The amount of Mn used according to the invention is in the range of 0.65 to 3.0% by weight, preferably in the range of 1.00 to 2.50% by weight, more preferably in the range of 1.20 to 2.20% by weight. Lower Mn content should be combined with higher W and CAnd vice versa to ensure sufficient hardenability of the steel.

Tungsten is very effective in retarding the high temperature diffusion control transition in steel. It delays the formation of ferrite and pearlite by extending the incubation time for ferrite and pearlite transformation. In other words, W increases the hardenability of the steel. It is important that W is in solid solution with iron for this hardenability improving effect. This is ensured by: austenitizing the steel sufficiently above A_c3The temperature is maintained for a suitable duration. In this respect, the presence of ferrite and/or pearlite in the microstructure was observed to be detrimental to the mechanical properties of the target microstructure according to the invention. The amount of W used in the present invention is more than 0.10% by weight and at most 0.60% by weight, preferably in the range of 0.10 to 0.50% by weight, more preferably in the range of 0.13 to 0.50% by weight, still more preferably in the range of 0.13 to 0.30% by weight. The amount of W should not be too high, since it will increase the alloying costs too much compared to the obtained advantages, and it should not be too low, since it will not be effective in providing the metallurgical effect described above.

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

No silicon is added and is not required to exert the desired metallurgical effect in the present invention. The amount of Si used in the present 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 is advantageous for avoiding the formation of ferrite and/or pearlite during press quenching. The amount of Cr used in the present invention is at most 0.10 wt%, preferably at most 0.05 wt%, and more preferably at most 0.009 wt%, the latter because larger amounts of Cr may result in the formation of Cr-containing carbides that may deteriorate the mechanical properties.

Molybdenum is added to improve the hardenability of the steel and to promote the formation of bainite. The amount of Mo used according to the present invention is at most 0.10 wt%, preferably at most 0.05 wt%, and more preferably at most 0.009 wt%, lower amounts being preferred, as higher amounts of Mo will significantly increase the alloying costs.

Aluminum is added to deoxidize the steel. The amount of Al is at most 0.10% by weight, preferably at most 0.05% by weight, more preferably at most 0.04% by weight. If more aluminum is added, some ferrite may be formed during pressure quenching, resulting in deterioration of mechanical properties.

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

Phosphorus is known to widen the critical zone temperature range of steel. P is also an element that can be used to maintain the desired retained austenite. However, P may deteriorate the workability of the steel. According to the invention, P should be present in an amount of at most 0.030% by weight, preferably at most 0.015% by weight.

It is desirable to minimize sulfur to reduce harmful non-metallic inclusions. S forms sulfide-based inclusions, such as MnS, which initiate cracks and deteriorate workability. Therefore, it is desirable to reduce the amount of S as much as possible. According to the invention, the amount of S is at most 0.025 wt.%, preferably at most 0.010 wt.%.

When titanium is present, it forms TiN precipitates to scavenge N at high temperatures as the steel melt cools. Formation of TiN inhibits B formation at lower temperatures₃N₄Making B (if present) more effective. Stoichiometrically, when B is added, the ratio of Ti to N (Ti/N) should be added>3.42. According to the invention, the amount of titanium is less than or equal to 0.02% by weight.

Niobium may have the effect of forming strengthening precipitates and refining the microstructure. Nb increases strength by grain refinement and precipitation hardening. Grain refinement leads to a more uniform microstructure and thus to improved hot stamping behaviour, in particular when high local strains are introduced. The fine uniform microstructure also improves the bending behaviour. The amount of Nb used in the present invention is not more than 0.01% by weight.

Vanadium may be added to form V (C, N) precipitates in order 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%, lower amounts being preferred for cost reasons and since V may lead to the formation of complex carbides with the micro-alloying elements, which formation may reduce the ductility characteristics of the product.

Boron is used to increase the hardenability of the steel sheet, and is to further increase the effect of stably securing the strength after quenching. According to the invention, B is present in amounts of ≤ 0.0005 wt.%.

