EP2627790B1

EP2627790B1 - Method of hot forming a steel blank and the hot formed part

Info

Publication number: EP2627790B1
Application number: EP11772882.4A
Authority: EP
Inventors: Shangping Chen; Christiaan Theodorus Wilhelmus Lahaye
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2010-10-12
Filing date: 2011-10-10
Publication date: 2014-10-08
Anticipated expiration: 2031-10-10
Also published as: WO2012048841A8; CN103154279A; JP2013545890A; US20130192726A1; EP2627790A1; CN103154279B; WO2012048841A1

Description

The invention relates to a method of hot forming a steel blank into a hot formed article having very high mechanical properties, such as an automotive part, to an article thus hot formed and to a steel strip, sheet or blank for use in a thermomechanical treatment process, in particular for use in a hot forming method according to the invention.
Recent progress in developing advanced high strength steels (AHSS) enables automotive manufacturers to increase vehicle's safety and crashworthiness of automotive parts and to reduce the weight of car bodies. Despite AHSS's high strength and light weight, inferior formability, poor ductility and toughness, compared to conventional steels, have become major hurdles for the application of AHSS to complex automotive parts such as A- or B-pillars reinforcements.
In attempts to solve the formability problem, hot (press) forming (hot stamping or press hardening) has been developed. By hot (press) forming, the strength of the steel article is improved through fast cooling after heating it to a temperature range where an austenitic phase exists and through the phase transformations of the austenite to harder phases such as bainite and martensite.
The basics of the hot forming technique and steel compositions adapted to be used were for the first time described in GB1490535 . Afterwards a lot of Carbon - Manganese - Boron based steels have been developed for use in hot press forming.
A typical steel used for the hot press forming is based on a composition system of 22MnB5, i.e. 0.22% of C, 1.2% of Mn, maximum 50 ppm of B, specified in EN10083. Hot press forming of 22MnB5 steel can produce complex parts such as bumpers and pillars with ultrahigh strength, minimum springback, and reduced sheet thickness. The tensile strength of boron steels is up to 1600 MPa, which is far above that of the highest-strength conventional cold stamping steels. However the ductility of total elongation A5 is less than 6%.
To improve the crash-resistance of the steel, the ductility and toughness should be further increased. Advanced high strength steels with TRIP (transformation induced plasticity) added multiphase microstructure are a solution. These steels are characterized by a ferritic-bainitic microstructure with retained austenite, which undergoes martensitic transformation during forming of products, additionally contributing to the strengthening of a ready part.
US2008/0286603 A1 has disclosed a steel sheet that exhibits an ultra-high strength after hot press forming followed by rapid cooling, and an enhanced yield strength after painting. The steel sheet has a composition (expressed in % by weight) comprising 0.1 to 0.5 C, 0.01 to 1.0 Si, 0.5 to 4.0 Mn, 0.1 or less P, 0.03 or less S, 0.1 soluble Al, 0.01 to 0.1 N, 0.3 or less W, and the balance being Fe and other inevitable impurities. Further a hot-pressed part made of the steel sheet and a method for manufacturing the hot-pressed part are disclosed. The hot-pressed part achieves a high increment in yield strength after heat treatment for painting while ensuring an ultra-high tensile strength. The steel provided according to this US patent application has an ultimate tensile strength (UTS) of 1500 MPa with the total elongation being less than 8%.
From US2008/0251160 A1 a high-strength cold-rolled steel sheet having an uniform elongation is known, including (in wt.%): 0.10 - 0.28 C; 1.0 - 2.0 Si; and 1.0 - 3.0 Mn. The structure has the space factors: 30 - 65% of bainitic ferrite; 30 - 50% of polygonal ferrite; and 5 - 20% of residual austenite. The desired microstructures were obtained according to the following process. After hot rolling and cold rolling, the steel sheet was heated for soaking up to a temperature which is equal to or higher than the A3 transformation point (A3), then cooled down temporarily to a temperature Tq expressed by the formula A3-250 (°C) ≤ Tq ≤ A3-20 (°C) at an average cooling rate of 1 - 10 °C/s, and then quenched from this temperature down into a bainitic transformation temperature range at an average cooling rate of 11 °C/s or faster. The tensile properties of the steel after such a heat treatment are typically, UTS = 1000 MPa and total elongation less than 15%.
US2008/0308194 A1 has disclosed a process for manufacturing a part made of steel having a multiphase microstructure. The composition of steel comprises, in % by weight: C 0.01 - 0.50, Mn 0.5 - 3.0, Si 0.001 - 3.0, Al 0.005 - 3.0, Mo ≤ 1.0, Cr ≤ 1.5, P ≤ 0.10, Ti ≤ 0.20, V ≤ 1.0 and optionally, one or more elements such as: Ni ≤ 2.0, Cu ≤ 2.0, S ≤0.05 and Nb ≤0.15, the balance of the composition being iron and inevitable impurities. In the process the steel sheet is heated to reach a soaking temperature Ts above Ac1 but below Ac3 and is held at this soaking temperature Ts for a soaking time ts so that the steel has an austenite content equal to or greater than 25 % by area; the steel blank is transferred into a forming tool for hot forming; and the part obtained is cooled within the tool at a cooling rate V to obtain a multiphase microstructure comprising ferrite, martensite or bainite and retained austenite, and ferrite is homogeneous in each of the regions.
Ryu H.-B et al., in a paper "Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel" (published in METALLURGICAL & MATERIALS TRANSACTIONS A, Vol. 33A, No. 9, 2002, p. 2811-2816), has reported a process of hot forming a steel specimen into a deformed article. This known process comprises - after austenizing for 5 minutes at 1000 °C - compression deformation at 920 °C. After this hot forming step is completed at this temperature, the deformed specimens are cooled at rates ranging from 10 - 35 °C/s to a simulated coiling temperature in the range of 420 - 480 °C. At this temperature the specimens are slowly cooled to 420 °C over a 2-minute period to complete bainitic transformation. Finally the specimens are air cooled to ambient temperature.
From EP1686195 A1 a heat treatment procedure - without deformation - for an already drawn wire is known. This heat treatment procedure comprises an intermediate isothermal transformation period of 60 - 3600 seconds at a temperature between Ms - 50 °C and Bs.
JP2003193193 teaches steel sheet having a two phase microstructure comprising bainite/bainitic ferrite as a main phase and austenite as a second phase. The manufacturing process comprises an isothermal heat treatment at 200 - 450 °C for 1 - 3000 seconds.
In order to meet the requirements of the automotive industry there is an ongoing demand for steel articles having improved mechanical properties.
It is an object of the present invention to provide a manufacturing method of such steel articles, as well as steel compositions, in particular suitable for use in this method, more particularly advanced high strength TBF (TRIP-aided bainitic ferrite) steel products suitable for hot press forming as a Boron steel alternative for automotive applications.
Another object of the present invention is to provide a multiphase microstructured steel resulting in simultaneously improved strength and ductility.
Yet a further object is to provide a steel having an ultimate tensile strength higher than 1400 MPa, such as about 1500 Mpa. Still another object is to provide a steel having a total elongation higher than 8%, such as about 12%.
According to a first aspect the invention provides a method of hot forming a steel blank into an article, such as an automotive part, as defined in claim 1, wherein the method comprises the following steps:

