Patent Application
20230257862
The present invention relates to a manufacturing method of a steel strip, a spot welded joint and the use of said steel strip or said spot welded joint. This invention is particularly well suited for the automotive industry due to the improvement of the Liquid Metal Embrittlement (LME) resistance property of advanced high strength steels.
In order to reduce a vehicle's weight, high strength steels are used in the automotive industry, in particular for structural parts. Such steel grades comprise alloying elements to greatly improve their mechanical properties.
During their manufacture, before coating, full hard steels undergo an annealing step which increases their strength-ductility balance. In this step, the steel is heated and maintained above its recrystallization temperature in a controlled atmosphere and then cooled to a galvanizing temperature for zinc coating on the steel surface by hot dip galvanizing method.
For example, a common practice is to heat the full hard steel from ambient temperature to a recrystallisation temperature (heating step) and then hold this temperature (soaking step). Both steps are performed in an atmosphere comprising for example 5% by volume of H2 along with 95% N2, having a dew point of −20° C. or higher. Then the steel is rapidly cooled to a desired temperature.
In the heating and soaking sections, above around 700° C., the dew point is controlled in such a way that the oxygen present in the high dew point atmosphere in the furnace diffuses into the steel sub-surface at a higher rate as compared to the diffusion of oxide forming steel alloying elements such as Manganese (Mn), Aluminum (Al), Silicon (Si) or Chromium (Cr) towards steel surface.
Presence of C along with other oxide forming steel alloying elements such as Mn, Si, Cr and Al lead to at least two types of reaction.
Firstly, as represented in
Secondly, as represented in
In a subsequent process step, these steels are usually coated by another metal or metallic alloy, such as a zinc-based coating, to improve their properties such as corrosion resistance, phosphatability, etc. The metallic coatings can be deposited by hot-dip method or electroplating method. The hot dip zinc-based coating also known as hot dip galvanizing usually contains around 0.1 to 0.4 in weight percent of aluminium. Said aluminium preferentially reacts with iron and forms an inhibition layer between the steel/coating interface. This inhibition layer is principally made of Fe and Al and forms Fe2Al5-xZnx(0<x<1), an intermetallic compound. Said inhibition layer may contain some Zn atoms.
When use in the automotive industry, the zinc coated steel sheets are usually welded together by the Resistance Spot Welding (RSW) method. During this process, liquid zinc or liquid zinc alloy penetrates the steel subsurface area and causes Liquid Metal Embrittlement (LME) of steel. It leads to a decrease of the steel ductility and causes early failure.
Concerning the decarburized layer, the thicker the decarburized layer, the better the resistance against LME. However, the decarburized layer deteriorates the mechanical properties of the steel. It is mainly due to formation of soft ferrite phase in the steel subsurface area. The decarburized layer thickness has to be controlled in such a way that it provides excellent LME resistance along with satisfying a target mechanical property. Overall, annealing atmosphere needs to be controlled in such a way that it produces an optimal depth of decarburized layer satisfying both excellent LME resistance as well as targeted mechanical properties. The purpose of this invention is to provide a solution solving the aforementioned problems.
The present invention provides a method for the manufacture of a coated steel sheet coated with a zinc-based or an aluminium-based coating, comprising:
Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.
To illustrate the invention, various embodiment and trials of non-limiting example will be described, particularly with reference to the following figures:
The invention relates to a method for the manufacture of a coated steel sheet coated with a zinc-based or an aluminium-based coating, comprising:
0.01≤Al≤1.0%,
0.07≤C≤0.50%,
0.3≤Mn≤5.0%,
V<0.2%,
0.01≤Si≤2.45%,
0.35≤Si+Al≤3.5
N≤0.01%,
P<0.02%,
S≤0.01%
B≤0.004%,
Co≤0.1%,
0.001≤Cr≤1.00%,
Cu≤0.5%,
0.001≤Mo≤0.5%,
Nb≤0.1%,
Ni≤1.0%,
Ti≤0.1%,
the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration,
In the following paragraphs, the scope of the claimed invention will be discussed and explained.
The provisioned steel has the claimed composition for the following reasons:
The steels may optionally contain elements such as Nb, B, Ni, Ti, Cu, Mo and/or Co for the following reasons.
Boron can optionally be contained in steel in quantity comprised below or equal to 0.004% by weight. By segregating at the grain boundary, B decreases the grain boundary energy and is thus beneficial for increasing the resistance to liquid metal embrittlement.