Nitrogen has an effect similar to C. N combines appropriately with titanium to form TiN precipitates. The amount of N according to the invention is at most 0.010% by weight. 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 to 0.005 wt%.

Oxygen: deoxidation of the steel product is required because oxygen reduces various properties such as tensile strength, ductility, toughness and/or weldability. Therefore, the presence of oxygen should be avoided. According to the invention, the amount of O is at most 0.01% by weight, preferably at most 0.005% by weight.

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 sulfur-containing inclusions and minimize the amount of elongated inclusions. However, the presence of CaS inclusions will still lead to inhomogeneities in the matrix; it is therefore desirable to reduce the amount of S.

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

It is preferred when the zinc-based coating is a coating comprising: 0.2-5.0 wt.% Al, 0.2-5.0 wt.% Mg, optionally up to 0.3 wt.% of one or more additional elements, the balance being zinc and unavoidable impurities. The additional element may 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 (spangles).

Preferably, the total amount of additional elements in the zinc alloy is at most 0.3 wt%. For typical applications, these small amounts of additional elements do not alter the properties of the coating or bath to any significant extent.

When one or more additional elements are present in the zinc alloy coating, each additional element is preferably present in an amount of up to 0.03 wt%, preferably each additional element is present in an amount of up to 0.01 wt%. Usually only additional elements are added in order to prevent dross formation in a bath with molten zinc alloy for hot dip galvanization or to form spangles in the coating.

The hot-stamped part produced from a steel strip, sheet or blank according to the invention has a microstructure comprising at most 50% by volume of bainite and the remainder being martensite. Preferably, the microstructure comprises at most 40% by volume bainite, the remainder being martensite. More preferably, the microstructure comprises at most 30% by volume bainite, the remainder being martensite. The presence of bainite is only applicable to the slow cooling rates encountered during pressure quenching. Typical cooling rates for the blank during press quenching are greater than about 30 deg.c/s. Above a cooling rate of 60 ℃/s, a fully martensitic microstructure is formed. In this case, martensite provides high strength, while softer bainite improves ductility. The small strength difference between martensite and bainite helps maintain high bendability due to the lack of a weak phase interface.

The hot-stamped parts according to the invention exhibit excellent mechanical properties. The part has a Tensile Strength (TS) of at least 745MPa, preferably at least 1070MPa, more preferably at least 1300MPa, and also has a tensile strength of at most 1400 MPa.

Suitably, the Total Elongation (TE) of the part is at least 5%, preferably 5.5%, more preferably at least 6% and most preferably at least 7%, and/or the Bending Angle (BA) at 1.0mm thickness is at least 78 °, preferably at least 100 °, more preferably at least 115 °, more preferably at least 130 ° and most preferably at least 140 °.

It is clear that the steel product according to the invention shows excellent impact energy absorption.

The invention also relates to the use of a hot-stamped part as described above as a structural part in the body-in-white (body-in-white) of a vehicle. Such parts are made from the steel strip, sheet or blank of the invention. These parts have a combination of high strength, high ductility and high bendability. In particular, the steel of the present invention is very attractive for parts in the form of structural parts for vehicles, since it exhibits excellent impact energy absorption and, in turn, opportunities for size reduction and weight reduction based on crashworthiness, compared to the use of conventional hot-stamped boron steel and cold-stamped multi-phase steel.

The invention also relates to a method for producing a part according to the invention.

The invention therefore 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 a billet according to any one of claims 1 to 3 or a preformed part produced from said billet to a temperature T₁And during a time period t₁During which the heated blank is held at T₁Wherein T is₁Higher than A of steel_c3Temperature and wherein t₁At most 10 minutes;

(b) at the time of delivery t₂During which the heated blank or preformed part is transferred to a hot stamping tool, during which time the temperature of the heated blank or preformed part is from a temperature T₁Is lowered to a temperature T₂Wherein the delivery time t₂At most 20 seconds;

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

(d) cooling a part to below M of steel in a hot stamping tool_fTemperature, using a cooling rate of at least 30 ℃/s.

