Patent Grant
8066829
This application is a continuation of U.S. Ser. No. 12/834,162 filed Jul. 12, 2010, which claims priority to PCT/IB08/000,079 filed Jan. 15, 2008.
The present invention relates to methods of manufacturing hot stamped products prepared from coated steels and to various uses of the invention products such as in spot welding.
In recent years the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive industry. Fabrication of such parts or products may include the successive following main steps:
This is illustrated for example by U.S. Pat. No. 6,296,805, incorporated herein by reference.
Thanks to an alloying of the pre-coating with the steel substrate, which has the effect of creating intermetallic alloys with high melting temperature, the blanks having such coating may be heated in a temperature range where austenitizing of the metallic substrate takes place, allowing further hardening by quenching.
Heat treatments of the blanks in view of the intermetallic alloying of the coating and austenitizing of the substrate are most frequently performed in furnaces. The thermal cycles experienced by the blanks include first a heating phase whose rate is a function of parameters such as furnace temperature settings, travelling speed, blank thickness, heating process, and coating reflectivity. After this heating phase, thermal cycles generally include a holding phase, whose temperature is the regulation temperature of the furnace.
Parts or products obtained after heating, hot stamping and rapid cooling display very high mechanical resistance and may be used for structural applications, for example for automotive industry applications. These parts must be frequently welded with others and high weldability is required. This means that:
EP1380666 discloses also a process including hot stamping of Al-coated steel sheets for the fabrication of welded structural members. But the weldability needs to be further improved.
There remains a need for a process making possible to prepare stamped parts or products which are very suitable to spot welding, which are easy to paint and which display good corrosion resistance.
The inventors have discovered that certain coated steels in which a base steel strip or sheet is at least partially coated (sometimes termed “pre-coated,” this prefix indicating that a transformation of the nature of the pre-coating will take place during heat treatment before hot stamping or forming) on at least one side with a coating of either aluminum or an aluminum alloy and in which the coating has a defined thickness, are conveniently formed into shaped parts after heating in particular conditions, and thereby display particular improved weldability.
The inventors have also discovered that particular good weldability of aluminized and hot stamped parts is associated with a special succession of coating layers on the parts, proceeding from steel substrate outwards, and a controlled fraction of porosities in these layers.
The inventors have also discovered that this special disposal of layers is associated to specific heating conditions.
It is an object of the present invention to provide novel hot stamped parts which are prepared from a pre-coated steel.
It is another object of the present invention to provide novel articles of manufacture, such as a motor vehicle, which contain such stamped parts.
It is another object of the present invention to provide novel methods of making stamped parts displaying high weldability.
These and other objects, which will become apparent during the following detailed description.
The invention is implemented with certain pre-coated steel strips, which comprise a strip of base steel and a pre-coating of aluminum or an aluminum alloy on at least a part of one side of the strip of the base steel. For many applications, the strip or sheet of base steel may comprise any type of steel which may be coated with either aluminum or an aluminum alloy. However, for certain applications, such as a structural part of an automobile, it is preferred that the strip of base steel comprises a steel for providing ultra high strength on the part, higher than 1000 MPa. In such cases, it is particularly preferred that the strip of base steel comprises a boron steel.
The strip can derive, by reason of its processing, from a hot-rolling mill, and possibly may be cold-rerolled again depending on the final thickness desired. Preferred thicknesses are 0.7 to 3 mm. Typically, the strip of base steel will be stored and transported in the form of a coil both before and after the formation of the coating.
An example of a preferred steel for the strip of base steel is one having the following composition by weight:
Particularly preferably, the composition by weight of the steel in the strip of base steel is the following:
An example of preferred commercially available steel for use in the strip of base steel is 22MnB5.
Chromium, manganese, boron and carbon may be added, in the composition of the steel according to the invention, for their effect on hardenability. In addition, carbon makes it possible to achieve high mechanical characteristics thanks to its effect on the hardness of the martensite.
Aluminum is introduced into the composition, to perform deoxidation in the liquid state and to protect the effectiveness of the boron.
Titanium, the ratio of the content of which with respect to the nitrogen content should be in excess of 3.42, is introduced for example in order to prevent combining of the boron with the nitrogen, the nitrogen being combined with titanium.
The alloying elements, Mn, Cr, B, make possible a hardenability allowing hardening in the stamping tools or the use of mild hardening fluids limiting deformation of the parts at the time of thermal treatment. In addition, the composition according to the invention is optimized from the point of view of weldability. Additions of Ni and Cu, up to 0.1%, may also be performed.
The steel may undergo a treatment for globularization of sulfides performed with calcium, which has the effect of improving the fatigue resistance of the sheet.
The strip of base steel is coated (or pre-coated, this prefix indicating that a transformation of the nature of the pre-coating will take place during heat treatment before stamping) with either aluminum or an aluminum alloy, preferably with hot-dip. A typical metal bath for an Al—Si coating generally contains in its basic composition by weight, from 8% to 11% silicon, from 2% to 4% iron, the remainder being aluminum or aluminum alloy, and impurities inherent in processing. Silicon is present in order to prevent the formation of a thick iron-metallic intermetallic layer which reduces adherence and formability. Other alloying elements useful with aluminum herein include iron, and calcium, between 15 and 30 ppm by weight, including combinations of two or more thereof with aluminium. Typical composition of Al—Si coating is: Al-9.3% Si-2.8% Fe. Invention coatings are not limited to these compositions, however.