d) cooling a heated steel blank to form an article during hot forming, starting at a starting temperature T2 above Ar3 to an interrupt temperature T3 in the range of 400 - 550 °C at a cooling rate V2 of at least 25 °C/s;
e) immediately further cooling the article from the interrupt temperature T3 to ambient temperature at a cooling rate V3 of 0.2 - 10 °C/s
wherein the interrupt temperature T3 and cooling rate V3 are selected such that the article thus obtained has a multiphase microstructure comprising by volume fraction:
- 55 - 90% of bainitic ferrite
- 5 - 15% of retained austenite
- 5 - 30% martensite.

The steel blank as a starting material for executing the method according to the invention can be obtained by standard casting processes, or indirectly from steel strip or sheet material. In hot rolling the cast ingot, the preheating temperature may be about 1100 - 1250 °C. Traditional hot rolling passes and rolling conditions may be implemented to roll the steel ingot to a sheet product of about 3 to 5 mm in thickness. The finish rolling temperature is about 850 - 880 °C. After hot rolling, the steel sheet is cooled at an average cooling rate of 5 - 50 °C/s to a coiling temperature between 550 and 700 °C. An excess of martensite or bainite is formed at a coiling temperature lower than 550 °C, resulting in an excessive increase in the strength of the hot-rolled steel sheet. The excessively increased strength acts as a load during subsequent cold rolling for the production of a cold-rolled steel sheet causing problems, such as a poor appearance. During cold rolling the coiled hot-rolled sheet is pickled and cold-rolled. Cold rolling can also be carried out under standard conditions at a reduction of about 30 - 75%. However, in view of prevention of uneven recrystallization, the cold rolling reduction is preferably controlled to range from 40 to 70%. After cold rolling, the sheet has a thickness of about 1 to 2 mm according to product requirements. After cold rolling the sheet may be decoiled and blanked to proper size suitable for hot forming according to the invention. A cold rolled sheet is a preferred starting product in the method according to the invention.
In the present method of cooling during hot forming, the two-step cooling pattern and the interrupt cooling temperature are properly controlled. Advantageously the heated steel blank used in step d) has an austenite structure. In a preferred embodiment for obtaining such austenite structure the method according to the invention also comprises the steps of

a) heating the steel blank to an austenitizing temperature T1 above Ac3, preferably in the range of Ac3 + 20 °C to Ac3 + 60 °C, more preferably at a heating rate of 10 - 25 °C/s;
b) soaking the steel blank in said range, preferably during a soaking time t1 of 1 - 5 minutes;
c) optionally transferring the heated and soaked blank to a hot forming facility.

