KR101360734B1 - Galvanized steel sheet having excellent coatibility and coating adhesion and method for manufacturing the same - Google Patents

Abstract

The method of manufacturing a hot-dip galvanized steel sheet having excellent plating property and plating adhesion, which is an aspect of the present invention, includes preparing a steel sheet, and reducing the atmosphere at which the prepared steel sheet is controlled at a dew point of -30 to 15 ° C. Heating to 750 ~ 850 ℃, a first cracking step of reacting the heated steel sheet with 200 ~ 3000 ppm of oxygen, a second cracking step of maintaining the holding steel sheet in a reducing atmosphere, after the cracking step, the holding steel sheet Cooling step and forming a galvanized layer on the cooled steel sheet. In addition, another aspect of the present invention, the hot-dip galvanized steel sheet having excellent plating property and plating adhesion, the base steel sheet, the reduced Fe layer formed on the base steel sheet and the zinc plated layer formed on the reduced Fe layer, the base steel sheet The surface portion of the oxide may be included in the longitudinal direction with a maximum length of 1㎛ or less.

Description

Hot-dip galvanized steel sheet with excellent plating property and adhesiveness and its manufacturing method {GALVANIZED STEEL SHEET HAVING EXCELLENT COATIBILITY AND COATING ADHESION AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a hot-dip galvanized steel sheet and a method for manufacturing the same widely used in automobiles, building structures and home appliances.

Recently, as the safety regulations of automobiles are strengthened and as part of environmentally-friendly efforts to reduce the emission of greenhouse gases, demand for high strength and light weight of automobile steel sheets is increasing. To this end, research is being actively conducted on DP (Dual Phase) steels and TRIP (Transformation Induced Plasticity) steels containing a large amount of non-plating elements such as Si, Mn or Al.

The hot-dip galvanized steel sheet is a plated layer formed by immersing a steel plate in an Al-containing galvanizing bath, thereby forming an alloying inhibitor layer called Fe ₂ Al _{5 at} the interface between the base steel plate and the galvanized layer, and suppressing the alloying. The layer serves to increase the adhesion between the steel sheet and the galvanized layer. However, in the case of a high-strength steel containing a large amount of Si, Mn or Al, Si, Mn or Al diffuse to the surface of the steel sheet to form an oxide in a portion where the alloying suppression layer is not formed, , Thereby causing peeling of the plating layer.

In order to solve this problem, Patent Literature 1 has been proposed, but the surface of the steel sheet is inevitably oxidized in the process of oxygen penetration into the base steel sheet and there is a limit to suppress the oxide of Si, Mn or Al formed at this time plateability It does not completely improve. In addition, although Patent Document 2 has been proposed, an oxide of Si, Mn or Al formed at the interface between the Fe oxide and the base steel sheet during the reduction annealing has a layer form, causing a plating peeling phenomenon in which the plating layer is dropped after galvanizing. There is a problem.

Therefore, there is an urgent need for a technology capable of ensuring excellent plating properties and plating adhesion of high strength hot-dip galvanized steel sheets.

United States Patent Publication 2008-0053576 Japanese Patent Publication No. 2005-154856

With respect to hot-dip galvanized steel sheet, there is a need for a method capable of ensuring excellent plating properties and plating adhesion by suppressing the surface concentration and oxidation of Si, Mn or Al, which are non-plating elements contained in steel, during annealing.

The method of manufacturing a hot-dip galvanized steel sheet having excellent plating property and plating adhesion, which is an aspect of the present invention, includes preparing a steel sheet, and reducing the atmosphere at which the prepared steel sheet is controlled at a dew point of -30 to 15 ° C. Heating at 750 to 850 ° C., a first cracking step of reacting the heated steel plate with 200 to 3000 ppm of oxygen, a second cracking step of maintaining the base steel sheet in a reducing atmosphere, and after the cracking step, the base steel sheet Cooling step and forming a galvanized layer on the cooled steel sheet.

In addition, another aspect of the present invention, the hot-dip galvanized steel sheet having excellent plating property and plating adhesion, the base steel sheet, the reduced Fe layer formed on the base steel sheet and the zinc plated layer formed on the reduced Fe layer, the base steel sheet The surface portion of the oxide may be included in the longitudinal direction with a maximum length of 1㎛ or less.

