CN109881060B

CN109881060B - Si-containing corrosion-resistant magnesium alloy and preparation method thereof

Info

Publication number: CN109881060B
Application number: CN201910233490.2A
Authority: CN
Inventors: 杜军; 韩帅; 冉合迎
Original assignee: South China University of Technology SCUT
Current assignee: South China University of Technology SCUT
Priority date: 2019-03-26
Filing date: 2019-03-26
Publication date: 2021-05-14
Anticipated expiration: 2039-03-26
Also published as: CN109881060A

Abstract

The present invention belongs toIn the technical field of magnesium alloy, a corrosion-resistant magnesium alloy containing Si and a preparation method thereof are disclosed. The corrosion-resistant magnesium alloy containing Si comprises the following components in percentage by weight: 6-10% of Sn; 0.7-1% of Si; 0.3-1% of RE; mg for the rest; and RE is Nd and/or Y. The invention also discloses a preparation method of the corrosion-resistant magnesium alloy. The corrosion-resistant magnesium alloy adopts Nd and Y modified eutectic Mg₂Si phase, improving eutectic Mg₂Si phase morphology, coarse Chinese character-like Mg₂Si is changed into a point shape or a rod shape, the structure distribution is uniform, and the corrosion resistance of the Si-containing magnesium alloy is obviously improved. The intermediate alloy adopted by the invention has low cost and simple processing technology, and is easy to realize industrialized mass production.

Description

Si-containing corrosion-resistant magnesium alloy and preparation method thereof

Technical Field

The invention belongs to the technical field of magnesium alloy, and particularly relates to a corrosion-resistant magnesium alloy containing Si and a preparation method thereof.

Background

The magnesium alloy has the advantages of low density, high specific strength and specific stiffness, high specific elastic modulus, good thermal conductivity, excellent machining performance and the like, has wide application prospect in the fields of aerospace, automobiles, electronics, national defense and military industry and the like, and is known as a green and environment-friendly and ecological metal material in the 21 st century. Therefore, magnesium alloy has become an ideal lightweight material to replace steel and aluminum alloy. However, compared with steel and aluminum alloy, magnesium alloy has low absolute strength and poor high-temperature performance, particularly because the electrode potential of Mg element is low, the corrosion resistance of magnesium alloy is generally poor, the application of magnesium alloy is greatly limited, and the development of magnesium alloy with high corrosion resistance is very important for expanding the application of magnesium alloy. Patent application with publication number CN106282706A discloses a rare earth corrosion resistant magnesium alloy. The corrosion-resistant magnesium alloy disclosed by the patent application has simple components and excellent corrosion resistance, but is added with a large amount of noble metal elements (Ag) and rare earth elements (Nd and Y), so that the cost is high, and the actual production is not facilitated.

The alloying method has become the most common and effective means for improving the high-temperature performance, the mechanical property, the corrosion resistance and the like of the magnesium alloy. The Sn element has the advantages of low price, easy alloying, simple and convenient processing technology and the like. The solid solubility of Sn in Mg is greatly changed along with the temperature, and the Sn is a typical alloying element with precipitation strengthening effect. In addition, Sn can form high melting point Mg in Mg₂And the Sn phase is dispersed and distributed on the crystal boundary, so that the crystal boundary can be effectively pinned to prevent dislocation sliding, and the room temperature and high temperature performance of the magnesium alloy can be improved. Similar to Sn element, Si element has the advantages of low cost and good physical property,very low solubility of Si in Mg and formation of a second phase Mg₂Si has the characteristics of low density, high melting point, high elastic modulus and the like, and is an effective reinforcing phase for improving the high-temperature performance, particularly the high-temperature creep resistance of the magnesium alloy. Therefore, Mg — Sn — Si based magnesium alloys are heat-resistant magnesium alloys that have received much attention in recent years. However, Si-containing magnesium alloys, particularly under ordinary casting conditions, contain Mg₂Si is often in the shape of coarse dendrites, polygons and Chinese characters, and the matrix is seriously cut, so that the mechanical property and the corrosion resistance of the magnesium alloy are reduced, and the application of the magnesium alloy is seriously influenced. Therefore, Mg is controlled and improved₂The shape, size and distribution of the Si phase are the key points for improving the corrosion resistance of the Si-containing magnesium alloy. The corrosion resistance of the magnesium alloy is improved by adding alloying elements to improve the microstructure, and the application range of the magnesium alloy can be effectively expanded.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention aims to provide a low-cost Si-containing magnesium alloy with excellent corrosion resistance and a preparation method thereof.