These austenitizing steps of the method for obtaining an austenite structure are performed by heating steel sheets, strips of blanks e.g. in a furnace or in a hot forming facility itself, to a temperature T1 above Ac3, more preferably in the range of Ac3 + 20 °C to Ac3 + 60 °C. Preferably the heating rate is in the range of 10 - 25 °C/s. This heating step is followed by soaking at a temperature above Ac3, preferably in the range of Ac3 + 20 °C to Ac3 + 60 °C, for a short period of time, preferably 1 - 5 min (Ac3 being the temperature at which transformation of ferrite into austenite is completed in hypoeutectoid steel, upon heating). The austenitizing temperature (T1) and soaking time are selected to ensure complete dissolution of carbides. If applicable, then the steel blank thus soaked is transferred to the hot forming device such as a hot forming press. If applicable, the transferring time is controlled, usually to a short transfer time such as less than 10 seconds to prevent cooling of the blank below the starting temperature T2 of the next hot forming step. The starting temperature T2 for hot forming and simultaneous cooling should be above the Ar3 point (typically in the range of 780-830 °C) to prevent any ferrite phase transformation, e.g. during transferring. (Ar3 being the temperature at which austenite begins to convert to ferrite upon cooling a steel.) During hot forming the blank is deformed and at the same time cooled down to an interrupt quenching temperature T3 in the range of 400 to 550 °C at a cooling rate V2 higher than 25 °C/s to avoid formation of proeutectoid ferrite and pearlite. If the cooling rate V2 is less than 25 °C/s, pearlite may be generated during quenching. Immediately, i.e. without holding the blank for a predetermined time at a temperature of about T3, such as isothermal austempering at T3, the steel blank is allowed to cool down further to room temperature at a further cooling rate V3 of 0.2 - 10 °C/s. The higher cooling rates above 5 °C/s within said range are advantageous for achieving a high total elongation (At). The range of 0.5 - 5 °C/s is preferred in view of high Ultimate Tensile Strength (UTS). The combined adjustment of the interrupt temperature T3 and the further cooling rate V3 in the final step effectively controls the volume fraction of the various phases in the final microstructure of the article thus manufactured. The interrupt quenching temperature T3 is in the bainitic transformation range. If T3 is too high, generally above 550 °C, some pearlite structure may form during the following slow cooling step at cooling rate V3. If T3 is too low, below Ms point, bainitic structure may be obtained in an insufficient amount in the final microstructure. If V3 is too fast, too much martensite is obtained in the final microstructure, and it is impossible to ensure sufficient elongation. If V3 is too small, too less martensite is obtained in order to guarantee the high strength. Moreover, the combination of T3 and V3 is selected such that the article obtained comprises a multiphase microstructure comprising by volume fraction 55 - 90% of bainitic ferrite, 5 - 15% retained austenite and 5 - 30% martensite. A low martensite content within this range, typically below 10 %, offers a relatively high total elongation. A relatively high UTS is achieved if the multiphase microstructure comprises 55 - 85% bainitic ferrite, 5 - 15% retained austenite and 10 - 30% martensite. Generally the higher the interrupt temperature T3, the lower V3 has to be in order to get this multiphase microstructure:
Preferably the steel, in particular TBF steel, in the article as manufactured, is characterized by a multiphase structure comprising, more preferably consisting of, carbide free bainitic ferrite, carbon enriched retained austenite and a relatively small amount of martensite. In the multiphase structure, the mother-phase structure (matrix) comprises fine plates of essentially carbide free bainitic ferrite. The formation of such a microstructure is due to the fact that the precipitation of cementite during bainitic transformation is suppressed by alloying the steel with a sufficient amount of Si and/or Al, which have very low solubility in cementite and greatly retards growth of cementite from austenite. Carbon that is rejected from the bainitic ferrite enriches the residual austenite, thereby transforming to martensite during cooling after bainitic transformation or stabilizing it down to room temperature.
