EP4308736A1

EP4308736A1 - Steel strip, sheet or blank and method for producing a hot-formed part or a heat-treated pre-formed part

Info

Publication number: EP4308736A1
Application number: EP22718071.8A
Authority: EP
Inventors: Radhakanta RANA
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2021-03-17
Filing date: 2022-03-17
Publication date: 2024-01-24
Also published as: WO2022195024A1; CN117120636A; KR20230157997A

Abstract

This invention relates to a steel strip, sheet or blank for producing hot-formed parts or a heat-treated pre-formed part, and to a method for hot-forming a steel blank or heat-treating a pre-formed part into an article and the use thereof, the steel strip, sheet or blank having the following composition, in wt.%: - C 0.07-0.20; - Mn 0.5-2.0; - Si 0.3-1.5; - Mo 0.1-1.0; and optionally one or more of the elements selected from: - Al <0.1; - Cr at most 0.050; - Cu <0.2; - N <0.01; 15 - P <0.04; - S <0.025; - O <0.01; - Ti <0.10; - V <0.15; the remainder being iron and inevitable impurities.

Description

STEEL STRIP, SHEET OR BLANK AND METHOD FOR PRODUCING A HOT-FORMED PART OR A HEAT-TREATED PRE-FORMED PART

Field of the invention

This invention relates to a steel strip, sheet or blank for producing a hot-formed part or a heat-treated pre-formed part, and to a method for hot-forming a steel blank or heat-treating a pre-formed part into an article and the use thereof.

Background of the invention

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 or 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 these reduce crash performance in service.

In hot-stamping or 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-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 high yield strength and ultimate tensile strength for anti- intrusive crashworthiness due to their fully martensitic microstructure achieved by pressquenching, but at the same time they display a low bendability and ductility which in turn result 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 behaviour 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 yield strength, bendability and fracture toughness, and in turn excellent crash energy absorption.

It is noted that the processes of hot-stamping, hot press forming, press hardening and hot-forming are considered synonymous in the context of this invention. These terms relate to a process in which steel sheets are heated up to the austenite range, hot-formed, and then die-quenched to form martensite structures. There exist two main variants:

1) Indirect Process: a blank is formed, trimmed, and formed in cold condition. The preformed part is later heated and quenched to obtain the required properties. In a variant, the preformed part can be given a hot calibration to achieve the final shape in the hot press after heating and quenched.

2) Direct Process: the unformed blank is heated in a furnace, formed in hot condition in a die (i.e. in hot-forming press), and quenched in the die to achieve the required properties.

Objectives of the invention

It is 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 excellent ultimate tensile strength, yield strength, bendability, ductility and fracture toughness, thereby providing an excellent crash energy absorption when compared to conventional cold-formed and hot-formed steels.

It is another object 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.

Description of the invention

It has now been found that these objects can be established when use is made of a low alloy steel strip, sheet or blank that contains, in addition to manganese, a relatively high amount of silicon and molybdenum, and that contains no boron and niobium as alloying elements. Alloying elements are elements that are deliberately added or allowed in the steel to provide a desirable effect. Boron and niobium are deemed not to provide a desirable effect in the steel according to the invention. Any boron or niobium that is present in the steel is to be qualified as inevitable impurities resulting from the Basic Oxygen Steelmaking (BOS) or the Electric Arc Furnace (EAF) process. In the context of this invention inevitable or unavoidable impurities mean that the low alloy steel does not contain any deliberately added elements other than those specified, and that the inevitable or unavoidable impurities are present only as a result of a technical or economical inability to remove them completely from the steel melt during the BOS- or EAF-process.

Accordingly, the present invention relates to a steel strip, sheet or blank for producing hot-formed parts or a heat-treated pre-formed part is provided, which comprises the following composition in weight % (wt.%):

• C 0.07-0.20;

• Mn 0.5-2.0;

• Si 0.3-1.5;

• Mo 0.1-1.0; and optionally one or more of the elements selected from:

• Al <0.1;

• Cr at most 0.050;

• Cu <0.2;

• N <0.01;

• P <0.04;

• S <0.025;

• 0 <0.01;

• Ti <0.10;

• V <0.15, the remainder being iron and inevitable impurities.

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

Examples of automotive components that can be made from these steels are the front and back longitudinal bars and the B-pillar. For the front longitudinal bar, currently a cold-formed dual phase steel (e.g. DP800) is used and for the B-pillar a hot- formed 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-forming and then the hybrid blank (known as laser welded blank or LWB in abbreviation) is formed 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.