According to the present method, it was found that by stamping the heated blank into a part as described above, a part of complex shape with enhanced mechanical properties can be obtained. In particular, the part exhibits excellent impact energy absorption compared to a multi-phase steel using conventional hot-stamped boron steel and cold-stamped steel, and thus allows opportunities for size reduction and weight reduction based on crashworthiness.

After cooling the part to below M_fAfter the temperature of the temperature, the part may be further cooled to room temperature, for example in air, or may be forcibly cooled to room temperature.

In the method according to the present invention, the billet to be heated in step (a) is provided as an intermediate for the subsequent step. The steel strip or sheet used to produce the blank may be obtained by standard casting processes. In a preferred embodiment, the steel strip or sheet is cold rolled. The steel strip or sheet may be suitably cut into steel blanks. Preformed steel parts may also be used. The preformed part may be partially or fully formed into the desired geometry, preferably at ambient temperature.

Heating a steel blank to a temperature T in step (a)₁Duration t₁. Preferably, in step (a), the temperature T₁A of steel_c3The temperature is 40-100 ℃ higher and/or the temperature T₂Higher than A_r3And (3) temperature. When T is₁Ratio A_c3The steel is heated to 40-100 ℃ for a period of time t₁Complete or almost complete austenitization is achieved and cooling during step (b) is fully possible. When the microstructure is a homogeneous austenitic microstructure, formability is enhanced.

Preferably, the time period t₁At least 1 minute and at most 7 minutes. Too long a time period t₁Coarse austenite grains can result, which degrades the final mechanical properties.

The heating means used in step (a) may be, for example, an electric or gas furnace, a resistance heating device, an infrared induction heating device.

In step (b), at a delivery time t₂During which the heated steel blank or preformed part is transferred to the hot stamping tool, at said transport time t₂During which the temperature of the heated steel blank or pre-formed part is driven from the temperature T₁Is lowered to a temperature T₂Wherein the delivery time t₂At most 20 seconds. Time t₂Is the time required to transport the heated blank from the heating apparatus to the hot stamping tool (e.g., press) and until the hot stamping apparatus is shut down. During the transfer, the blank or the preformed part is cooled and/or controlled by natural airFrom which temperature T the effect of other available cooling methods can be₁Cooling to a temperature T₂. The heated blank or preformed part may be transferred from the heating apparatus to the hot stamping tool by an automated robotic system or any other transfer method. Can also combine T₁、t₁And T₂To select the time t₂In order to control the microstructure evolution of the steel at the start of hot stamping and quenching. Suitably, t₂Equal to or less than 12 seconds, preferably t₂Equal to or less than 10 seconds, more preferably t₂Equal to or less than 8 seconds, and most preferably equal to or less than 6 seconds. In step (b), the blank or preformed part may be cooled at a cooling rate V of at least 10 ℃/s₂From temperature T₁Cooling to temperature. V₂Preferably in the range of 10-15 deg.C/s. When the blank or preformed part should be pre-cooled, the cooling rate should be higher, for example at least 20 ℃/s, up to 50 ℃/s or higher.

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

In step (d), the formed part in the hot stamping tool is cooled to below M of the steel_fTemperature of temperature, using a cooling rate V of at least 30 ℃/s₃. Preferably, the cooling rate V in step (d)₃In the range of 30 to 150 deg.C/s, more preferably in the range of 30 to 100 deg.C/s.

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

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

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

In fig. 1, the horizontal axis represents time T, and the vertical axis represents temperature T. The time T and the temperature T are shown diagrammatically in fig. 1. No numerical value can be derived from fig. 1.

