KR20100055389A - Process for manufacturing a galvannealed steel sheet by dff regulation - Google Patents

Abstract

The invention deals with a process for manufacturing a hot-dip galvannealed steel sheet having a TRIP microstructure, and comprising, by % by weight, 0.01 <= C <= 0.22%, 0.50 <= Mn <= 2.0%, 0.5 < Si <= 2.0%, 0.005 <= 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, said process comprising the steps consisting in:-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 comprising Si, Mn and Al,-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.

Description

Production method of alloyed galvanized steel sheet by DFF control {PROCESS FOR MANUFACTURING A GALVANNEALED STEEL SHEET BY DFF REGULATION}

The present invention relates to a method for producing an alloyed hot-dip galvannealed steel sheet having a TRIP microstructure.

In order to meet the demand for lighter power-driven ground vehicle structures, TRIP steel (term TRIP stands for transformation-induced plasticity) combines very high mechanical strength with very high levels of deformability. It is known. TRIP steels have a microstructure comprising ferrite, residual austenite and optionally martensite and / or bainite, and thus may have a tensile strength of 600 to 1,000 MPa. Steels of this kind are widely used for the production of energy-absorbing parts such as structural and safety parts such as longitudinal members and reinforced parts.

Before being transported to an automobile manufacturer, the steel sheet is coated with a zinc-based coating, which is usually done by hot dip galvanizing to increase the corrosion resistance. After removal from the zinc bath, the galvanized steel sheet is often annealed to improve alloying of the iron and zinc coatings of the steel (so-called galvanized zinc). Coatings of this kind made of zinc-iron alloys provide better weldability than zinc coatings.

Most TRIP steels are obtained by adding large amounts of silicon to the steel. Silicon stabilizes ferrite and austenite at room temperature and prevents residual austenite from decomposing to form carbides. However, TRIP steel sheets comprising more than 0.2% by weight of silicon are difficult to galvanize because silicon oxide is formed on the surface of the steel sheet during annealing just before coating. Such silicon oxides exhibit poor wettability with respect to molten zinc and degrade the plating performance of the steel sheet.

To solve this problem, the use of TRIP steels with a low silicon content (less than 0.2% by weight) is known. However, this has the significant disadvantage that a high level of tensile strength, ie a tensile strength of about 800 MPa can be achieved only when the carbon content is increased. However, this has the effect of lowering the mechanical resistance of the welding point.

On the other hand, the alloying rate during the alloying galvanization process becomes very slow, regardless of the TRIP steel composition, due to the external selective oxidation acting as a diffusion barrier to iron, and the temperature of the alloying galvanizing must be increased. The increase in temperature of the galvanized alloy is detrimental to the preservation of the TRIP effect due to the decomposition of residual austenite at high temperatures. In order to preserve the TRIP effect, large amounts of molybdenum (greater than 0.15% by weight) must be added to the steel, as a result of which the precipitation of carbides can be delayed. However, this affects the cost of the steel sheet.

In fact, since the residual austenite is transformed into martensite under the influence of deformation, the TRIP effect is observed when the TRIP steel sheet is deformed, and the strength of the TRIP steel sheet increases.

Therefore, it is an object of the present invention to eliminate the above drawbacks and to alloy hot-dip galvanized steel sheets having a TRIP microstructure exhibiting high silicon content (greater than 0.5% by weight) and high mechanical properties. It is proposed a method of ensuring wettability and absence of uncoated parts, thus ensuring good adhesion and good surface appearance of the zinc alloy coating on the steel sheet and also preserving the TRIP effect.

A first subject of the invention is a process for the production of an alloyed hot dip galvanized steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite and / or bainite.

Composition is in weight percent,

0.01 ≤ C ≤ 0.22%

0.50 ≤ Mn ≤ 2.0%

0.5 <Si ≤ 2.0%

0.005 ≤ Al ≤ 2.0%

Mo <0.01%

Cr ≤ 1.0%

P <0.02%

Ti ≤ 0.20%

V ≤ 0.40%

Ni ≤ 1.0%

Nb ≤ 0.20%

Providing a steel sheet, wherein the balance of the composition is an inevitable impurity due to iron and smelting;

A layer of iron oxide is formed on the surface of the steel sheet and Si oxide, Mn oxide, Al oxide, a composite oxide comprising Si and Mn, a composite oxide comprising Si and Al, a composite oxide comprising Al and Mn, and Si, Oxidizing the steel sheet so that an internal oxide of at least one type of oxide selected from the group consisting of complex oxides including Mn and Al is formed;

Reducing the oxidized steel sheet to reduce the iron oxide layer;

Hot-dip galvanizing the reduced steel sheet to form a zinc-based coated steel sheet; And

Alloying the zinc-based coated steel sheet to form an alloyed galvanized steel sheet

It is a method of producing an alloyed hot dip galvanized steel sheet comprising a.

