JPS5931569B2 - Manufacturing method of low thermal expansion coefficient high descending point non-magnetic steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a non-magnetic steel with a low thermal expansion coefficient and high yield point, which has a thermal expansion coefficient as low as or lower than that of ferrite steel, a high yield point, and a low magnetic permeability. The purpose of this invention is to provide a method for manufacturing non-magnetic steel at low cost that does not increase even after mechanical stress. Yield strength at room temperature (O8
The present invention relates to a preferable method for manufacturing non-magnetic steel having a characteristic of 2% resistance) of 36 Ky/m4 or more.

Fields that require the use of iron-based non-magnetic materials in structural components and other parts of recently developed ultra-high-speed land transportation systems such as magnetic levitation high-speed trains (maglev trains) and plasma generators for nuclear fusion reactors (Q). is gradually expanding and demand is increasing.

However, as iron-based nonmagnetic steel, approximately appropriate nonmagnetism can be obtained by basically selecting the composition to have an austenitic structure, and a typical example thereof is austenitic stainless steel. Also, there are other Ha
dfield steel (0.9~1.3%C111~14%M
Mn-Cr steel (e.g. DINX40Mn, Cr18
steel), Mn-Cr-Ni steel (e.g. DINX55Mn,
Ni, Crl4 steel), Mn-C, -Ni-V steel (for example, DINX45Mn, Ni, C, V1376 steel), etc. are also known as low carbon high manganese nonmagnetic steels. However, a common problem with non-magnetic steels as mentioned above is that the coefficient of thermal expansion is at the level of ordinary carbon steel (160~1.3X10~5
/'C) is significantly higher than (186-169×10
-57'C), which poses a major obstacle in its application to structures. In particular, in cases where dimensional accuracy and deformation caused by expansion due to temperature changes during use, such as the structure of the magnetically levitated linear moke car described above, are important, it is desirable that the coefficient of thermal expansion be as low as possible. Although the conventional non-magnetic steels mentioned above have high tensile strength, their yield points are extremely low, so when used as structures, Q is often undesirable. The objective can be achieved to some extent by increasing the amount of C or adding a large amount of alloying elements such as Cr, Ni, and V. However, increasing the amount of C increases the coefficient of thermal expansion. In addition, adding large amounts of alloying elements such as Cr, Ni, and V has the disadvantage of significantly increasing the price. The present invention was created after repeated studies in view of the above-mentioned circumstances, and is applicable to guideway structures, roadbed reinforcing bars, nuclear fusion reactor structures, etc. in the above-mentioned magnetic levitation high-speed railways. In particular, we have improved the high thermal expansion coefficient and low yield strength, which are the shortcomings of conventional non-magnetic steel, and created a material with a thermal expansion coefficient comparable to that of ordinary steel (an average thermal expansion coefficient of 0 to 100°C). 1.0~1.3X
10-5/゜c), and its yield point (0.2% yield strength) is as high as 36Kp/M4, making it possible to produce non-magnetic steel with a magnetic permeability of 1.1 or less even when cold-worked at low cost. It was successfully manufactured. That is, in the present invention, C: 0.5%
less than, S1: less than 2 inches, Mn: 20-30%, N: 0
．． Contains less than 0.04%, and between Mn and C, Mn(e) > 16XC(d)+18...
・I Mn(d)>-12XC(to)+21.5...
・・・・・・It is possible to satisfy the relationship of the inequality II even when falling down, and if necessary, the steel ingot or billet containing Cr in a range of less than 2%, with the balance consisting of Fe and unavoidable impurities, can be made into 1220 Hot rolling is performed after heating to below ℃G), and the rolling finish temperature (FT) is set to FT('C)<800+400
By satisfying the inequality XC (to) O, 0℃
The average coefficient of thermal expansion between 100℃ and 1.25Xi0-5
Low thermal expansion coefficient of 7'c or less, and yield point (0.2%
A high manganese nonmagnetic steel having a high yield point of 36K9A4 or higher (yield strength) is obtained.

The reasons for limiting the ingredients in the present invention as described above are as follows. That is, C is an important element that stabilizes austenite, and the more it increases, the more other austenite stabilizing elements can be saved, and it is also effective as an element for strengthening austenitic steel. , C per 0.1%, it is possible to increase the yield strength by 1.8Kv and increase the tensile strength by 2.2KS'/.

Therefore, it is preferable that this C be contained to a certain extent, but on the other hand, if this C is too large, hot workability may be inhibited, and an even larger amount of Mn may be added to maintain the thermal expansion coefficient to the level desired by the present invention.
This is not economically preferable because it requires 0.5%, and furthermore, it worsens machinability, so it is set to less than 0.5%. Next, Mn
is an element that is cheaper and more effective as an austenite stabilizing element than other elements, and the stability of austenite in high Mn steel is determined by the balance between the basic C content and Mn content. is as shown in Figure 2 (high C
Kaede stabilizes with low Mn.

