CN114134411A

CN114134411A - Spheroidized annealed steel for low-temperature-resistant high-strength ball screw and manufacturing method thereof

Info

Publication number: CN114134411A
Application number: CN202111186104.2A
Authority: CN
Inventors: 翟蛟龙; 樊启航; 邵淑艳; 李芸; 张魁; 陈泽雷; 余苹; 俞金锦; 刘雯
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2021-10-12
Filing date: 2021-10-12
Publication date: 2022-03-04
Anticipated expiration: 2041-10-12
Also published as: WO2023061185A1; DE112022000269T5; JP2024522326A; CN114134411B; US20240254577A1

Abstract

The invention relates to low-temperature-resistant high-strength spheroidized annealed steel for a ball screw, which comprises the following chemical components in percentage by mass: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: less than or equal to 0.025 percent, P less than or equal to 0.025 percent, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, less than or equal to 0.05% of Al, less than or equal to 0.0010% of Ca, less than or equal to 0.003% of Ti, less than or equal to 0.0010% of O, less than or equal to 0.04% of As, less than or equal to 0.03% of Sn, less than or equal to 0.005% of Sb, less than or equal to 0.002% of Pb, and the balance of Fe and inevitable impurities. The manufacturing process of the steel for the ball screw comprises the steps of electric furnace or converter-external refining-VD or RH vacuum degassing-continuous casting-continuous rolling-shearing or saw cutting-heap cooling-spheroidizing annealing-finishing-beating and warehousing. The steel for the ball screw has high yield strength (not less than 1380MPa) and tensile strength (not less than 1500MPa), and simultaneously has far-exceeding low-temperature toughness of the bearing steel (Charpy impact energy AKU at minus 40 ℃ below zero)₂≥27J)。

Description

Spheroidized annealed steel for low-temperature-resistant high-strength ball screw and manufacturing method thereof

Technical Field

The invention relates to the technical field of bar alloy steel, in particular to steel for processing a low-temperature-resistant high-strength ball screw and a manufacturing method thereof.

Background

In mechanical equipment, a ball screw is an indispensable transmission part for power and displacement transmission, and according to the difference of service environments, the ball screw which is used in some extreme environments needs to have high precision and high wear resistance of the traditional screw and also needs to meet the service requirement of keeping high toughness in severe environments such as high wind, great wave and severe cold in the two polar regions of the earth.

The traditional ball screw uses high-carbon chromium bearing steel, such as GCr15 brand, and the like, the material can only meet the service requirement of contact rigidity with a steel ball after quenching and tempering, the toughness under a low-temperature environment cannot meet the service requirement of an extreme environment, and the axial expansion and contraction performance of the material is a main factor causing the final grinding precision of the ball screw to be not up to the standard because the heat treatment deformation of the high-carbon bearing steel is difficult to control. In addition, because the carbon content of the steel is high, the grinding processing performance after quenching is poor, and the occurrence rate of processing quality problems such as grinding cracks is high.

Disclosure of Invention

The invention provides a novel low-temperature-resistant high-strength steel for a ball screw and a production method thereof, so that the surface of a processed ball screw product has ultrahigh hardness, strength and wear resistance under an extreme low-temperature condition, and also has ultrahigh low-temperature toughness, the dimensional stability in the processing and using processes is good, and the operation precision of the final screw service process is ensured.

In order to achieve the above object, the steel for a ball screw according to the present invention has mechanical properties at the following levels or requirements:

the requirements for non-metallic inclusions in the steel are given in table 1 below:

TABLE 1

The mechanical properties of the steel after thermal refining (e.g. 880 ℃ oil quenching +450 ℃ water cooling) are shown in table 2:

TABLE 2

Mechanical Properties	Yield strength	Tensile strength	Elongation percentage	-40 ℃ impact energy AKU₂
						≥1380MPa	≥1500MPa	≥9％	≥27J

Hardness of steel material: the hardness of J9mm was not less than 58HRC (hardness at a depth of 9mm from the surface was not less than 58HRC) when the hardenability of the end was examined by JIS G0561 method.

