Coated steel strips and using the same, stamping blanks prepared from the same, stamped products prepared from the same, and articles of manufacture which contain such a stamped product
10 claims: 6 independent, 4 dependent
- 1Zastrzeżenia patentowe 1. Wyrób z wytłaczanej powlekanej stalowej blachy, który obejmuje:(a) taśmę ze stali bazowej mającą pierwszą stronę i drugą stronę, przy czym stal w taśmie ma skład obejmujący, wagowo: 0,15% < węgiel < 0,5% 0,5% < mangan < 3% 0,1% < krzem < 0,5% 0,01% < chrom < 1% tytan < 0,2% aluminium < 0,1% fosfor < 0,1% siarka < 0,05% 0,0005% < bor < 0,08%, oraz dodatkowo obejmuje żelazo oraz zanieczyszczenia związane z obróbką;i (b) powłokę na co najmniej jednej ze wspomnianej pierwszej strony wspomnianej taśmy ze stali bazowej oraz wspomnianej drugiej strony wspomnianej taśmy ze stali bazowej, gdzie (i) wspomniane powlekanie zachodzi wskutek interdyfuzji między wspomnianą stalą bazową a wstępną powłoką z aluminium lub stopu aluminium, (ii) wspomniane powlekanie zachodzi w kierunku od stali bazowej na zewnątrz, - (a) Warstwa interdyfuzyjna ma następujący skład wagowy: 86-95%Fe, 4-10%Al, 0-5%Si - (b) Warstwa pośrednia ma następujący skład wagowy: 39-47% Fe, 53-61%Al, 0-2%Si - (c) Warstwa międzymetaliczna - (d) Warstwa powierzchniowa ma następujący skład wagowy: 39-47% Fe, 53-61%Al, 0-2%Si;gdzie wspomniane warstwy (c) i (d) zajmują co najmniej 90% poziomu odpowiadającego wspomnianym rozważanym warstwom, i gdzie mniej niż 10% warstwy (c) jest obecne na skrajnej powierzchni wyrobu.
- 2Wyrób z wytłaczanej powlekanej stalowej blachy według zastrzeżenia 1, w którym wspomniana powłoka ma grubość większą niż 30 mikrometrów.
- 3Wyrób z wytłaczanej powlekanej stalowej blachy według któregokolwiek z zastrzeżeń 1 albo 2, gdzie wspomniana warstwa (a) ma grubość mniejszą niż 15 mikrometrów.
- 4Wyrób z wytłaczanej powlekanej stalowej blachy według któregokolwiek z zastrzeżeń 1 do 3, w którym stal w taśmie ma skład obejmujący, wagowo:0,20% < węgiel < 0,5% 0,8% < mangan < 1,5% 0,1% < krzem < 0,35% EP 3 290 199 B1 0,01% < chrom < 1% tytan < 0,1% aluminium < 0,1% fosfor < 0,05% siarka < 0,03% 0,0005% < bor < 0,01%, oraz dodatkowo obejmuje żelazo oraz zanieczyszczenia związane z obróbką;i
- 5Wyrób z wytłaczanej powlekanej stalowej blachy według któregokolwiek z zastrzeżeń 1 do 4, w którym stal obejmuje 20 ppm lub mniej siarki.
- 6Wyrób z wytłaczanej powlekanej stalowej blachy według któregokolwiek z zastrzeżeń 1 do 4, w którym stosunek tytanu do azotu w stali w % wagowych przekracza 3,42.
- 7Wyrób z wytłaczanej powlekanej stali według któregokolwiek z zastrzeżeń 1 do 6, w którym wstępna powłoka aluminium albo stopu aluminium wagowo zawiera od 8% do 11% krzemu, od 2% do 4% żelaza, a pozostałość stanowi aluminium oraz zanieczyszczenia z obróbki.
- 8Wyrób z wytłaczanej powlekanej stali według któregokolwiek z zastrzeżeń 1 do 7, w którym warstwa międzymetaliczna ma następujący skład, wagowo:62-67%Fe, 30-34%Al, 2-6%Si.
- 9Lądowy pojazd silnikowy obejmujący wyrób z wytłaczanej powlekanej stali według któregokolwiek z zastrzeżeń 1 do 8. EP 3 290 199 Β1 Fig. 1 (d) Warstwa powierzchniowa (c) Międzymetaliczna warstwa bogata w Al (b) Warstwa pośrednia (a) Warstwa interdyfuzyjna Stalowy substrat
- 1010pm Międzymetaliczna warstwa bogata w Al Warstwa interdyfuzyjna Stalowy substrat Fig.2 10pm EP 3 290 199 Β1 Fig. 3 10μπ> Fig. 4 10 pm EP 3 290 199 Β1 Fig. 5 Zawartość siarki (10-3 %wag.) Fig.6 EP 3 290 199 Β1 Temperatura (°C) Całkowity czas przebywania w piecu łącznie z ogrzewaniem (min.) Fig. 7 EP 3 290 199 B1 ODNOŚNIKI CYTOWANE W OPISIE Lista odnośników cytowanych przez zgłaszającego ma na celu wyłącznie pomoc dla czytającego. Nie stanowi części dokumentu patentu europejskiego. Pomimo, że dołożono największej 5 staranności przy jej tworzeniu, nie można wykluczyć błędów lub przeoczeń i EUP nie ponosi żadnej odpowiedzialności w tym względzie. Dokumenty patentowe cytowane w opisie • US 6296805 B [0003] · EP 1380666 A [0003]
Independent claims10
172 paragraphs in 9 sections, as filed
DESCRIPTION
FIELD OF THE INVENTION [0001] The present invention of extruded products prepared from coated steels, and the various uses of the products of the invention, for example in spot welding, etc.
[0002] Additional advantages and other features of the present invention will be set forth in part in the following description, and in part will become apparent to those skilled in the art upon reviewing the information below, or may be learned in the practice of the present invention. The advantages of the present invention can be realized and achieved as indicated in detail in the appended claims. It will be understood that the present invention may have different and different embodiments, and several details thereof may be modified in various obvious aspects, all without departing from the present invention. The description should be regarded as illustrative and not restrictive.