While not bound by a particular theory of operation, the inventors believe that several of the benefits of the invention are first related to a specific range of pre-coating thickness tp of 20 to 33 micrometers:
The pre-coated steel sheets or strips are then cut into blanks, and submitted to heat treatments in furnace prior to hot stamping, in order to obtain products or parts. The inventors have discovered that very good welding properties are achieved if the coating obtained on parts or products made out of blanks having undergone intermetallic alloying, austenitizing and hot stamping, displays distinctive features. It must be pointed out that this coating is different from the initial pre-coating, since the thermal treatment causes an alloying reaction with the steel substrate which modifies both the physico-chemical nature and the geometry of the pre-coating: in this regard, the inventors have discovered that particularly good weldability of aluminized and hot stamped parts is associated with the following succession of coating layers on the parts, proceeding from steel substrate outwards:
The inventors have also discovered that particular good weldability is obtained with a limited quantity of porosities in the coating layers, as will be detailed below.
In a preferred embodiment, the layers are as follows:
In a preferred embodiment the total thickness of layers (a) to (d) is greater than 30 micrometers.
In another preferred embodiment, the thickness of layer (a) is less than 15 micrometers.
The inventors have discovered that high weldability is especially obtained when layers (c) and (d) are essentially continuous; the character of essential continuity of these layers is defined in the following manner: the layers may be fully continuous. But they may be fragmented in some areas due to layer parts coming from lower or upper levels. According to the invention, this fragmentation must be limited, i.e. layers (c) and (d) must occupy at least 90% of their respective level. High weldability is obtained when less than 10% of layer (c) is present at the extreme surface of the part. Without being bound by a theory, it is thought that this particular layer disposal, in particular layer (a) and layers (c) and (d) influence the resistivity of the coating both by their intrinsic characteristics and by the effect of roughness. Thus, current flow, heat generation at the surfaces, and nugget formation in the initial stage of spot welding are affected by this particular arrangement.
This favorable layer disposition is obtained for example when aluminum- or aluminum alloy pre-coated steel sheets, whose thickness range from, e.g., 0.7 to 3 mm, are heated for 3 to 13 minutes (this dwell time includes the heating phase and the holding time) in a furnace without special atmosphere is heated to a temperature of 880 to 940° C. The invention does not require a furnace with a controlled atmosphere. Other conditions leading to such favorable layer dispositions are found in
Particularly preferred conditions are:
For sheets of total thicknesses greater or equal to 0.7 mm, and less than or equal to 1.5 mm, the preferred treatment conditions: (furnace temperature, total dwell time in the furnace) are illustrated in
For sheets of total thicknesses greater than 1.5 mm, and less than or equal to 3 mm, the preferred treatment conditions: (furnace temperature, total dwell time in the furnace) are illustrated in
The heating rate Vc is comprised between 4 and 12° C./s for producing a favorable alloyed layer disposition. Vc, depending in particular of furnace settings, is defined as the mean heating rate between 20 and 700° C. experienced by the pre-coated steel blank in the preheated furnace. The inventors have discovered that the control of Vc in this particular range allows to influence the nature and the morphology of the alloyed layers which are formed. It is here underlined that the heating rate Vc is different from the mean heating rate, which is the heating rate between room temperature and furnace holding temperature.
The inventors have discovered in a surprising manner that special heating conditions are particularly favourable for the formation of alloyed layers, leading to less porosities formation. Without being bound by a theory of the invention, it is believed that the formation of the preferred alloyed layers takes place in a particular temperature range due to the particular kinetics of alliation in this range: in this respect, it has been discovered that the control of the heating rate in the particular temperature range between 500 and 700° C. (designated here as Vc′) is especially important and that the value of Vc′ has to be comprised between 1.5 and 6° C./s.
When Vc′ is lower than 1.5° C./s, there is a risk that the kinetics of oxidation, resulting from the interaction of oxygen of the furnace atmosphere with the pre-coating surface, competes with the kinetics of alliation between the steel substrate and the pre-coating. Thus, the desired alloyed layer disposal is not obtained. Furthermore slow heating rate V′c causes a too high quantity of porosities in the coating.
When Vc′ is higher than 6° C./s, the intermetallic layer (c) has a tendency to be present in more than 10% at the extreme surface of the part, thus reducing weldability. When Vc′ is comprised between 1.5 and 6° C./s, the character of essential continuity of layers (c) and (d) is fully ensured.
Without being bound by a theory, it is thought that the porosity formation and its influence on weldability, may be explained as follows:
During spot welding of welding products, current flows initially around the porosities, which collapse progressively due to pressure and temperature elevation. Thus, the current flows through a coating whose some properties may change discontinuously, which in turn may lead to increased sparking and splashings during the welding operation.