The invention realizes the specific technical scheme of the performance:

the spheroidizing annealing steel for the low-temperature-resistant high-strength ball screw comprises the following chemical components in percentage by mass: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: less than or equal to 0.025 percent, P less than or equal to 0.025 percent, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, less than or equal to 0.05% of Al, less than or equal to 0.0010% of Ca, less than or equal to 0.003% of Ti, less than or equal to 0.0010% of O, less than or equal to 0.04% of As, less than or equal to 0.03% of Sn, less than or equal to 0.005% of Sb, less than or equal to 0.002% of Pb, and the balance of Fe and inevitable impurities.

The design of the chemical components is as follows:

1) determination of C content

C is an element necessary to ensure wear resistance, and carbon in steel increases hardness and strength by increasing martensite transformation ability, thereby improving wear resistance. However, a C content exceeding 0.77% significantly increases crack sensitivity and decreases low-temperature toughness. The content of the invention is controlled to be 0.40-0.70%.

2) Determination of the Si content

Si is a deoxidizer in the steel-making process and improves the hardness, strength, elastic limit and yield ratio of steel in a solid solution strengthening mode. It can reduce the diffusion speed of C in ferrite, make the carbide precipitated during tempering not easy to aggregate, and raise the tempering softening resistance of steel material. In addition, Si reduces oxidation during frictional heating and increases the cold deformation hardening rate of steel to improve the wear resistance of the material. However, too high Si content decreases low temperature toughness. The invention controls the content of Si to be 1.20-1.80%.

3) Determination of Mn content

Mn is an element effective for strengthening steel as a deoxidizing element in the steel making process, and plays a role in solid solution strengthening to make up for the strength loss caused by the reduction of the C content in steel. And Mn can improve the hardenability of steel and the hot workability of steel. Mn can eliminate the influence of S (sulfur): mn can form MnS with a high melting point with S in steel smelting, thereby weakening and eliminating the adverse effect of S. The Mn content is 1.60%, which significantly reduces the toughness of the steel. The Mn content of the invention is controlled to be 1.00-1.60%.

4) Determination of the Cr content

Cr is a carbide-forming element and can improve the hardenability, wear resistance and corrosion resistance of the steel. Part of Cr in the steel replaces iron to form alloy cementite, so that the tempering stability of the steel is improved; part of the ferrite is dissolved in the ferrite to generate solid solution strengthening, thereby improving the strength and the hardness of the ferrite. However, the Cr content is too high, and is easily bonded to carbon in steel to form large carbides, which reduce the contact fatigue life of the steel. In summary, the Cr content of the present invention is determined to be in the range of 0.80 to 1.20%.

5) Determination of Al content

Al is a deoxidizer in the smelting process, and except for reducing dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride inclusions to refine grains. However, when the Al content exceeds 0.05%, the fluidity of the molten steel is greatly reduced, and the difficulty of casting is increased. The Al content of the invention is determined to be less than or equal to 0.05 percent.

6) Determination of Ni content

Ni exists in a solid solution form in the steel, and in the component system of the invention, Ni can reduce the fault energy and remarkably improve the low-temperature impact property of the steel, but too high Ni can cause too high content of residual austenite in the steel, reduce the strength and increase the cost. The Ni content of the invention is determined to be 0.10-0.60%.

7) Determination of the Cu content

The Cu element can form fine precipitates during tempering, so that the strength of the steel is improved, and the Cu element is beneficial to improving the corrosion resistance of the steel under an extreme environment. But too high Cu easily causes grain boundaries to weaken and cause cracking. The Cu content of the invention is determined to be 0.30-0.80%.

8) Determination of Mo content

Mo can refine the crystal grains of steel, improve hardenability and heat strength, and maintain sufficient strength and creep resistance at high temperature. And simultaneously, the brittleness of the alloy steel caused by tempering can be inhibited. However, the molybdenum alloy belongs to a heavy alloy, and in order to control the cost and achieve the expected effect, the content of Mo is determined to be 0.10-0.40%.