BACKGROUND OF THE INVENTION [0003] In recent years, the use of precoated steels in hot extrusion processes for forming parts has become very important, especially in the automotive industry. The manufacture of such parts may include the following main steps:
- Pre-coating steel sheets by immersion in molten metal
- Cutting and trimming to obtain blanks
- Heating of semi-finished products in order to obtain the preparation of a steel alloy for the substrate with a pre-coating, as well as steel austenitization
- Hot forming followed by rapid cooling of the parts to obtain dominant martensitic structures
See, for example, US 6,296,805. EP-A 1 380 666 relates to aluminum-coated steel sheet for car parts, wherein the coating includes an inner layer and an outer layer with several intermediate layers sandwiched therebetween.
[0004] By forming a steel alloy with a precoat, resulting in intermetallic alloys with a high melting point, blanks having such a coating can be heated in a temperature range in which the austenitization of the metal substrate occurs, allowing further quenching through rapid cooling.
[0005] Heat treatment of semi-finished products due to the formation of intermetallic coating alloy and austenitization of the substrate is most often performed in furnaces, where the semi-finished products are moved on rollers. The thermal cycles to which the blanks undergo first include the heating phase, whose speed is a function of parameters such as the thickness of the blanks, furnace temperature, travel speed and coating reflection coefficient. After this heating phase, thermal cycles generally include a waiting phase whose temperature is the furnace control temperature. However, there are problems during the operation of the furnace: the rollers can be contaminated by metallic deposits, which are formed as a result of pre-coating the blanks. If these deposits are excessive, the rollers need to be maintained and performance reduced.
[0006] Parts obtained after heating and rapid cooling show very high mechanical strength and can be used for structural applications, for example for
EP 3 290 199 B1 applications in the automotive industry. These parts must often be welded with others and high weldability is required. It means that:
- The welding operation should be able to be carried out in a sufficiently wide working range to ensure that any drift of nominal welding parameters does not affect the weld quality. For resistance welding, which is very common in the automotive industry, the welding range is determined by a combination of parameters: one of the most important is the welding current I and the force F applied to the part during welding. The right combination of these parameters helps ensure that insufficient weld core diameter (due to insufficient intensity or insufficient force) is not obtained and that weld metal eczema does not occur.
- Welding should also be carried out in such a way as to obtain a high mechanical strength of the weld. This mechanical resistance can be assessed by tests such as shear stress tests or transverse tensile tests.
[0007] There is a need for coated steels that can conveniently be used to manufacture shaped parts in an extrusion process. There is also a need for coated steels that can be used to manufacture shaped parts by means of an extrusion process that are suitable for welding. There is also a need for methods for producing such coated steels and extruded parts.
SUMMARY OF THE INVENTION [0008] The inventors have noticed that some coated steels in which the base steel strip is at least partially coated (sometimes referred to as precoated, this prefix indicating that the transformation of the nature of the precoating will take place during the heat treatment before extrusion) on at least one side with an aluminum or aluminum alloy coating and wherein the coating has a predetermined thickness and is preferably substantially uniform, are preferably formed after heating into shaped parts by extrusion and are conveniently welded. In addition, the inventors have noted that the problem of roller contamination described above generally results from an insufficient degree of intermetallic alloy formation between the substrate and the metal precoat. In addition, it was noted that the location of roll contamination corresponds to zones of semi-finished products in contact with the rollers in which the thickness of the metallic pre-coating locally exceeds the average thickness. Although this is not related to any particular theory, it is believed that if the precoating is locally too thick, the intermetallic melt forming is insufficient and the precoating melts, contaminating the rollers. In this way, the inventors have noticed that controlling the uniformity of the pre-layer thickness throughout the entire sheet within the given tolerances is an important factor for achieving the desired degree of intermetallic alloy formation, enabling improved resistance to subsequent melting of the coating during roll movement.
[0009] The inventors have also noted that particularly good weldability of aluminized and hot stamped parts is associated with a particular order of coating layers on the parts, starting from the steel substrate outwards.
[0010] The inventors also noted that the specific combination of transfer time between the heating furnace and the extrusion die, the degree of deformation during extrusion,
The extrusion temperature, the cooling rate of the product during extrusion, leads to the production of parts having a fully homogeneous martensitic structure and that the increase in plasticity or energy absorption of the parts after extrusion is obtained by lowering the sulfur level below the critical value, with these two benefits being obtained with or without the aluminum coating / aluminum alloy according to the invention and other coatings.
OBJECTS OF THE INVENTION [0011] Accordingly, in view of the above, one object of the present invention is to provide new precoated steel strips that can be conveniently machined to form extruded blanks.
[0012] Another object of the present invention is to provide new pre-coated steel strips or sheets that can conveniently be extruded in part.
[0013] Another object of the present invention is to provide new coated steels that can be conveniently formed in part by hot extrusion.
[0014] Another object of the present invention is to provide new methods for producing such coated steel.
[0015] Another object of the present invention is to provide new extruded blanks that are manufactured from such coated steel.
[0016] Another object of the present invention is to provide new methods for producing such extruded blanks.
[0017] Another object of the present invention is to provide new extruded parts that are manufactured from such coated steel.
[0018] Another object of the present invention is to provide new methods for producing such extruded parts.
[0019] Another object of the present invention is to provide new products such as a motorized vehicle that includes such extruded parts.
[0020] Another object of the present invention is to provide new extruded parts.
[0021] Another object of the present invention is to provide new methods for producing welded extruded parts.
[0022] Another object of the present invention is to provide new products such as a motorized vehicle that includes such welded extruded parts.
[0023] Another object of the present invention is to provide new welded coated steels and welded extruded blanks.
[0024] Another object of the present invention is to provide new methods for producing such welded coated steels and welded extruded blanks.
[0025] Listed and other items that will become apparent in the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS [0026]
EP 3 290 199 B1
Figure 1 shows the coated part according to the invention after heat treatment and extrusion. Subsequent layers of coating on the steel substrate are: (a) interdiffusion layer; (b) an intermediate layer; (c) an intermetallic layer; and (d) a surface layer. This arrangement is particularly advantageous for further welding of parts.
Figure 2 shows a coating of a steel substrate after heat treatment and extrusion which does not correspond to the invention. Subsequent layers (interdiffusion layer and intermetallic layer) give poorer results in spot welding.
Figure 3 shows the microstructure of a steel part, hot stamped and cooled in conditions not according to the invention.
Figure 4 shows the microstructure of a steel part, hot stamped and cooled according to a preferred set of conditions according to the invention.