The advantages of this type of microstructure are manifold. The bainitic ferrite in TBF steel is present in the form of plates with an ultra fine grain size, usually the length ranging up to about 15 micrometer at most and the thickness ranging up to 0.3 micrometer at most (typically ~10 µm long and ~0.2 µm thick). Furthermore the strength and ductility are improved simultaneously. The high flow stresses are due to the small thickness of the bainitic laths and to the absence of proeutectoid polygonal ferrite contrary to existing TRIP (transformation induced plasticity) steels. It is assumed that the retained austenite offers an additional TRIP effect, and that it is useful for improving total elongation. The presence of martensite islands leads to a high strength and to high instantaneous hardening rates. If the synergy effect is secured between transformation induced plasticity attained by residual austenite and use of fine plate bainitic ferrite in TBF steel, a dramatic improvement of elongation of TBF steel is attained. Toughness and crash performance are improved as well.
Moreover, to reach a high strength and a high ductility of the steel, the constituents of the various phases in the final microstructure preferably comprise, by volume fraction, bainitic ferrite: 55 - 90%, retained austenite: 5 - 15% and martensite: 5 - 30%.
The multiphase microstructure according to the invention can be obtained by designing steel with a specific composition and by applying careful control of the thermomechanical treatment process as discussed above.
Thus, in other words in a first aspect a preferred embodiment of the present invention relates to a method of hot forming a steel blank into an article, wherein the method comprises heating the steel blank to an austenitizing temperature T1, advantageously in the range of Ac3 + 20 °C and Ac3 + 60 °C, and soaking the steel blank in the austenitizing temperature range, such that carbides are completely dissolved; introducing the heated and soaked blank in a hot forming device, such as a hot press, while preventing ferrite phase transformation; hot forming the steel blank to form a shaped article starting at a starting temperature T2 above Ar3 and cooling the article thus being hot formed to an interrupt temperature T3 in the bainitic transformation range at a cooling rate such that formation of proeutectoid ferrite and pearlite is avoided; and further cooling the article from the interrupt temperature T3 to ambient temperature at a cooling rate such that the volume fraction of bainitic ferrite is 55 - 90%, of retained austenite 5 - 15%, and of martensite 5 - 30% in the article thus manufactured.
Advantageously, the steel comprises the following elements (in wt. %):C 0.15 - 0.45, Si 0.6 - 2.5, Mn 1.0 - 3.0, Mo 0 - 0.5, Cr 0 - 1.0, P 0.001 - 0.05, S < 0.03, Ca < 0.003, Ti 0.1 or less and V 0.1 or less and the balance Fe and other inevitable impurities. Optionally, the steel may also contain Al less than 1.5%, partially replacing the same amount of Si, provided that the sum of Si and Al is in the range of 1.2 - 2.5%. Preferably, the amounts of Mn and Cr satisfy Mn + Cr ≤ 3%, while also preferably, the amounts of C and Mo satisfy C + 1/3Mo ≤ 0.45% for better control of the multiphase microstructure.
Hereinbelow the functions of each element in the steel composition is described. Content is represented as % by weight of the total composition.
C is an element for securing high strength, and for securing retained austenite. C is added in an amount of 0.15% or more to form the desired multiphase microstructure to achieve ultra-high strength and ductility. Meanwhile, when the C content exceeds 0.45%, it is difficult to obtain the multiphase microstructure through the method according to the invention comprising a contiguous cooling subprocess. Moreover, there is a great possibility that the toughness and weldability of the steel sheet will be deteriorated. C is preferably present in an amount of 0.2 - 0.4%, more preferably 0.2 - 0.35%.
Mn is one of the main elements in the steel composition according to the invention. The functions of Mn include stabilizing the austenite and obtaining the desired multiphase microstructure. If Mn is less than 1.0%, the effects are not sufficiently marked. Whereas if the content exceeds 3%, a fully martensite structure is easily created. As a result, the steel is hardened and embrittled during press forming. Besides, 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. The Mn content is limited to the range of 1.0 - 3.0%, preferably 1.5 - 2.5% and more preferably 1.6 - 2.5%, even more preferably 1.7 - 2.4%.
Si is an element effective for reinforcing a solid solution, and is useful for suppressing production of carbide due to decomposition of retained austenite. To obtain high ductility, toughness and formability, the formation of carbides (either transition carbide or cementite) should be avoided as much as possible. Si suppresses the precipitation of brittle cementite during bainite formation, and hence results in an improvement in formability and toughness. A minimum of 1.0% Si is needed to form carbide free bainite. However, Si is also known to deteriorate galvanizability due to the formation of oxides adherent to the steel substrate. Therefore, the upper limit of Si is controlled below 2.5%. The Si content is advantageously limited to the range of 1.2 - 2.5% and more preferably to the range of 1.4 - 2.0%.
