JP7326247B2 - Steel strip, sheet or blank for producing hot formed parts, parts and method of hot forming blanks into parts - Google Patents

Description

The present invention relates to steel strip, sheet or blanks for producing hot formed parts; hot formed parts; and methods of producing hot formed parts.

There is an increasing demand for steel alloys that enable lighter weight automotive components to reduce fuel consumption while at the same time improving passenger protection.

To meet the demands of the automotive industry in terms of improved mechanical properties, e.g. improved tensile strength, energy absorption, workability, ductility and toughness, cold forming and cold forming to produce steels meeting these demands. Hot forming processes have been developed.

In the cold forming process, steel is formed into a product near room temperature. Steel products produced in this way are, for example, dual-phase (DP) steels with a ferrite-martensite microstructure. Although these DP steels exhibit high ultimate tensile strength, their bendability and yield strength are low, which is undesirable due to poor crash performance.

In the hot forming process, the steel is heated above the recrystallization temperature and quenched to obtain the desired material properties, usually by martensitic transformation. The basis for hot forming techniques and steel compositions suitable for use therefor has previously been described in GB 1490535.

The steel usually used for hot forming is 22MnB5 steel. This boron steel can be heated in a furnace and is austenitized, typically at 870-940° C., transferred from the furnace to a forming tool, stamped to the desired part shape, and the part cooled at the same time. An advantage of boron steel parts produced in this way is that they exhibit high ultimate tensile strength associated with anti-intrusive crashworthiness due to their fully martensitic microstructure. At the same time, they exhibit low ductility and bendability, which limits toughness and results in low impact energy absorptive crashworthiness.

Fracture toughness measurements are a convenient means of indicating the crash energy absorption of steels. High fracture toughness parameters generally result in good crash behavior.

In view of the above, it should be apparent that there is a need for steel components that exhibit excellent ultimate tensile strength, as well as excellent ductility and bendability, and thus excellent impact energy absorption.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a hot formable steel strip, sheet or blank for a component which is superior when compared to conventional cold formed and hot formed steel. It has a combination of ultimate tensile strength, ductility and bendability, which provides excellent impact energy absorption.

Another subject of the invention is to provide a hot formed part manufactured from said steel strip, sheet or blank and the use of said hot formed part as a structural part of a vehicle. .

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

FIG. 1 is a schematic diagram of an embodiment of the method according to the invention. FIG. 2 shows a cross-section of a drop tower for axial crush testing.