Chromium can be present with a content below or equal to 1.00% by weight. Chromium permits to delay the formation of pro-eutectoid ferrite during the cooling step after holding at the maximal temperature during the annealing cycle, making it possible to achieve higher strength level. Its content is limited to 1.00% by weight for cost reasons and to prevent excessive hardening.
Copper can be present with a content below or equal to 0.5% by weight for hardening the steel by precipitation of copper metal.
Molybdenum in quantity below or equal to 0.5% by weight is efficient for increasing the hardenability and stabilizing the retained austenite since this element delays the decomposition of austenite.
Nickel can optionally be contained in steel in quantity below or equal to 1.0% by weight so to improve the toughness.
Titanium and Niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates. However, when the Nb amount is above 0.1% and/or Ti content is greater than 0.1% by weight, there is a risk that an excessive precipitation may cause a reduction in toughness, which has to be avoided.
P and S are considered as a residual element resulting from the steelmaking. P can be present in an amount below or equal to 0.04% by weight. S can be present in an amount below or equal to 0.01% by weight.
Preferably, the chemical composition of the steel does not include Bismuth (Bi). Indeed, without willing to be bound by any theory, it is believed that if the steel sheet comprises Bi, the wettability decreases and therefore the coating adhesion.
For a proper understanding of the exposed invention, few terms will be defined. The dew point is the temperature to which air must be cooled to become saturated with water vapor. In the steelmaking, Ac1 corresponds to the temperature at which the Austenite start to form during heating. Ms corresponds to the temperature at which, upon rapid cooling, Austenite starts to form Martensite.
The several steps of the process can take place in furnaces as represented in
The pre-heating step generally occurs after the steel has been cold-rolled also known as Full Hard condition. During this pre-heating, the steel sheet is heated from room temperature to a temperature T1 between 550° C. and Ac1+50° C. in a non-oxidizing atmosphere. It can be done in any heating means able to heat the steel at a temperature T1 without producing iron oxide or a in limited amount. For example, this step can be done in a RTF (Radiant Tube Furnace) having an atmosphere made up of N2, H2 and unavoidable impurities, in an heating by induction mean or in a DFF (Direct-Fired Furnace) having an atmosphere having an air/combustible gas ratio <1. However, it is possible in a DFF comprising several zones, e.g. 5 zones, to have a ratio air/combustible gas >1 in the last or the two last zones.
During the heating step, the steel sheet is heated from a temperature T1 to a recrystallisation temperature T2 between 720° C. and 1000° C. in an atmosphere A1, comprising between 0.1 and 15% by volume of H2 with the balance made up of an inert gas, H2O, O2 and unavoidable impurities having a dew point DP1 between −10° C. and +30° C. Nitrogen can be used as an inert gas.
During the soaking step, the steel sheet is heated at said recrystallisation temperature T2 in an atmosphere A2, comprising between 0.1 and 15% by volume of H2 with the balance made up of an inert gas, H2O, O2 and unavoidable impurities having a dew point DP2 between −30° C. and 0° C., said dew points DP1 being higher than said dew point DP2. Nitrogen can be used as inert gas.
The atmospheres A1 and A2 can be achieved by using preheated steam and incorporating the N2—H2 gases in a furnace equipped with pyrometer, H2 and dew point detectors in the different sections monitoring the H2, atmosphere dew point and temperature.
The cooling can be achieved in an atmosphere comprising 20 to 50% of H2 along with N2. This gas mixture has been blown on the steel surface using a high-speed fan. The cooling can also be achieved by any other cooling means such as cooling rolls.
In the following part, without being bound by any theory, the physical phenomenon in the heating and soaking steps will be explained.
In the heating step, the gradual increase of temperature along with the comparatively high dew point permits to have a high pO2 (partial pressure of oxygen) leading to the diffusion of the oxygen into the steel. This increased oxygen diffusion has two major consequences. Firstly, it permits deep decarburization of the steel sub-surface by the reaction with interstitial element carbon. Secondly, oxygen reacts with substitutional oxide forming elements such as Mn, Si, Al and Cr and forms internal oxide in the steel sub-surface area which reduces the amount of alloying element available to form surface oxides. Those internal oxides preferentially form on the grain boundary area due to a faster diffusion of these alloying elements.