According to one aspect of the present invention, it is possible to suppress the surface concentration and oxidation of Si, Mn or Al to improve the plating properties and plating adhesion of the hot-dip galvanized steel sheet.

Figure 1 (a) is a schematic diagram of a manufacturing method of a conventional hot-dip galvanized steel sheet.
Figure 1 (b) is a schematic diagram of a manufacturing method of another conventional hot-dip galvanized steel sheet.
Figure 1 (c) is a schematic diagram of a method of manufacturing a hot-dip galvanized steel sheet which is an embodiment of the present invention.

The present inventors studied a method for providing a hot-dip galvanized steel sheet excellent in plating properties and plating adhesion, in order to suppress the hard plating property by the annealing oxide produced on the surface of the steel sheet in the heating section during heat treatment of the steel sheet , Fe oxide is formed on the surface of steel sheet by injecting an appropriate amount of oxygen into the crack section, and then reduced in a reducing atmosphere to form reduced iron on the surface of steel sheet, whereby Si, Mn or Al contained in steel It has been recognized that oxides can be discontinuously formed at the surface layer in the form of a root so as to prevent the peeling phenomenon of the plating layer which can be caused by the layer-type oxide after galvanizing, and also through the oxidation-reduction process. Fe oxide is formed on the surface of the steel sheet and then reduced to form reduced iron. Month is aware of the fact that the plating property improvement, led to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and like reference numerals designate like parts throughout the specification.

As shown in Fig. 1 (a), conventionally, when the base steel sheet 1 is heated, Si, Mn, Al internal oxides 4 are formed inside the base steel sheet, and Si, Mn, Al on the surface thereof. Oxide (3) is formed, after the reduction to form a galvanized layer 6, the unplated section appeared. In addition, as shown in Fig. 1 (b), by supplying oxygen during the heat treatment of the holding steel sheet 1 to form a Fe oxide layer (2) on the surface of the holding steel sheet, to ensure a reduced Fe layer (5) during reduction Although the zinc plating layer 6 was formed, it was also difficult to prevent the phenomenon that a plating layer peeled at this time. Therefore, one side of the present invention, as shown in Figure 1 (c), to maintain the high dew point when heating the steel sheet 1 to form Si, Mn, Al internal oxide (4) inside the steel sheet, In the case of performing the process of forming the galvanized layer 6 after injecting oxygen into the crack section to form the Fe oxide layer 2 and then forming the reduced Fe layer 5 on the surface of the steel sheet by reduction annealing. It is possible to greatly improve the plating adhesion.

Hereinafter, a method of manufacturing a hot-dip galvanized steel sheet which is one aspect of the present invention will be described in detail.

First, prepare the steel sheet. The steel sheet may include at least 0.5% by weight of one or two or more selected from the group consisting of Si, Mn and Al. However, the present invention can be applied even when the content is lower than the above, and steel grades containing a large amount of the elements are only limited to these lower limits because unplating or plating peeling may be a problem. In addition, the upper limit is not particularly limited, and due to the Si, Mn, Al, and the like, all kinds of steels that are easily caused by unplating or plating peeling may be included.

Thereafter, the prepared steel sheet is heated. According to one aspect of the invention, it is important to control the dew point during the heating. If the dew point is low, Si, Mn or Al contained in the steel is concentrated to the surface of the steel sheet to form annealing oxide, while if the high degree point, oxygen penetrates into the steel sheet to form Si, Mn or Al oxide inside the steel sheet. They serve to suppress the concentration on the steel sheet surface. In addition, the dew point is controlled to an appropriate temperature range of the heating section for internal oxidation, and the diffusion coefficient of oxygen is changed according to the temperature of the section maintained at the high dew point, thereby determining the flux of oxygen. When the flux of oxygen is high, oxygen penetrates into the steel sheet and internal oxidation occurs, so the maximum temperature of the heating section maintained at a high dew point is an important variable for determining the internal oxidation.