9) Determination of Ca content

The Ca content increases the number and size of the spot-like oxides in the steel, and since the spot-like oxides have high hardness and poor plasticity, they are not deformed when the steel is deformed, and voids are easily formed at the interface, deteriorating the properties of the steel. Meanwhile, smelting cost control is combined. The range of the Ca content of the invention is determined to be less than or equal to 0.001 percent.

10) Determination of the Ti content

The damage of Ti to steel is caused by the residual Ti in the steel in the form of titanium nitride and titanium carbonitride inclusions. The hard and angular inclusions seriously affect the fatigue life of the material, and particularly, the harm of the titanium-containing inclusions is particularly prominent under the conditions that the purity is obviously improved and the quantity of other oxide inclusions is very small. Meanwhile, smelting cost control is combined. The range of the Ti content of the invention is determined to be less than or equal to 0.003 percent.

11) Determination of the O content

The oxygen content represents the total amount of oxide inclusions, the limitation of the oxide brittle inclusions influences the service life of a finished product, and a large number of tests show that the reduction of the oxygen content is obviously beneficial to improving the purity of steel, particularly reducing the content of the oxide brittle inclusions in steel. Meanwhile, smelting cost control is combined. The oxygen content of the invention is determined to be less than or equal to 0.0010 percent.

12) P, S determination of content

P is highly segregated in steel during solidification, and P dissolves in ferrite to distort and coarsen crystal grains and increase cold shortness. Meanwhile, smelting cost control is combined. The range of the P content of the invention is determined to be less than or equal to 0.025 percent. S causes the steel to generate hot brittleness, reduces the ductility and the toughness of the steel, and combines the smelting cost control. The range of the S content of the invention is determined to be less than or equal to 0.025 percent.

13) Determination of As, Sn, Sb, Pb content

As, Sn, Sb, Pb and other trace elements belong to low-melting-point nonferrous metals, and exist in steel, which causes the appearance of soft spots and uneven hardness on the surface of parts, so that the As, the Sn, the Sb, the Pb and other trace elements are regarded As harmful elements in the steel and are combined with smelting cost control. The content ranges of the elements are determined to be less than or equal to 0.04 percent of As, less than or equal to 0.03 percent of Sn, less than or equal to 0.005 percent of Sb and less than or equal to 0.002 percent of Pb.

The manufacturing process of the steel for the ball screw comprises the steps of electric furnace or converter-external refining-VD or RH vacuum degassing-continuous casting-continuous rolling-shearing or saw cutting-heap cooling-spheroidizing annealing-finishing-beating and warehousing.

The main production process is characterized in that:

1. high-quality molten iron, scrap steel and raw and auxiliary materials are adopted to reduce the content of harmful elements in the molten steel. The deoxidation in the refining process is enhanced, the residual aluminum content in steel is ensured, and concentrated advanced deoxidation and VD or RH vacuum degassing treatment are carried out by utilizing good dynamic conditions in molten steel, so that non-metallic inclusions are fully floated and the lower gas content is controlled. After vacuum degassing, soft argon blowing is carried out for a long time, so that impurities are ensured to float sufficiently, and meanwhile, the quantity of the impurities in the steel is reduced by carrying out anti-oxidation protection in the whole continuous casting process. In addition, a control technology of reducing the pollution of foreign impurities to molten steel by using high-quality refractory materials is selected, and the control to the production process is strengthened.

2. The continuous casting process adopts the electromagnetic stirring and soft reduction technology and adopts low superheat degree pouring, so that the component segregation of the continuous casting blank is effectively improved and reduced, particularly, after advanced equipment such as solidification tail end electromagnetic stirring and soft reduction is added, the density of a casting blank solidification structure is improved, the center porosity and shrinkage cavity of the casting blank are effectively controlled, the distance between secondary dendrite arms is obviously improved, the center equiaxial crystal rate is obviously improved, and crystal grains are refined, so that the quality of the casting blank is obviously improved, and the component segregation is reduced.