It has now been found that these objectives can be achieved using low-alloy steels containing relatively small amounts of niobium and boron in addition to carbon, manganese, chromium, titanium and nitrogen.
The present invention includes the following inventions.
[1] % by weight of the following composition:
C: 0.03-0.17
Mn: 0.65-2.50
Cr: 0.2 to 2.0
Ti: 0.01-0.10
Nb: 0.01-0.10
B: 0.0005 to 0.005
N: 0.01 or less
In some cases
Si: 0.1 or less
Mo: 0.1 or less
Al: 0.1 or less
Cu: 0.1 or less
P: 0.03 or less
S: 0.025 or less
O: 0.01 or less
V: 0.15 or less
Ni: 0.15 or less
Ca: 0.15 or less
one or more elements selected from
has
The balance is iron and unavoidable impurities,
A steel strip, sheet or blank for the production of hot formed parts having a Ti/N greater than or equal to 3.42.
[2] C: 0.05 to 0.17, preferably 0.07 to 0.15, and/or
Mn: 1.00 to 2.10, preferably 1.20 to 1.80, and/or
Cr: 0.5 to 1.7, preferably 0.8 to 1.5, and/or
Ti: 0.015 to 0.07, preferably 0.025 to 0.05, and/or
Nb: 0.02 to 0.08, preferably 0.03 to 0.07, and/or
B: 0.0005 to 0.004, preferably 0.001 to 0.003, and/or
N: 0.001-0.008, preferably 0.002-0.005
Ca: 0.01 or less
The steel strip, sheet or blank according to [1], wherein
[3] The steel according to [1] or [2], wherein the total amount of Mn and Cr is less than 2.7, preferably 0.5 to 2.5, more preferably 2.0 to 2.5 strips, sheets or blanks.
[4] Mn, Cr and B, (B × 1000) / (Mn + Cr) is in the range of 0.185 to 2.5, preferably 0.2 to 2.0, more preferably 0.5 to 1 Steel strip, sheet or blank according to any one of [1] to [3], used in an amount ranging from .5.
[5] Steel strip, sheet or blank according to any one of [1] to [4], provided with a zinc-based coating or an aluminum-based coating or an organic-based coating.
[6] The zinc-based coating contains 0.2 to 5.0% by weight Al, 0.2 to 5.0% by weight Mg, and optionally one or more of 0.3% by weight or less. Steel strip, sheet or blank according to [5], which is a coating comprising additional elements, the balance being zinc and incidental impurities.
[7] A hot-formed part made from the steel strip, sheet or blank according to any one of [1] to [6], having a tensile strength of 750 MPa or more, preferably 800 MPa or more, More preferably hot formed parts above 900 MPa and below 1400 MPa.
[8] Total elongation (TE) is 5% or more, preferably 5.5% or more, more preferably 6% or more, most preferably 7% or more, and/or a bending angle at a thickness of 1.0 mm The hot formed part of [7], wherein (BA) is 100° or greater, preferably 115° or greater, more preferably 130° or greater, most preferably 140° or greater.
[9] the component has a microstructure;
the microstructure contains 60% or less bainite and the balance is martensite;
Hot formed component according to [7] or [8], wherein the microstructure preferably comprises 50% or less bainite, more preferably 40% or less bainite.
[10] Use of the hot-formed part according to any one of [7] to [9] as a structural part of a vehicle body-in-white.
[11] A method of hot forming a steel blank or preformed part into a part, comprising:
a. heating the blank according to any one of [1] to [3] or a preformed part made from said blank to a temperature T1 and holding said heated blank at temperature T1 for a time t1 wherein the temperature T1 is higher than the Ac3 temperature of the steel and the time t1 is 10 minutes or less;
b. conveying the heated blank or the preformed part to a hot forming tool during a conveying time t2, wherein the temperature of the heated blank or the preformed part increases during the conveying time a step in which the temperature is lowered from T1 to T2 during t2, and the conveying time t2 is 20 seconds or less;
c. hot forming the heated blank or the preformed part into a part; and
d. cooling the part in the hot forming tool to a temperature below the Mf temperature of the steel at a cooling rate of 30° C./s or higher;
method including.
[12] The method according to [11], wherein the temperature T1 in step (a) is 50-100°C higher than the Ac3 temperature and/or the temperature T2 is higher than the Ar3 temperature.
[13] The time t1 in step (a) is 1 minute or more and 7 minutes or less, and/or the time t2 in step (b) is 12 seconds or less, preferably 2 to 10 seconds, [11] Or the method according to [12].
[14] Any of [11] to [13], wherein the component is cooled in step (d) at a cooling rate in the range of 30 to 150°C/s, preferably 30 to 100°C/s. or one of the methods described.
[15] At least one component according to any one of [7] to [9] and/or at least one component manufactured according to the method according to any one of [11] to [14] vehicle equipped with

Accordingly, the present invention provides, in weight percent, the following composition:
C: 0.03-0.17
Mn: 0.65-2.50
Cr: 0.2 to 2.0
Ti: 0.01-0.10
Nb: 0.01-0.10
B: 0.0005 to 0.005
N: 0.01 or less In some cases,
Si: 0.1 or less Mo: 0.1 or less Al: 0.1 or less Cu: 0.1 or less P: 0.03 or less S: 0.025 or less O: 0.01 or less V: 0.15 or less Ni: 0.15 or less Ca: 0.05 or less having one or two or more elements selected from
The balance is iron and unavoidable impurities,
It relates to a steel strip, sheet or blank for producing hot-formed parts, having Ti/N equal to or greater than 3.42.

Hot formed parts made from steel strip, sheet or blank according to the present invention have improved tensile strength, combined ductility and bendability and thus impact toughness when compared to conventional hot formed boron steels. showed that.

In connection with this steel we have two car parts in mind: the front longitudinal bar and the B-pillar. The front longitudinal bars currently use cold formed duplex steel (DP800) and the B-pillars use hot stamped 22MnB5 steel. The energy absorption of DP steel is lower and the use of high strength steel (maximum tensile strength higher than 800 MPa) reduces weight through downgauging and passenger safety through higher crash energy absorption. It will be possible to improve On the other hand, one of the solutions currently used for the B-pillar is to use two types of steel: ultra-high strength (approximately 1500 MPa) steel 22MnB5 on top and low strength (500 MPa) steel on the bottom. is. The two steel blanks are joined by laser welding before hot stamping and the composite blank is stamped into the B-pillar. By using this solution, the upper part resists penetration during impact and the lower part absorbs energy due to its high ductility. The present invention offers the potential for better performance and weight savings. That is, the high-strength steel of the present invention can replace the underlying low-strength steel due to its higher energy absorption performance.