In one aspect of the invention it is preferable to control the dew point to -30 ~ 15 ℃. Based on the Wagner model, thermodynamic data show that the boundary dew point between external and internal oxidation in an atmosphere of 10% H ₂ -N ₂ is about -11.2 ° C, and the dew point in which Fe is oxidized in an atmosphere of 5% H ₂ -N ₂ is 18 ° C. Because it has been reported in the literature. When the dew point is lower than -30 ° C, the flux of Si, Mn or Al contained in the steel to diffuse to the surface is larger than the flux of oxygen into the steel sheet, and these Si, Mn or Al diffuse to the surface of the steel sheet to form a layered Si. , Mn or Al oxides are formed to infer the wettability of zinc as well as to cause the plating layer to fall off after galvanizing. On the other hand, when the dew point exceeds 15 ° C, not only Si, Mn or Al are oxidized, but Fe is also a condition in which oxidation can occur, and the flux of oxygen penetrating into the steel sheet is reduced to form Fe oxide, so that internal oxidation is sufficiently sufficient. It won't happen. Afterwards, Si, Mn or Al, which is concentrated on the surface of the steel sheet in the oxidation-reduction process, forms a layered oxide directly under Fe oxide, causing a phenomenon that the plating layer and the base iron interface are peeled off after galvanizing.

At this time, the heating temperature is preferably 750 ~ 850 ℃. This is to control the high dew point, and if the heating temperature is low in the section maintained at the high dew point, the diffusion coefficient of oxygen is low, so the flux of oxygen that penetrates into the steel sheet becomes small and thus internal oxidation does not occur, so the high dew point It is preferable to control the maximum temperature of the heating section to be controlled to 750 ℃ or more. In addition, the upper limit of the heating temperature is preferably controlled to 850 ° C in consideration of the material characteristics and the crack process, which is a post-pore bottom.

After the heating step, it is preferable to perform the cracking step. The cracking stages can be divided into the first and second stages, and are referred to as first and second cracking stages, respectively.

In the first cracking step, it is preferable to react the heated steel sheet with oxygen of 200 to 3000 ppm. If the oxygen concentration is less than 200 ppm, the thickness of the Fe oxide formed on the surface of the steel sheet is so thin that volume shrinkage occurs with discontinuous iron in the form of discontinuous heat treatment, thereby preventing the surface thickening of Si, Mn or Al in the steel. As a result, the alloying inhibitory layer that improves the wettability of zinc does not develop uniformly over the entire surface of the steel sheet, and thus is not effective in improving plating properties. On the other hand, if the concentration of oxygen supplied to the crack section electricity exceeds 3000 ppm, the risk of hydrogen exploding by reacting with oxygen during the conversion to the reducing heat treatment atmosphere after forming the Fe oxide cannot be excluded. If the oxide is formed thick Fe oxide that can not be reduced remains in the form of a layer may cause plating peeling after galvanizing. Therefore, the concentration of oxygen to oxidize the surface of the steel sheet to 200 ~ 3000 ppm to the level that reduced iron can be formed evenly on the surface of the steel sheet so that the alloying suppression layer can be formed well without causing the risk of explosion and plating peeling off. It is desirable to. However, in order to improve the effect of this invention further, it is more preferable to control the said oxygen concentration to 500-2500 ppm.

In addition, the supplied oxygen is preferably supplied at an air-fuel ratio of 1.0 ~ 1.4. The air-fuel ratio means the ratio of air and fuel.When the air-fuel ratio is less than 1.0, the amount of oxygen in the direct heating furnace is small so that surface oxidation does not occur sufficiently. When the air-fuel ratio exceeds 1.4, the amount of oxygen remaining after combustion increases the thickness of the oxide layer. It is preferable to control to 1.0 to 1.4 because is too thick because the oxide layer remains after the reduction annealing, which may lead to peeling of the plating layer after galvanizing.

After the first cracking step, a second cracking step may be performed. The second cracking step is preferably carried out in a reducing atmosphere. Fe oxide formed on the base steel sheet is reduced by the second cracking step of the reducing atmosphere to form a reduced Fe layer on the base steel sheet. In addition, the reducing atmosphere gas is preferably a mixed gas composed of 3 to 20% hydrogen and the balance nitrogen. In addition, when the hydrogen content of the reducing gas is less than 3%, the reduction of the iron oxide layer may not occur sufficiently, which may cause peeling of the plating layer by the residual oxide layer. If the hydrogen content exceeds 20%, there is no problem. Because of this increase, it is desirable to limit the hydrogen content to 3 to 20%.

