PROCESS FOR MANUFACTURING A GALVANNEALED STEEL SHEET BY DFF REGULATION

Description

The present invention relates to a process for manufacturing a hot-dip galvannealed steel sheet having a TRIP microstructure.

To meet the requirement of lightening power-driven ground vehicle structures, it is known to use TRIP steels (the term TRIP standing for transformation-induced plasticity), which combine very high mechanical strength with the possibility of very high levels of deformation. TRIP steels have a microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite, which allows them to achieve tensile strength from 600 to 1000 MPa. This type of steel is widely used for production of energy-absorbing parts, is such as for example structural and safety parts such as longitudinal members and reinforcements.

Before the delivery to car-makers, steel sheets are coated with a zinc-based coating generally performed by hot-dip galvanizing, in order to increase the resistance to corrosion. After leaving the zinc bath, galvanized steel sheets are often submitted to an annealing which promotes the alloying of the zinc coating with the iron of the steel (so-called galvannealing). This kind of coating made of a zinc-iron alloy offers a better weldability than a zinc coating.

Most of TRIP steel sheets are obtained by adding a large amount of silicon to steel. Silicon stabilizes the ferrite and the austenite at room temperature, and prevents residual austenite from decomposing to form carbide. However, TRIP steel sheets containing more than 0.2% by weight of silicon, are galvanized with difficulty, because silicon oxides are formed on the surface of the steel sheet during the annealing taking place just before the coating. These silicon oxides show a poor wettability toward the molten zinc, and deteriorate the plating performance of the steel sheet.

To solve this problem, it is known to use TRIP steel having low silicon content (less than 0.2% by weight). However, this has a major drawback: a high level of tensile strength, that is to say about 800 MPa, can be achieved only if the content of carbon is increased. But, this has the effect to lower the mechanical resistance of the welded points.

On the other hand, alloying speed during the galvannealing process is strongly slowed down whatever the TRIP steel composition because of external selective oxidation acting as a diffusion barrier to iron, and the temperature of the galvannealing has to be increased. The increase of the temperature of the galvannealing is detrimental to the preservation of the TRIP effect because of the decomposition of the residual austenite at high temperature. In order to preserve the TRIP effect, a large quantity of molybdenum (more than 0.15% by weight) has to be added to the steel, so that the precipitation of carbide can be delayed. However, this has an effect on the cost of the steel sheet.

Indeed, the TRIP effect is observed when the TRIP steel sheet is being deformed, as the residual austenite is transformed into martensite under the effect of the deformation, and the strength of the TRIP steel sheet increases.

The purpose of the present invention is therefore to remedy the aforementioned drawbacks and to propose a process for hot-dip galvannealing a steel sheet having a high silicon content (more than 0.5% by weight) and a TRIP microstructure showing high mechanical characteristics, that guarantees a good wettability of the surface steel sheet and no non-coated portions, and thus guarantees a good adhesion and a nice surface appearance of the zinc alloy coating on the steel sheet, and that preserves the TRIP effect.

The first subject of the invention is a process for manufacturing a hot-dip galvannealed steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite, said process comprising the steps consisting in:

• • providing a steel sheet whose composition comprises, by weight:

0.01≦C≦0.22%

0.50≦Mn≦2.0%

0.5<Si≦2.0%

0.0055≦Al≦2.0%

Mo<0.01%

Cr≦1.0%

P<0.02%

Ti≦0.20%

V≦0.40%

Ni≦1.0%

Nb≦0.20%,

• • the balance of the composition being iron and unavoidable impurities resulting from the smelting,
  • oxidizing said steel sheet in order to form a layer of iron oxide on the surface of the steel sheet, and to form an internal oxide of at least one type of oxide selected from the group consisting of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide comprising Si and Al, complex oxide comprising Al and Mn, and complex oxide comprising Si, Mn and Al is formed,
  • reducing said oxidized steel sheet in order to reduce the layer of iron oxide,
  • hot-dip galvanising said reduced steel sheet to form a zinc-based coated steel sheet, and
  • subjecting said zinc-based coated steel sheet to an alloying treatment to form a galvannealed steel sheet.