3. According to the invention, smelting raw materials are sequentially subjected to electric furnace or converter smelting, LF refining, RH or VD vacuum degassing and continuous casting to obtain a continuous casting square billet with a specification of 390 multiplied by 510mm and above, wherein the continuous casting square billet conforms to the chemical composition of a steel product; the continuous casting slab is subjected to pit-entering slow cooling to prevent the continuous casting slab from cracking, the slow cooling time is not less than 48 hours, then the continuous casting slab is sent into a heating furnace with neutral or weak oxidizing atmosphere for heating and cogging into an intermediate slab with the thickness of 200 x 200 mm-300 x 300mm, the heating temperature is 1000 ℃ and 1250 ℃, the heating time is more than 5 hours, the cogging temperature during cogging rolling is 1000-1200 ℃, the rolling temperature is not less than 800 ℃, the cogging rolling compression ratio is more than 5, the intermediate slab is subjected to pit-entering slow cooling, the pit-entering temperature is not less than 500 ℃, and the slow cooling time is not less than 48 hours.

Then the intermediate blank is sent into a heating furnace to be rolled into target steel, and the specific rolling process comprises the following steps: the temperature of the preheating section is controlled to be 650-900 ℃, the temperature of the heating section is controlled to be 1000-1250 ℃, the temperature of the soaking section is controlled to be 1000-1250 ℃, and in order to ensure that the blank is fully and uniformly heated, the total heating time is more than 2 hours. The rolling start temperature is controlled to be 1000-1200 ℃, the rolling temperature of is controlled to be above 800 ℃, and the stack cooling is carried out after the rolling is finished.

In order to ensure the dimensional accuracy stability of steel materials when the ball screw is manufactured, the steel materials need to be spheroidized and annealed, and a three-section annealing process is innovatively used as follows:

firstly, the temperature is kept at 805 +/-10 ℃ for 7 hours, so that in a two-phase region of ferrite and austenite, partial cementite is dissolved in the austenite (secondary cementite), and a matrix (ferrite + secondary cementite ═ pearlite) is provided with a cementite particle for subsequent nucleation, so that the dynamic balance is achieved. Different from hypereutectoid steel such as bearing steel GCr15, the hypoeutectoid steel related to the invention needs water mist cooling to increase the supercooling driving force for balling, and then is subjected to 5+4.5 hours of staged isothermy to ensure that cementite (secondary cementite in austenite) is fully precipitated in a spherical form, (the two-stage isothermy is used for controlling the spheroidization size, the isothermal spheroidization temperature is too high, the balls are too large, the isothermal spheroidization temperature is too low, and the spheroidization rate is too low), and the cementite cannot grow too large (generally 0.1-0.5 mu m), (0.3-0.5 mu m)) to influence the dimensional stability of the subsequent heat treatment processing process of the ball screw.

And finally, carrying out subsequent straightening and flaw detection on the annealed bar product to obtain a target bar product.

Compared with the prior art, the invention has the advantages that:

1) different from the traditional GCr15 bearing steel, the chemical composition is optimized, so that the hardenability, the yield strength and the temper softening resistance of the steel are obviously improved, and the cracking tendency is small.

2) Compared with the traditional GCr15 bearing steel, the coarse spherical cementite (generally 1-3 mu m) is superior, the spherical cementite in the delivery state of the steel exists in a uniform and finer (generally 0.1-0.5 mu m) spheroidization state, the spheroidization rate reaches over 95 percent, and the rest structure is ferrite. The structure distortion energy is small, the heat treatment deformation is small in the process of processing the lead screw product, the size precision is high, and the precision use requirement of the ball screw can be met.

3) The traditional GCr15 bearing steel has very large low-temperature brittleness (summer specific impact energy AKU at-40 ℃ generally)₂Less than 10J), the steel for the ball screw has the low-temperature toughness (summer specific impact energy AKU at minus 40 ℃) far exceeding that of the bearing steel under the conditions of high yield strength (more than or equal to 1380MPa) and tensile strength (more than or equal to 1500MPa)₂≥27J)。

Drawings

FIG. 1 is a texture map of spheroidizing annealing in example 1 of the present invention;

FIG. 2 is a structure diagram of spheroidizing annealing according to example 2 of the present invention;

FIG. 3 is a microstructure diagram of spheroidizing annealing in example 3 of the present invention.

Fig. 4 is a three-stage spheroidizing process diagram according to an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to examples.