The purpose of the invention is realized by the following technical scheme:

the corrosion-resistant magnesium alloy containing Si comprises the following components in percentage by weight:

and RE is Nd and/or Y.

When RE is Nd and Y, the weight content of Nd is greater than that of Y, preferably the weight content of Nd is greater than or equal to 1.5 times the weight content of Y, and more preferably the weight content of Nd is greater than or equal to 2 times the weight content of Y.

Preferably, when RE is Nd and Y, 1.5 times the weight of Y < the weight of Nd < 10 times the weight of Y.

The preparation method of the Si-containing corrosion-resistant magnesium alloy comprises the following steps:

(1) melting pure magnesium: melting the magnesium ingot in a protective atmosphere to obtain a pure magnesium melt;

(2) alloying: adding a magnesium-silicon intermediate alloy, pure Sn and a magnesium-RE intermediate alloy into the melt obtained in the step (1), melting, stirring, standing and preserving heat to obtain a magnesium alloy melt; RE in the magnesium-RE intermediate alloy is Nd or Y; the magnesium-RE intermediate alloy is magnesium-Nd intermediate alloy and/or magnesium-Y intermediate alloy;

(3) alloy casting: and (3) carrying out slag drawing and casting on the magnesium alloy melt obtained in the step (2) to obtain the Si-containing corrosion-resistant magnesium alloy.

And (2) adding a magnesium-silicon intermediate alloy and pure Sn into the pure magnesium melt obtained in the step (1), melting and uniformly mixing, then adding a magnesium-RE intermediate alloy for modification treatment, stirring after melting, standing and preserving heat to obtain the magnesium alloy melt.

The mixing is to stir for 1-2 min after the materials are fully melted; the melting temperature in the melting and uniformly mixing process is 730-770 ℃;

the temperature of the modification treatment is 730-770 ℃; the stirring time after melting is 1-2 min, and the standing and heat preservation time is 8-12 min.

In the step (2), the magnesium-silicon intermediate alloy is Mg-3% Si (3% refers to the mass percentage of Si element in the magnesium-silicon intermediate alloy); the magnesium-RE intermediate alloy is Mg-20% RE intermediate alloy, namely Mg-20% Y and/or Mg-20% Nd intermediate alloy.

The melting temperature in the step (1) is 730-770 ℃.

The protective atmosphere in the step (1) is SF₆And N₂The mixed gas of (3); mixed volume ratio of SF₆:N₂＝2:98。

The melting and heat preservation temperature in the step (2) is 730-770 ℃.

The step (3) of casting refers to casting in a preheated carbon steel mold; the preheating temperature of the die is 180-210 ℃.

The basic principle of the invention is as follows:

the addition of Sn element into Mg-Si alloy improves the second phase Mg₂The size and distribution of Si, and Sn dissolved in the alpha-Mg matrix participate in the film forming process, the components of the film are magnesium oxide and tin oxide, and in addition, Mg with uniform distribution₂The Sn phase can play a role of a barrier in the corrosion process, and the corrosion rate is reduced. Thin paperThe addition of the earth elements (Nd, Y) also improves the corrosion resistance of the Mg-Si alloy. On one hand, the rare earth element influences the electrochemical corrosion process of the magnesium alloy, and after the rare earth element is added, a Si-RE-rich phase is generated in a matrix and can be used as a weak cathode to reduce the corrosion driving force, so that the occurrence of galvanic corrosion is inhibited. On the other hand, in the Mg-Si alloy modified with the rare earth element, eutectic Mg₂The Si phase is changed from a thick Chinese character shape to a more uniform and fine point shape or rod shape. During the corrosion process, as the alpha-Mg matrix is dissolved, the second phase gradually accumulates and forms a continuous phase on the surface of the matrix, and meanwhile, corrosion products gradually accumulate to form a corrosion product film, and the second phase and the continuous phase can effectively serve as a physical barrier to prevent further penetration of corrosion ions (such as Cl-) and inhibit further corrosion. Therefore, after the Sn and RE elements are added, the corrosion resistance of the Mg-Si alloy is obviously improved.