In order to obtain an alloyed hot dip galvanized steel sheet having a TRIP microstructure according to the present invention, a steel sheet comprising the following elements is provided:

0.01 to 0.22% by weight of carbon. This element is essential for obtaining good mechanical properties, but it should not be present in too large an amount so as not to degrade wettability. In order to promote hardenability, obtain sufficient yield strength (R _e ) and form stabilized residual austenite, the carbon content should not be less than 0.01% by weight. The bainite transformation takes place from the austenitic microstructure formed at high temperatures, and ferrite / bainite lamellae are formed. Due to the very low solubility of carbon in ferrite as compared to austenite, carbon of austenite is rejected between the lamellars. Due to the silicon and manganese, there is almost no precipitation of carbides. Thus, without precipitation of any carbides, the interlamellar austenite gradually increases in carbon. As such carbon increases, austenite is stabilized, that is, martensite transformation of this austenite does not occur upon cooling to room temperature.

0.50 to 2.0% by weight manganese. Manganese improves the hardenability, making it possible to achieve high yield strength (R _e ). Manganese promotes the formation of austenite, thereby lowering the martensite transformation start temperature (Ms) and contributing to stabilizing austenite. However, in order to prevent segregation, the steel needs not to have too high manganese content, which can be proved during the heat treatment of the steel sheet. Moreover, excessive addition of manganese forms a thick inner manganese oxide layer that causes brittleness and may result in insufficient adhesion of the zinc-based coating.

More than 0.5% by weight, preferably more than 0.6% by weight and not more than 2.0% by weight of silicon. Silicon improves the yield strength (R _e ) of the steel. This element stabilizes ferrite and residual austenite at room temperature. Silicon suppresses the precipitation of cementite upon cooling from austenite and significantly inhibits the growth of carbides. This is due to the fact that the solubility of silicon in cementite is very low and that silicon increases the activity of carbon in austenite. Thus, any cementite nuclei formed are surrounded by silicon-rich austenite regions and are discharged to the precipitate-matrix interface. This silicon-rich austenite is also carbonaceous and slows the growth of cementite due to the reduced diffusion due to the reduced carbon gradient between the cementite and adjacent austenite regions. Therefore, the addition of such silicon contributes to stabilizing residual austenite in an amount sufficient to obtain the TRIP effect. During the annealing step to improve the wettability of the steel sheet, a composite oxide comprising silicon and / or manganese and / or aluminum is formed and dispersed under the surface of the steel sheet. However, excessive addition of silicon forms a thick internal silicon oxide layer and possibly a complex oxide comprising silicon and / or manganese and / or aluminum, causing brittleness and insufficient adhesion of the zinc-based coating. have.

0.005 to 2.0% by weight of aluminum. Like silicon, aluminum stabilizes ferrite and increases the formation of ferrite when the steel sheet is cooled. Silicon does not dissolve well in cementite and in this regard can be used to avoid precipitation of cementite and to stabilize residual austenite when maintaining the steel at bainite transformation temperature. A minimum amount of aluminum is required to deoxidize the steel.

Molybdenum less than 0.01, preferably up to 0.006% by weight. In the conventional method, addition of Mo is required to prevent carbide precipitation during reheating 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 in the case of the conventional galvanized steel sheet containing no internal oxide at all. As a result, since there is no need to delay the bainite transformation as has been done during the alloying process of the conventional galvanized steel sheet, the content of molybdenum can be reduced and can be made less than 0.01% by weight.

Up to 1.0% by weight of chromium. When galvanizing steel, the chromium content should be limited to avoid surface appearance problems.

Phosphorus less than 0.02% by weight, preferably less than 0.010% by weight. Phosphorus, together with silicon, increases the stability of residual austenite by inhibiting precipitation of carbides.

Up to 0.20% by weight of titanium. Titanium improves the yield strength (R _e ), but in order to avoid degradation of toughness, the content of titanium should be limited to 0.20% by weight.