Figure 5 shows the effect of sulfur on the bending angle of parts after hot extrusion.
Figure 6 shows the effect of sulfur on the energy initiating the cracking of parts after hot extrusion.
Figure 7 shows furnace temperature conditions as a function of total oven residence time for sheets with a total thickness of 0.7-1.5 to 1.5-3 mm that provide particularly advantageous welding coatings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0027] As noted above, the inventors observed that some coated steels in which the base steel is at least partially precoated on at least one side with an aluminum or aluminum alloy coating, and wherein the precoating has a certain thickness and is substantially uniform, they are conveniently formed into shaped parts by extrusion, forming one basis for the invention.
[0028] In the context of the present invention, the terms first side (or side 1) and second side (or side 2) of a strip or sheet metal, etc. of base steel refer to two bulky opposite surfaces whose surface area is defined by the length and width of the strip base steel. In contrast, the side edges of the base steel strip are two small opposite surfaces that have a surface area defined by the length and thickness of the strip. The upper and lower edges of the base steel strip are two small opposite surfaces whose surface area is defined by the width and thickness of the strip. Below, tp is the thickness of the precoat at any location considered on pages 1 and 2 of the sheet or blank. In particular, for coated sheets on both sides 1 and 2, tp1 means thickness on side 1 and tp2 means thickness on side 2.
[0029] According to a highly preferred embodiment, tp is controlled in a precise range, expressed by (tpmin, tpmax), to improve the resistance to roll contamination. The thickness is preferably controlled both in the longitudinal (or rolling) direction of the strip or sheet metal, as well as in the transverse direction.
[0030] Regarding the problem of roll contamination, it is particularly important to check the pre-coating on the sheet or blank side that is in direct contact with the rollers. Because the steel sheet coating step can be followed by various operations (e.g., hot coating, which provides coated pages 1 and 2),
Careful checking of the precoating on both sides of the sheet is preferred. For example, after any cooling, machining, cutting, punching etc. pages 1 and 2 may not be easily identifiable. However, if you use the tp control on both sides of the sheet being coated (first side and second side), you do not need to track pages 1 and 2, because neither side will contaminate the roll. In addition, it is not necessary to cut the sheet metal in order to obtain a preferred smaller sheet that exhibits greater uniformity in the thickness of the precoating, thereby providing a sheet that is pre-coated by, for example, immersion in molten metal. In other words, important benefits are achieved when the minimum and maximum coating thickness on the first side (tpmin1, tpmax1) and the minimum and maximum coating thickness on the other side (tpmin2, tpmax2) of the steel sheet or blank are controlled. Coated by immersion in molten metal, steels are the preferred steels here. However, regardless of the coating method, the thickness of the pre-coating on one or both sides of the sheet can be measured and monitored continuously directly on the coating line after the coating operation. This can be done by means of devices known per se, such as thickness measuring instruments based on x-ray absorption. At any time, the thickness measurement at a given location can be performed, for example, over an area of several hundred mm<sup>2</sup>, which represents the dimension of the irradiated zone by X-rays.
[0031] In a preferred embodiment, many such devices are arranged at different distances in the transverse direction of the tape to obtain a thickness profile of the precoating along the width of the tape.
[0032] The inventors have noticed that the resistance to contamination or contamination of the rolls in the furnaces is improved when the minimum and maximum thickness of at least one of the first side (tpmin1, tpmax1) and the other side (tpmin2, tpmax2) respectively are 20 and 33 micrometers (micrometer is the same as microns and is a metric unit of length equal to one millionth of a meter). In other words, in a preferred embodiment, at any place on at least one surface of the sheet or blank, the thickness tp of the precoat is preferably from 20 to 33 microns, including 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 and 32 micrometers, and all ranges and subranges between them, as well as all numbers between each integer value listed (e.g., 22.34 micrometers). In the case of coating by immersion in molten metal, precise control of this precoating thickness range can be realized on the working line, for example by means of a gas blowing nozzle system after coating, for example after the strip or sheet has been removed from the bath, and by the flatness of the strip. The number, geometry and position of the nozzles and flow rates are one of the main parameters of precise thickness control tp. Given the present disclosure, one skilled in the art can control the thickness of the precoat as described herein without undue effort.
The invention relates to tapes manufactured in industrial conditions, i.e. in which the control of the thickness of the pre-coating is effective over a wide surface of the tape, i.e. with a length greater than 100 m and a width greater than 600 mm. In this way, blank or trimmed blanks exhibit very high uniformity in precoating thickness, and the oven heat treatment settings need not be changed to accommodate any changes in this thickness.
[0033] Without referring to any particular theory of operation, the inventors believe that several of the advantages of the invention relate to the aforementioned precoating thickness range, such as the following:
- For a pre-coat thickness of less than 20 microns, the alloy layer that is formed when heating the blank has insufficient roughness. Thus, the adhesion of subsequent painting is low on this surface and corrosion resistance is reduced.
- If the precoat thickness is greater than 33 micrometers at a given location of the sheet, there is a risk that the thickness difference between this location and some other places where the precoat is thinner becomes too important: the oven heat treatment settings can be adjusted to a smaller the thickness of the initial coating, but not thicker. Thus, the alloying reaction with the intermetallic alloying may not be sufficient, because the average diffusion distance of the components in the precoat becomes much smaller than the local thickness of the precoat. As a result, alloying will be much more difficult in the outer (or surface) part, especially at high heat rates.
[0034] Thus, in a first embodiment, the present invention provides coated steel strips that include a base steel strip and an aluminum or aluminum alloy precoating on at least part of one side of the base steel strip. In many applications, the base steel strip can contain any type of steel that can be coated with aluminum or an aluminum alloy. However, in some applications, such as the car's structural part, it is preferred that the base steel strip contains very high strength steel (UHSS). In such cases, it is particularly preferred that the base steel strip comprises boron steel.
[0035] The inventors have also noticed that good welding results are achieved when the coating obtained on parts made of semi-finished products that have undergone intermetallic alloying, austenitization and hot extrusion has special features. It should be emphasized that this coating differs from the initial precoat because the heat treatment causes the alloying reaction with the steel substrate, which modifies both the physicochemical character and the geometry of the precoat: in this regard, the inventors have found that particularly good weldability of aluminized and extruded hot parts is related to the following order of coating layers on the parts, starting from the steel substrate outwards:
- (a) Interdiffusion layer,
- (b) Transition layer,
- (c) Intermetallic layer,
- (d) Surface layer.