At the end of the heating step, the steel sub-surface area comprises:
Those values are only given to get an order of magnitude. Parameters such as the heating time, temperature at the end of the heating, steel carbon content as well as the dew point which determines the pO2 influence the thickness of said complete as well as partially decarburized layers.
In the soaking step, as compared to the heating step, the temperature is higher, but the dew point is lower. It has several effects on the steel sub-surface area.
Due to the comparatively lower dew point at the soaking section, the amount of oxygen is also lower and thus can only diffuse to a limited (smaller) depth into the steel sub-surface area causing a decarburization reaction in a limited depth of steel sub-surface area. In the meantime, carbon atoms diffuse from the bulk to the carbon depleted area of the steel sub-surface area (partially decarburized layer followed by decarburized layer). In fact, carbon atoms present in the partially decarburized area diffuse into the decarburised area and the partially decarburized area is back filled with the carbon atoms from the bulk. Thus, it produces a decarburized layer very close to steel surface. The said decarburization reaction depends on several factors such as the soaking temperature, the dew point (pO2), the soaking duration and the amount of carbon present in the bulk steel.
Consequently, at the end of the soaking step, the steel sub-surface area comprises:
Those values are only given to get an order of magnitude.
Due to a higher partial pressure of oxygen (pO2) in the heating section, higher amount of O2 can easily diffuse in the steel sub-surface area and forms internal oxide and thus trap the Si, Mn, Cr, Al into much deeper in the sub-surface area. This phenomenon occurs in early stage of recrystallization in the heating section. In the soaking section, mostly grain growth and formation of large ferrite grains in the steel sub-surface area occur.
Due to the formation of internal oxides deeper into the steel sub-surface area followed by the grain growth, a ferrite layer free from internal oxides has been formed at the steel surface. This layer can easily react with the aluminium in the coating bath during galvanizing and forms a satisfying inhibition layer.
Contrary to the state of the art, in this annealing process, the dew point of the heating step is higher than of the soaking step permitting improvement of the steel properties in terms of liquid metal embrittlement (LME) resistance as previously explained. Apparently, the invention also has the advantage to produce a controlled depth of complete decarburized layer, having a carbon weight-percent of less than 5 percent of the carbon weight-percent of the bulk steel.
Preferably, the dew point DP2 is between −25° C. and +10° C. Preferably, the dew point DP2 is between −20° C. and 0° C. Preferably, the dew point DP2 is between −25° C. and −5° C. Even more preferably, the dew point is between −25° C. and −5° C.
Preferably, said cooling step, said steel sheet is cooled down to a temperature T3 between Ms and Ms+150° C. and maintained at T3 for at least 40 seconds in an atmosphere A3 comprising between 1 and 30% by volume of H2 and an inert gas, having a dew point DP3 below or equal to −40° C. Even more preferably, said temperature T3 is between Ms+10° C. and Ms+150° C. This permits to have a partitioned microstructure.
Preferably, after said cooling step iv), said steel sheet is further cooled down to a temperature TQT between (Ms−5° C.) and (Ms−170° C.) and undergoes then a reheating step v) wherein said steel sheet is reheated up to a temperature T4 between 300 and 550° C. during 30 s to 300 s. Such step is also known as a partitioning step. Even more preferably, said steel sheet is optionally held at TQT for a duration comprised between 2 and 8 s. Even more preferably, said steel sheet is reheated up to a temperature T4 between 330 and 490° C.
Preferably, after said cooling step iv) and said reheating step v), an equalizing step vi) said steel strip is heated at a temperature between 300° C. and 500° C. in an atmosphere A4 comprising between 1 and 30% by volume of H2 and at least an inert gas, having a dew point DP4 below or equal to −40° C.
Preferably, said steel sheet in step A) has at least in weight percent: 0.001≤Cr+Mo≤1.000%.
Preferably, said heating and soaking steps last between 100 and 500 seconds. Preferably, in said heating and soaking steps, the atmosphere A1 and A2 comprise between 3 and 8% by volume of H2.
Preferably, said DP1 is between 5° C. and 40° C. higher than DP2. Even more preferably, said DP1 is between 10° C. and 30° C. higher than DP2.
Preferably, in said step C) said coating is done by electroplating or hot-dip coating.