Al is also an element useful for suppressing production of carbide due to decomposition of, particularly, retained austenite. Partial replacement of Si by a same amount of Al has been shown to effectively retard cementite formation without a detrimental effect on hot-dip coatability in TBF steels. However, a high concentration of Al leads to higher possibility of polygonal ferrite to be generated, which is less effective than fine plate ferrite in view of strength. A full substitution of Si by an equivalent amount of Al leads to a marked deterioration of the strength-ductility balance. If added, the amount of Al is limited to 1.5% or smaller.
P is an element useful for maintaining desired retained austenite, and its effect is exerted by an amount of P of 0.001 % or larger, more preferably 0.005% or larger, but P may deteriorate the workability of the steel when it is added in an excess amount. Accordingly, the P content is preferably limited to 0.05% or less.
S is a harmful element which forms sulfide based inclusions such as MnS, which initiates crack formation, and deteriorates processibility. Therefore, it is desirable to reduce the amount of S as much as possible. Accordingly, S is controlled to 0.03% or smaller.
Mo and Cr serve to improve the hardenability of the steel and facilitate the formation of bainite ferrite. At the same time, they are elements having similar effectiveness useful for stabilizing retained austenite. Therefore, Mo and Cr are very effective for process control. It is advantageous that each of them is contained at 0.05% or larger. However, when each of them is added excessively, the effect is saturated and a higher addition is not economical. Therefore, the amount of Mo is 0.5% or smaller, and the amount of Cr is 1% or smaller.
Ti and V have the effect of forming strengthening precipitates and refining microstructure. The amount of each of them is 0.1% or smaller, preferably 0.05% or smaller.
Ca is an element effective for controlling a form of sulfide in the steel, and improving processibility. It is recommended that Ca is contained at 0.0003% or more. However, when it is added excessively, the effect is saturated. Therefore, the preferred amount is 0.0003 - 0.003%.
Regarding a practical manufacturing process, the heat treatment described above may be carried out by heating and cooling in a continuous annealing facility (CAL), hot press forming facility, salt bath or the like.
Advantageously the first combined deformation/cooling step d) is performed in a hot forming facility. In a preferred embodiment the second cooling step e) is performed in air or in stock outside the hot forming facility.
Preferably the steel of the formed article has an ultimate tensile strength (UTS) of at least 1400 MPa, advantageously at least 1500 MPa, more preferably at least 1600 MPa, and most preferably at least 1700 MPa.
Advantageously the steel of the formed article has a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, and most preferably at least 14%.
The heat treatment processes can be simply performed by applying the hot (press) forming in a standard hot forming facility with only modification of the simultaneous cooling process, including cooling interrupt temperature and velocities. After austenitizing, the heat blank is inserted into the die sets of a hot press, in which the blank is shaped and cooled. The transfer time (e.g. 5 to 10 s) between furnace and the press forming tools is controlled to secure that the starting deformation temperature is higher than Ar3. The cooling rate V2 of the steel part in the forming tool depends on the deformation and on the quality of the contact between the tool and the steel blank. The forming tools may be cooled for example by using circulation of a liquid to ensure that the cooling rate is high enough (>25 °C/s) during quenching in the press mould. The interrupt cooling temperature can be controlled by the time for separation of the pressing tools. The formed article is then removed from the mould and cooled to room temperature in air. The formed articles, e.g. sheets, may also be stacked and then cooled to ambient temperature in air.
After cooling down to room temperature, the parts will be built in the car body structure. Then the paintbake process is performed. The paintbake cycle does not affect the properties.
In a second aspect the present invention provides a steel article, preferably formed according to the method of the present invention, wherein the steel has a microstructure comprising by volume fraction:

55 - 90% of bainitic ferrite
5 - 15% of retained austenite
5 - 30% martensite.

The details of the composition and microstructure as discussed above in view of the method according to the invention, in particular the advantageous and/or preferred embodiments are similarly applicable to the second aspect of the invention. Preferably the steel has an Ultimate Tensile Strength of at least 1400 MPa, advantageously at least 1500 MPa, preferably at least 1600 MPa, more preferably at least 1700 MPa and/or a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, most preferably at least 14%.
According to a third aspect the present invention provides a steel strip, sheet or strip for use in a thermomechanical treatment, in particular a hot forming method according to the invention described above, having a composition, in weight %:

C: 0.15 - 0.45

Si: 0.6 - 2.5

Mn: 1.0 - 3.0

Al: 0 - 1.5

Mo: 0 - 0.5

Cr: 0 - 1.0

P: 0.001 - 0.05

S: < 0.03

Ca: < 0.003

Ti: < 0.1

V: < 0.1

the balance being Fe and inevitable impurities,
wherein Si + Al = 1.2 - 2.5%.
Preferably the relationships Mn + Cr ≤ 3% and C + 1/3Mo ≤ 0.45% are applied.

In preferred compositions of the steel strip, sheet or blank according to the third aspect of the invention comprises, the alloying elements are present in, expressed in weight %,:

C:	0.2 - 0.4, preferably 0.2 - 0.35 and/or
Si:	0.8 - 2.0, preferably 1.2 - 1.8 and/or
Mn:	1.5 - 2.5, preferably 1.7 - 2.4 and/or
Mo:	0.05 - 0.5 and/or
Cr:	0.05 - 1:0 and/or
P:	0.005 - 0.05 and/or
Ca:	00003 - 0.003.

The articles obtained from the strip, sheet or blank exhibit a high tensile strength by rapid cooling after heat treatment and achieve a high increment in yield strength after heat treatment, especially for painting. Based on these advantages, excellent impact properties of the steel article according to the present invention are attained. In addition, the steel article according to the present invention advantageously exhibits good adhesion to a coating layer. Furthermore, other advantages of the steel article according to the present invention are good surface appearance and superior corrosion resistance after painting.
A schematic representation of a practical embodiment of the method according to the invention is shown in Fig. 1, showing a temperature vs. time plot. A steel blank is heated at a heating rate of 15 °C to the austenitizing temperature T1 in the range of Ac3 + 20 °C to Ac3 + 60 °C and soaked at that temperature during a soaking time t1. The thus heated and soaked blank is transferred from the furnace to the hot forming facility, during which cooling by air occurs to some extent. Care is taken that the temperature T2 is not decreased below Ar3 before hot press forming of the blank. After hot press forming the blank thus formed is cooled down to the interrupt temperature T3 at a rate of more than 25 °C. Then air cooling is carried out.
The invention is further illustrated by the following examples.

Steels having compositions A-F as specified in Table 1 were cast to ingots of about 25 kg (100x110x330 mm). The ingots were reheated and then roughly hot rolled to slabs having a thickness of 40 mm. Then finish hot rolling was applied with the following process parameters: preheating at 1200°C for 30 min; multipass rolling to thickness 4 mm (40-27-18-12-8-6-4 mm); and finishing rolling temperature 860 ± 20 °C. Controlled cooling at a rate 30 °C/s to 600 °C in a run-out-table was carried out in order to simulate the commercially used coiling process. After the coiling process the steel plates contain a microstructure consisting of ferrite and pearlite and have a tensile strength less than 700 MPa. The plate was then cold rolled from 4 to 1 mm sheet.