Compared with the prior art, the invention has the following advantages and beneficial effects:

(1) the corrosion-resistant magnesium alloy adopts Nd and Y modified eutectic Mg₂Si phase, improving eutectic Mg₂Si phase morphology, coarse Chinese character-like Mg₂Si is changed into a point shape or a rod shape, and the structure is uniformly distributed;

(2) the invention obviously improves the corrosion resistance of the Si-containing magnesium alloy by an alloying method;

(3) the intermediate alloy adopted by the invention has low cost and simple processing technology, and is easy to realize industrialized mass production.

Drawings

FIG. 1 is an optical microstructure of the Mg-0.8% Si alloy in comparative example 1;

FIG. 2 is a macroscopic corrosion morphology of the Mg-0.8% Si alloy of comparative example 1;

FIG. 3 is a microscopic corrosion morphology of the Mg-0.8% Si alloy of comparative example 1;

FIG. 4 is an optical microstructure of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1;

FIG. 5 is a macroscopic corrosion morphology map of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1;

FIG. 6 is a micro-erosion profile of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1.

Detailed Description

For a better understanding of the present invention, the present invention will be further described below with reference to examples and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Comparative example 1: mg-0.8% Si alloy

The raw materials used in this comparative example included high purity magnesium, a Mg-3% Si master alloy. The alloy comprises the following elements in percentage by weight: 0.8 percent of Si and the balance of Mg.

Preparation of Mg-0.8% Si alloy:

(1) melting high-purity magnesium: heating a high-purity magnesium ingot (the purity is 99.95%) to a molten state, wherein the melting temperature is 750 ℃, and obtaining a pure magnesium melt;

(2) alloying: adding Mg-3% Si intermediate alloy into the melt obtained in the step (1), and stirring for 1min after the melt is completely melted to make the components uniform; stirring, standing and preserving heat for 10 min;

(3) alloy casting: and (3) performing slag drawing on the magnesium alloy melt treated in the step (2), and then casting into a carbon steel mold preheated at 200 ℃ to obtain the Mg-0.8% Si alloy. The whole process is carried out under a protective atmosphere (the protective gas is SF)₆And N₂Mixed gas, the volume ratio of the two is 2%: 98%). The dosage of the high-purity magnesium ingot and the Mg-3% Si intermediate alloy is calculated according to the weight percentage of elements in the Mg-0.8% Si alloy.

In order to characterize the structure and performance characteristics of the above alloys, the as-cast structure of the alloys was observed using an optical microscope (model: Leica DFC); observing the macroscopic corrosion morphology of the sample by using a digital camera; microscopic corrosion morphology observation of the sample is carried out by utilizing a scanning electron microscope (model: Merlin); the polished sample was subjected to potentiodynamic polarization curve test using an electrochemical workstation (model: SP-150) with a scanning range of. + -. 0.3V of open-circuit potential, a scanning rate of 1mV/s, a test temperature of 25 ℃ and data analysis using EC-Lab software. In addition, the etching full immersion test was carried out in accordance with GB10124-1988, in which the specimen size was Φ 30X 5 (. + -. 1) mm, the etching conditions were 3.5% NaCl solution, and the immersion time was 24 hours. The test results are shown in Table 1.

FIG. 1 is an as-cast optical microstructure of the Mg-0.8% Si alloy of comparative example 1. As shown in FIG. 1, the Mg-0.8% Si alloy structure is mainly composed of an alpha-Mg phase and primary Mg₂Si phase and alpha-Mg + Mg₂A eutectic structure of Si phase. Wherein eutectic Mg₂Si phase is in the shape of thick Chinese character, nascent Mg₂The Si phase is surrounded by an alpha-Mg halo, and the alpha-Mg halo is surrounded by a biphasic eutectic structure. FIG. 2 is a macroscopic corrosion morphology of the Mg-0.8% Si alloy of comparative example 1, and it can be seen that the sample suffered severe pitting, a large number of corrosion pits were generated on the surface of the substrate, and the integrity of the alloy was destroyed. FIG. 3 is a microscopic corrosion morphology of the Mg-0.8% Si alloy of comparative example 1, showing that the α -Mg matrix is dissolved and a large number of corrosion holes appear on the surface of the sample. The alloy is measured to have the weight loss rate of 6.68mg/cm in the immersion test²H, self-etching current density of 145.1 μ A-cm-²The corrosion resistance of the alloy is extremely poor.