In the second cracking step, it is preferable to control the dew point to -35 ° C or less. When the dew point exceeds -35 ° C in the process of reduction heat treatment of the Fe oxide, oxygen penetrates in the depth direction of the base steel sheet to form an oxide having a longitudinal length exceeding 1 μm in the surface layer of the steel sheet, thereby processing the steel sheet in the process of processing the steel sheet. It is preferable to control it at -35 degrees C or less because peeling may arise in a layer and plating adhesiveness may be inferior.

In addition, the first cracking step, the second cracking step is preferably carried out at 750 ~ 850 ℃ to ensure excellent material properties such as tensile strength or elongation of the steel. If the temperature is lower than 750 ° C. during the cracking, internal oxidation does not occur and only Fe oxide is formed on the surface of the steel sheet, so that plating peeling occurs due to Si, Mn, or Al oxide present in a layer form at the interface between the plating layer and the steel sheet after the galvanizing process. do. On the other hand, if the temperature at the time of cracking is higher than 850 ° C., the fuel and energy consumption used to increase the temperature of the annealing furnace increases, and the second steel recrystallization prevents obtaining a steel sheet of a material having excellent tensile strength or elongation, and thus the surface of the steel sheet. Fe oxide is formed thicker than the proper thickness may cause the plating peeling phenomenon due to the remaining Fe oxide present in the plating layer and the steel plate interface after galvanizing.

Thereafter, the base steel sheet can be cooled. The method of cooling here is not specifically limited, Any method may be used.

The cooled steel sheet may be immersed in a plating bath to form a plating layer. The method for forming the plating layer is not particularly limited. However, the temperature of the plating bath is preferably controlled to 440 ~ 460 ℃, if the temperature of the plating bath is less than 440 ℃ the viscosity of the plating bath is increased to reduce the mobility of the roll (roll) winding the steel sheet It causes slippage between the steel sheet and the roll, causing defects in the steel sheet. In addition, when the temperature of the plating bath exceeds 460 ℃ to promote the dissolution of the steel sheet to accelerate the generation of dross in the form of Fe-Zn compound to generate an unplated. Therefore, in order to minimize the occurrence of defects of the steel sheet, it is preferable to control the temperature of the plating bath to 440 ~ 460 ℃.

After the plating step, the hot-dip galvanized steel sheet may be additionally alloyed and heat treated. By controlling the alloying heat treatment temperature to 480 ° C. or higher, the Fe content can be sufficiently secured in the galvanized layer, and by controlling the temperature to 600 ° C. or lower, the powdering phenomenon of the plating layer falling off during the processing of excessive Fe content in the plating layer can be prevented. have.

Hereinafter, the hot-dip galvanized steel sheet which is another aspect of the present invention will be described in detail.

The hot dip galvanized steel sheet may include a base steel sheet. As described above, the base steel sheet may include at least 0.5% by weight of one or two or more selected from the group consisting of Si, Mn and Al. However, the present invention can be applied even when the content is lower than the above, and steel grades containing a large amount of the elements are only limited to these lower limits because unplating or plating peeling may be a problem.

An oxide may be formed in the surface layer of the base steel sheet. The surface layer portion is within 1 μm in the direction of the base steel sheet from the interface between the base steel sheet and the reduced Fe layer. The oxide may have a maximum length of 1 μm or less in the longitudinal direction. This is because when the oxide formed on the surface layer of the base steel sheet has a maximum length exceeding 1 μm in the longitudinal direction, peeling occurs in the oxide layer during the forming of the steel sheet, thereby resulting in poor plating adhesion. However, in order to further improve the effect of the present invention, the maximum length of the oxide in the longitudinal direction is more preferably controlled to 0.5 µm or less.

As described above, the role of the oxide is formed in the form of a root in the surface layer of the steel sheet so that the plating layer does not fall off after galvanizing. In addition, the type of the oxide is not particularly limited, but the oxide may be one or two or more of Si-based oxides, Mn-based oxides, and Al-based oxides.