The steel chemistry and the hot-forming thermal cycle are the most critical steps to achieve the desired microstructures, properties and performance. Most of the alloying elements (C, Mn, Mo) deliberately added in the steel of invention are to increase the hardenability so that ferritic and/or pearlitic transformation during press cooling of the formed part are avoided. The presence of ferrite and pearlite are detrimental to achieve high strength, high bendability and high toughness and therefore undesired. However, a small amount of ferrite is permissible, as shown in this invention. Bainite which does not have high strength difference with martensite can also be optionally present which has been utilised in this invention in the martensitic matrix to improve the toughness and bendability, and thus energy absorption capacity, while not adversely affecting the desired strength level.

Carbon is added as the main strengthening element and also to promote hardenability. In martensite, C increases strength by distorting the cubic BCC lattice and converting it to a BCT lattice. In bainite which is a mixture of ferrite and carbides, C strengthens the ferrite by solid solution hardening. According to the invention the carbon content is 0.07 to 0.20 wt.%.

Manganese and silicon are added also to increase the hardenability. In addition, Mn causes solid solution hardening as well. Mn below the limit will give insufficient hardenability and above the limit will make the steel difficult to process - as for example in terms of rolling, hot dip galvanising. Si also causes solid solution strengthening.

The hardenability of the alloy is further increased by adding small amount of Mo to the steel. Mo in small amount in steel is very effective in delaying ferritic, pearlitic and bainitic transformations. Mo also refines the grain size. In an embodiment the Mo content in the steel is at most 1.0 wt.%. In another embodiment, the Mo content in the steel is at most 0.80 wt.%, and preferably is at most 0.50 wt.%. In some embodiment the Mo content in the steel is at most 0.20 wt.%. The Mo content in the steel is preferably kept higher than 0.1 wt.%. If Mo content is less than 0.1 wt.%, its hardenability effect may not be sufficient to give the desired microstructure for this invention. On the other hand, very high Mo content is also not necessary for two reasons: firstly, Mo in higher amounts of addition can precipitate as metal carbides during the slow cooling rates if encountered during press quenching. This would reduce the hardenability effect of Mo, which is obtained when it is in the solid solution. Secondly, Mo is an expensive alloying element and therefore addition of Mo above the amount required for hardenability is undesirable as it can cause loss of Mo from solution in the form of precipitates. Any other element present in the steel are unavoidable elements which are there as a consequence of scrap melting and steel making. These are not deliberately added and hence their effects are negligible. As for example, some amount of Al will always be present in the steel as a consequence of its use as deoxidizer during steelmaking. Similarly, some residual amounts of S and P will also be there even after using modern iron and steelmaking to avoid them. However, for the invention to work best the total amount of undesirable elements should be below 0.1 wt.%, preferably below 0.05 wt.%.

The alloys, having their chemistries carefully designed as described above, are processed through the individual process steps 1 through 10 given below, and then subjected to the hot-forming thermal cycle as shown in Figures la and lb - intercritical reheating or austenitizing reheating respectively. Austenitizing reheating is done above the Ac3 temperature and intercritical reheating is done at a temperature chosen in between Acl and Ac3, where Acl is the temperature where austenite starts to form during heating of the steel and Ac3 is the temperature where the ferrite to austenite transformation completes during heating.

According to the invention the chromium content in the steel is at most 0.050 wt.%, and preferably it is present only as an inevitable impurity, i.e. it is not added as an alloying element. In an embodiment the chromium content in the steel is at most 0.040 wt.%, and preferably is at most 0.020 wt.%. in some embodiment the chromium content in the steel is at most 0.010 wt.%. In an embodiment the chromium content in the steel is lower than 0.0001 wt %, preferably there is no chromium present in the steel according to the invention.

In an embodiment the copper content in the steel is at most 0.050 wt.%, and preferably it is present only as an inevitable impurity, i.e. it is not added as an alloying element. Preferably there is no copper present in the steel according to the invention.

Preferably titanium and/or niobium and/or boron and/or vanadium are present only as an inevitable impurity, which means that Ti and/or Nb is at most 0.005 wt.%, V is at most 0.010 wt.% and B is at most 0.0005 wt.% (i.e. 5 ppm). In an embodiment the niobium content in the steel is lower than 0.0001 wt %, preferably there is no niobium present in the steel according to the invention.

According to a further embodiment of the invention, the steel strip, sheet or blank is provided which comprises the following composition in wt.%:

• C 0.07 - 0.18 and/or,

• Mn 0.8 - 1.8 and/or,

• Si 0.5 - 1.0 and/or,

• Mo 0.15 - 0.5.

Although these elements all have an influence on the properties and processability of the steel, it should be noted that the elements can be varied independently. A change in the preferred ranges for one or more of these four elements does not necessitate a corresponding change in any of the others.