In fig. 1, a steel blank or a pre-formed part is (pre-) heated to above a at a specific (pre-) heating rate_c1The austenitizing temperature of (a). Once A is exceeded_c1The (pre) heating rate is then reduced until the blank or the preformed part reaches a value higher than A_c3The temperature of (2). The strip, sheet or blank is then held at that particular temperature for a period of time. Subsequently, the heated blank is transferred from the furnace to a hot stamping tool, during which the blank is cooled to some extent by air. The blank or preformed part is then thermoformed into a part and cooled (or quenched) at a cooling rate of at least 30 ℃/sec. At a level of M lower than that of steel_fAfter the temperature of the temperature, the hot stamping tool is opened and the shaped article is cooled to room temperature.

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

-A_c1: the temperature at which austenite begins to form during heating.

-A_c3: the temperature at which the ferrite to austenite transformation ends during heating.

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

-M_s: the temperature at which the transformation of austenite to martensite starts during cooling.

-M_f: the temperature at which the transformation of austenite to martensite ends during cooling.

The invention will be illustrated by the following non-limiting examples.

Examples

Steel composition A (according to the invention)

Steel blanks having dimensions of 220mm x 110mm x 1.5mm were prepared from cold-rolled steel sheets having the compositions shown in table 1. These steel billets were subjected to hot stamping hot cycling in a Hot Dip Annealing Simulator (HDAS) and were subjected to hot stamping hot cycling by Schuler SMG GmbH&Kg in a hot stamping press (hereinafter SMG press). HDAS is used for slower cooling rates (30-80 ℃/s), while SMGThe press was used for the fastest cooling rate (200 ℃/s). The steel blanks were heated again to 900 deg.C (at A) in a nitrogen atmosphere_c350 ℃ above and 940 ℃ (at A)_c3Above 90 ℃ C.) of₁And incubated for 5 minutes to minimize surface degradation. The blank is then subjected to transfer cooling, the temperature dropping 120 ℃ in 10 seconds, so that the cooling rate V₂Is about 12 ℃/s, then at the following cooling rate V₃Cooling to 160 ℃: 30. 40, 50, 60, 80, 200 ℃/s. From the heat treated samples, longitudinal tensile specimens having a gauge length of 50mm and a width of 12.5mm (Euronorm a50 specimen geometry) were prepared and tested with quasi-static strain rates. The microstructure was characterized from the RD-ND plane. Bending specimens (40 mm. times.30 mm. times.1.5 mm) from parallel and transverse to the rolling direction were prepared from each condition and tested by the three-point bending test as described in the VDA 238-100 standard until fracture. The sample having the bending axis parallel to the rolling direction was determined as a longitudinal (L) bending sample, and the sample having the bending axis perpendicular to the rolling direction was determined as a vertical (T) bending sample. The measured bend angle at 1.5mm thickness was also converted to the angle at 1.0mm thickness (the square root of the original bend angle x the original thickness). For each type of test, three measurements were made and the average from the three tests is given for each condition.

For selected conditions (SMG press samples, reheated at 940 ℃), J-integrated fracture toughness and falling tower axial impact tests were performed. Compact tensile specimens were prepared from both the machine and transverse directions according to the NFMT76J standard for fracture toughness testing. For transverse samples, the crack extends in the rolling direction and the load is transverse to the rolling direction, and for longitudinal samples the opposite is true. The test specimens were tested at room temperature according to ASTM E1820-09. The pre-cracks are introduced by fatigue loading. Final testing was performed using tensile loading using flexural plates to maintain stress in the plane of the sheet. Three tests were carried out for each condition and, according to the guidelines in the BS7910 standard, the minimum of three equivalents (MOTE values) of the different fracture toughness parameters is given.

A brief description of the fracture toughness parameters is given below. CTOD being the crack tipThe end opening displacement and is a measure of the extent to which the crack opens at failure (if brittle) or maximum load. J is the J integral and is a measure of toughness that takes into account energy, and is therefore calculated from the area under the curve, up to failure or maximum load. KJ is a stress intensity factor determined from J integration using the established expression, given as KJ ═ J (E/(1- ν)²))]^0.5Where E is young's modulus (═ 207GPa), and v is poisson's ratio (═ 0.03). K_qUnder a load P_qThe value of the stress intensity factor measured below, where P is determined by taking the elastic slope of the load line_qThen take the line with the smaller slope of 5% and apply P_qDefined as the load at the intersection of the line and the load line.