See, for example, Figure 1. In a preferred embodiment, said layers are as follows:
- (a) Interdiffusion layer, preferably of medium hardness (e.g. HV50g between 290 and 410, HV50g means hardness measured under a pressure of 50 grams). This layer has the following weight composition: 86-95% Fe, 4-10% Al, 0-5% Si
- (b) Intermediate layer (HV50g about 900 - 1000 e.g. +/- 10%)) This layer has the following weight composition: 39-47% Fe, 53-61% Al, 0-2% Si
EP 3 290 199 B1
- (c) Intermetallic layer with an HV50g hardness of about 580-650, e.g. +/- 10%) In a preferred embodiment, this layer has the following weight composition: 62-67% Fe, 30-34% Al, 2-6% Si
- (d) Surface layer (HV50g about 900 - 1000 e.g. +/- 10%)) This layer has the following weight composition: 39-47% Fe, 53-61% Al, 0-2% [0036] In a preferred embodiment the total thickness of layers (a) to (d) is greater than 30 microns.
In a further preferred embodiment, the thickness of layer (a) is less than 15 microns, for example 14, 12, 10, 8, 6, 4, 2 or 1 micron, and all numbers, ranges and subranges in between, including all numbers between each value recorded as an integer (e.g. 13.84 microns).
[0038] The inventors have noticed that high weldability is particularly achieved when layers (c) and (d) are substantially continuous (i.e. they occupy a level of at least 90% of the layer concerned), and when less than 10% of layer (c) occurs on the extreme surface of the part. Without wishing to be bound by theory, it is believed that this particular distribution of the layer, in particular layers (a) and layers (c) and (d) affects the resistivity of the coating both through their characteristic properties and by the roughness effect. Thus, current flow, surface heat generation and weld core formation at the initial stage of spot welding depend on this particular arrangement.
[0039] This advantageous layer distribution is obtained, for example, when steel sheets pre-coated with aluminum or aluminum alloy, whose thickness range is, for example, 0.7 to 3 mm, are heated for 3 to 13 minutes (this residence time includes the heating phase and waiting time) in an oven heated to a temperature of 880 to 940 ° C. Other conditions leading to such favorable layer distributions are found in Figure 7:
For sheets with a total thickness greater than or equal to 0.7 mm and less than or equal to 1.5 mm, the preferred machining conditions: (oven temperature, total oven time) are shown in Figure 7 according to the conditions placed within the ABCD diagram.
[0040] For sheets with a thickness greater than 1.5 mm and less than or equal to 3 mm, the preferred processing conditions: (oven temperature, total oven time) are shown in Figure 7 in the EFGH diagram.
[0041] The heating rate Vc is in the range between 4 and 12 ° C / s to produce the preferred distribution of alloy layers. In this context, the heating rate reflects the increase in temperature that is felt by the pre-coated steel when placed in a preheated oven. Vc is defined as the average heating rate between 20 and 700 ° C. The inventors have noticed that Vc control in this exact range is a key factor because it directly controls the nature and morphology of the alloy layers formed. It is emphasized here that the heating rate Vc is different from the average heating rate, which is the heating rate between room temperature and oven temperature. Rates 6, 7, 8, 9, 10 and 11 ° C / s are included, as are all numbers, ranges and subranges between them, and contain all numbers between each integer value listed (e.g., 7.7 ° C / s). In this regard, all the conditions set out in Figure 7 are indicated. The following conditions are particularly preferred:
(for thickness 0.7-1.5mm)
EP 3 290 199 B1
- 930 ° C, from 3 minutes to 6 minutes;
- 880 ° C, from 4 minutes 30 seconds to 13 minutes (for thickness 1.5 to 3 mm)
- 940 ° C, from 4 minutes to 8 minutes;
- 900 ° C, from 6 minutes 30 seconds to 13 minutes [0042] A particular advantage is due to the pre-coatings, whose thickness is between 20 and 33 microns, because this thickness range gives a favorable layer distribution, and the uniformity of the thickness of the pre-coating is associated with homogeneity of the coating formed after alloying.
[0043] The heated blanks are then transferred to a die, hot extruded to obtain a part or article, and cooled at a rate greater than 30 ° C / s. The cooling rate is defined here as the average rate between the exit of the heated blank from the oven and 400 ° C.
[0044] The base steel strip is coated with aluminum or an aluminum alloy. Commercial pure aluminum is known in the art as type 2 aluminum, while 5 to 11% by weight silicon aluminum alloys are known in the art as type 1 aluminum. Silicon is present to prevent the formation of a thick iron-metallic intermetallic layer, which reduces adhesion and deformability. Other alloying elements useful in the present invention with aluminum include iron in an amount of 2.5 to 3% by weight and calcium in an amount of 15 to 30 ppm by weight, including combinations of two or more of aluminum.
[0045] A typical metal bath for an Al-Si coating usually contains, in its basic composition based on the weight of the composition, from 8% to 11% silicon, from 2% to 4% iron, the rest being aluminum or an aluminum alloy, and unavoidable impurities from treatment. A typical composition of the Al-Si coating is as follows: Al-9.3% Si-2.8% Fe. However, the coatings according to the invention are not limited to the listed compositions.
[0046] The base steel used herein may be any which can be coated by a conventional coating technique. For example, the base steel strip may be any hot rolled strip, such as those produced by hot rolling a steel plate (with or without subsequent cold rolling). Typically, the base steel strip will be stored and transported in a coil both before and after coating.
[0047] The claimed steel composition for the base steel strip is one that has the following weight composition:
0.15% <carbon <0.5%
0.5% <manganese <3%
0.1% <silicon <1%
0.01% <chromium <1% titanium <0.2% aluminum <0.1% phosphorus <0.1% sulfur <0.05%
0.0005% <boron <0.010%,
The residue comprises, consists essentially of or comprises iron and impurities inherent in processing. The use of such steel ensures very high mechanical strength after heat treatment, and the aluminum-based coating ensures high corrosion resistance.