In an embodiment the carbon content is at least 0.10 wt.%, preferably at least 0.12 wt.%.

In an embodiment the Mo content is 0.21 wt.% or higher and/or 0.39 wt.% or lower. Preferably the Mo content is 0.29 wt.% or lower. The production of a hot-formed part or a heat-treated pre-formed part can be done with or without a protective metallic coating layer present on the steel strip, sheet or blank. To prevent piece-wise coating of the finished part it is preferable to provide the steel strip, sheet or blank with the protective metallic coating layer prior to the forming operation. Providing a strip with such a metallic coating is preferably performed in a hot-dip coating process. The ultimate goal of using a coating on the steel strip is to enhance the corrosion resistance of the formed article in service.

So, according to a further embodiment of the invention, the steel strip, sheet or blank according to the invention is provided with a metallic coating layer such as zinc or a zinc alloy coating layer, such as a zinc-alloy coating layer comprising 0.3-4.0 wt.% Mg and 0.3-6.0 wt.% Al; optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities; the remainder being zinc. Preferably the alloying element contents in the zinc-alloy coating layer shall be 1.0 - 2.0 % Magnesium and 1.0 -3.0 %. Aluminium, optionally at most 0.2% of one or more additional elements, unavoidable impurities and the remainder being zinc. In an even more preferred embodiment the zinc alloy coating comprises at most 1.6% Mg and between 1.6 and 2.5% Al, optionally at most 0.2% of one or more additional elements, unavoidable impurities and the remainder being zinc.

In another embodiment the steel strip, sheet or blank is provided with a metallic coating layer such as a (commercially pure) aluminium layer or an aluminium alloy layer. A typical metal bath for a hot dip coating such an aluminium layer comprises of aluminium alloyed with silicon e.g. aluminium alloyed with 8 to 11 wt.% of silicon and at most 4 wt.% of iron, optionally at most 0.2 wt.% of one or more additional elements such as calcium, unavoidable impurities, the remainder being aluminium. Silicon is present in order to prevent the formation of a thick iron-intermetallic layer which reduces adherence and formability. Iron is preferably present in amounts between 1 and 4 wt.%, more preferably at least 2 wt.%.

In order to obtain the desired mechanical properties the hot-formed part or heat- treated pre-formed part produced from the steel strip, sheet or blank has a martensitic microstructure comprising from 0 to at most 70 vol.% bainite, and preferably at most 60 vol.% bainite. In this application, for the sake of clarity the martensitic microstructure is defined as a microstructure comprising 0 to at most 70 vol.% bainite, preferably at most 60 vol.% bainite.

According to a further embodiment of the invention, the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to the invention has a martensitic microstructure comprising at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite, more preferably only trace amounts or no ferrite at all. For the sake of avoiding confusion: the term 'ferrite' as used herein above refers to the BCC ferrite (a) structure, and does not refer to acicular ferrite or ferritic bainite or upper bainite, as these are considered to be bainitic microstructures in the context of this invention. The ferrite occurs if T1 is chosen in the intercritical range.

According to a further embodiment of the invention, the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank has a martensitic microstructure comprising at most 70 vol.% bainite, preferably at most 60 vol.% bainite and at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite.

According to a further embodiment of the invention, the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot-formed part has a tensile strength of at least 700 MPa, preferably of at least 750 MPa.

According to a further embodiment of the invention, the hot-formed part or heat- treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot-formed part having a total elongation, TE, of at least 6% with respect to a gauge length of 50 millimetre (as per the EN 10002 standard) and a bending angle, BA, at 1.0 mm thickness (as per the VDA 238-100 standard) of at least 100°, preferably at least 115°.

Preferably the finished article, optionally after some post-processing to make the article suitable for it, is subjected to a paint baking treatment.

According to a further embodiment of the invention, a method for hot-forming a steel blank or heat-treating a pre-formed part into an article comprising the steps of:

A):

• heating the steel blank, the steel blank being according to any one of the claims of 1-4, to a temperature T1 and holding the heated blank at T1 during a time period tl, wherein T1 is in the range of Ac3 - 20°C to Ac3 + 100°C, and wherein tl is at most 15 minutes;

• transferring the heated steel blank to a hot-forming press during a transport time t2 during which the temperature of the heated steel blank decreases from temperature T1 to a temperature T2, wherein T2 is above Arl of the steel and wherein the transport time t2 is at most 15 seconds;

• hot-forming the heated steel blank into an article; and

• cooling the article in the hot-forming press to a temperature below the Mf temperature of the steel blank with a cooling rate, CR, of at least 20°C/s; or