Examples 1 to 3 show the chemical composition and the production method of the steel for a ball screw according to the present invention, respectively, and compare them with a commercially available GCr15 bearing steel.

The chemical compositions (wt%) of the examples are shown in tables 2 and 3

TABLE 2

TABLE 3

The inclusions of the steels of the examples are shown in Table 4

TABLE 4

The mechanical properties (880 ℃ oil quenching +450 ℃ water cooling) of the examples are compared in Table 5

TABLE 5

The end hardenability data of the steels of the examples are shown in Table 6

TABLE 6

	Examples	J9mm(HRC)
			The invention	1	59.48
The invention	2	59.65
			The invention	3	59.91
GCr15	4	43.33

Referring to fig. 1-3, the microstructure of the steel material of each example is different from the coarse spherical cementite of the conventional GCr15 bearing steel, the cementite of the steel material in the delivery state of the invention exists in a uniform and finer (generally 0.1-0.5 μm) spheroidization state, the spheroidization rate reaches more than 95%, and the rest of the structure is ferrite. The structure distortion energy is small, the heat treatment deformation is small in the process of processing the lead screw product, the size precision is high, and the precision use requirement of the ball screw can be met.

The production process of the steel for a ball screw of each example was electric furnace or converter-external refining-VD or RH vacuum degassing-continuous casting billet squaring into intermediate billet-intermediate billet heating-rolling into material-spheroidizing annealing-finishing-beating and warehousing.

During smelting, high-quality molten iron, scrap steel, raw and auxiliary materials, high-quality deoxidizer and refractory materials are selected. In the production process of the electric furnace/converter, the tapping terminal C of the three examples is respectively controlled to be 0.05-0.25%, the terminal P is required to be less than or equal to 0.025%, and the continuous casting superheat degree is controlled to be within 15-35 ℃.

The slabs of the respective examples were subjected to the cogging rolling process as shown in table 7 below.

TABLE 7

And (3) conveying the intermediate blank into a heating furnace to be rolled into a target round bar, wherein the specific rolling process comprises the following steps: the temperature of the preheating section is controlled to be 650-900 ℃, the temperature of the heating section is controlled to be 1000-1250 ℃, the temperature of the soaking section is controlled to be 1100-1200 ℃, and in order to ensure that the blank is fully and uniformly heated, the total heating time is 2 hours or more. The rolling initial rolling temperature is controlled to be 900-1100 ℃, the rolling temperature of is controlled to be more than 800 ℃, and the steel is slowly cooled after rolling is finished, so that AlN particles in the steel are fine, uniform and fully separated out, crystal grains are refined, the condition of mixed crystals of the steel is prevented, and the steel is cooled in a heaped mode after rolling is finished.Spheroidizing annealing is carried out on the rolled finished bar The treatment and process are shown in the three-section spheroidizing process diagram. And (4) carrying out flaw detection treatment on the bar products subjected to spheroidizing annealing, and finally, forging and warehousing.

As can be seen from tables 2, 3, 4, 5 and 6, the steel for a low temperature resistant high strength ball screw according to the above embodiments of the present invention has significantly better control levels of harmful elements such as oxygen, titanium and non-metallic inclusions than the conventional GCr15 bearing steel. Particularly, in the aspect of mechanical properties, after the same quenching and tempering process, the yield strength, the tensile strength, the low-temperature impact and the temper softening resistance of the invention are obviously superior to those of the traditional GCr15 bearing steel, the yield strength is improved by more than 400MPa, the tensile strength is improved by 300MPa, the low-temperature impact property is improved by more than 30J, and the hardness is improved by more than 10 HRC. The hardenability is also obviously better than that of the traditional GCr15 bearing steel.

Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that modifications and variations of the present invention are possible to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The spheroidized annealing steel for the low-temperature-resistant high-strength ball screw is characterized by comprising the following chemical components in percentage by mass: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: less than or equal to 0.025 percent, P less than or equal to 0.025 percent, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, less than or equal to 0.05% of Al, less than or equal to 0.0010% of Ca, less than or equal to 0.003% of Ti, less than or equal to 0.0010% of O, less than or equal to 0.04% of As, less than or equal to 0.03% of Sn, less than or equal to 0.005% of Sb, less than or equal to 0.002% of Pb, and the balance of Fe and inevitable impurities.