[0048] A particularly preferred weight composition of steel in the base steel strip is as follows:
0.15% <carbon <0.25%
0.8% <manganese <1.8%
0.1% <silicon <0.35%
0.01% <chromium <0.5% titanium <0.1% aluminum <0.1% phosphorus <0.1%
0.002% <boron <0.005%, the residue contains, consists essentially of or comprises iron and impurities inherent in processing.
[0049] The tape preferred here is 100 m long and 600 mm wide. The preferred thickness is 0.7 to 3 mm.
[0050] Even more preferably, in the sheet metal composition, the weight ratio of titanium content relative to nitrogen content exceeds 3.42, which is considered a level at which boron can no longer combine with nitrogen.
[0051] An example of a preferred commercially available steel for use in a base steel strip is 22MnB5.
[0052] Chromium, manganese, boron and carbon can be added to the steel compositions of the invention because of their effect on hardenability. In addition, carbon allows achieving good mechanical characteristics due to its effect on martensite hardness.
[0053] Aluminum is incorporated into the composition to perform liquid deoxidation and to safeguard boron efficiency.
[0054] Titanium, whose content relative to the nitrogen content should exceed 3.42, is introduced, for example, to avoid joining boron with nitrogen, whereby nitrogen combines with titanium.
[0055] The alloying elements, Mn, Cr, B, impart hardenability, enabling curing in extrusion tools or the use of mild curing fluids, limiting deformation of parts during heat treatment. In addition, the composition of the invention is optimized for weldability.
[0056] Steel in sheet metal can be processed to globulate sulfides with calcium, which has an effect on improving the fatigue resistance of the sheet.
[0057] As mentioned above, ultra high strength can be provided using the coated and hot stamped steel sheets of the invention. This high level of strength is sometimes associated with limited ductility. In applications requiring higher plasticity, in particular when bending ability is required of a part or article, the inventors have noticed that increased ductility can be obtained when the sulfur content is carefully controlled: when the sulfur level in the base steel is less than or equal to 0.002% (20 ppm), the bending angle may be greater than 60 ° and an increased result is obtained
The toughness and tear strength of the parts that have undergone heat treatment and extrusion. Preferred levels include 20, 18, 15, 13, 10, 8, 5, 2, etc. ppm sulfur. In fact, this advantage is generally applicable to steel and is not limited to coated steel or steel coated with Al or Al alloy. Referring to no particular theory, analyzing the causes of premature failure of some parts during bending operations, the inventors observed that the damage is initiated on sulfide inclusions. It is therefore believed that the decohesion between inclusions and the martensitic or bainito-martensitic matrix acts as stress concentration factors and triggers further propagation of cracks in the plastic mode.
[0058] The disclosure also discusses a method for producing parts, starting from the coated sheet of the invention and then cut into a blank, the coating of which after forming the blank is subjected to a temperature increase at a rate exceeding 4 ° C / second but lower than 12 ° C / second. The heating rate Vc is defined as the average rate between 20 and 700 ° C.
[0059] The invention also relates to the use of hot rolled steel sheet, which can then be cold rolled and coated, for structural and / or anti-burglary or substructural parts for a land motor vehicle, such as, for example, a bumper, door reinforcement, spokes wheels etc.
[0060] The sheet according to the invention described above can originate, as a result of its treatment, from a hot rolling mill and optionally it can be cold wound again depending on the desired final thickness. It is then coated with an aluminum-based coating, e.g. by immersion in a bath containing, in addition to an aluminum / alloy source, e.g. from 8% to 11% silicon and from 2% to 4% iron, the sheet has high mechanical strength after heat treatment and high corrosion resistance, as well as good ability to paint and glue.
[0061] The coating is preferably controlled as above and has in particular the function of protecting the base plate against corrosion under various conditions. The thermal treatment used in the hot molding process or after molding allows for good mechanical properties that can exceed 1500 MPa for mechanical strength and 1200 MPa for the yield strength. The final mechanical properties are regulated and depend in particular on the martensitic fraction of the structure, on the carbon content of steel and on heat treatment. During the heat treatment carried out on the finished part or during the hot shaping process, the coating forms a layer with significant resistance to abrasion, wear, fatigue, impact, and also provides good corrosion resistance and good paint and glue. The coating avoids various surface preparation operations such as for heat treated steel sheets without any coating.
[0062] The steel sheet can be pre-coated by dipping, after pickling, in an aluminum bath containing, for example, only aluminum or aluminum and from 8% to 11% silicon and 2% to 4% iron, or only from 2% to 4% iron, or even in an aluminum bath containing preferably from 9% to 10% silicon and from 2% to 3.5% iron. Aluminum can be aluminum as such or an aluminum alloy.
[0063] In an embodiment of the sheet coating by immersion in a metal bath containing an aluminum alloy containing about 90% aluminum, the coating layer comprises a first layer
EP 3 290 199 B1 in contact with the steel surface. This layer, in direct contact with the sheet surface, is strongly molten with iron.
[0064] The second coating layer, on top of the first, contains about 90% aluminum and may contain silicon and a small amount of iron, depending on the composition of the bath.
[0065] The first alloy layer may crack when the sheet is subjected to high stress during the cold forming operation in the manufacture of the part.
[0066] According to the invention, after forming the part, the coating is subjected to a temperature increase at a speed exceeding 4 ° C / second. This increase in temperature allows the aluminum to be melted quickly, which fills the cracks that occur during the shaping operation.
[0067] During heat treatment, the base coating, for example aluminum, is transformed into an alloying layer with iron containing various phases depending on the heat treatment and having a significant hardness that may exceed 600 HV50.
[0068] Another advantage of the invention is that the diffusion of iron in the coating starts at a high temperature. In this way, better cohesion is achieved between the coating and steel in the sheet metal. In a further embodiment of the invention, the thermal treatment can be carried out locally in strongly deformed zones.
[0069] According to the invention, the sheet, in its delivered condition, in a roll or in a sheet metal, whose thickness can be in the range from 0.25 mm to 15 mm, has good forming properties and good corrosion resistance, as well as good ability to for painting or gluing. Preferably, the steel sheet or blank has a thickness of less than 3 mm, because the cooling rates that can be obtained after quenching are high and help to achieve martensitic structures.
[0070] The steel sheet, the coated article, shows considerable corrosion resistance in the delivered condition, during molding and heat treatment, as well as during the use of the finished part.