A reduced Fe layer may be formed on the base steel sheet. Since the reduced Fe layer is a Fe layer and is a Fe layer formed by redox-reduction, in the present invention, the reduced Fe layer is only referred to as a reduced Fe layer, but is not limited thereto. The Fe layer is improved in plating property by controlling the alloying inhibitor layer to be well developed during the galvanizing process.

In addition, a zinc plating layer may be formed on the reduced Fe layer. The zinc plated layer may be a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer. In addition, the component type, plating amount, etc. of the said galvanized layer are not specifically limited in this invention.

Through this, the internal oxide formed on the surface layer of the base steel sheet serves as a root, to prevent the peeling of the plating layer, it is possible to control the alloying suppression layer is well developed in the galvanizing process by the reduced Fe layer.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

The steel sheet having a total of Si, Mn, and Al contents of 3.1% by weight was cold rolled, and the surface of the steel sheet was cleaned by degreasing and pickling. In order to perform the heating step, the nitrogen gas containing 5% hydrogen was blown, and the dew point in the heating section and the maximum temperature maintained at the high dew point in the section were changed as shown in Table 1 below.

Thereafter, in order to perform the first cracking step, the air-fuel ratio was blown to the oxygen concentration shown in Table 1 in the range of 1.0 to 1.4 to form Fe oxide on the surface of the steel sheet and oxidized at 800 ° C. for 30 seconds. In order to perform the second cracking step, the oxygen supply was cut off, the nitrogen gas containing 5% hydrogen was blown, and the annealing process was performed at 800 ° C. for 30 seconds in a reducing atmosphere while maintaining the dew point below -35 ° C. . Lastly, in order to perform the hot dip galvanizing process, the steel plate was immersed in the plating bath for 3 seconds, and then the coating amount deposited on the surface was maintained at 60 g / m 2 through air wipping.

In addition, in order to evaluate the plating property of the steel plate after the plating process, the coating area ratio of the zinc plated layer to the total surface area of the plated was measured, and the values thereof are shown in Table 1 below. For the cross-section observation, the specimen was cut into 15 × 15 mm 2, the cross section was polished, and the plating layer was observed with a scanning electron microscope (SEM). In addition, in order to measure the plating adhesion of the steel sheet, a bending test was performed after bending the specimen having a size of 30 × 80 mm 2 at 180 ° angle.

Depending on the material properties of the steel sheet, 0T or 1T bending was performed in a range where the material did not break. When the transparent vinyl tape was attached to the bending part and then peeled off, the plating layer appeared as 'peeling', and when the plating layer did not come out at all, it was described together in Table 1 below as 'non-peeling'.

division Heating section
Temperature (℃) dew point
(℃) Oxygen concentration
(ppm) Zinc plated layer
Coverage area ratio (%) Plated
Adhesiveness Inventory 1 800 8 1200 99 Non-peel Inventive Example 2 750 -10 700 97.5 Non-peel Inventory 3 730 0 1000 98 Non-peel Honorable 4 775 -18 950 95 Non-peel Inventory 5 830 -7 850 98.5 Non-peel Inventory 6 810 -13 500 93 Non-peel Comparative Example 1 790 -16 100 87 Non-peel Comparative Example 2 600 -2 900 96 Exfoliation Comparative Example 3 740 28 1050 97 Exfoliation Comparative Example 4 820 -34 1500 98.5 Exfoliation Comparative Example 5 790 0 5000 98 Exfoliation

As shown in Table 1, Inventive Examples 1 to 6 satisfy the conditions controlled by the present invention for both the maximum temperature of the heating section and the first crack stage oxygen concentration value, so that the coating area ratio of the zinc plated layer is all greater than 97%. It is very excellent in the properties, there is no peeled off part can be confirmed that the plating adhesion is also excellent.

On the contrary, in Comparative Example 1, although the internal oxidation occurred due to the dew point of -16 ° C in the heating section and the peeling of the plating layer did not occur after the galvanization, the oxygen concentration supplied to the electricity of the crack section did not meet the conditions of the present invention. In the process of reducing annealing due to the formation of extremely thin Fe oxide on the surface, the reduced iron was not uniformly distributed throughout the steel sheet. As a result, Si, Mn or Al contained in the steel is concentrated on the surface to form an oxide in the form of a layer, so that the alloying inhibiting layer is not uniformly developed after galvanizing, and the coating area ratio of the galvanizing layer is only 87%, resulting in inferior plating. It was.