Preferably, the steel strip, sheet or blank for producing hot formed parts as described above has, in weight percent, the following composition:
C: 0.05 to 0.17, preferably 0.07 to 0.15, and/or Mn: 1.0 to 2.1, preferably 1.2 to 1.8, and/or Cr: 0.0. 5-1.7, preferably 0.8-1.5, and/or Ti: 0.015-0.07, preferably 0.025-0.05, and/or Nb: 0.02-0. 08, preferably 0.03 to 0.07, and/or B: 0.0005 to 0.004, preferably 0.001 to 0.003, and/or N: 0.001 to 0.008, preferably 0.002 to 0.005,
In some cases
Si: 0.1 or less, preferably 0.05 or less,
Mo: 0.1 or less, preferably 0.05 or less,
Al: 0.1 or less, preferably 0.05 or less,
Cu: 0.1 or less, preferably 0.05 or less,
P: 0.03 or less, preferably 0.015 or less,
S: 0.025 or less, preferably 0.01 or less,
O: 0.01 or less, preferably 0.005 or less,
V: 0.15 or less, preferably 0.05 or less,
Ca: having one or two or more elements selected from 0.01 or less,
The balance is iron and unavoidable impurities.

Carbon is added to ensure good mechanical properties. C is added in an amount of 0.03% by weight or more to achieve high strength and enhance hardenability of the steel. If too much carbon is added, the toughness and weldability of the steel sheet can be degraded. Therefore, the amount of C used according to the present invention is in the range 0.03-0.17% by weight, preferably in the range 0.05-0.17% by weight, more preferably 0.07-0.15% by weight. is in the range of

Manganese is used to increase hardenability and provide solid-solution strengthening. The Mn content is 0.65% by weight or more to achieve adequate substitutional solid-solution strengthening and adequate quench hardenability, as well as prevent segregation of Mn during casting. Minimize, yet maintain a sufficiently low carbon equivalent for automotive resistance spot welding technology. Additionally, Mn is an element useful for lowering the Ac3 temperature. A higher Mn content is advantageous in lowering the temperature required for hot press molding. If the Mn content exceeds 2.5% by weight, the weldability and hot and cold rolling properties of the steel sheet deteriorate, which can affect the workability of the steel. The amount of Mn used according to the invention is in the range 0.65-2.5 wt%, preferably in the range 1.0-2.1 wt%, more preferably in the range 1.2-1.8 wt%. is.

Chromium improves the hardenability of steel and helps prevent the formation of ferrite and/or pearlite during press hardening. In this regard, the presence of ferrite and/or pearlite in the microstructure has been found to be detrimental to the mechanical properties of the microstructure of interest in the present invention. The amount of Cr used in the present invention is in the range of 0.2-2.0 wt%, preferably in the range of 0.5-1.7 wt%, more preferably in the range of 0.8-1.5 wt%. Range.

Preferably manganese and chromium are used in amounts such that Mn+Cr is less than 2.7, Mn+Cr preferably in the range 0.5 to 2.5, more preferably in the range 2.0 to 2.5. is.

Titanium is added to form TiN precipitates during molten steel cooling and to remove N at high temperatures. The formation _of TiN inhibits the formation of _B3N4 at low temperatures, so that B, which is also an essential element for the present invention, becomes more effective. Stoichiometrically, the ratio of addition of Ti to N (Ti/N) should be greater than 3.42. According to the invention, the amount of titanium is in the range 0.01-0.1% by weight, preferably in the range 0.015-0.07% by weight, more preferably in the range 0.025-0.05% by weight. Range.

Niobium has the effect of forming strengthening precipitates and refining the microstructure. Nb increases strength through grain refinement and precipitation hardening. Grain refinement produces a more homogeneous microstructure and improves hot forming behavior, especially when highly localized strain is introduced. A fine and homogeneous microstructure also improves the bending behavior. The amount of Nb used in the present invention is in the range of 0.01-0.1 wt%, preferably in the range of 0.02-0.08 wt%, more preferably in the range of 0.03-0.07 wt%. Range.

Boron is an important element for enhancing the hardenability of a steel sheet and further increasing the effect of stably ensuring the strength after hardening. According to the invention, B is in the range of 0.0005-0.005% by weight, preferably in the range of 0.0005-0.004% by weight, more preferably in the range of 0.001-0.003% by weight.

Nitrogen has a similar effect as C. N properly combines with titanium to form TiN precipitates. The amount of N according to the invention is less than or equal to 0.01% by weight. Preferably, the amount of N is in the range 0.001-0.008% by weight. Preferably N is present in an amount ranging from 0.002 to 0.005% by weight.

According to the present invention, Mn, Cr and B are such that (B×1000)/(Mn+Cr) is in the range of 0.185 to 2.5, preferably in the range of 0.2 to 2.0, more preferably 0 It is used in an amount such that it ranges from 0.5 to 1.5. The (B×1000)/(Mn+Cr) ratio applied according to the invention achieves adequate hardenability of the steel.