Example 1 Mg-8% Sn-0.7% Si-1.0% Y alloy

The raw materials used in this example included high purity magnesium, Mg-3% Si master alloy, high purity Sn, and Mg-20% Y master alloy. The alloy comprises the following elements in percentage by weight: 0.7% of Si, 8% of Sn, 1.0% of Y and the balance of Mg. The usage of high-purity magnesium, Mg-3% Si intermediate alloy, high-purity Sn and Mg-20% Y intermediate alloy (20% refers to the mass percentage of Y in the intermediate alloy) is calculated according to the weight percentage of the elements of the alloy.

Preparation of Mg-8% Sn-0.7% Si-1.0% Y alloy:

(1) melting high-purity magnesium: heating the high-purity magnesium ingot to a molten state, wherein the melting temperature is 770 ℃, and obtaining a pure magnesium melt;

(2) alloying: sequentially adding Mg-3% Si intermediate alloy, high-purity Sn and Mg-20% Y intermediate alloy into the melt obtained in the step (1), and stirring for 1min after the melt is completely melted to ensure that the components are uniform; stirring, standing and keeping the temperature for 8 min;

(3) alloy casting: and (3) carrying out slag drawing on the magnesium alloy melt treated in the step (2), then casting into a carbon steel mold preheated at 180 ℃, and naturally cooling to obtain the Mg-8% Sn-0.7% Si-1.0% Y alloy. The whole process is carried out under a protective atmosphere (the protective gas is SF)₆And N₂Mixed gas, the volume ratio of the two is 2%: 98%).

To characterize the texture and performance characteristics of this example, the alloy was subjected to texture observation, corrosion topography observation, electrochemical curve measurement, and etch full immersion testing. The test and test methods are consistent with comparative example 1. The test results are shown in Table 1.

FIG. 4 is an as-cast optical microstructure of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1. As shown in FIG. 4, a new black Mg phase is precipitated along the grain boundaries₂Sn, and the surrounding black area is an Sn-rich area. In comparison with comparative example 1, Mg in the alloy₂Si phase is thinned, and the thick Chinese character shape is changed into a point shape or a rod shape, and the distribution is more uniform. FIG. 5 is a macroscopic corrosion morphology of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1, which shows that after the Sn element is added, the corrosion sample is blue gray, the integrity of the sample after corrosion is good, the surface of the substrate is partially corroded, and the corrosion area is small and the depth is shallow. FIG. 6 is a microscopic corrosion morphology of the Mg-8% Sn-0.7% Si-1.0% Y alloy of example 1, as shown in the figure, the substrate is slightly corroded, and the corrosion morphology is laminar or filiform. The alloy has a weight loss rate of 0.43mg/cm in a soaking test²H, self-etching current density of 33.8 μ A-cm-². The corrosion rate decreased by more than an order of magnitude, only about 1/15 times the corrosion rate of the alloy in the comparative example, i.e., the corrosion resistance increased by a factor of 15. The self-corrosion current density is reduced by 76.7%. The alloying method of the invention has obvious effect on the modification of the alloy structure and can obviously improve the corrosion resistance of the Si-containing magnesium alloy.

Example 2 Mg-8% Sn-0.7% Si-1.0% Nd

The raw materials used in this example included high purity magnesium, an Mg-3% Si master alloy, high purity Sn, and an Mg-20% Nd master alloy. The alloy comprises the following elements in percentage by weight: 0.7 percent of Si, 8 percent of Sn, 1.0 percent of Nd and the balance of Mg.

