JP2001262223A - METHOD FOR PRODUCING HIGH PURITY Fe-Cr ALLOY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high purity Fe-Cr alloy the C content of which is reduced to <=0.003% by decarburizing annealing by controlling the dew point Dp of the atmosphere and heat treating temperature T. SOLUTION: When subjecting an Fe-Cr alloy sheet containing >=0.03 mass % C to decarburizing annealing in a hydrogen atmosphere or in a hydrogen- nitrogen atmosphere, heat treating temperature T( deg.C) is held within the range of 700 to 1,200 deg.C, and the following inequality is satisfied between the dew point Dp( deg.C) of the atmosphere and the heat treating temperature T to reduce the C content to <=0.003 mass %: 0.0625T-86.9>Dp>-0.04T-46. In this way, the formation of an oxidized film checking decarburizing reaction is suppressed, and the diffusing and oxidizing reaction of C in the steel is promoted, thus, decarburizing can be performed to a level of extra-low carbon of <=0.003%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-purity Fe--Cr alloy having a C content reduced to 0.003 mass% by decarburizing annealing.

[0002]

2. Description of the Related Art A high-purity Fe-Cr alloy, particularly an ultra-low carbon high-purity Fe-Cr alloy, is a material having excellent corrosion resistance, mechanical properties and magnetic properties. High-purity Fe-Cr alloys are generally produced by a refining process, specifically, a converter-vacuum degassing process in which coarse decarburization is performed in a converter and then decarburization is performed to a target carbon concentration by vacuum degassing. I have. In addition to the refining process, F
By keeping the e-Cr alloy at a temperature of 850 to 1200 ° C. in a hydrogen atmosphere for 5 minutes to 20 hours, the C content is reduced to 0.01.
A method of reducing the content to 5% by mass or less (Japanese Patent Application Laid-Open No. 49-63618).
A method of lowering the C and N contents to 0.01% by mass or less by maintaining the temperature at 00 ° C. for 5 minutes to 20 hours (Japanese Patent Laid-Open No.
No. 0-79419) is also known.

[0003]

In the converter-vacuum degassing process, a long-time decarburization treatment is required at the time of vacuum degassing in order to decarbonize to a target carbon concentration, which significantly reduces the working efficiency and production cost. Increase. On the other hand, in the conventional atmosphere heat treatment process, there is a limit to the reduction of C content.
In the method of JP-A-9-63618, at least 0.005% by mass, and in the method of JP-A-50-79419, at least 0.
It is only reduced to 004% by mass. However,
In order to improve corrosion resistance, mechanical properties, magnetic properties, and the like, it is necessary to reduce the amount of C to at least 0.003% by mass or less.
For example, the magnetic flux density B ₁ tends to increase as the C amount decreases, and rapidly increases when the C amount is 0.003% by mass or less (FIG. 1). Viewed from the relationship between the C content and flux density B _1, in the conventional atmospheric heat treatment process seen it may not be sufficiently high purity.

[0004]

SUMMARY OF THE INVENTION The present invention has been devised in order to solve such a problem. An oxide film which inhibits the decarburization reaction on the surface of the Fe--Cr alloy by changing the oxygen potential of the heat treatment atmosphere to an oxygen film is provided. By maintaining it at a level that does not form, the C content is reduced to 0.003% by mass or less, and high-purity Fe—Cr having excellent corrosion resistance, mechanical properties, and magnetic properties.
The purpose is to obtain an alloy.