In order to obtain the hot-dip galvannealed steel sheet having a TRIP microstructure according to the invention, a steel sheet comprising the following elements is provided:

• • Carbon with a content between 0.01 and 0.22% by weight. This element is essential for obtaining good mechanical properties, but it must not be present in too large amount in order not to tear the weldability. To encourage hardenability and to obtain a sufficient yield strength R_e, and also to form stabilized residual austenite the carbon content must not be less than 0.01% by weight. A bainitic transformation takes place from an austenitic microstructure formed at high temperature, and ferrite/bainite lamellae are formed. Owing to the very low solubility of carbon in ferrite compared with austenite, the carbon of the austenite is rejected between the lamellae. Owing to silicon and manganese, there is very little precipitation of carbide. Thus, the interlamellar austenite is progressively enriched with carbon without any carbides being precipitated. This enrichment is such that the austenite is stabilized, that is to say the martensitic transformation of this austenite does not take place upon cooling down to room temperature.
  • Manganese with a content between 0.50 and 2.0% by weight. Manganese promotes hardenability, making it possible to achieve a high yield strength R_e. Manganese promotes the formation of austenite, contributes to reducing the martensitic transformation start temperature Ms and to stabilizing the austenite. However, it is necessary to avoid the steel having too high a manganese content in order to prevent segregation, which may be demonstrated during heat treatment of the steel sheet. Furthermore, an excessive addition of manganese causes the formation of a thick internal manganese oxide layer which causes brittleness, and the adhesion of the zinc based coating will not be sufficient.
  • Silicon with a content of more than 0.5% by weight, preferably more than 0.6% by weight, and less or equal to 2.0% by weight. Silicon improves the yield strength R_e of the steel. This element stabilizes the ferrite and the residual austenite at room temperature. Silicon inhibits the precipitation of cementite upon cooling from austenite, considerably retarding the growth of carbides. This stems from the fact that the solubility of silicon in cementite is very low and the fact that silicon increases the activity of the carbon in austenite. Thus, any cementite nucleus that forms will be surrounded by a silicon-rich austenitic region, and rejected to the precipitate-matrix interface. This silicon-enriched austenite is also richer in carbon, and the growth of the cementite is slowed down because of the reduced diffusion resulting from the reduced carbon activity gradient between the cementite and the neighbouring austenitic region. This addition of silicon therefore contributes to stabilizing an amount of residual austenite sufficient to obtain a TRIP effect. During the annealing step to improve the wettability of the steel sheet, internal silicon oxides and complex oxide comprising silicon and/or manganese and/or aluminum are formed and dispersed under the surface of the sheet. However, an excessive addition of silicon causes the formation of a thick internal silicon oxide layer and possibly complex oxide comprising silicon and/or manganese and/or aluminum which causes brittleness and the adhesion of the zinc based coating will not be sufficient.
  • Aluminum with a content between 0.005 and 2.0% by weight. Like the silicon, aluminum stabilizes ferrite and increases the formation of ferrite as the steel sheet cools down. It is not very soluble in cementite and can be used in this regard to avoid the precipitation of cementite when holding the steel at a bainitic transformation temperature and to stabilize the residual austenite. A minimum amount of aluminum is required in order to deoxidize the steel.
  • Molybdenum with a content less than 0.01% by weight, and preferably not exceeding 0.006% by weight. Conventional process requires the addition of Mo to prevent carbide precipitation during re-heating after galvanizing. Here, thanks to the internal oxidation of silicon, manganese and aluminum, the alloying treatment of the galvanized steel sheet can be performed at a lower temperature than that of conventional galvanized steel sheet comprising no internal oxide. Consequently, the content of molybdenum can be reduced and be less than 0.01% by weight, because it is not necessary to delay the bainitic transformation as it is the case during the alloying treatment of conventional galvanized steel sheet.
  • Chromium with a content not exceeding 1.0% by weight. The chromium content must be limited in order to avoid surface appearance problems when galvanizing the steel
  • Phosphorus with a content not exceeding 0.02% by weight, and preferably less than 0.010% by weight. Phosphorus in combination with silicon increases the stability of the residual austenite by suppressing the precipitation of carbides.
  • Titanium with a content not exceeding 0.20% by weight. Titanium improves the yield strength of R_e, however its content must be limited to 0.20% by weight in order to avoid degrading the toughness.
  • Vanadium with a content not exceeding 0.40% by weight. Vanadium improves the yield strength of R_e by grain refinement, and improves the weldability of the steel. However, above 0.40% by weight, the toughness of the steel is degraded and there is a risk of cracks appearing in the weld zones.
  • Nickel with a content not exceeding 1.0% by weight. Nickel increases the yield strength of R_e. Its content is generally limited to 1.0% by weight because of its high cost.
  • Niobium with a content not exceeding 0.20% by weight. Niobium promotes the precipitation of carbonitrides, thereby increasing the yield strength of R_e. However, above 0.20% by weight, the weldability and the hot formability are degraded.