B) :

• heating the pre-formed part produced from the steel blank, the steel blank being according to any one of the claims of 1-4, to a temperature T1 and holding the pre-formed part at T1 during a time period tl, wherein T1 is in the range of Ac3 - 20°C to Ac3 + 100°C, and wherein tl is at most 15 minutes;

• transferring the heated pre-formed part to a hot-forming press during a transport time t2 during which the temperature of the pre-formed part decreases from temperature Tl to a temperature T2, wherein T2 is above Arl of the steel and wherein the transport time t2 is at most 15 seconds;

• hot-forming the pre-formed part into an article; and

C):

• heating the pre-formed part produced from the steel blank, the steel blank being according to any one of the claims of 1-4, to a temperature Tl and holding the pre-formed part at Tl during a time period tl, wherein Tl is in the range of Ac3 - 20°C to Ac3 + 100°C, and wherein tl is at most 15 minutes;

• cooling the article to a temperature below the Mf temperature of the steel blank with a cooling rate, CR, of at least 20°C/s;

Arl is the temperature at which the austenite to ferrite transformation completes during cooling of the steel and Mf is the martensite transformation finish temperature.

It is important to note that the chemical composition is suitable to produce a part with a microstructure along the thermo-mechanical routes A, B or C which all benefit from the specific chemical composition designed in this invention. The difference between the three different thermo-mechanical routes lies in the combination or separation of the mechanical and the thermal part of the thermo-mechanical route.

Route A: heating a steel blank to achieve the austenite or austenite + ferrite state, deform it to its final shape while it is in the defined state (> Arl) and cool it to transform the austenite into the desired microstructure (direct hot-forming of the article), or

Route B: cold deform a steel blank to an intermediate shape, heating the intermediate shape to achieve the austenite or austenite + ferrite state, deform it to its final shape while it is in the defined state (hot calibration step) and cool it to transform the austenite into the desired microstructure (indirect hot-forming with hot calibration step of the article), or

Route C: cold deform a steel blank to a pre-formed part having its final shape, heat the pre-formed part to achieve the austenite or austenite + ferrite state and cool it to transform the austenite into the desired microstructure (indirect hot-forming of the article)

It is noted that the cooling of the article could also be performed outside the hot- forming press in Route A or B, but it is preferable to cool it in the hot-forming press for better control of the cooling process. Route A is very suitable for use in combination with an aluminium or aluminium-silicon coated steel substrate, whereas Route B and C are more suitable if a zinc or zinc-alloy coated steel substrate is used.

The process preceding the three different thermo-mechanical methods (A, B and C) comprises the following steps:

1. Steel making in a BOS-plant or an electric arc furnace (EAF)

2. Continuous casting a thick slab, a thin slab or a strip

3. Reheating or homogenising the thick slab, thin slab or cast strip

4. Hot-rolling to the final hot-rolled thickness

5. Run-out-table cooling and coiling of the hot-rolled strip

6. Pickling

7. Cold-rolling

8. Continuous annealing or (optionally) galvannealing after coating (step 9)

9. Coating (optional - i.e. the steel strip can be coated &. uncoated): hot dip galvanizing, electrogalvanizing, aluminizing (Al or Al alloy) or chemical conversion treatment

10. Blanking

These process steps are commonly known. It is well within the scope of the skilled persons' abilities to determine the appropriate process conditions to provide with a steel blank suitable for the processes according to the invention and these preceding process steps are not limiting factors for the invented steel chemistry to give the desired effects. It is preferable that the steel making step is performed in a BOS-plant.

While the above are the complete process steps to obtain a coated steel strip for making blanks suitable for the hot-forming process claimed in this invention, the blanks for carrying out the hot-forming process can be prepared from any of the following states of the steel:

• from hot-rolled condition: after step 6

• from cold-rolled condition: after step 7

• from uncoated annealed condition: after step 8 • from coated annealed condition: after step 9 The chemical composition range of the steel in this invention has been designed in such a way that blanks made out of any of the above intermediate conditions will give the desired microstructure and the mechanical properties after any of the Routes A, B or C.

According to the invention T1 is in the range of Ac3 - 20°C to Ac3 + 100°C. However, according to a further embodiment of the invention T1 is preferably Ac3 +50°C or lower. Preferably T1 is Ac3 or higher, and more preferably T1 is Ac3 + 20°C or higher. The increasingly tighter temperature ranges allow a more controlled conditioning of the austenitic microstructure prior to the deformation or the start of the cooling, should this be required. E.g. excessive or abnormal austenite grain growth is prevented.