Vanadium up to 0.40% by weight. Vanadium improves the yield strength (R _e ) by grain refinement and improves the wettability of the steel. However, at more than 0.40% by weight, the toughness of the steel deteriorates and there is a risk of cracking in the weld zone.

Up to 1.0 weight percent nickel. Nickel increases the yield strength (R _e ). The content of nickel is generally limited to 1.0% by weight, due to its high cost.

Up to 0.20% by weight of niobium. Niobium improves the precipitation of carbonitrides, thereby increasing the yield strength (R _e ). However, at more than 0.20% by weight, weldability and high temperature formability deteriorate.

The balance of the composition is iron and other elements that are normally expected to be found and which are impurities that occur as a result of smelting steel, where their proportions do not affect the desired properties.

The steel sheet having the above composition is first oxidized and then reduced before being hot dip galvanized and heat treated in a bath of molten zinc to form the alloyed galvanized steel sheet.

The purpose is to form an oxidized steel sheet having an outer layer of iron oxide of controlled thickness that will protect the steel from the selective external oxidation of silicon, manganese and aluminum while the steel sheet is annealed prior to hot dip galvanizing.

The oxidation of the steel sheet is characterized in that the iron oxide layer does not contain a superficial oxide selected from the group consisting of silicon oxide, manganese oxide, aluminum oxide, silicon and / or a composite oxide comprising manganese and / or aluminum. Under conditions that can be formed on the surface. During this step, an internal selective oxidation of silicon, manganese and aluminum takes place under the iron oxide layer and a deep depletion zone of silicon, manganese and aluminum is formed which minimizes the risk of selective oxidation on the surface when further reduction takes place. Therefore, silicon oxide, manganese oxide, aluminum oxide, a composite oxide containing Si and Mn, a composite oxide containing Si and Al, a composite oxide containing Mn and Al, and a composite oxide containing Si, Mn and Al An internal oxide layer of at least one type of oxide selected from the group consisting of is formed.

Oxidation is carried out by heating the steel sheet from ambient temperature to a heating temperature T1 of 680-800 ° C. in an atmosphere of a direct flame furnace, preferably containing air and fuel at an air-fuel ratio of 1 to 1.2. desirable.

When the temperature T1 is higher than 800 ° C, the iron oxide layer formed on the surface of the steel sheet contains manganese coming out of the steel, and the wettability is impaired. If the temperature T1 is less than 680 ° C., the internal oxidation of silicon, manganese and aluminum is not promoted and the galvanizability of the steel sheet is insufficient.

If the atmosphere has an air-fuel ratio of less than 1, the oxidation on the surface of silicon, manganese and aluminum takes place, and thus silicon oxide, manganese oxide, aluminum oxide and silicon and / or manganese and / or aluminum, possibly in combination with iron oxide The layer on the surface of the oxide selected from the group consisting of complex oxides is formed, the wettability is impaired. However, if the air-fuel ratio exceeds 1.2, the iron oxide layer is too thick and will not be fully reduced. Therefore, wettability is also impaired.

When exiting the flame furnace directly, the oxidized steel sheet is reduced under conditions such that iron oxide can be fully reduced to iron. This reduction step can be carried out in a radiant tube furnace or a resistance furnace. Thus, the oxidized steel sheet is preferably heat-treated in an atmosphere containing more than 15% by volume of hydrogen and the balance being nitrogen and unavoidable impurities. In fact, if the content of hydrogen in the atmosphere is less than 15% by volume, the iron oxide layer may be insufficiently reduced and the wettability is impaired.

The oxidized steel sheet is heated from a heating temperature T1 to a soaking temperature T2, then cracks at the cracking temperature T2 for a cracking time t2, and finally cooled from the cracking temperature T2 to a cooling temperature T3.

The said crack temperature T2 becomes like this. Preferably it is 770-850 degreeC. When the steel sheet is at the temperature T2, a dual phase microstructure of ferrite and austenite is formed. If T2 is above 850 ° C., the volume ratio of austenite increases too much, and external selective oxidation occurs at the surface of the steel. If T2 is less than 770 ° C, the time required for forming a sufficient volume ratio of austenite is too long.