The drop tower axial crash test was conducted under SMG-press conditions with a 200kg load and a loading speed of 50 km/hour, with the load impacting a crash box with a closed top-hat geometry (fig. 2) made from a larger sheet at a height of 500mm (transverse to the rolling direction). The cross-sectional dimensions of the falling tower are given in mm in fig. 2 (t ═ 1.5mm, R₀3 mm). A back plate of 100mm width was spot-welded to the contour to prepare a crash box.

For some selected conditions, paint bake thermal cycles (180 ℃ for 20 minutes) were also given to the samples and the tests were performed as would be directly reflected from the results.

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

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

In Table 1, the chemical composition of steel compositions A-C is indicated, in weight%.

In table 2, the transformation temperatures of steel composition a are shown.

The results of the various tests are given in tables 3 to 8.

In Table 3, the cooling rates V are shown for various₃Steel composition ofYield Strength (YS), Ultimate Tensile Strength (UTS), Uniform Elongation (UE), and Total Elongation (TE) of thing A. Further, table 3 shows a microstructure composed of martensite (M) and bainite (B). It will be clear from Table 3 that at different cooling rates V₃Ultimate tensile strength greater than 740MPa is achieved.

In Table 4, the cooling rates V are shown at different cooling rates₃The steel composition A obtained thereafter had a Bending Angle (BA) at a thickness of 1.0 mm. It is clear from table 4 that high bending angles of more than at least 130 ° are achieved for both the longitudinal (L) and transverse (T) orientations.

In table 5 is shown various mechanical properties of steel composition a after it has been subjected to a hot stamping and baking treatment simulating the paint baking treatment used during automotive manufacturing. The steel composition A was heated to 900 ℃ for 5 minutes and then cooled after transfer at a V of 200 ℃/s₃And (6) cooling. The baking treatment was carried out at 180 ℃ for 20 minutes. It will be clear from table 5 that approximately the same minimum levels of Yield Strength (YS), Ultimate Tensile Strength (UTS), Ultimate Elongation (UE), Total Elongation (TE) and Bend Angle (BA) were also achieved after steel composition a had been subjected to a baking treatment. This means that in the manufacture of automobiles after paint baking, the required performance will be ensured under service conditions.

In Table 6, 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 comparing the contents of tables 4 and 6, it will be immediately clear that the steel part according to the invention (steel composition A) constitutes a major improvement in bendability when compared to the conventional cold formed steel product DP1000 (steel composition B) and the conventional hot stamped steel product 22MnB5 (steel composition C).

It is also clear from table 7 that the fracture toughness parameter of the steel part according to the invention (steel composition a) is also higher than that of the billet made from DP1000 (steel composition B).

In table 8, the collision behavior of steel compositions a and B is shown. As is clear from table 8, the collision behavior of the steel composition a is better than that of DP1000 (steel composition B) under the hot-pressing condition and the hot-pressing and baking condition. The baking conditions were the same as described above. The crash box of steel composition a did not show any signs of cracking after the test, whereas the crash box of DP1000 (steel composition B) showed severe cracking at the folds. In addition, steel composition a exhibited a higher energy absorbing capacity.

The high and improved crash behaviour of the hot-stamped steel composition a according to the invention is due to the higher bending angle and the higher fracture toughness properties when compared to conventional steel products of similar strength. In this respect, it was observed that during a collision, the steel component needs to be folded, which is determined by its bendability, while on the other hand the energy absorption capacity before failure is determined by its fracture toughness parameter.

In view of the above, it will be clear to the skilled person that the steel product according to the invention constitutes a significant improvement over conventionally known cold and hot stamped steel products.