According to a further embodiment of the invention tl is in the range of 3 minutes to 12 minutes to provide optimal microstructure following cooling. If tl is less than 3 minutes, then the microstructure during reheating at Tl does not reach equilibrium and dissolution of all the alloying elements in the matrix is not complete. This can result in poor mechanical properties due to inhomogeneous microstructure after cooling in the hot-forming press. On the other hand, when tl is longer than 12 minutes, excessive grain growth during reheating at Tl will occur resulting in a coarse final microstructure which will deteriorate the final mechanical properties.

Preferably the cooling rate CR of the article is in the range of 30-200°C/s to provide optimal microstructure. In an embodiment of the invention the cooling rate CR of the article is at least 40°C/s, preferably at least 50 °C/s, more preferably at least 60 °C/s and even more preferably at least 80 °C/s. The higher the cooling rate, the microstructure of the steel contains more martensitic phase and lesser bainitic phase, optionally together with the small amount of ferrite. Below a cooling rate of 30°C/s, pro- eutectoid ferrite and/or pearlite will form during cooling that will deteriorate the mechanical properties. On the other hand, a cooling rate above 200°C/s is not necessary from a metallurgical viewpoint of achieving the microstructure (i.e. martensite from austenite), which is also practically always not possible to achieve.

Optionally the method according to the invention includes a paint baking treatment, used after painting hot-formed component, for instance an additional ageing treatment at about 170-200°C for 20 minutes.

According to a third aspect the invention is also embodied in the use of an article manufactured according to the method of the invention, wherein the resulting article preferably is an automotive body or chassis part.

The invention is also embodied in a hot-formed part produced according to the invention and/or an article manufactured according to the method of the invention.

Examples

Example 1 As substrates the materials according to table 1 were used. The inventive steel was cast and processed into cold-rolled strips with a gauge of 1.5 mm through reheating the cast steel to 1200°C, hot-rolling (Finish Rolling Temperature 900°C) to a final hot- rolled thickness of 4 mm. After finish rolling the steel was cooled on the run-out table at 25 °C/s to 700°C and simulated for coil cooling (i.e. coiling). After pickling the hot- rolled strips were cold-rolled to 1.5 mm. The comparative steels of DP1000 and DP800, which are two cold-formable commercial steels, were also processed to the same condition (cold-rolled 1.5 mm gauge) and continuously annealed to achieve their mechanical properties as specified in the well-known industry specifications such as in VDA 239-100.

Table 1: Steel chemistry in wt.% (balance Fe and inevitable impurities).

*impurity level

The Acl, Ac3 and Ms and Mf of the inventive steel were determined by dilatometry, shown in Table 2 (Ms = martensite transformation start temperature). Dilatometry experiments were performed on the cold-rolled samples of 10 mm x 5 mm x 1.5 mm dimensions (length along the rolling direction). The samples were heated to 950°C at a rate of 15°C/s, held there for 5 min. and quenched at 150°C/s to room temperature to determine these transformation temperatures. Since hardenability of steels is relevant for hot-forming, the continuous cooling transformation (CCT) diagrams of the steels were also determined from dilatometry. For the purpose, the samples were heated at a rate of 15°C/s to 950°C, soaked for 5 min. and then cooled to room temperature at the following different cooling rates: 1, 5, 10, 30, 40, 50, 60 and 100 °C/s. In addition, all the samples were characterized for their Vickers hardness (HV10) and optical microstructures to determine the phase fractions by image analysis. The obtained CCT diagram is presented in figure 2. This diagram can be used to predict the microstructure as a function of the chosen cooling rate.

Table 2: Critical temperatures measured by dilatometry.

Blanks of dimensions, 220 mm x 110 mm x 1.5 mm, were prepared from the cold- rolled material and were subjected to reheating to 900°C (10°C below Ac3) and 940°C (30°C above Ac3) and were soaked for 5 min. in a nitrogen atmosphere to minimize surface degradation, transferred resulting in a temperature drop of 120°C in 10s and then subjected to cooling to about 160°C in the following rates: 30, 40, 50, 60, 80, 200°C/s. From the heat-treated samples, A50 tensile specimens along the rolling direction were prepared and tested with quasi-static strain rate (EN10002 standard). Microstructures were characterized from the RD-ND planes (RD and ND stand for rolling direction and normal direction respectively). Microstructures were quantified by an Image Analysis software after etching the samples with different etchants: 2 vol.% nital, 10 vol.% sodium metabisulphite and Le Pera reagents. 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 according to the VDA 238-100 standard. The samples with bending axis parallel to the rolling direction are identified as longitudinal (L) bending specimens whereas those with bending axis perpendicular to the rolling direction are denoted as perpendicular (T) bending specimens. The measured bending angles at 1.5 mm thickness were converted to the angles for 1 mm thickness using a suitable formula (= original bending angle at 1.5 mm thickness x square root of original thickness of 1.5 mm). For each type of tests, three measurements were done and the average values from three tests are presented for each condition.