[0005] In order to achieve the object, the production method of the present invention is characterized in that, when decarburizing annealing a Fe-Cr alloy plate containing 0.03 mass% or more of C in a hydrogen atmosphere or a hydrogen-nitrogen atmosphere, heat treatment is performed. The temperature T (° C.) is maintained in the range of 700 to 1200 ° C., and the following relationship is established between the dew point D _P (° C.) of the atmosphere and the heat treatment temperature T, and the C content is reduced to 0.003% by mass or less. It is characterized by the following. 0.0625T−86.9> D _P > −0.04T−
46

Further, Fe-
Plate thickness t (mm) of Cr alloy plate and heat treatment holding time H
It is preferable to set (hour). 96 ≧ H ≧ a · t + b (when 9 ≧ t ≧ 1.55 · lnT-9.79) 96 ≧ H ≧ a · (1.55 · lnT-9.79) + b (when t <1.55 · lnT-9.79) a = 82.92−10.9 · lnT, b = 157.41−23.1 · lnT

[0007]

The decarburization in a hydrogen atmosphere or a hydrogen-nitrogen atmosphere proceeds by a solid-phase / gas-phase reaction between C in steel and oxygen or water vapor in the atmosphere, and therefore is promoted as the dew point of the atmosphere increases. Therefore, it is important to control the dew point of the atmosphere for high decarburization. The present inventors, as a result of the dew point and a heat treatment temperature of the atmosphere is to investigate and study the effect on the decarburization reaction, the dew point D _P (° C.) and a heat treatment temperature D _P between T (℃)> -0. When the relationship of 04 × T-46 was established, it was found that C in the steel became an oxidized region and the decarburization reaction proceeded as shown in FIG. However, in a region where a Cr oxide is generated, the generated chromium oxide hinders the decarburization reaction. In the Cr reduction region, the dew point D _P and the heat treatment temperature T are D _P <0.0625T−
This is an area satisfying 86.9.

Accordingly, in order to efficiently decarburize the Fe—Cr alloy, the difference between the dew point D _P and the heat treatment temperature T is D _P > −
0.04 T-46 and D _P <0.0625T-86.9
To satisfy. The decarburization reaction is also affected by the heat treatment temperature T. At a heat treatment temperature T of less than 700 ° C., the diffusion reaction is slow and the decarburization reaction hardly occurs. Conversely, if the heat treatment temperature T exceeds 1200 ° C., it becomes difficult to construct a heat treatment furnace that can withstand high temperatures.

[0009] The decarburization reaction of an Fe-Cr alloy is also affected by the diffusion of C in steel. The diffusion of C in steel is promoted as the heat treatment temperature T increases and the plate thickness t (mm) decreases. The range of the sheet thickness t and the heat treatment holding time H (hour) at which the C content becomes 0.003% by mass or less after the Fe-13% Cr alloy having the initial C content of 0.030% by mass is heat-treated in a hydrogen atmosphere. investigated. As a result, as shown in FIG. 3, the higher the heat treatment temperature T, the wider the range of the sheet thickness t and the heat treatment holding time H at which the C content becomes 0.003% by mass or less. The thinner the thickness t, the more advantageous the reduction of the C content. However, if the thickness t falls below a certain value, the heat treatment temperature T at which the C content falls below 0.003% by mass hardly changes. It was approximated by a straight line.

The relationship shown in FIG. 3 is generalized by regression calculation as shown in FIG. The upper limit of the heat treatment holding time H is set to 96 since a long time decreases productivity and economic efficiency.
Set to time. The sheet thickness t takes into account the unwinding of the Fe-Cr alloy and the winding of the decarburized high-purity Fe-Cr alloy,
The upper limit was set to 9 mm. Under these conditions, the C content: 0.
The heat treatment holding time H satisfying 003% by mass or less is shown in FIG.
96 ≥ H ≥ at + b (when 9 ≥ t ≥ 1.55 lnT-9.79) 96 ≥ H ≥ a-(1.55 lnT-9.79) + b (t <1.55 lnT-9.79 A) = 82.92−10.9 · lnT, b = 157.41−23.1 · lnT

[0011] Fe-Cr decarburized annealing according to the present invention
The alloy contains Cr: 8.0 to 30.0% by mass and C: 0.1
Fe-Cr alloy containing 0% by mass or less. If the Cr content is less than 8.0% by mass, the required corrosion resistance cannot be obtained.
Conversely, if the Cr content exceeds 30.0% by mass, toughness and manufacturability decrease. On the other hand, if the C content exceeds 0.10% by mass, the heat treatment time for reducing C to 0.003% by mass or less becomes longer, and the productivity decreases.