[0071] The presence of the coating during the heat treatment of the part makes it possible to prevent decarburization of the base metal as well as oxidation. This is an undeniable advantage, especially for hot forming. In addition, heating the treated part does not require a controlled atmosphere furnace to prevent decarburization.
[0072] Heat treatment of the metal in the sheet metal includes heating at a temperature in the range of Ac1, the initial austenitic transition temperature during heating, for example 750 ° C and 1200 ° C, in an oven for a period depending on the temperature to be achieved and the thickness blank. The composition is optimized to limit grain growth during heat treatment. If a completely martensitic structure is desired, the waiting temperature should be higher than Ac3, e.g. 840 ° C, the temperature of the total austenitic transformation. After maintaining the standby temperature, cooling should follow the final desired structure.
[0073] The semi-finished products are then transferred from the furnace to the extrusion press. When the time elapsing between the exit of the semi-finished products from the furnace and the entry into the extrusion press is more than 10 seconds, a partial transformation into austenite is possible: if it is desired to obtain a full martensitic structure, the transfer time between exit from the furnace and extrusion should be less than 10 seconds.
[0074] The inventors also noticed that the achievement of a fully martensitic structure is related to the amount of deformation in the hot forming operation: the amount of local deformation due to hot forming is closely related to the shape of the part or product and may exceed local or 50% in some specific regions. The inventors have found that when the local stress exceeds the critical value of 10%, the cooling rate must be high enough to achieve complete martensitic transformation. Otherwise, the bainitic transformation can occur to a large extent instead of the martensitic transformation. Therefore, there is a risk that heterogeneous structure will appear on parts with complex shape, where some locations are much more deformed than others. In this regard, the inventors have provided evidence that, in parts of the parts where the stress is higher than 10%, the cooling rate must be increased above 50 ° C / s to guarantee complete martensitic transformation. The cooling rate is defined as the average speed between the exit of the hot blank and the furnace, up to 400 ° C.
[0075] However, it is also possible to obtain ferritobainitic or ferritic-martititic structures by heating them at a temperature in the range between Ac1, for example 750 ° C and Ac3, for example 840 ° C, followed by appropriate cooling. Depending on the level of resistance to be achieved and the thermal treatment used, one or more of these components are / are present in varying proportions.
[0076] The modulation of heat treatment parameters allows, for a given composition, different levels of hot and cold sheet resistance depending on the desired thickness. For the highest levels of resistance, the structure consists mainly of martensite.
[0077] Steel is particularly suitable for the production of structural and anti-burglary elements.
[0078] The invention thus allows the production of hot or cold rolled steel sheets of the desired thickness, coated and providing wide forming possibilities, which after heat treatment carried out on the finished part allows obtaining mechanical strength exceeding 1000 MPa, with significant resistance to impact, fatigue, abrasion and wear, while maintaining good corrosion resistance, as well as good welding, painting and gluing ability.
[0079] The present invention will now be further described by means of certain embodiments which are not intended to be limiting.
EXAMPLES
Example 1:
[0080] In the first embodiment, cold rolled steel sheet, 1.9 mm thick, containing by weight: 0.23% carbon, 1.25% manganese, 0.017% phosphorus, 0.002% sulfur, 0.27% silicon, 0.062% aluminum, 0.021% copper, 0.019% nickel, 0208% chromium, 0.005% nitrogen, 0.038% titanium, 0.004% boron, 0.003% calcium, was pre-coated with an alloy based on 9.3% silicon, 2.8 % iron and the remainder is aluminum and unavoidable impurities. Depending on the manufacturing conditions, namely the setting of the blowing devices on the operating line, sheets 120 m long and 650 mm wide with different thickness ranges were produced.
EP 3 290 199 B1
- Sheet A (according to the invention): the thickness tpi and tp2 on each side of the sheet was controlled to be in the range (20-33) micrometers, in each position of the two sheet surfaces, both in the longitudinal (or displacement) direction and section. The measurement was carried out continuously using measuring instruments for thickness measurements based on X-ray emissions. At any time, the measuring site was a round zone with a radius of about 20 mm. The sheets were then cut into 1.2 x 0.5 blanks <sub>m</sub>2<sub>.</sub>
- Sheet B (reference): on these sheets, the thickness of the precoating had greater variation, because the thickness tp1 and tp2 on both sides of the sheet was in the range (30-45) micrometers. Blanks cut from these sheets have the same thickness of pre-coating.
[0081] The blanks were then heated in an oven at T = 920 ° C. The heating time was 3 minutes, with a waiting time of 4 minutes. The microstructure is fully austenitic. The blanks were then transferred from the furnace to an extrusion press. When the time elapsing between the exit of the blanks from the furnace and the transfer to the extrusion press is more than 10 seconds, partial transformation into austenite is possible, thereby reducing the mechanical resistance of the extruded part.
[0082] The blanks were then directly cooled without hot extrusion to improve possible remelting of the coating. [0083] In the A series, no melting of the precoating was found. Formation of the intermetallic alloy between the precoat and the steel substrate took place completely.
[0084] In the B series, the precoat was mainly melted, but some remelting was found, especially in the case of earlier thicker precoat sites. This partial remelting of the aluminum precoat contributes to the progressive contamination of the rollers in the furnace. The sheets according to the invention do not contribute to this progressive accumulation of dirt on the rollers.
Example 2:
[0085]
i) Conditions according to the invention: in the second embodiment, a cold-rolled steel sheet, 1.2 mm thick, 120 m long and 650 mm wide, having the same composition and the same initial coating as in Example 1 was prepared. The sheets were then cut into blanks that were heated at 920 ° C for 6 minutes, this time including the heating phase and waiting time. The heating rate Vc between 20 and 700 ° C was 10 ° C / s. The semi-finished products were finally hot pressed and cooled to obtain full martensitic structures.
The parts obtained after the hot extrusion are covered with a coating of 40 microns thickness, which has a four-layer structure, illustrated in Figure 1. From the steel substrate, the layers are as follows:
- (a) Interdiffusion layer or intermetallic layer, 17 microns thick. This layer consists of two sub-layers. HV50g hardness ranges from 295 to 407, and the average composition is as follows: 90% Fe, 7% Al, 3% Si.
EP 3 290 199 B1
- (b) A transition layer, darker in appearance, 8 microns thick. This layer has a hardness of 940HV50g and the average weight composition: 43% Fe, 57% Al, 1% Si.