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 in the transverse to the rolling direction, whereas the opposites apply 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 Of Three Equivalents value (MOTE value) for different fracture toughness parameters is 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 by 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/(l-v²))]⁰·⁵ where E is the Young's modulus (= 207 GPa) and v is the Poisson's ratio (= 0.3). 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 with a load of 200 kg and a loading speed of 50 km/hour for the load to hit the crash boxes from a 10 m height. Crash profiles of 500 mm height (transverse to the rolling direction) and 100 mm width were made. The back plates of 100 mm width of the same material were spot-welded to the profiles to prepare the crash boxes.

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

The results are presented in Tables 3 to 7. Table 3 shows that the ultimate tensile strength (UTS) greater than 700 MPa was achieved for all the cooling rates. The presence of small amount of ferrite and some amount bainite increased the tensile ductility. The yield strength (YS) increases with increasing cooling rate for both reheating temperatures since the amount of martensite increased in the microstructure with cooling rate. Microstructures are either fully martensitic or a mixture of martensite, bainite and small amount of ferrite. High bending angles, greater than at least 115° at 1 mm thickness are also achieved as shown in Table 4 for both the specimen orientations. It is evident that small amount of ferrite can be used in the hard matrix microstructure to improve the tensile elongation (both the uniform elongation (Au) and the total elongation (A50)), without sacrificing bendability. The same minimum levels of these properties are also achieved after a baking treatment as shown in Table 4.

Comparing the bending angles of DP800 and DP1000 shown in Table 5 with those of the invented steel in Table 4 and 5 confirm that higher bendability was achieved in the invented steel compared to the standard cold-forming steels. The fracture toughness parameters of the invented steel are also higher than that of DP1000 and DP800 steels (Table 6). Table 7 presents that the crash behaviour of the invented steel both in hot pressed as well as hot pressed and baked condition is better than DP1000 and DP800. The crash boxes of the invented steel did not show any indication of cracking after the tests.

The high performance of the invented steel (crash behaviour) in comparison with the available steels of similar strength is due to the higher bending angle and higher fracture toughness properties. During a crash, the component needs to fold, to be able to absorb energy without fracture, which is determined by its bendability. On the other hand, the energy absorption capability before failure is determined by its fracture toughness parameters. The improvements in these properties of the invented steel have been possible by virtue of the inventive steel chemistry design that provided the suitable microstructures, as defined in this invention, through the defined hot-forming processes.

It is noted that similar favourable results as described above are obtainable when a blank is produced from the steel in the as-hot-rolled condition, in the uncoated annealed condition or in the coated annealed condition. Table 3: Tensile properties and microstructure.

Table 4: Bending angles.

Table 5: Tensile properties and bendability. hot pressed (900°C reheating with 200°C/s cooling rate) and baked.

Table 6: Fracture toughness parameters. Table 7: Summary of Crash test results Example 2

Two inventive steels were cast and processed into cold-rolled strips with a gauge of 1.5 mm through reheating the cast steel to 1200°C, hot-rolling (Finish Rolling Temperature 900°C) to a final hot-rolled thickness of 4 mm. After finish rolling the steel was cooled on the run-out table at 25 °C/s to 700°C and simulated for coil cooling (i.e. coiling). After pickling the hot-rolled strips were cold-rolled to 1.5 mm. The chemical compositions and the critical phase transformation temperatures are given in Table 8 and Table 9 respectively.

Table 8: Steel chemistry in wt.% (balance Fe and inevitable impurities). *impurity level

Table 9: Critical temperatures measured by dilatometry.

Blanks of dimensions, 220 mm x 110 mm x 1.5 mm, were prepared from the cold- rolled material and were first annealed continuously at 720, 750 and 780 °C with total times of 458, 250 and 172 seconds in HNx atmosphere in a hot-dip annealing simulator (HDAS). Then, the annealed blanks were subjected to a hot-forming thermal cycle in the HDAS apparatus, reheating to 910°C (above Ac3) at a rate of 15°C/s and were soaked for 5 min., simulated for transfer cooling resulting in a temperature drop of 120°C in 10s and then subjected to cooling to room temperature with a cooling rate of 30°C/s. The hot-forming thermal cycles were applied in a nitrogen atmosphere to minimize surface degradation of the samples.