In order to achieve the desired TRIP effect, sufficient austenite must be formed during the cracking step, and as a result, sufficient residual austenite is maintained during the cooling step. Cracking is performed during time t2, and time t2 is preferably 20 to 180 seconds. If the time t2 is greater than 180 seconds, the austenite grains become coarse and the yield strength R _{e of the} steel after formation is limited. Moreover, the hardenability of the steel is low. However, if the steel sheet is cracked for a time t2 of less than 20 seconds, the ratio of austenite formed is not sufficient, and sufficient residual austenite and bainite are not formed upon cooling.

The reduced steel sheet is finally cooled at a cooling temperature T3 close to the temperature of the bath in order to avoid cooling or reheating the bath of molten zinc. Therefore, it is preferable that T3 is 460-510 degreeC. Therefore, a zinc based coating having a homogeneous microstructure can be obtained.

When the steel sheet is cooled, it is hot dip in a molten zinc bath whose temperature is preferably 450 to 500 ° C. This bath contains 0.08 to 0.135% by weight of dissolved aluminum, with the balance being zinc and inevitable impurities. Aluminum is added to the bath to deoxidize the molten zinc and to make it easier to control the thickness of the zinc-based coating. Under such conditions, precipitation of the delta phase (FeZn ₇ ) is induced at the interface between the steel and the zinc-based coating.

When leaving the bath, the steel sheet is wiped off by projection of the gas to adjust the thickness of the zinc-based coating. This thickness, which is usually 3 to 10 μm, is determined in accordance with the required corrosion resistance.

The hot dip galvanized steel sheet is finally heat treated so that a coating made of a zinc-iron alloy is obtained by diffusing iron from the steel to the zinc of the coating. This alloying treatment can be carried out by maintaining the steel sheet for a crack time t4 of 10 to 30 seconds at a temperature T4 of 460 to 510 ° C. Thanks to the absence of external selective oxidation of silicon, manganese and aluminum, this temperature T4 is lower than conventional alloying temperatures. For that reason, large amounts of molybdenum are not required for the steel, and the content of molybdenum in the steel can be limited to less than 0.01% by weight. If the temperature T4 is less than 460 ° C, alloying of iron and zinc is impossible. If the temperature T4 is higher than 510 ° C, due to unwanted carbide precipitation, it becomes difficult to form stable austenite, and the TRIP effect cannot be obtained. The time t4 is adjusted so that the average iron content in the alloy is from 8 to 12% by weight, which is a good compromise between the improvement of the weldability of the coating and the limitation of powdering during molding.

1 is a photograph of sample A before the annealing step after the preheating step, and FIG. 2 is a photograph of sample B before the annealing step after the preheating step.

In the following, the present invention is explained by way of examples given as non-limiting explanations and with reference to FIGS. 1 and 2.

The test was carried out using samples A and B having a thickness of 0.8 mm made of steel of the composition given in Table 1 below.

Samples A and B are preheated from ambient temperature (20 ° C.) to 750 ° C. directly in a flame furnace. Samples A and B are then annealed continuously in a radiant furnace, heated from 750 ° C. to 800 ° C., then cracked at 800 ° C. for 60 seconds, and finally cooled to 460 ° C. The atmosphere in the radiant furnace contains 30% by volume of hydrogen, the balance being nitrogen and inevitable impurities.

After cooling, samples A and B are hot-dipped galvanized in a molten zinc bath containing 0.12% by weight of aluminum and the balance being zinc and inevitable impurities. The temperature of the bath is 460 ° C. After the zinc-based coating was wiped off with nitrogen and cooled, the thickness of the zinc-based coating was 7 μm.

First, the objective is to compare the wettability and adhesion of these samples when the air-fuel ratio in the flame furnace changes directly. The air-fuel ratio is 0.90 for sample A and 1.05 according to the present invention for sample B. The results are shown in Table 2.

Wetting is visually contrasted by the operator. In addition, the adhesion of the coating is also visually controlled after the 180 ° bending test of the sample.

Table 1: Samples A and B chemical composition in weight percent, balance of the composition being iron and inevitable impurities (Samples A and B).

1 is a photograph of sample A before the annealing step after the preheating step, and FIG. 2 is a photograph of sample B before the annealing step after the preheating step.

The aim is to show the effect of internal selective oxidation of silicon and manganese on the alloying temperature. Therefore, the temperature of the alloying process applied to sample B in order to obtain the alloy galvanized steel sheet according to the present invention is compared with the alloying temperature of sample A.