Preparation of Mg-8% Sn-0.7% Si-1.0% Nd:

(2) alloying: sequentially adding Mg-3% Si intermediate alloy, high-purity Sn and Mg-20% Nd intermediate alloy into the melt obtained in the step (1), and after the intermediate alloy is completely melted, manually stirring for 1min to ensure that the components are uniform; stirring, standing and keeping the temperature for 8 min;

(3) alloy casting: and (3) carrying out slag drawing on the magnesium alloy melt treated in the step (2), then casting into a carbon steel die preheated at 180 ℃, and naturally cooling to obtain Mg-8% Sn-0.7% Si-1.0% Nd. The whole process is carried out under a protective atmosphere (the protective gas is SF)₆And N₂Mixed gas, the volume ratio of the two is 2%: 98%).

The as-cast optical microstructure of this example was similar to that in fig. 4 (example 1). After alloying, a new black Mg phase is segregated at the grain boundary₂Sn and Sn-rich regions. Mg (magnesium)₂The Si phase is refined and uniformly distributed. The macro and micro-scale corrosion morphology of the Mg-8% Sn-0.7% Si-1.0% Nd alloy is similar to that in FIGS. 5 and 6 (example 1), the matrix is only slightly corroded locally, and the corrosion morphology is lamellar or filiform. The alloy has a weight loss rate of 0.61mg/cm in a soaking test²H, self-etching current density of 51.5 μ A-cm-². The corrosion rate decreased by an order of magnitude, only about 1/10 times the corrosion rate of the alloy in the comparative example, i.e., the corrosion performance increased by a factor of 10. The self-corrosion current density is reduced by 64.5 percent. The alloying method of the invention has obvious effect on the modification of the alloy structure and can obviously improve the corrosion resistance of the Si-containing magnesium alloy.

Example 3 Mg-6% Sn-1.0% Si-0.7% Nd-0.3% Y

The raw materials used in this example included high purity magnesium, Mg-3% Si master alloy, high purity Sn, Mg-20% Nd, and Mg-20% Y master alloy. The alloy comprises the following elements in percentage by weight: 1.0% of Si, 6% of Sn, 0.7% of Nd, 0.3% of Y and the balance of Mg.

Preparation of Mg-6% Sn-1.0% Si-0.7% Nd-0.3% Y:

(1) melting high-purity magnesium: heating the high-purity magnesium ingot to a molten state, wherein the melting temperature is 730 ℃, and obtaining a pure magnesium melt;

(2) alloying: sequentially adding Mg-3% Si intermediate alloy, high-purity Sn, Mg-20% Nd and Mg-20% Y intermediate alloy into the melt obtained in the step (1), and manually stirring for 2min after the melt is completely melted to ensure that the components are uniform; stirring and standing for 12 min;

(3) alloy casting: and (3) carrying out slag drawing on the magnesium alloy melt treated in the step (2), then casting into a carbon steel die preheated at 210 ℃, and naturally cooling to obtain the alloy Mg-6% Sn-1.0% Si-0.7% Nd-0.3% Y. The whole process is carried out under a protective atmosphere (the protective gas is SF)₆And N₂Mixed gas, the volume ratio of the two is 2%: 98%).

The as-cast optical microstructure of this example was similar to that in fig. 4 (example 1). After alloying, in a crystal black new phase Mg₂Sn segregates at grain boundaries and is surrounded by a Sn-rich region. Mg (magnesium)₂The Si phase has obvious thinning effect, and is changed from thick Chinese character shape into point shape or short rod shape and is uniformly distributed. The macro and micro-scale corrosion morphology of the Mg-6% Sn-1.0% Si-0.7% Nd-0.3% Y alloy is similar to that in FIGS. 5 and 6 (example 1), the matrix is only slightly corroded locally, and the corrosion morphology is lamellar or filiform. The alloy has a weight loss rate of 0.37mg/cm in a soaking test²H, self-etching current density of 32.3 μ A cm^-2. The corrosion rate decreased by more than an order of magnitude, only about 1/17 times the corrosion rate of the alloy in the comparative example, i.e., the corrosion performance increased by a factor of 17. The self-corrosion current density is reduced by 77.7%. The alloying method of the invention has obvious effect on the modification of the alloy structure and can obviously improve the corrosion resistance of the Si-containing magnesium alloy.