- (c) An intermetallic layer looking like a light phase, 8 microns thick, showing hardness of 610HV50g, with an average composition of 65% Fe, 31% Al, 4% Si
- (d) Darker surface layer, 7 microns thick, 950 HV50g, with an average composition of 45% Fe, 54% Al, 1% Si.
Layers (c) and (d) are quasi-continuous, i.e. they occupy a level corresponding to at least 90% of the layer concerned. In particular, layer (c) does not reach the extreme surface except exceptions. In any case, layer (c) covers less than 10% of the extreme surface.
ii) Reference conditions: on the other hand, blanks with the same base material and pre-coated parts were heated in an oven in different conditions: Blanks were heated to 950 ° C for 7 minutes, including the heating phase. The heating rate Vc was 11 ° C / s. These conditions correspond to the degree of alloying that is more important than under (i)
- In this coating, the bright intermetallic layer (c) is not continuous and appears dispersed in the coating. About 50% of this layer occurs on the extreme surface of the part. Furthermore, the 10-micrometer interdiffusion layer in contact with the steel substrate is thinner than in the previous case in Figure 1.
[0086] Spot resistance welding was carried out in two situations: i) and ii):
- (i): Coating with quasi-continuous layers (c) and (d), layer (c) covers less than 10% of the extreme surface
- (ii): Coating with mixed and discontinuous layers, layer (c) covers more than 10% of the extreme surface. [0087] Spot welding was carried out by applying two parts and joining them under the following conditions:
- Pressure and electrode pressure during welding: 4000 N
- Press time: 50 periods
- Welding and waiting times: respectively 18 periods [0088] In all conditions, an appropriate intensity range has been determined to achieve:
- no splashes during welding
- acceptable weld core size.
[0089] For conditions i), the weldability range, expressed as current, is 1.4kA. For conditions ii) the weldability range is extremely small.
[0090] Thus, it can be seen that the coating according to the invention gives much more satisfying results.
Example 3:
[0091] In a third embodiment, the cold-rolled steel sheet of Example 1 was cut into blanks 500x500 mm2, which were heated at 920 ° C for 6 minutes, then hot extruded and cooled, under such conditions, that two different cooling rates were obtained:
EP 3 290 199 B1
- (A): Cooling rate: Va = 30 ° C / s
- (B): Cooling rate: Vb = 60 ° C / s [0092] Due to the shape of the parts, different levels of deformation were formed during hot extrusion ε. In particular, some high stress zones exhibit deformation levels higher than 30%.
- As shown in Figure 3, metallographic observations reveal that when ε> 10%, partial bainitic or ferritic transformation occurs at chilled parts Va = 30 ° C / s, mainly at earlier austenite grain boundaries. On the other hand, the cooled portions Vb = 60 ° C / s exhibit a fully martensitic microstructure as illustrated in the Figure. 4. The latter structures show better mechanical resistance and high homogeneity in the case of mechanical pressure.
[0093] Thus, even in products or parts where the stress is greater than 10%, the use of the cooling according to the invention ensures microstructural and mechanical homogeneity.
Example 4:
[0094] In a fourth embodiment, steel castings with different sulfur contents were developed. These steels were then hot rolled and then cold rolled into 2.2 mm thick sheet metal. The sulfur content ranges from 11 ppm (0.0011%) to 59 ppm (0.006%). In addition to sulfur, these various steel castings include: 0.24% carbon, 1.17% manganese, 0.01% phosphorus, 0.25% silicon, 0.045% aluminum, 0.01% copper, 0.02% nickel , 0.2% chromium, 0.04% titanium, 0.003% boron, 0.002% calcium, the rest being iron and unavoidable impurities.
[0095] Said sheets were pre-coated with an aluminum based alloy comprising 9.3% silicon, 2.8% iron, the remainder being aluminum and unavoidable impurities.
[0096] The sheets were then cut into blanks, which were heated at 950 ° C for 5 minutes, then hot extruded and cooled to obtain a full martensitic structure. Mechanical resistance exceeded 1450 MPa.
[0097] Samples were extracted in a transverse rolling direction and subjected to a bend test with alternating bending modes. The inventors have shown that the critical bending angle (angle at break) is closely related to the sulfur content of the steel: when the sulfur content is less than 0.002%, the bending angle exceeds 60 °, which indicates higher ductility and energy absorption. [0098] The compact specimens for the tensile test were also extracted in the transverse direction of rolling to measure the tear resistance, i.e. energy, which is necessary to initiate or propagate an existing crack. The results, shown in Figure 6, indicate that initiation energy higher than 18 Joules is achieved when the sulfur content is less than 0.002% by weight.
[0099] Since the mentioned features of high strength, high energy absorption and weldability are required in the automotive industry, parts or products made according to the invention will be used profitably for such applications.
[00100] Although the above description is clear with respect to the understanding of the invention, the following terms used in the following list of preferred embodiments and claims have the following assigned meanings to avoid any misunderstanding:
pre-coating - a material (Al or Al alloy) applied or placed on at least part of the strip or sheet metal etc. of base steel to form the base coat / composite; the composite has not been subjected to an alloying reaction between the Al or Al alloy material being applied and the base steel alloy forming - the reaction between the precoat and the base steel to produce at least one intermediate layer with a different composition from the base steel as well as the precoat. The melt forming reaction occurs during the heat treatment immediately preceding hot extrusion. The alloying reaction affects the overall thickness of the precoat. The alloy forming reaction forms the following layers: (a) interdiffusion, (b) intermediate, (c) intermetallic and (d) surface as described above;
precoated steel - precoat / base composite that has not undergone an alloying reaction between the applied material and the base steel;
coating - the initial coating after having undergone an alloying reaction between the initial coating and the base steel. In the claimed invention, the coating comprises the above-described (a) interdiffusion, (b) intermediate, (c) intermetallic, and (d) surface layers;
coated steel or article - precoated steel or article that has undergone an alloying reaction between the precoat and the base steel. In the claimed invention, the coated steel is a strip or sheet metal, etc., of a base steel covered with a coating according to the invention comprising (a) interdiffusion, (b) intermediate, (c) intermetallic and (d) surface layers as described above;
blank - shape cut from the tape.
article - extruded blank [0101] The above description of the invention provides a method and a process for its manufacture and use, so that any person skilled in the art is able to make and apply the above, this disclosure being set forth in particular in relation to the appended claims which form part of the original description. The above expressions selected from the group consisting of, selected from and similar include mixtures of certain materials. Terms such as (it) and the like, as used herein, are open terms meaning at least, unless otherwise stated. All references, patents, applications, tests, standards, documents, publications, brochures, texts, articles etc. are listed here for information. Where a limit or number range is given, extreme values are also included. In addition, all values and subranges within a numerical limit or range are explicitly included as if they were explicitly stored. The above description has been presented to enable a person skilled in the art to make and use the invention and is provided in the context of the particular application and its requirements.