From the heat-treated samples, A50 tensile specimens with 190 mm total length, 20 mm width, 60 mm parallel length and 50 mm gauge length were prepared. The length of the specimens were along the rolling direction. The tensile tests were done at a quasi-static strain rate following EN 10002 standard. Microstructures were characterized from the RD-ND planes (RD and ND stand for rolling direction and normal direction respectively). Microstructures were quantified by an Image Analysis software after etching the samples with different etchants: 2 vol.% nital, 10 vol.% sodium metabisulphite and Le Pera reagents. Bending specimens (40 mm x 30 mm x 1.5 mm) from both parallel to rolling direction and perpendicular to rolling direction were prepared from each of the conditions and tested till fracture by three-point bending test according to the VDA 238-100 standard. These samples with bending axis parallel to the rolling direction are identified as longitudinal (L) bending specimens. The measured bending angles at 1.5 mm thickness were converted to the angles for 1 mm thickness using a suitable formula (= original bending angle at 1.5 mm thickness x square root of original thickness of 1.5 mm). For each type of tests, three measurements were done and the average values from three tests are presented for each condition.

The tensile properties of the steels after various annealing heat treatments are presented in Table 10 and 11. After annealing the steels achieved a mixed microstructure of ferrite matrix with pearlite. This combination of phases led to a soft condition before hot forming. The soft condition is characterized by low yield strength, low ultimate tensile strength and high total elongation. This soft condition is suitable for blanking od the sheets before hot forming. Steel A had the following range of tensile properties after annealing : yield strength of 361 to 420 MPa; ultimate tensile strength of 579 to 942 MPa; total elongation A50 of 11.0 to 23.7%. Steel B possessed the following range of tensile properties after hot forming : yield strength of 334 to 449 MPa; ultimate tensile strength of 541 to 902 MPa; total elongation A50 of 11.8 to 30.5%.

After hot forming following the mentioned thermal cycle before, all the samples of Steel A achieved a microstructure of 95 to 100 vol.% martensite + 5 to 0 vol.% bainite, and the samples of steel B achieved a microstructure of 90 to 95 vol.% martensite and 10 to 5 vol.% bainite.

The tensile properties of steel A and steel B after hot forming are presented in Table 12 and Table 13 respectively, and bendability results are in Table 14 and Table 15 respectively for steel A and steel B. It is clear from Table 12 and Table 13 that the ultimate tensile strength (UTS) after hot forming is greater than 700 MPa for both the steels after all the annealing conditions (in the range of 1072 to 1280 MPa for steel A and 850 to 1149 MPa for steel B). The yield strength (YS) values are also high (719 to 883 MPa for steel A and 559 to 783 MPa for steel B). The total elongation values are also above 6% (6.4 to 10% for steel A and 7.3 to 14.7% for steel B). The bending angles at 1 mm thickness in Table 14 and Table 15 are also higher than 100° for most of the conditions (101.0 to 137.4° for steel A and 112.6 to 140.2° for steel B). The high UTS values were caused by the predominantly martensitic matrix after hot forming in both the steels. In these examples, the predominantly single phase martensite gave high bending angles because of absence of any substantial weak interfaces from other phases. The minimum values of total elongation of 6% was guaranteed due to small specified amounts of bainite present in the microstructures. Steel B showed higher total elongation and bendability because of slightly higher fractions of bainite in the martensitic matrix. The achieved bending angles in steel A and steel B were far greater than those achieved in comparative DP800 and DP1000 steels in Table 5. This particular feature, combined with high UTS and YS will give higher crash energy absorption in these invented steels than the comparative steels. Table 10: Tensile properties after annealing of the cold-rolled Steel A.

Table 11: Tensile properties after annealing of the cold-rolled Steel B. Table 12: Tensile properties after annealing and hot forming of Steel A for different annealing conditions.

Table 13: Tensile properties after annealing and hot forming of Steel B for different annealing conditions. Table 14: Bendability after annealing and hot forming of Steel A for different annealing conditions.

Table 15: Bendability after annealing and hot forming of Steel B for different annealing conditions.

Brief description of the drawings

The invention will now be explained by means of the following, non-limiting figures. Figure la : Temperature sequence with reheating between Acl and Ac3; Figure lb

Temperature sequence with reheating above Ac3. Some critical temperatures and process stages are indicated.

Figure 2: CCT-diagram of the inventive steel in Table 2.

Figure 3: Experimental details of the direct hot-forming experiments. Same temperature schedule is used for indirect hot-forming at a different timescale. Some critical temperatures, cooling rates, durations and process stages are indicated.