The amounts of Si, Mo, Al, Cu, P, S, O, V, Ni and Ca, if present, should all be low.

Silicon is also added to enhance hardenability and suitable substitutional solid solution strengthening. The amount of Si used in the present invention is 0.1% by weight or less, preferably 0.5% by weight or less.

Aluminum is added to deoxidize the steel. The Al content is 0.1% by weight or less, preferably 0.05% by weight or less.

Molybdenum is added to improve the hardenability of steel and promote the formation of bainite. The amount of Mo used according to the invention is 0.1% by weight or less, preferably 0.05% by weight or less.

Copper is added to improve hardenability and increase the strength of steel. When present, Cu is used in accordance with the present invention in an amount of 0.1 wt.% or less, preferably 0.05 wt.% or less.

P is known to extend the intercritical temperature range of the steel transformation interval. P is also an element useful for maintaining desired retained austenite. However, P may reduce the workability of steel. According to the invention, P should be present in an amount of 0.03% by weight or less, preferably 0.015% by weight or less.

The amount of sulfur should be minimized to reduce harmful non-metallic inclusions. S forms sulfide-based inclusions such as MnS, which cause cracks and reduce workability. Therefore, it is desirable to reduce the amount of S as much as possible. According to the invention, the amount of S is less than or equal to 0.025% by weight, preferably less than or equal to 0.01% by weight.

Steel products need to be deoxidized because oxygen reduces various properties such as tensile strength, ductility, toughness and/or weldability. Therefore, the presence of oxygen should be prevented. According to the invention, the amount of O is less than or equal to 0.01 wt%, preferably less than or equal to 0.005 wt%.

Vanadium may be added to form V(C,N) precipitates to strengthen steel products. The amount of vanadium, if present, is no more than 0.15 wt%, preferably no more than 0.05 wt%.

Nickel may be added in amounts up to 0.15% by weight. Ni can be added to increase the strength and toughness of steel.

Calcium may be present in an amount of 0.05 wt.% or less, preferably 0.01 wt.% or less. Ca is added to spheroidize sulfur-containing inclusions and minimize the amount of elongated inclusions. However, the presence of CaS inclusions still leads to matrix heterogeneity. Therefore, it is best to reduce the amount of S.

According to a preferred embodiment, 1000 ^* B divided by the sum of Mn and Cr should be between 0.185 and 2.5, preferably between 0.5 and 1.5. This restriction improves the hardenability of the steel.

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 forming process.

The zinc-based coating contains 0.2-5.0 wt% Al, 0.2-5.0 wt% Mg, and optionally one or more additional elements up to 0.3 wt%. It is preferred that the coating contain zinc and the balance being zinc and incidental impurities. Additional elements can be selected from the group comprising Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr, or Bi. Pb, Sn, Bi, and Sb are commonly added to form spangles.

Preferably, the total amount of additional elements in the zinc alloy is 0.3% by weight or less. These small amounts of additional elements do not change the coating or bath properties to any significant extent for normal use.

Preferably, when one or more additional elements are present in the zinc alloy coating, each is present in an amount no greater than 0.03 wt%, preferably no greater than 0.01 wt%. Additional elements are usually added only to prevent dross formation in baths containing molten zinc alloys for hot dip galvanizing or to cause spangle formation in the coating layer.

Hot-formed parts produced from steel strip, sheet or blank according to the invention have a microstructure which contains up to 60% bainite and the balance martensite. Preferably, the microstructure comprises no more than 50% by volume bainite, the balance being martensite. More preferably, the microstructure comprises no more than 40% by volume bainite, the balance being martensite. Martensite provides high strength, while the softer bainite improves ductility. The slight difference in strength between martensite and bainite helps maintain high bendability due to the lack of weak phase interfaces.

Hot-formed parts according to the invention exhibit excellent mechanical properties. The tensile strength (TS) of the part is 750 MPa or more, preferably 800 MPa or more, more preferably 900 MPa or more and 1400 MPa or less.

Suitably, the total elongation (TE) of the part is 5% or more, preferably 5.5% or more, more preferably 6% or more, most preferably 7% or more, and/or the thickness of the part The bend angle (BA) at 1.0 mm is 100° or more, preferably 115° or more, more preferably 130° or more, most preferably 140° or more.

It will be evident that the steel product according to the invention exhibits excellent impact energy absorption.

The invention also relates to the use of such hot-formed parts as structural parts for body-in-white vehicles. The part is made of steel strip, sheet or blank according to the invention. These parts have high strength, high ductility and high bendability. Especially the part as a vehicle structural part is very attractive because of its excellent crash energy absorption compared to the use of conventional hot formed boron steel and cold formed multi-phase steel. , and thus show downgauge and weight saving opportunities based on impact resistance.