2. The spheroidized annealed steel for a low temperature resistant high strength ball screw according to claim 1, characterized in that: after the steel is subjected to quenching and tempering, the yield strength is more than or equal to 1380MPa, the tensile strength is more than or equal to 1500MPa, the elongation is more than or equal to 9 percent, and the Charpy impact energy AKU at the temperature of minus 40 DEG C₂27J or more, the terminal hardenability is checked by JIS G0561 method, and the hardness of J9mm is 58HRC or more.

3. The spheroidized annealed steel for a low temperature resistant high strength ball screw according to claim 1, characterized in that: the cementite exists in a 0.1-0.5 mu m spheroidized state in the delivery state of the steel, the spheroidization rate reaches over 95 percent, and the rest of the structure is ferrite.

4. A method for manufacturing spheroidized annealed steel for a low-temperature resistant high-strength ball screw according to claim 1, characterized in that: the method comprises the following steps:

firstly, smelting a smelting raw material by an electric furnace or a converter, LF refining, RH or VD vacuum degassing and continuous casting in sequence to obtain a continuous casting square billet with the specification of 390 multiplied by 510mm and more, wherein the continuous casting square billet conforms to the chemical composition of a steel finished product;

secondly, performing pit slow cooling on the continuous casting billet for not less than 48 hours, and then sending the continuous casting billet into a heating furnace in a neutral or weak oxidizing atmosphere for heating and cogging into an intermediate billet with the thickness of 200 x 200 mm-300 x 300 mm;

step three, the intermediate blank is sent into a heating furnace to be rolled into target steel;

step four, then carrying out three-stage annealing on the rolled steel plate;

and fifthly, carrying out subsequent straightening and flaw detection on the annealed bar product to obtain a target bar product.

5. The method for manufacturing spheroidized annealed steel for a low-temperature-resistant high-strength ball screw according to claim 4, characterized in that: selecting high-quality molten iron, scrap steel and raw and auxiliary materials during smelting in the first step, selecting high-quality deoxidizer and refractory material, controlling the tapping end point C to be 0.05-0.25% and the end point P to be less than or equal to 0.025% in the production process of an electric furnace/converter, adopting electromagnetic stirring and soft pressing technology in the continuous casting process, and adopting low superheat degree for pouring, wherein the superheat degree of continuous casting is controlled to be within 15-35 ℃.

6. The method for manufacturing spheroidized annealed steel for a low-temperature-resistant high-strength ball screw according to claim 4, characterized in that: the heating temperature in the step two is 1000-.

7. The method for manufacturing spheroidized annealed steel for a low-temperature-resistant high-strength ball screw according to claim 4, characterized in that: the specific rolling process of the third step is as follows: the temperature of the preheating section is controlled to be 650-900 ℃, the temperature of the heating section is controlled to be 1000-1250 ℃, the temperature of the soaking section is controlled to be 1000-1250 ℃, in order to ensure that the blank is fully and uniformly heated, the total heating time is more than 2 hours, the rolling start temperature is controlled to be 1000-1200 ℃, the finish rolling temperature is controlled to be more than 800 ℃, and the blank is cooled in a piling way after the rolling is finished.

8. The method for manufacturing spheroidized annealed steel for a low-temperature-resistant high-strength ball screw according to claim 4, characterized in that: the three-stage annealing process in the fourth step specifically comprises the following steps: firstly, the rolled steel is kept at 805 +/-10 ℃ for 7 hours, so that partial cementite is dissolved in austenite, a matrix is provided with cementite particles for subsequent nucleation, dynamic balance is achieved, then water mist cooling is carried out, the temperature is kept at 745 +/-10 ℃ for 5 hours, the temperature is kept at 690 +/-10 ℃ for 4.5 hours after 1 hour of furnace cooling, and finally the rolled steel is cooled at 500 +/-10 ℃ for about 5 hours and taken out of the furnace.