Increased spot weldability is observed when the coating resulting from interdiffusion contains, in surfacic fraction, less than 10% of porosities. For a given area representative of the coating, this fraction is the total surface occupied by porosities, as referred to the area of the coating.
Special good weldability is experienced when the superficial layer has a controlled compacity, which means that the superficial layer (d) contains less than 20% porosities: this fraction is the surface of porosities in the superficial layer (d), as referred to the area of this superficial layer.
A special advantage arises from pre-coatings whose thickness is comprised between 20 and 33 micrometers, since this thickness range yields favorable layer disposal, and since the homogeneity of the pre-coating thickness is associated to an homogeneity of the coating formed after alliation treatment.
Heated blanks are thereafter transferred from the furnace to a die, hot stamped in a press to obtain a part or product, and cooled at a rate Vr of more than 30° C./s. The cooling rate Vr is defined here as the mean rate between the exit of the heated blank from the furnace, down to 400° C. In these conditions, austenite formed at high temperature mainly transform into martensitic or martensitic-bainitic structures with high strength.
In a preferred embodiment, the elapsed time between the exit of the heated blank and the introduction of the blank in the hot stamping press is not more than 10 seconds. Otherwise, a partial transformation from austenite is susceptible to appear: if obtaining a full martensitic structure is desired, the transfer time between the exit of the furnace and stamping should be less than 10 s.
The coating obtained has in particular the function of protecting the basic sheet against corrosion in various conditions. At the time of thermal treatment performed on a finished part or at the time of a hot-shaping process, the coating forms a layer having a substantial resistance to abrasion, wear, fatigue, shock, as well as a good resistance to corrosion and a good capacity for painting and gluing. The coating makes it possible to avoid different surface-preparation operations such as for steel sheets for thermal treatment not having any coating.
The thermal treatment applied at the time of a hot-forming process or after forming makes it possible to obtain high mechanical characteristics which can exceed 1500 MPa for mechanical resistance and 1200 MPa for yield stress. The final mechanical characteristics are adjustable and depend in particular on the martensite fraction of the structure, on the carbon content of the steel and on the thermal treatment.
The invention also concerns the use of a hot-rolled steel sheet which then can be cold-rolled and coated, for structural and/or anti-intrusion or substructure parts for a land motor vehicle, such as, for example, a bumper bar, a door reinforcement, a wheel spoke, etc.
The present invention will now be further described by way a certain exemplary embodiments which are not intended to be limiting.
i)—Conditions according to the invention: in an example of implementation, a cold rolled steel sheet, 1.2 mm thick, has been fabricated: it contains 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, 0.208% chromium, 0.005% nitrogen, 0.038% titanium, 0.004% boron, 0.003% calcium. The sheet has been pre-coated with an aluminum-based alloy with composition 9.3% silicon, 2.8% iron, the remainder being aluminum and unavoidable impurities. The thickness on each side of the sheet was controlled to be within the range (20-33) micrometers.
The sheets were afterwards cut into blanks which were heated at 920° C. for 6 mn, this time including the heating phase and the holding time. Heating rate Vc between 20 and 700° C. was 10° C./s. The heating rate Vc′ between 500 and 700° C. was 5° C./s. No special control of furnace atmosphere was performed. The blanks were transferred from the furnace to a press in less than 10 s, hot stamped and quenched in order to obtain full martensitic structures.
The parts obtained after hot-stamping are covered by a coating, 40 micrometers thick, which has a four layer structure. Starting from the steel substrate, the layers are the following:
Layers (c) and (d) are quasi-continuous, i.e. occupying at least 90% of the level corresponding to the considered layer. In particular, layer (c) does not reach the extreme surface except very exceptionally. Anyway, this layer (c) occupies less than 10% of the extreme surface.
A small number of porosities were observed in the coating, their surfacic fraction in this coating being lower than 10%. The surfacic fraction of porosities in the superficial layer (d) is lower than 20%.
ii) Conditions of reference: blanks with the same base material and pre-coating were furnace-heated in different conditions: The blanks were heated to 950° C. for 7 minutes, this time including the heating phase. Heating rate Vc was 11° C./s. Heating rate Vc′ between 500 and 700° C. was 7° C./s. These conditions correspond to a degree of alloying which is more important than in conditions (i)
Resistance spot welding was performed in the two situations i) and ii):
Resistance spot welding was performed by superposing two parts and joining them in the following conditions:
In each condition, the suitable intensity range was determined for obtaining:
Tensile tests were also performed to assess the weldability range.
While the above description is clear with regard to the understanding of the invention, the following terms as used in the following list of preferred embodiments and claims have the following noted meanings in order to avoid any confusion:
The above written description of the invention provides a manner and process of making and using it such that any person skilled in this art is enabled to make and use the same, this enablement being provided in particular for the subject matter of the appended claims, which make up a part of the original description.
Thus, the present invention provides, among other things, the following preferred embodiments:
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| Number | Date | Country | |
|---|---|---|---|
| 20110214475 A1 | Sep 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12834162 | Jul 2010 | US |
| Child | 13118077 | US | |
| Parent | PCT/IB2008/000079 | Jan 2008 | US |
| Child | 12834162 | US |