The invention also relates to a method of manufacturing a component according to the invention.

The invention therefore also provides a method of hot forming a steel blank or a preformed part into a part, comprising:
a. heating said blank or a preformed part produced from said blank to a temperature T1 and holding said heated blank at temperature T1 for a time t1, said temperature T1 being the Ac3 higher than the temperature and the time t1 is 10 minutes or less;
b. conveying the heated blank or the preformed part to a hot-forming tool during a conveying time t2, wherein the heated blank or the preformed part is is lowered from temperature T1 to temperature T2 during the transfer time t2, and the transfer time t2 is 20 seconds or less;
c. hot forming said heated blank or said preformed part into a heated article; and d. cooling the part in the hot forming tool to a temperature below the Mf temperature of the steel at a cooling rate (V3) of 30° C./s or more.

In accordance with the present method, it has been found that by forming a heated blank into a part as described above, complex shaped parts with improved mechanical properties can be obtained. In particular, the resulting parts exhibit superior impact energy absorption compared to the use of conventional hot-formed boron steels and cold-formed multiphase steels, thus impact-based downsizing. Gives opportunities for gauge and lightening.

After cooling the part below the Mf temperature, the part can be further cooled to room temperature, for example in air, or forced to cool to room temperature.

In the method according to the invention the blank heated in step (a) is provided as an intermediate for the subsequent steps. The steel strip or sheet from which the blank is made can be obtained by standard casting processes. In a preferred embodiment the steel strip or sheet is cold rolled. Steel strips or sheets can suitably be cut into steel blanks. Preformed steel parts can also be used. The preformed part can be partially or wholly formed into the desired shape, preferably at ambient temperature.

The steel blank is heated to temperature T1 during time t1 in step (a). Preferably, in step (a) the temperature T1 is 50-100° C. above the Ac3 temperature of the steel and/or the temperature T2 is above the Ar3 temperature. If the temperature T1 is 50-100° C. higher than the Ac3 temperature, the steel is completely or nearly completely austenitized within time t1 and cooling during step (b) is readily possible. Formability is improved when the microstructure is a homogeneous austenitic microstructure.

Preferably, the time t1 is 1 minute or more and 7 minutes or less. If the time t1 is too long, the austenite grains become coarse and the final mechanical properties may deteriorate.

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

In step (b) the heated steel blank or preformed part is transferred to a hot forming tool during transfer time t2. The temperature of the heated steel blank or preformed part drops from temperature T1 to temperature T2 during the transfer time t2, which is less than or equal to 20 seconds. Time t2 is the time required for the heated blank to be transferred from the heating device to the hot forming tool (eg press) and the hot forming device to be closed. During transport, the blank or preformed part can be cooled from temperature T1 to temperature T2 by the action of natural air cooling and/or other available cooling methods. The heated blank or preformed part can be transferred from the heating device to the forming tool by an automated robotic system or any other transfer method. Time t2 can also be selected in combination with temperature T1, time t1 and temperature T2 to control the evolution of the steel microstructure at the beginning of forming and quenching. Suitably, time t2 is 12 seconds or less, preferably 10 seconds or less, more preferably 8 seconds or less, most preferably 6 seconds or less. In step (b), the blank or preformed part can be cooled from temperature T1 to temperature at a cooling rate V2 of 10° C./s or more. Velocity V2 is preferably in the range of 10-15° C./s. If the blank or preformed part needs to be precooled, the cooling rate should be higher, for example ≧20° C./s and ≦50° C./s, or even more.

In step (c) the heated blank or preformed part is formed into a part having the desired shape. The molded parts are preferably structural parts of vehicles.

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

The present invention provides an improved method of introducing the desired bainite phase into the steel microstructure during hot forming operations. The method enables the production of hot formed steel parts that exhibit an excellent combination of high strength, high ductility and high bendability.

One or more steps of the method according to the invention are carried out in a controlled inert atmosphere of hydrogen, nitrogen, argon or other inert gas to prevent oxidation and/or decarburization of the steel. It is possible.

FIG. 1 is a schematic diagram of an embodiment of the method according to the invention.

FIG. 2 shows a cross-section of the drop tower for axial crush testing.

In FIG. 1, the horizontal axis represents time t and the vertical axis represents temperature T. In FIG. Time t and temperature T are shown schematically in FIG. No values can be derived from FIG.