In addition, in Comparative Example 2, since the maximum temperature maintained at the high dew point in the heating section did not meet the conditions of the present invention, the diffusion coefficient of oxygen was low, so that the flux of oxygen penetrating into the steel sheet was little, so that the internal oxidation hardly occurred. After the oxidation-reduction process, the alloying suppression layer was uniformly formed after the zinc plating by the reduced iron formed on the surface, and the coating area ratio of the zinc plating layer was 96%, and the plating property was excellent. The phenomenon that the plating layer fell off at the steel plate interface occurred.

In Comparative Example 3, the dew point in the heating section exceeds the conditions of the present invention, so that not only the oxidation of Si, Mn, or Al but also the oxidation of Fe is satisfied, Fe oxide is simultaneously formed on the surface of the steel sheet. The flux of oxygen that penetrates internally decreases so that internal oxidation does not occur sufficiently. Accordingly, the reduced iron formed on the surface of the steel sheet during the oxidation-reduction process uniformly forms an alloying suppression layer after galvanizing, so that the coating area ratio of the galvanized layer was 97%, and the plating property was excellent. However, since the internal oxidation did not occur sufficiently, Si, Mn or Al concentrated in the surface of the steel sheet during the heat treatment process formed a layered Si, Mn or Al oxide formed on the interface between the plating layer and the steel sheet, the plating layer was dropped.

In addition, in Comparative Example 4, the dew point in the heating section did not meet the conditions of the present invention, and the flux of oxygen penetrating into the steel sheet was small, so that internal oxidation did not occur, and Si, Mn, or Al was concentrated on the surface of the steel sheet to form an annealed oxide. It was. Since the reduced iron formed through oxidation-reduction helped to form an alloying inhibitor layer in order to have excellent wettability of zinc in the galvanizing process, the coating area ratio of the zinc plated layer reached 98.5%, but the layer-type Si already formed at the interface between the plated layer and the steel sheet. , Mn or Al oxide caused a phenomenon that the plating layer is peeled off.

Finally, in Comparative Example 5, the oxygen concentration supplied to the crack section electricity exceeded the conditions of the present invention, so that the thickness of the Fe oxide layer formed on the surface of the steel sheet became thicker than necessary, so that the residue could not be reduced during the reduction annealing process. Since Fe oxide is present, plating peeling by Fe oxide in a layer form occurs at the interface between the plating layer and the steel plate after galvanizing.

1. steel sheet,
2. Fe oxide layer,
3.Si, Mn, Al oxides,
4. Si, Mn, Al internal oxides,
5. reduced Fe layer,
6. Galvanized layer.

Claims (9)

Preparing a base steel sheet; Heating the prepared base steel sheet to a controlled reducing atmosphere at a dew point of −30 to 15 ° C. and to 750 to 850 ° C .; A first cracking step of reacting the heated steel sheet with 200 to 3000 ppm of oxygen; A second cracking step of maintaining the steel sheet in a reducing atmosphere; Cooling the steel sheet after the cracking step; And forming a galvanized layer on the cooled steel sheet.
The method according to claim 1, wherein the first cracking step is a method of manufacturing a hot-dip galvanized steel sheet having excellent plating and plating adhesion carried out at 750 ~ 850 ℃.
The method according to claim 1, wherein the first cracking step is a method of manufacturing a hot-dip galvanized steel sheet excellent in plating property and plating adhesion carried out at an air-fuel ratio of 1.0 to 1.4.
The method according to claim 1, wherein the second cracking step is a method of manufacturing a hot-dip galvanized steel sheet excellent in plating property and plating adhesion carried out at 750 ~ 850 ℃.
The method according to claim 1, wherein the second cracking step is a method of manufacturing a hot-dip galvanized steel sheet having excellent plating and plating adhesion carried out in a mixed gas atmosphere consisting of 3 to 20% hydrogen and the balance nitrogen.
The method of claim 1, wherein the second cracking step is performed in a reducing atmosphere controlled at a dew point of −35 ° C. or lower.
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