Table 1: chemical composition, steels A, B and C (wt%)

	C	Mn	Si	Al	Nb	B	Cr	W	Ti	N	Balance of
												A	0.15	2.0	0.003	0.04	<0.001	<0.0001	0.009	0.28	0.001	0.0045	Fe + impurities
B	0.15	2.3	0.1	0.033	0.01	-	-		0.015	0.0035	Fe + impurities
												C	0.23	1.25	0.2	0.03	-	0.003	-		-	0.004	Fe + impurities

Table 2: transformation temperature, Steel composition A

A_c1(℃)	A_c3(℃)	M_s(℃)	M_f(℃)
				740	850	451	232

Table 3: mechanical Properties and microstructure of Steel composition A

Table 4: bending Angle of Steel composition A

T₁(℃)	V₃(℃/s)	BA(1.5mm)	BA(1.5mm)	BA(1.0mm)	BA(1.0mm)
								L sample (°)	T sample (. degree.)	L sample (°)	T sample (. degree.)
900	30	96.1	120.6	117.6	147.8
						900	40	95.2	116.3	116.6	142.5
900	50	90.1	112.6	110.4	138
						900	60	88.3	108.8	108.1	133.3
900	80	83.8	115.3	102.6	141.2
						900	200	64.2	98.2	78.7	120.3
940	30	97.8	114.4	119.8	140.1
						940	40	94.6	114.4	115.8	140.2
940	50	109.7	111.2	134.3	136.2
						940	60	98	113	120	138.4
940	80	100.3	102.1	122.8	125
						940	200	88.7	93.1	108.6	114.1

TABLE 5 mechanical properties of Steel composition A after bake hardening

YS(MPa)	UTS(MPa)	UE(％)	TE(％)	BA(1.5mm)	BA(1.5mm)	BA(1.0mm)	BA(1.0mm)
								L sample (°)	T sample (. degree.)	L sample (°)	T sample (. degree.)
1085	1315	3.3	6.6	76.2	100	93.3	122.5

TABLE 6 mechanical properties, Steel compositions B (DP1000) and C (22MnB5)

Table 7: fracture toughness parameters for Steel compositions A-C

Table 8: collision 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 in% by weight:

C:<0.20,

Mn:0.65-3.0,

W:0.10-0.60,

and optionally comprising 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 balance being iron and unavoidable impurities.

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

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 ≦ 0.009, preferably ≦ 0.005, and/or

Al: 0.05 or less, preferably 0.04 or less,

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 comprising: 0.2-5.0 wt.% Al, 0.2-5.0 wt.% Mg, optionally up to 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 745MPa, preferably at least 1070MPa, more preferably at least 1300MPa, 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 a thickness of 1.0mm 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, having a microstructure comprising at most 50% bainite, the balance being martensite, preferably at most 40% bainite, more preferably at most 30% bainite.

8. Use of a hot-stamped part according to any of claims 5 to 7 as a 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, the method comprising the steps of:

(e) heating a billet according to any one of claims 1 to 3 or a preformed part produced from said billet to a temperature T₁And during a time period t₁During which the heated blank is held at T₁Wherein T is₁Higher than A of steel_c3Temperature and wherein t₁At most 10 minutes;

(f) at the time of delivery t₂During which the heated blank or preformed part is transferred to a hot stamping tool, during which time the temperature of the heated blank or preformed part is from a temperature T₁Is reduced to a temperature T₂Wherein the delivery time t₂At most 20 seconds;

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

(h) cooling a part to below M of steel in a hot stamping tool_fTemperature and cooling rate of at least 30 ℃/s.

10. The method of claim 9, wherein in step (a), the temperature T₁Ratio A_c3The temperature is 40-100 ℃ higher and/or the temperature T₂Higher than A_r3And (3) temperature.

11. According to the claimsThe method of claim 9 or 10, wherein the time period t in step (a)₁Is at least 1 minute and at most 7 minutes, and/or the time period t in step (b)₂Is at most 12 seconds, preferably said time period t₂Between 2 seconds and 10 seconds.

12. A method according to any one of claims 9 to 11, wherein in step (d) the part is cooled at a cooling rate in the range 30-150 ℃/s, preferably 30-100 ℃/s.

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