In FIG. 1, a steel blank or preformed part is (re)heated to an austenitizing temperature above Ac1 at a specified (re)heating rate. Above the Ac1 temperature, the (re)heating rate is reduced until the blank or preformed part reaches a temperature above the Ac3 temperature. The strip, sheet or blank is then held at this particular temperature for a period of time. The heated blank is then transferred from the furnace to a hot forming tool, during which some air cooling of the blank occurs. The blank or preformed part is then hot formed into a part and cooled (or quenched) at a cooling rate of 30° C./s or higher. After reaching a temperature below the Mf temperature of the steel, open the hot forming tool and allow the formed article to cool to room temperature.

Various temperatures used throughout this application are described below.
Ac1: Temperature at which austenite begins to form during heating.
Ac3: Temperature at which ferrite transforms to austenite during heating.
Ar3: Temperature at which transformation from austenite to ferrite begins during cooling.
Ms: temperature at which transformation from austenite to martensite begins during cooling.
Mf: Temperature at which transformation from austenite to martensite ends during cooling.

The invention is illustrated by the following non-limiting examples.

Steel with composition A (according to the invention) Steel blanks with dimensions 220 mm x 110 mm x 1.5 mm were made from cold-rolled steel sheets with the composition shown in Table 1. These steel blanks were subjected to hot forming thermal cycles in a melt annealing simulator (HDAS) and an SMG press. HDAS was used for slower cooling rates (30-80° C./s) and SMG press was used for the fastest cooling rates (200° C./s). The steel blanks were reheated to temperatures T1 of 900° C. (36° C. above Ac3 temperature) and 940° C. (76° C. above Ac3 temperature), respectively, for 5 minutes under a nitrogen atmosphere to minimize surface degradation. Soaked . The blank is then transfer cooled with a temperature drop of 120° C. in 10 seconds, thus a cooling rate V2 of about 12° C./s, followed by the following cooling rates V3: 30, 40, 50, 60 , 80, 200°C/s to 160°C. Longitudinal tensile specimens with a gauge length of 50 mm and a width of 12.5 mm (A50 specimen geometry) were prepared from the heat treated samples and tested at quasi-static strain rates. The microstructure was characterized from the RD-ND plane. Bending specimens (40 mm x 30 mm x 1.5 mm) parallel and transverse to the rolling direction were prepared from each condition and tested to failure by the 3-point bending test described in the VDA238-100 standard. Specimens with a bending direction parallel to the rolling direction were identified as longitudinal (L) bend specimens, and specimens with a bending direction perpendicular to the rolling direction were designated as perpendicular (T) bend specimens. The bend angles measured at 1.5 mm thickness were also converted to angles at 1 mm thickness (=original bend angle x square root of original thickness). For each type of test, triplicate measurements were made and the mean values from triplicate tests are shown for each condition.

J-integral fracture toughness tests and drop tower axial crush tests were performed for selected conditions (reheated SMG press specimens at 940° C.). Compact tensile specimens conforming to the NFMT76J standard were prepared in both longitudinal and transverse directions for fracture toughness testing. For transverse specimens the crack runs along the rolling direction and the loading is transverse to the rolling direction whereas for longitudinal specimens it is the opposite. Specimens were tested at room temperature according to ASTM E1820-09 standard. A pre-crack was introduced by fatigue loading. Final testing was performed by tensile loading using an anti-buckle plate to maintain in-plane stress on the sheet material. Three tests were performed for each condition, showing the minimum values of three equivalents (MOTE values) for various fracture toughness parameters according to the guidelines of the BS7910 standard.

A brief description of the fracture toughness parameters is provided below. CTOD is crack tip opening displacement, a measure of the degree to which a crack opens at either failure (for brittle failure) or maximum load. J is the J-integral, an energy-aware measure of toughness, so it is calculated from the area under the curve to failure or maximum load. KJ is the stress intensity factor determined from the J-integral using the established formula given by KJ=[J(E/(1-v ² ))] ^0.5 . where E is Young's modulus (=207 GPa) and v is Poisson's ratio (=0.03). Kq is the stress intensity factor value measured at the load Pq. Here, Pq is obtained by choosing the slope of the loading line that is in the elastic state, then drawing a straight line with a slope that is 5% smaller, and defining Pq as the load at which this straight line intersects the loading line. It is determined.

Axial crush tests of drop towers were carried out in SMG pressed conditions, applying a load with a weight of 200 kg and a load rate of 50 km/h into a crash box with a closed top hat shape (Fig. 2). Impacts were made at a height of 500 mm (transverse to the rolling direction). The cross-sectional dimensions of the drop tower are given in millimeters in FIG. 2 (t=1.5 mm, R ₀ =3 mm). To prepare the crash box, a 100 mm wide backplate was spot welded to the profile.

At some selected conditions, the samples were also subjected to a paint bake thermal cycle, and the tests were conducted to reflect directly from the results.