Figure 4: Graphical presentation of the mechanical property data in Table 3 after annealing at (a) 900 or (b) 940°C. Left Y-axis - Open square: tensile strength (Rm), open circles: yield strength (Rp); Right Y-axis - Open triangles: uniform elongation (Au), open diamonds: total elongation (A50), closed circles yield ratio. Figure 5: Graphical presentation of the bending angle data in Table 4 after annealing at (a) 900 or (b) 940°C. Closed line T=transverse direction, dashed line L=longitudinal direction.

Figure 6: Schematic drawing of the three thermo-mechanical treatment routes A, B and C. CRC=Cold-rolled coil, pfp =pre-formed part.

Claims

1 A steel strip, sheet or blank for producing hot-formed parts or a heat-treated pre formed part having the following composition in wt.%:

• C 0.07- 0.20;

• Mn 0.5-2.0;

• Si 0.3-1.5;

• Mo 0.1-1.0; and optionally one or more of the elements selected from:

• Al <0.1;

• Cr at most 0.050;

• Cu <0.2;

• N <0.01;

• P <0.04;

• S <0.025;

• O <0.01;

• Ti <0.10;

• V <0.15; the remainder being iron and inevitable impurities.

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

• C 0.07 - 0.18 and/or,

• Mn 0.8 - 1.8 and/or,

• Si 0.5 - 1.0 and/or,

• Mo 0.15 - 0.5.

3. The steel strip, sheet or blank according to any one of the claims 1-2, wherein the steel strip, sheet or blank is provided with

• a metallic coating layer wherein the metallic layer comprises zinc or a zinc alloy coating layer, such as a zinc-alloy coating layer comprising 0.3-4.0 wt.% Mg and 0.3-6.0 wt.% Al; optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities; the remainder being zinc, or

• with a metallic coating layer wherein the metallic layer comprises an aluminium layer or aluminium alloy layer, such as aluminium alloyed with silicon e.g. aluminium alloyed with 8 to 11 wt.% of silicon and at most 4 wt.% of iron, optionally at most 0.2 wt.% of one or more additional elements such as calcium, unavoidable impurities and the remainder being aluminium.

4. Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the part has a martensitic microstructure comprising from 0 to at most 70 vol.% bainite, preferably 60 vol.% bainite.

5. Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the part has a martensitic microstructure comprising at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite

6. Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the part has a martensitic microstructure comprising at most 70 vol.% bainite, preferably at most 60 vol.% bainite and at most 15 vol.% ferrite, preferably at most 10 vol.% ferrite, preferably at most 5 vol.% ferrite.

7. Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot- formed part has a tensile strength of at least 700 MPa, preferably of at least 750 MPa.

8 Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims, wherein the hot- formed part has a total elongation, TE, of at least 6% with respect to a gauge length of 50 millimetre according to EN 10002 standard and a bending angle, BA, at 1.0 mm thickness of at least 100°, preferably at least 115° according to VDA 238-100 standard.

9. Hot-formed part or heat-treated pre-formed part produced from a steel strip, sheet or blank according to any one of the preceding claims wherein the part is subjected to a paint baking step.

10. A method for hot-forming a steel blank or heat-treating a pre-formed part into an article comprising the steps of: A) : heating the steel blank, the steel blank being according to any one of the claims of 1-4, to a temperature T1 and holding the heated blank at T1 during a time period tl, wherein T1 is in the range of Ac3 - 20°C to Ac3 + 100°C, and wherein tl is at most 15 minutes;

• transferring the heated steel blank to a hot-forming press during a transport time t2 during which the temperature of the heated steel blank decreases from temperature Tl to a temperature T2, wherein T2 is above Arl of the steel and wherein the transport time t2 is at most 15 seconds;

• hot-forming the heated steel blank into an article; and

• cooling the article in the hot-forming press to a temperature below the Mf temperature of the steel with a cooling rate, CR, of at least 20°C/s; or

B) :

• hot-forming the pre-formed part into an article; and

C):

• cooling the article in the hot-forming press to a temperature below the Mf temperature of the steel with a cooling rate, CR, of at least 20°C/s;

11. The method according to claim 10, wherein Tl is in the range from Ac3 -20°C to Ac3 +50°C.

12. The method according to any one of claims 10-11, wherein tl is in the range of 3 minutes to 12 minutes.

13. The method according to claims 10-12, wherein the article is cooled with a cooling rate, CR, in the range of 30-200°C/s.

14. Use of an article manufactured according to any one of claims 10-13, wherein the resulting article is an automotive body or chassis part.

15. A vehicle comprising a hot-formed part according to any one of claims 5-9 and/or an article manufactured according to the method of anyone of claims 10-13.