And the hot dip galvanized sample B is heated to 480 degreeC, and it hold | maintains at this temperature for 19 second, and alloying process is carried out. The inventor has confirmed that the TRIP microstructure of the alloyed hot dip galvanized steel sheet obtained according to the present invention was not lost by this alloying treatment.

In order to obtain alloying of the zinc-based coating of Sample A, it needs to be heated to 540 ° C. and held at this temperature for 20 seconds. With such a treatment, the inventors found that carbide precipitation occurred, the residual austenite was no longer maintained during cooling to room temperature, and the TRIP effect was lost.

Claims (13)

A process for producing an alloyed hot-dip galvanized steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite and / or bainite,
Composition is in weight percent,
0.01 ≤ C ≤ 0.22%
0.50 ≤ Mn ≤ 2.0%
0.5 <Si ≤ 2.0%
0.005 ≤ Al ≤ 2.0%
Mo <0.01%
Cr ≤ 1.0%
P <0.02%
Ti ≤ 0.20%
V ≤ 0.40%
Ni ≤ 1.0%
Nb ≤ 0.20%
Providing a steel sheet comprising a remainder of the composition is an inevitable impurity due to iron and smelting;
A layer of iron oxide is formed on the surface of the steel sheet and Si oxide, Mn oxide, Al oxide, a composite oxide comprising Si and Mn, a composite oxide comprising Si and Al, a composite oxide comprising Al and Mn, and Si, Oxidizing the steel sheet so that an internal oxide of at least one type of oxide selected from the group consisting of complex oxides comprising Mn and Al is formed;
Reducing the oxidized steel sheet to reduce the iron oxide layer;
Hot-dip galvanizing the reduced steel sheet to form a zinc-based coated steel sheet; And
Alloying the zinc-based coated steel sheet to form an alloyed galvanized steel sheet
A manufacturing method of an alloyed hot dip galvanized steel sheet comprising a. The method for producing an alloyed hot dip galvanized steel sheet according to claim 1, wherein the steel sheet contains P <0.010% by weight. The method for producing an alloyed hot dip galvanized steel sheet according to claim 1 or 2, wherein the steel sheet contains Mo ≦ 0.006% by weight. The oxidation of the steel sheet according to any one of claims 1 to 3, wherein the steel sheet is heated from an ambient temperature to a temperature T1 in a direct flame furnace in an atmosphere containing air and fuel at an air-fuel ratio of 1 to 1.2. The manufacturing method of the alloying hot-dip galvanized steel plate performed. The method for producing an alloyed hot dip galvanized steel sheet according to claim 4, wherein the temperature T1 is 680 to 800 ° C. The method of any one of claims 1 to 5, wherein the reduction of the oxidized steel sheet consists of a heat treatment performed in an atmosphere containing more than 15% by volume of hydrogen and the balance being nitrogen and an unavoidable impurity. A process for producing an alloyed hot dip galvanized steel sheet comprising a heating step from a temperature T1 to a cracking temperature T2, a cracking step for a cracking time t2 at the cracking temperature T2, and a cooling step from the cracking temperature T2 to a cooling temperature T3. . The method for producing an alloyed hot dip galvanized steel sheet according to claim 6, wherein the crack temperature T2 is 770 to 850 ° C. The method for producing an alloyed hot dip galvanized steel sheet according to claim 6 or 7, wherein the crack time t2 is 20 to 180 seconds. The method for producing an alloyed hot dip galvanized steel sheet according to any one of claims 6 to 8, wherein the cooling temperature T3 is 460 to 510 ° C. The method for producing an alloyed hot dip galvanized steel sheet according to any one of claims 5 to 9, wherein the reduction is performed in a radiation tube furnace or a resistance furnace. The hot-dip galvanizing according to any one of claims 1 to 10, wherein the reduced steel sheet is subjected to hot-dip galvanizing by hot-plating the reduced steel sheet in a molten bath containing 0.08 to 0.135% by weight of aluminum and the balance being zinc and inevitable impurities. , Method for producing alloyed hot dip galvanized steel sheet. The method for producing an alloyed hot dip galvanized steel sheet according to claim 11, wherein the melting bath has a temperature of 450 to 500 ° C. The alloying hot dip galvanizing according to any one of claims 1 to 12, wherein the zinc-based coated steel sheet is subjected to the alloying treatment by heating the zinc-based coated steel sheet at a temperature T4 of 460 to 510 ° C for a crack time t4 of 10 to 30 seconds. Method of manufacturing steel sheet.

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