Steels with Compositions B and C (Not According to the Invention) For comparison reasons, a commercially available cold-formed CR590Y980T-DP (steel with composition B, commonly known as DP1000 steel) was also tested. This is because the steel with composition B has a similar strength level as the steel blank according to the invention. In addition, for comparison reasons, a standard hot-formed 22MnB5 steel product (steel with composition C) was also tested.

Table 1 specifies the chemical composition in weight percent of steels with compositions A-C.

Table 2 shows the transformation temperatures of steels with composition A.

The results of various tests are shown in Tables 3-8.

Table 3 shows the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE) and total elongation (TE) for steel with composition A after various cooling rates V3. Additionally, Table 3 shows the microstructure in terms of martensite (M) and bainite (B). From Table 3 it is clear that an ultimate tensile strength of over 800 MPa was achieved at various cooling rates V3.

Table 4 shows the bending angles (BA) at a thickness of 1.0 mm for steels with composition A obtained after different cooling rates V3. From Table 4 it is clear that excellent bend angles of at least over 130° were achieved in both the longitudinal (L) and transverse (T) directions.

Table 5 lists various mechanical properties for steel with composition A after subjecting the steel with composition A to hot forming and a baking process simulating the baking finish process used during automobile manufacturing. Indicated. A steel with composition A is heated to 900° C., soaked for 5 minutes and then cooled at V3 at 200° C./s followed by transport cooling. The baking treatment was performed at 180° C. for 20 minutes. From Table 5, approximately the same minimum levels of yield strength (YS), ultimate tensile strength (UTS), ultimate elongation (UE), total elongation (TE) and bend angle (BA) are obtained for steel with composition A. It is clear that this is achieved even after being subjected to This means that the properties required in automobile manufacturing after baking finish are guaranteed under the conditions of use.

Table 6 shows various mechanical properties for steel with composition B (DP1000) and steel with composition C (22MnB5). These steels with compositions B and C were tested under the same test conditions as the steel with composition A. Comparing the contents of Tables 4 and 6, it can be seen that the steel parts according to the invention (steel with composition A) are better than the conventional cold formed steel product DP1000 (steel with composition B) and the conventional hot formed steel. It becomes immediately apparent that it provides a significant improvement in terms of bendability compared to the product 22MnB5 (steel with composition C).

From Table 7 it is also clear that the fracture toughness parameters of the steel parts according to the invention (steel with composition A) are also higher than those of blanks made from DP1000 (steel with composition B).

Table 8 shows the crushing behavior of steels with compositions A and B. From Table 8 it is clear that the crushing behavior of steel with composition A is superior to that of DP1000 (steel with composition B) under both hot pressing and hot pressing and baking conditions. . Baking conditions are the same as described above. The crash box of steel with composition A showed no signs of cracking after testing, while the crash box of DP1000 (steel with composition B) showed severe cracking at the fold. Furthermore, steel with composition A exhibits a higher energy absorption capacity.

The improved crushing behavior of the hot-formed steel with composition A according to the invention when compared to conventional steel products of similar strength results in better bending angle properties and better fracture toughness properties. It is due to In this respect, during impact, the steel part has to be bent, which is determined by its bendability. On the one hand, the energy absorption performance before failure is determined by the fracture toughness parameter.

In view of the above, it will be apparent to those skilled in the art that the steel product according to the present invention offers significant improvements over previously known cold-formed and hot-formed steel products.

Claims (19)