Table 1. Chemical compositions (in wt. %) of TBF steel

Alloy	C	Si	Mn	Mo	Cr	Ti	P	S	Ac3 (°C)	Ms (°C)
A	0.31	1.47	1.66	-	-	0.01	0.03	0.01	863	334
B	0.40	1.48	1.67	-	-	0.01	0.03	0.01	848	306
C	0.29	1.76	2.39	-	-	0.01	0.03	0.01	879	313
D	0.22	1.57	2.35	0.25	-	0.01	0.03	0.01	890	340
E	0.30	1.50	2.40	0.25	-	0.01	0.03	0.01	876	318
F	0.30	1.50	2.00	0.25	0.5	0.01	0.03	0.01	874	315

V and Al were below detection level.

The critical temperatures such as Ac3 and Ms were determined by applying standard dilatometric analysis to facilitate the determination of the temperatures in the process according to the invention.
The resulting cold rolled sheets were subjected to a heat treatment by using a CASIM simulator. Specifically, the steel sheets were heated to 870 to 920°C at a rate of 15 °C/s, holding the sheets at this temperature for 2 min, then cooling at a rate of 50 °C/s to an interrupt temperature between 400 and 550 °C, thereafter, cooling at a rate of 0.2 to 10 °C/s to simulate different air cooling conditions. Tensile tests were conducted by applying JIS 5 tensile test specimen to measure tensile strength and elongation of the specimens with the required microstructures. The volume fraction of bainite and/or martensite in the microstructures was estimated by using metallographic characterisation in combination with dilatametric analysis. The volume fraction of the retained austenite was determined by using TEM. Other measurements are standard. The tensile test results and the invented alloys with the required the microstructure constituents are given in Table 2.

The experimental results indicate that the microstructures and the properties are essentially independent from the austenitizing temperature T1 as long as the T1 is above Ac3, advantageously between Ac3 + 20 °C and Ac3 + 60 °C. It is also proven that the transferring temperature T2 and the cooling rate V2 do not significantly affect the microstructures and properties as well provided that T2 is higher than Ar3 and V2 is equal to or more than 25 °C/s and preferably less than 100 °C/s. However, the microstructures and the properties are strongly dependent on the interrupt temperature T3 and the cooling rate V3. The C content and alloying element contents have a large effect on the selection of T3 and V3. From these results, the conditions for the required multiphase microstructures of the alloys according to the invention can be obtained by careful adjustment of T3 and V3. For alloys with higher C or higher Mo contents, the cooling rate V3 can be controlled in a lower range 0.25 to 2 °C/s; for alloys containing lower C or lower Mo, the cooling rate V3 can be controlled in a higher range 2 to 10° C/s. For a given alloy composition at a fixed T3 temperature, higher cooling rate will result in less bainitic ferrite, but relatively more martensite. Therefore higher strength will be obtained. Lower cooling rate will result in more bainitic ferrite, but relatively less martensite, then higher elongation will be achieved. The higher T3 temperature is, the relatively more bainitic ferrite will be in the final microstructure for a given cooling rate, resulting in high strength but lower elongation.

Table 2. Hardness and tensile properties of the alloys and their microstructures

Alloy	T3	V3	Final microstructure			HV5	YS	UTS	At
Alloy	(°C)	(°C/s)	BF(%)	M(%)	Ar(%)	HV5	(MPa)	(MPa)	(%)
A	450	4	65	23	12	435	1024	1503	10.5
A	500	8	76	16	10	410	943	1435	14
B	400	1	74	20	6	476	1035	1660	12
B	450	1	87	8	5	435	965	1420	14.2
C	450	1	77	14	9	460	1023	1572	12.5
C	500	1	85	8	7	455	987	1475	13.6
D	450	2	85	5	10	485	914	1451	12.5
E	450	0.5	65	25	10	491	1025	1796	9
E	550	0.5	68	23	9	480	1016	1693	8.9
F	500	0.5	80	12	8	475	997	1599	11
F	550	0.5	72	20	8	519	1011	1695	9.5

In the above table the following abbreviations are used:

BF: = Bainitic Ferrite
M: = Martensite
Ar: = Retained Austenite
HV5: = Vickers hardness measured at load of 5 kgf
YS: = Yield Strength
UTS: = Ultimate Tensile Strength
At: = Total Elongation