Example 4 Mg-10% Sn-1.0% Si-0.3% Nd-0.7% Y

The raw materials used in this example included high purity magnesium, Mg-3% Si master alloy, high purity Sn, Mg-20% Nd, and Mg-20% Y master alloy. The alloy comprises the following elements in percentage by weight: 1.0% of Si, 10% of Sn, 0.3% of Nd, 0.7% of Y and the balance of Mg.

Preparation of Mg-10% Sn-1.0% Si-0.3% Nd-0.7% Y:

(2) alloying: sequentially adding Mg-3% Si intermediate alloy, high-purity Sn, Mg-20% Nd and Mg-20% Y intermediate alloy into the melt obtained in the step (1), and manually stirring for 2min after the melt is completely melted to ensure that the components are uniform; stirring, standing and keeping the temperature for 12 min;

(3) alloy casting: and (3) carrying out slag drawing on the magnesium alloy melt treated in the step (2), then casting into a carbon steel die preheated at 210 ℃, and naturally cooling to obtain the alloy Mg-10% Sn-1.0% Si-0.3% Nd-0.7% Y. The whole process is carried out under a protective atmosphere (the protective gas is SF)₆And N₂Mixed gas, the volume ratio of the two is 2%: 98%).

The as-cast optical microstructure of this example was similar to that in fig. 4 (example 1). After alloying, black new phase Mg₂Sn segregates at grain boundaries and is surrounded by a Sn-rich region. Mg (magnesium)₂The Si phase is obviously thinned, and is changed into a point or short rod shape from a thick Chinese character shape and is uniformly distributed. The macro and micro-scale corrosion morphology of the Mg-10% Sn-1.0% Si-0.3% Nd-0.7% Y alloy is similar to that in FIGS. 5 and 6 (example 1), the matrix is only slightly corroded locally, and the corrosion morphology is lamellar or filiform. The alloy has a weight loss rate of 0.53mg/cm in a soaking test²H, self-etching current density of 47.8 μ A cm^-2. The corrosion rate decreased by more than an order of magnitude, only about 1/13 times the corrosion rate of the alloy in the comparative example, i.e., the corrosion performance increased by a factor of 13. The self-corrosion current density is reduced by 67.1%. The alloying method of the invention has obvious effect on the modification of the alloy structure and can obviously improve the corrosion resistance of the Si-containing magnesium alloy.

TABLE 1 Corrosion behavior of each of the alloys of comparative example 1 and examples 1-4

The embodiments of the present invention are not limited to the embodiments described above, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and they are included in the scope of the present invention.

Claims

1. A corrosion-resistant magnesium alloy containing Si is characterized in that: comprises the following components in percentage by weight:

Sn： 6～10%；

Si： 0.7～1%；

RE：0.3~1%；

mg for the rest;

the RE is Nd and Y; when RE is Nd and Y, the weight of 2 times of Y is less than or equal to that of Nd and the weight of 10 times of Y;

(2) alloying: adding a magnesium-silicon intermediate alloy and pure Sn into the pure magnesium melt obtained in the step (1), melting and uniformly mixing, then adding a magnesium-RE intermediate alloy for modification treatment, stirring after melting, standing and preserving heat to obtain a magnesium alloy melt; RE in the magnesium-RE intermediate alloy is Nd and Y; the magnesium-RE intermediate alloy is magnesium-Nd intermediate alloy and magnesium-Y intermediate alloy;

2. The Si-containing corrosion resistant magnesium alloy according to claim 1, wherein: in the specific step of the step (2), uniformly mixing means stirring for 1-2 min after fully melting; the melting temperature in the melting and uniformly mixing process is 730-770 ℃;

3. The Si-containing corrosion resistant magnesium alloy according to claim 1, wherein: in the step (2), the magnesium-silicon intermediate alloy is Mg-3% Si; the magnesium-RE intermediate alloy is Mg-20% RE intermediate alloy, namely Mg-20% Y and Mg-20% Nd intermediate alloy.

4. The Si-containing corrosion resistant magnesium alloy according to claim 1, wherein: the melting temperature in the step (1) is 730-770 ℃;

the protective atmosphere in the step (1) is SF₆And N₂The mixed gas of (1).

5. The Si-containing corrosion resistant magnesium alloy according to claim 1, wherein: the melting and heat preservation temperature in the step (2) is 730-770 ℃ independently;