[0012]

Examples: Cr: 13.12% by mass, C: 0.030% by mass, Si: 0.38% by mass, Mn: 0.38% by mass,
Fe containing 0.028% by mass of P and 0.01% by mass of S
30 kg of a Cr alloy was melted in vacuum, and Fe-Cr alloy sheets of various thicknesses were manufactured through the steps of forging, hot rolling, hot rolling annealing, surface grinding, cold rolling, finish annealing, and pickling.

Each of the obtained Fe—Cr alloy sheets was heat-treated in a hydrogen atmosphere, and the C content of the heat-treated Fe—Cr alloy sheet was analyzed. As can be seen from Table 1 showing the relationship between the heat treatment atmosphere and the C content, Test Nos. 1 to 16 which were subjected to the atmosphere heat treatment satisfying the conditions specified in the present invention all had a C content of 0.003 mass. % Or less. In contrast, in Test No. 17 to 20 out of the range of heat treatment temperature T and the dew point D _P is defined in the of the invention, C content was not reduced at all. In Test Nos. 21 to 25 in which the heat treatment holding time H was too short with respect to the plate thickness t, the C content after decarburization was 0.1%.
003% by mass. Further, Test Nos heat treatment temperature T is too low test No. 26 and the dew point D _P is too low 27
In the above, the C content was hardly reduced.

[0014]

[0015]

As described above, in the present invention, when the Fe—Cr alloy is decarburized by the atmosphere heat treatment, the decarburization temperature is controlled by controlling the atmosphere heat treatment temperature and dew point.
It enables decarburization to an extremely low carbon level of 003% by mass or less. The high-purity Fe-Cr alloy obtained in this way makes use of excellent corrosion resistance, mechanical properties and magnetic properties, and is used for construction, automobile, high corrosion resistance, high processing, high magnetic properties, electromagnetic properties, etc. Used in various fields. In addition, since it does not require a long-time treatment unlike the refining process, it is excellent in productivity.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the C content on the magnetic flux density B ₁ of an Fe—Cr alloy.

FIG. 2 is a graph showing a heat treatment temperature and a dew point range of a heat treatment atmosphere at which decarburizing annealing of an Fe—Cr alloy is possible.

FIG. 3. Fe—C after annealing at various heat treatment temperatures
Graph showing the range of plate thickness and heat treatment holding time in which the C content of the r alloy can be reduced to 0.003% by mass or less.

FIG. 4 is a graph showing a range of a sheet thickness and a heat treatment holding time in which the C content of the Fe—Cr alloy can be reduced to 0.003% by mass or less.

Claims (2)

[Claims] 1. Fe—C containing 0.03% by mass or more of C
When the r-alloy plate is decarburized and annealed in a hydrogen atmosphere or a hydrogen-nitrogen atmosphere, the heat treatment temperature T (° C.) is maintained in the range of 700 to 1200 ° C., and the dew point D _P (° C.) of the atmosphere and the heat treatment temperature T are maintained. A method for producing a high-purity Fe-Cr alloy, wherein the relationship of the following formula is satisfied, and the C content is reduced to 0.003 mass% or less. 0.0625T−86.9> D _P > −0.04T−46 2. The condition that the thickness t (mm) of the Fe—Cr alloy plate and the heat treatment holding time H (hour) satisfy the following equation:
The high purity Fe according to claim 1, wherein the -Cr alloy plate is decarburized and annealed.
-A method for producing a Cr alloy. 96 ≧ H ≧ a · t + b (when 9 ≧ t ≧ 1.55 · lnT-9.79) 96 ≧ H ≧ a · (1.55 · lnT-9.79) + b (when t <1.55 · lnT-9.79) a = 82.92−10.9 · lnT, b = 157.41−23.1 · lnT