The balance of the composition consists of iron and other elements that are usually expected to be found and impurities resulting from the smelting of the steel, in proportions that have no influence on the desired properties.

The steel sheet having the above composition is first subjected to an oxidation followed by a reduction, before being hot-dip galvanized in a bath of molten zinc and heat-treated to form said galvannealed steel sheet.

The aim is to form an oxidized steel sheet having an outer layer of iron oxide with a controlled thickness which will protect the steel from the selective outer oxidation of silicon, manganese and aluminum, while the steel sheet is annealed before the hot-dip galvanization.

Said oxidation of the steel sheet is performed under conditions that allow the formation, on the surface of the steel sheet, of a layer of iron oxide containing no superficial oxides selected from the group consisting of silicon oxide, manganese oxide, aluminum oxide, complex oxide comprising silicon and/or manganese and/or aluminum. During this step, internal selective oxidation of silicon, manganese and aluminum will develop under the iron oxide layer, and leads to a deep depletion zone in metallic silicon, manganese and aluminum which will minimize the risk of superficial selective oxidation when the further reduction will be achieved. A layer of an internal oxide of at least one type of oxide selected from the group consisting of silicon oxide, manganese oxide, aluminum oxide, complex oxide comprising Si and Mn, complex oxide comprising Si and Al, complex oxide comprising Mn and Al and complex oxide comprising Si, Mn and Al is thus formed.

The oxidation is preferably performed by heating said steel sheet from ambient temperature to a heating temperature T1 which is between 680 and 800° C., in a direct flame furnace where the atmosphere comprises air and fuel, with a ratio air-to-fuel preferably between 1 and 1.2.

When the temperature T1 is above 800° C., the iron oxide layer formed on the surface of the steel sheet will contain manganese coming from the steel, and the wettability will be impaired. If the temperature T1 is below 680° C., the internal oxidation of silicon, manganese and aluminum will not be favoured, and the galvanizability of the steel sheet will be insufficient.

An atmosphere having a ratio air-to-fuel less than 1 leads to the formation of superficial oxidation of silicon, manganese and aluminum, and thus a superficial layer of oxides selected from the group consisting of silicon oxide, manganese oxide, aluminum oxide, and complex oxide comprising silicon and/or manganese and/or aluminum, possibly in combination with iron oxide is formed, and the wettability is impaired. However, with a ratio air-to-fuel above 1.2, the layer of iron oxide is too thick, and will not be completely reduced. Thus, the wettability will also be impaired.

When leaving the direct flame furnace, the oxidized steel sheet is reduced in conditions permitting the achievement of the complete reduction of the iron oxide into iron. This reduction step can be performed in a radiant tube furnace or in a resistance furnace. Said oxidized steel sheet is thus heat treated in an atmosphere comprising preferably more than 15% by volume of hydrogen, the balance being nitrogen and unavoidable impurities. Indeed, if the content of hydrogen in the atmosphere is less than 15% by volume, the layer of iron oxide can be insufficiently reduced and the wettability is impaired.

Said oxidized steel sheet is heated from the heating temperature T1 to a soaking temperature T2, then it is soaked at said soaking temperature T2 for a soaking time t2, and is finally cooled from said soaking temperature T2 to a cooling temperature T3.

Said soaking temperature T2 is preferably between 770 and 850° C. When the steel sheet is at the temperature T2, a dual phase microstructure composed of ferrite and austenite is formed. When T2 is above 850° C., the volume ratio of austenite grows too much, and external selective oxidation occurs on the steel surface. But when T2 is below 770° C., the time required to form a sufficient volume ratio of austenite is too high.

In order to obtain the desired TRIP effect, sufficient austenite must be formed during the soaking step, so that sufficient residual austenite is s maintained during the cooling step. The soaking is performed for a time t2, which is preferably between 20 and 180 s. If the time t2 is longer than 180 s, the austenite grains coarsen and the yield strength R_e of the steel after forming will be limited. Furthermore, the hardenability of the steel is low. However, if the steel sheet is soaked for a time t2 less than 20 s, the proportion of austenite formed will be insufficient and sufficient residual austenite and bainite will not form when cooling.