EP 3 290 199 B1
Contents9
7 sheets
Sheet 1 Sheet 2 Sheet 3 Sheet 4 Sheet 5 Sheet 6 Sheet 7
80 members in 18 offices
Priority claims2
| Document | Office | Kind | Date |
|---|---|---|---|
| 06842403 | European Patent Office (EPO) | A | |
| 2006004019 | International Bureau of the World Intellectual Property Organization (WIPO) | W |
Members80
| Document | Office | Kind | |
|---|---|---|---|
| CA2668393A1 | Canada | A1 | |
| WO2008053273A1 | World Intellectual Property Organization (WIPO) | A1 | |
| MX2009004453A | Mexico | A | |
| EP2086755A1 | European Patent Office (EPO) | A1 | |
| KR20090086970A | Republic of Korea | A | |
| CN101583486A | China | A | |
| JP2010508438A | Japan | A | |
| RU2395593C1 | Russian Federation | C1 | |
| ZA200902611B | South Africa | B | |
| US2011165436A1 | United States of America | A1 | |
| US8307680B2 | United States of America | B2 | |
| US2013029172A1 | United States of America | A1 | |
| CA2668393C | Canada | C | |
| JP5334854B2 | Japan | B2 | |
| KR101327648B1 | Republic of Korea | B1 | |
| BRPI0622071A2 | Brazil | A2 | |
| CN101583486B | China | B | |
| US9708683B2 | United States of America | B2 | |
| US2017260603A1 | United States of America | A1 | |
| EP2086755B1 | European Patent Office (EPO) | B1 | |
| LT2086755T | Lithuania | T | |
| DK2086755T3 | Denmark | T3 | |
| PT2086755T | Portugal | T | |
| EP3290199A1 | European Patent Office (EPO) | A1 | |
| EP3290200A1 | European Patent Office (EPO) | A1 | |
| PL2086755T3 | Poland | T3 | |
| HUE036195T2 | Hungary | T2 | |
| EP3290199B1 | European Patent Office (EPO) | B1 | |
| US2019271055A1 | United States of America | A1 | |
| US2019271056A1 | United States of America | A1 | |
| US2019271057A1 | United States of America | A1 | |
| US2019271058A1 | United States of America | A1 | |
| US2019276912A1 | United States of America | A1 | |
| EP3587104A1 | European Patent Office (EPO) | A1 | |
| EP3587105A1 | European Patent Office (EPO) | A1 | |
| US10550447B2 | United States of America | B2 | |
| PL3290199T3This record | Poland | T3 | |
| US10577674B2 | United States of America | B2 | |
| US10590507B2 | United States of America | B2 | |
| US10597747B2 | United States of America | B2 | |
| HUE046945T2 | Hungary | T2 | |
| ES2754621T3 | Spain | T3 | |
| US10961602B2 | United States of America | B2 | |
| US11041226B2 | United States of America | B2 | |
| US2021262055A1 | United States of America | A1 | |
| EP3290200B1 | European Patent Office (EPO) | B1 | |
| DK3290200T3 | Denmark | T3 | |
| US2022002834A1 | United States of America | A1 | |
| PT3290200T | Portugal | T | |
| LT3290200T | Lithuania | T | |
| FI3290200T3 | Finland | T3 | |
| PL3290200T3 | Poland | T3 | |
| EP3587104B1 | European Patent Office (EPO) | B1 | |
| FI3587104T3 | Finland | T3 | |
| US11326227B2 | United States of America | B2 | |
| ES2912001T3 | Spain | T3 | |
| HUE057362T2 | Hungary | T2 | |
| PL3587104T3 | Poland | T3 | |
| EP4023433A1 | European Patent Office (EPO) | A1 | |
| EP3587105B1 | European Patent Office (EPO) | B1 | |
| HUE058929T2 | Hungary | T2 | |
| FI3587105T3 | Finland | T3 | |
| ES2929999T3 | Spain | T3 | |
| PL3587105T3 | Poland | T3 | |
| HUE060861T2 | Hungary | T2 | |
| KR101327648B9 | Republic of Korea | B9 | |
| US11939643B2 | United States of America | B2 | |
| US2024110256A1 | United States of America | A1 | |
| US12012640B2 | United States of America | B2 | |
| US2024309484A1 | United States of America | A1 | |
| EP3290199B2 | European Patent Office (EPO) | B2 | |
| US12454737B2 | United States of America | B2 | |
| EP2086755B2 | European Patent Office (EPO) | B2 | |
| FI3290199T4 | Finland | T4 | |
| DK2086755T4 | Denmark | T4 | |
| PL3290199T5 | Poland | T5 | |
| PL2086755T5 | Poland | T5 | |
| FI2086755T4 | Finland | T4 | |
| US20260035761A1 | United States of America | A1 | |
| ES2754621T5 | Spain | T5 |
Numbers
- Publication
- 3290199
- Application
- 17192410
Titles2
- English
- COATED STEEL STRIPS AND USING THE SAME, STAMPING BLANKS PREPARED FROM THE SAME, STAMPED PRODUCTS PREPARED FROM THE SAME, AND ARTICLES OF MANUFACTURE WHICH CONTAIN SUCH A STAMPED PRODUCT
- Polish
- Powlekane taśmy stalowe, wytwarzane z nich półfabrykaty do wytłaczania, wytwarzane z nich wytwory wytłaczane, oraz wytwarzane z nich wyroby, które zawierają taki wytwór wytłaczany
Classification
- CPC, 10
- C21D9/46
- B32B15/00
- B32B15/012
- C23C2/12
- C23C2/26
- C23C2/40
- Y10T428/12389
- Y10T428/12396
- Y10T428/12757
- C21D8/02
- IPC, 2
- B32B15 00
- C21D9 46