Claims

Method of hot forming a steel blank into an article, the method comprising the following steps:
d) cooling a heated steel blank to form an article during hot forming, starting at a starting temperature T2 above Ar3 to an interrupt temperature T3 in the range of 400 - 550 °C at a cooling rate V2 of at least 25 °C/s,
wherein the blank has the following composition in weight %: C: 0.15 - 0.45 Si: 0.6 - 2.5 Mn: 1.0 - 3.0 Al: 0 - 1.5 Mo: 0 - 0.5 Cr: 0 - 1.0 P: 0.001 - 0.05 S: < 0.03 Ca: < 0.003 Ti: < 0.1 V: < 0.1

the balance being Fe and inevitable impurities,
wherein Si + Al = 1.2 - 2.5%;

e) without holding the blank for a predetermined time at a temperature of T3 immediately further cooling the article from the interrupt temperature T3 to ambient temperature at a cooling rate V3 of 0.2 - 10 °C/s,
wherein the interrupt temperature T3 and cooling rate V3 are selected using the relation that the higher T3 is the lower V3 is, such that the article thus obtained has a multiphase microstructure comprising by volume fraction:
55 - 90% of bainitic ferrite

5 - 15% of retained austenite

5 - 30% martensite.
Method according to claim 1, wherein the blank is produced from a steel strip or sheet
Method according to claim 1 or 2, wherein Mn + Cr ≤ 3% and C + 1/3 Mo ≤ 0.45%.
Method according to anyone of the preceding claims, wherein the composition comprises, in weight%: C; 0.2 - 0.4 and/or Si: 0.8 - 2.0 and/or Mn: 1.5 - 2.5 and/or Mo: 0.05 - 0.5 and/or Cr: 0.05 - 1.0 and/or P: 0.005 - 0.05 and/or Ca: 0.0003 - 0.003.
Method according to any one of claims 1-4, wherein the composition comprises, in weight %: C: 0.2 - 0.35 and/or Si: 1.2 - 1.8 and/or Mn: 1.7 - 2.4.
Method according to any one of the preceding claims, wherein the bainitic ferrite is essentially carbide free and the retained austenite is carbon enriched.
Method according to any one of the preceding claims, wherein the grains of bainitic ferrite have a length of at most 15 µm and a thickness of at most 0.3 µm.
Method according to any one of the preceding claims, further comprising -prior to the hot forming step d) - the steps of
a) heating the steel blank to an austenitizing temperature T1 above Ac3, preferably in the range of Ac3 + 20 °C - Ac3 + 60 °C, preferably at a heating rate of 10 - 25 °C/s;

b) soaking the steel blank in said range, preferably during a soaking time of 1-5 minutes;

c) optionally transferring the heated and soaked blank to a hot forming facility.
Method according to claim 8, wherein step a) is performed in a continuous annealing facility, or a hot forming facility, or a salt bath or equivalent.
Method according to any one of the preceding claims, wherein step d) is performed in a hot forming facility.
Method according to any one of the preceding claims, wherein step e) is performed outside a hot forming facility in air.
Steel article hot formed according to the method of any one of the preceding claims, wherein the steel has a microstructure comprising by volume fraction:
55 - 90% of bainitic ferrite

5 - 15% of retained austenite

5 - 30% martensite, and
wherein the steel article has the following composition in weight %: C: 0.15 - 0.45 Si: 0.6 - 2.5 Mn: 1.0 - 3.0 Al: 0 - 1,5 Mo: 0 - 0.5 Cr: 0 - 1.0 P: 0.007 - 0.05 S: < 0.03 Ca: < 0.003 Ti: < 0.1 V: < 0.1

the balance being Fe and inevitable impurities,
wherein Si + Al = 1.2 - 2.5%.
Steel article according to claim 12, wherein the steel has an Ultimate Tensile Strength of at least 1400 MPa, advantageously at least 1500 MPa, preferably at least 1600 MPa, more preferably at least 1700 MPa and/or a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, most preferably at least 14%.
Steel article according to claim 12 or 13, wherein Mn + Cr ≤ 3% and C + 1/3 Mo ≤ 0.45%.
Steel article according to any one of claims 12-14, wherein, expressed in weight %,: C: 0.2 - 0.4, preferably 0.2 - 0.35 and/or Si: 0.8 - 2.0, preferably 1.2 - 18 and/or Mn: 1.5 - 2.5, preferably 1.7 -2.4 and/or Mo: 0.05 - 0.5 and/or Cr: 0.05 - 1.0 and/or P: 0.005 - 0.05 and/or Ca: 0.0003 - 0.003.