The reduced steel sheet is finally cooled at a cooling temperature T3 near the temperature of the bath of molten zinc, in order to avoid the cooling or the re-heating of said bath. T3 is thus preferably between 460 and 510° C. Therefore, a zinc-based coating having a homogenous microstructure can be obtained.

When the steel sheet is cooled, it is hot dipped in the bath of molten zinc whose temperature is preferably between 450 and 500° C. This bath can contain 0.08 to 0.135% by weight of dissolved aluminium, the balance being zinc and unavoidable impurities. Aluminum is added in the bath in order to deoxidize the molten zinc, and to make it easier to control the thickness of the zinc-based coating. In that condition, precipitation of delta phase (FeZn₇) is induced at the interface of the steel and of the zinc-based coating. When leaving the bath, the steel sheet is wiped by projection of a gas, in order to adjust the thickness of the zinc-based coating. This thickness, which is generally between 3 and 10 μm, is determined according to the required resistance to corrosion.

The hot-dip galvanized steel sheet is finally heat-treated so that a coating made of a zinc-iron alloy is obtained, by diffusion of the iron from steel to the zinc of the coating. This alloying treatment can be performed by maintaining said steel sheet at a temperature T4 between 460 and 510° C. for a soaking time t4 between 10 and 30 s. Thanks to the absence of external selective oxidation of silicon, manganese and aluminum, this temperature T4 is lower than the conventional alloying temperatures. For that reason, large quantities of molybdenum to the steel are not required, and the content of molybdenum in the steel can be limited to less than 0.01% by weight. If the temperature T4 is below 460° C., the alloying of iron and zinc is not possible. If the temperature T4 is above 510° C., it becomes difficult to form stable austenite, because of the unwished carbide precipitation, and the TRIP effect cannot be obtained. The time t4 is adjusted so that the average iron content in the alloy is between 8 and 12% by weight, which is a good compromise for improving the weldability of the coating and limiting the powdering while shaping.

The invention will now be illustrated by examples given by way of non-limiting indication and with reference to FIGS. 1 and 2.

Trial was carried out using samples A and B coming from 0.8 mm thick sheet manufactured from a steel sheet whose composition is given in table I.

Samples A and B are pre-heated from ambient temperature (20° C.) to 750° C., in a direct flame furnace. They are subsequently and continuously annealed in a radiant tube furnace, where they are heated from 750° to 800° C., then they are soaked at 800° C. for 60 s, and finally they are cooled to 460° C. The atmosphere in the radiant tube furnace comprises 30% by volume of hydrogen, the balance being nitrogen and unavoidable impurities.

After cooling, samples A and B are hot dip galvanized in a molten zinc-based bath comprising 0.12% by weight of aluminium, the balance being zinc and unavoidable impurities. The temperature of said bath is 460° C. After wiping with nitrogen and cooling the zinc-based coating, the thickness of the zinc-based coating is 7 μm.

First, the aim is to compare the wettability and the adherence of these samples, when the air-to-fuel ratio in the direct flame furnace fluctuates. The air-to-fuel ratio is 0.90 for sample A, and 1.05 according to the invention for sample B. The results are shown in table II.

The wettability is visually controlled by an operator. The adherence of the coating is also visually controlled after a 180° bending test of samples.

FIG. 1 is a photography of sample A after the pre-heating step and before the annealing step, and FIG. 2 is a photography of sample B after the pre-heating step and before the annealing step.

Then, the aim is to show the effect of the internal selective oxidation of silicon and manganese on the temperature of alloying. Thus, the temperature of alloying treatment applied to sample B in order to obtain a galvannealed steel sheet according to the invention is compared with the temperature of alloying of sample A.

Sample B which has been hot dip galvanized is then subjected to an alloying treatment by heating it to 480° C., and by maintaining it at this temperature for 19 s. The inventors have checked that the TRIP microstructure of the obtained hot dip galvannealed steel sheet according to the invention was not lost by this alloying treatment.

In order to obtain the alloying of the zinc-based coating of sample A, it is necessary to heat it to 540° C., and to maintain it at this temperature for 20 s

With such a treatment, the inventors have checked that carbide precipitation occurs, residual austenite is no more kept during cooling down to room temperature and that the TRIP effect has disappeared.