Hot-formed parts manufactured from steel strip, sheet or blank,
Said steel strip, sheet or blank has, in % by weight, the following composition:
C: 0.03-0.17
Mn: 0.65-2.50
Cr: 0.2 to 2.0
Ti: 0.01-0.10
Nb: 0.01-0.10
B: 0.0005 to 0.005
N: 0.01 or less In some cases,
Si: 0.1 or less Mo: 0.1 or less Al: 0.1 or less Cu: 0.1 or less P: 0.03 or less S: 0.025 or less O: 0.01 or less V: 0.15 or less Ni: 0.15 or less Ca: 0.15 or less having one or more elements selected from 0.15 or less, the balance being iron and unavoidable impurities, and Ti/N being 3.42 or more, where , Ti/N represents the mass ratio of the amount of Ti to the amount of N,
The hot-formed part has a tensile strength of 750 MPa or more and 1400 MPa or less,
the hot formed part has a microstructure;
A hot formed part wherein the microstructure comprises no more than 60% bainite and the balance martensite. 2. The hot formed part according to claim 1, wherein the total elongation (TE) is 5% or more and/or the bend angle (BA) at 1.0 mm thickness is 100[deg.] or more. Said steel strip, sheet or blank contains the following elements:
C: 0.05 to 0.17, and/or Mn: 1.00 to 2.10, and/or Cr: 0.5 to 1.7, and/or Ti: 0.015 to 0.07, and / or Nb: 0.02 to 0.08, and / or B: 0.0005 to 0.004, and / or N: 0.001 to 0.008,
3. A hot formed part according to claim 1 or 2, comprising Ca: 0.01 or less. Hot formed part according to any one of the preceding claims, wherein the total amount of Mn and Cr is less than 2.7. Mn, Cr and B are such that (B/(Mn+Cr))×1000 is 0 . Any one of claims 1 to 4, used in an amount ranging from 185 to 2.5, where B/(Mn+Cr) represents the mass ratio of the amount of B to the total amount of Mn and Cr A hot formed part as described in . Hot-formed component according to any one of the preceding claims, wherein the steel strip, sheet or blank is provided with a zinc-based coating or an aluminum-based coating or an organic-based coating. Said steel strip, sheet or blank is provided with a zinc-based coating, said zinc-based coating comprising 0.2-5.0 wt% Al, 0.2-5.0 wt% Mg, optionally , 0.3% by weight or less of one or more additional elements, the balance being zinc and unavoidable impurities. replaced parts. Steel strip, sheet or blank for producing hot formed parts according to claim 1 or 2,
Said steel strip, sheet or blank has, in % by weight, the following composition:
C: 0.03-0.17
Mn: 0.65-2.50
Cr: 0.2 to 2.0
Ti: 0.01-0.10
Nb: 0.01-0.10
B: 0.0005 to 0.005
N: 0.01 or less In some cases,
Si: 0.1 or less Mo: 0.1 or less Al: 0.1 or less Cu: 0.1 or less P: 0.03 or less S: 0.025 or less O: 0.01 or less V: 0.15 or less Ni: 0.15 or less Ca: 0.15 or less having one or two or more elements selected from
The balance is iron and unavoidable impurities,
A steel strip, sheet or blank having Ti/N equal to or greater than 3.42, where Ti/N represents the mass ratio of the amount of Ti to the amount of N. C: 0.05 to 0.17, and/or Mn: 1.00 to 2.10, and/or Cr: 0.5 to 1.7, and/or Ti: 0.015 to 0.07, and / or Nb: 0.02 to 0.08, and / or B: 0.0005 to 0.004, and / or N: 0.001 to 0.008,
9. Steel strip, sheet or blank according to claim 8, wherein Ca: 0.01 or less. 10. Steel strip, sheet or blank according to claim 8 or 9, wherein the total amount of Mn and Cr is less than 2.7. Mn, Cr and B are used in amounts such that (B/(Mn+Cr))×1000 is in the range of 0.185 to 2.5, where B/(Mn+Cr) is the total amount of Mn and Cr. Steel strip, sheet or blank according to any one of claims 8 to 10, representing the mass ratio of the amount of B. Steel strip, sheet or blank according to any one of claims 8 to 11, provided with a zinc-based coating or an aluminum-based coating or an organic-based coating. Said steel strip, sheet or blank is provided with a zinc-based coating, said zinc-based coating comprising 0.2-5.0 wt% Al, 0.2-5.0 wt% Mg, optionally , 0.3% by weight or less of one or more additional elements, the balance being zinc and incidental impurities. Use of the hot-formed part according to any one of claims 1 to 7 as a structural part of a vehicle body-in-white. A method for hot forming a steel blank or a preformed part into a hot formed part according to any one of claims 1 to 7, comprising :
a. Heating a blank according to any one of claims 8 to 13 or a preformed part made from said blank to temperature T1 and holding said heated blank at temperature T1 for a time t1 wherein the temperature T1 is higher than the Ac3 temperature of the steel and the time t1 is 10 minutes or less;
b. conveying the heated blank or the preformed part to a hot forming tool during a conveying time t2, wherein the temperature of the heated blank or the preformed part increases during the conveying time a step in which the temperature is lowered from T1 to T2 during t2, and the conveying time t2 is 20 seconds or less;
c. hot forming said heated blank or said preformed part into a part; and d. A method comprising cooling the part in the hot forming tool to a temperature below the Mf temperature of the steel at a cooling rate of 30°C/s or greater. A process according to claim 15, wherein the temperature T1 in step (a) is 50-100°C higher than the Ac3 temperature and/or the temperature T2 is higher than the Ar3 temperature. 17. A method according to claim 15 or 16, wherein the time t1 in step (a) is 1 minute or more and 7 minutes or less and/or the time t2 in step (b) is 12 seconds or less. A method according to any one of claims 15-17, wherein the part is cooled in step (d) with a cooling rate in the range of 30-150°C/s. A vehicle comprising at least one component according to any one of claims 1-7.

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