Preferably in said step C), said coating is done by hot-dip coating method and said steel strip is set at a temperature between 5° C. to 10° C. above a galvanizing bath, having an aluminium content between 0.15 and 0.40 weight percent, being maintained at a temperature between 450° C. to 470° C.
Preferably in said step C), said coating is done by hot-dip coating method and said steel strip is set at a temperature between 5° C. to 10° C. above a galvanizing bath, having an aluminium content between 0.09 and 0.15 weight percent, being maintained at a temperature between 450° C. to 470° C. and is then heated to a temperature between 470° C. and 550° C. after exiting said galvanizing bath. Such process steps permit production of a galvannealed steel strip.
As shown in
As illustrated in
Said internal oxide layer is on the exterior portion of the decarburised layer, closer to the inhibition layer as illustrated in
As illustrated in
The internal oxide layer cannot be thicker than the decarburised layer. Consequently, if the decarburised layer has a thickness of “x” μm, x being between 5 and 12 μm, the internal oxide layer has a thickness between 2 and “x” μm. Said internal oxide layer is on the exterior portion of the decarburised layer, closer to the inhibition layer as illustrated in
Preferably, said steel strip has a thickness between 0.5 mm and 3.0 mm.
Preferably, said steel strip has an ultimate tensile strength (UTS) greater than 900 MPa.
The invention also relates to a spot welded joint of at least two metal sheets comprising at least a steel sheet as previously described, said joint containing zero crack having a size above 100 μm.
Preferably, said spot welded joint comprises two or three metal sheets. Preferably, said spot welded joint comprises also an aluminium sheet or a steel sheet.
The invention also relates to the use of any previously described coated steel sheet or of any previously described spot welded joint for the manufacture of automotive vehicle.
The following section deals with experimental results exhibiting the improved surface and subsurface properties. The experiments have been performed on two different grades of steel (Steel A and Steel B) having a strip thickness between 1.4 to 1.6 mm.
The different experimental parameters are reported in Table 1.
A first set of experiments (A1 and A2*) was conducted to show the influence of the dew points difference in the heating and soaking sections on the decarburization behaviour of the steel, on a first steel grade (Steel A). The steel was annealed followed by galvanized in a Zn-0.20 wt. % Al coating bath as per the thermal cycles reported in
A second experiment (A3*) was conducted on Steel A. The steel was annealed followed by a galvanized in a Zn-0.129 wt. % Al coating bath as per the thermal cycles reported in
A third experiment (BP) was carried out on a different steel grade (Steel B). The steel was annealed followed by a galvanized in a Zn-0.20 wt. % Al coating bath as per the thermal cycles reported in
A fourth experiment (B2*) was also conducted on Steel B. The steel was annealed followed by a galvanized in a Zn-0.129 wt. % Al coating bath as per the thermal cycles reported in
Experiments A2*, A3*, B1* and B2* are according to the present invention wherein the dew point of the heating section is higher than of the soaking section.
The micrograph A2* of the steel subsurface area as per the present invention presents:
On the contrary, the micrograph A1 of the steel subsurface, as per the state of the art, shows only a steel bulk 18 and a partially decarburized layer 17 of around 45 μm. This comparison exhibits the benefits of the claimed method on the formation of a decarburized layer in the steel sub-surface area which is favourable in order to obtain the target mechanical as well as Liquid Metal Embrittlement resistance properties.
This experiment exhibits a preferable claimed method wherein DP1 is between 5° C. and 30° C. higher than DP2
As shown in
In experiments A3* and B2*, galvannealed coated Steel A and Steel B respectively were produced after galvanizing in Zn-0.129 wt. % Al bath followed by post coating heat treatment (also known as galvannealing treatment) at 480° C. for Steel A and 510° C. for Steel B.
The Liquid Metal Embrittlement (LME) susceptibility of above galvanized and galvannealed coated steel produced as per the thermal cycles reported in Table 1 were evaluated by resistance spot welding method on a steel produced in the condition of the A2*, A3*, B1* and B2* experiments. The type of the electrode was ISO Type B with a face diameter of 6 mm; the force of the electrode was 5 kN and the flow rate of water of was 1.5 g·min−1. The welding cycle has been reported in Table 2:
The LME crack resistance behaviour was evaluated using 3-layer stack-up condition. In this condition, three coated steel sheets were welded together by resistance spot welding as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/054322 | May 2020 | WO | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/053333 | 4/22/2021 | WO |