Therefore, the lower limit of Mn content is about 7% in high C steel, but it is necessary to keep the coefficient of thermal expansion low, which will be described later, so it is practically necessary to make it 20% or more. Furthermore, adding 30% or more of Mn increases the cost and also increases the difficulty in manufacturing, so 30% is set as the upper limit. However, as a result of multiple regression analysis on the coefficient of thermal expansion of about 30 steel types, it was found that C tends to increase the coefficient of thermal expansion, while Mn tends to decrease it, making the average coefficient of thermal expansion lower than that of ordinary steel. Normal 1.25 cutting 0-5'/'C (0~
The range in which the temperature is below 100°C (average) is as shown in equation (1) above, and this is illustrated by a... shown in Figure 1 above.
・A line or higher. In addition, C and Mn are both austenite stabilizing elements, and as they increase, magnetic permeability decreases, but multiple regression analysis was performed in the same manner as above to find the range in which stable non-magnetism can be obtained even after 20% cold working. The result obtained is as shown in the above-mentioned formula (II), and this is illustrated as the lines b...b shown in FIG. 1 above.

In other words, the coefficient of thermal expansion is 1.25X10-5/'C, which is the same as that of ordinary steel.
In order to obtain magnetic permeability of 1.1 or less even by cold working, the amount of C and Mn (in addition to the above-mentioned restrictions regarding this, formulas (I) and (II) must be satisfied at the same time). To summarize this and show the area of the present invention, it is the hatched area in Figure 1. However, the conventional high C-{JtMn nonmagnetic steel does not satisfy the thermal expansion coefficient and has a substantial The amounts of C and Mn are determined by the formula (I).In addition, in Figure 2 mentioned above, the amounts of C and Mn to obtain a stable austenite phase in the case of 20% cold working and 80% cold working are shown. The balance relationship is shown, and the case of 80% cold working is similar to that of 20% cold working, even if there is a slight deviation.

Regarding N, if it is less than 0.005%, the stabilization of austenite is likely to be lost, and if it exceeds 0.04%, the machinability of the steel will be impaired, so it is set at 0.005 to 0.04%.

Further, Ni, Cr, and V are effective elements for increasing the strength of austenitic Mn steel, but from the viewpoint of economic efficiency, it is preferable that Ni and Cr are each 2% or less, and V is 0.5% or less. Addition within this range does not impair the extremely low coefficient of thermal expansion, which is a feature of the present invention.

In addition, in the present invention (in this case, when hot rolling a steel ingot or slab having the above-mentioned composition), it is necessary to pay sufficient attention to the soaking and heating temperature.

In other words, Figure 3 shows the relationship between tensile temperature and high-temperature ductility when high-Mn austenitic steel was heated and subjected to a high-temperature tensile test. It has been shown that cracks are more likely to occur.
However, in the case of large steel ingots, component segregation becomes significant, so in actual operation it is desirable to heat the steel ingots at a lower temperature of 1220° C. or lower. Furthermore, regarding the rolling conditions, it is
It greatly affects the yield strength (yield strength) of austenitic steel, and the grain refining effect of rolling in the austenite low temperature range is very large.

That is, FIG. 4 shows the relationship between rolling finishing temperature and yield strength (0.2% proof stress), and 0.23C-21.
For 4Mn steel, below 900°C, 0.12C-27.
Regarding 4Mn steel, it is possible to achieve an increase in yield strength of over 10 Ky/ml by controlling the finishing temperature to 850°C or less. Apart from this, the present inventor has also studied many high Mn austenitic steels. The results are shown in Figure 5. Obtaining a yield strength of 36K9/1 or higher is determined by the balance with the reinforcing effect of carbon itself, and generally speaking, this is achieved by setting the finishing temperature to 800 to 950 degrees Celsius or less. This can be achieved by controlling the rolling finish temperature to approximately satisfy the following (self) equation. Finishing temperature FT ('0<800+400xC (a)...III Specific examples of the present invention, together with comparative examples thereof, will be explained as follows.

That is, the present inventors specifically prepared 25Ky steel ingots having the composition shown in Table 1 below, and applied the manufacturing conditions according to the present invention to these steel ingots as shown on the right side of Table 1. It was rolled under manufacturing conditions outside the present invention.

The properties of each steel number as mentioned above were measured and the results are shown in Table 2 below.

In other words, as seen in Group A steels, if the heating temperature is too high above 1220°C, flaws will occur in the steel plate.
This heating temperature is 1 in common for each group of A-D.
By setting the temperature to 220°C or less, one improvement in the production of high Mn steel sheets is achieved.
It is recognized that the surface flaws, which were one of the major problems, have been resolved. In addition, good yield strength is obtained by lowering the rolling finishing temperature, and D3 steel plate can obtain a preferable yield strength even when finished at 950°C, making it possible to significantly save on the addition of alloying elements. The rolling conditions at this time are such that the cumulative reduction rate below 1000°C increases continuously as the finishing temperature increases, for example, at a finishing temperature of 750°C.
The temperature was adjusted to approximately 60% at ℃. On the other hand, it was confirmed from multiple regression analysis Q that the average coefficient of thermal expansion α from 0 to 100° C. is expressed by the following formula m.

α=1.80+0.48C-0.03Mn-...
PJ and the constant thermal expansion coefficient line ζ calculated by this formula (IV) are as shown in FIG. Furthermore, as shown in Figures 7 and 8, the coefficient of thermal expansion hardly changes even when the Cr content or N content changes;
It was confirmed that the coefficient of thermal expansion of high-Mn steels is also substantially dominated by the amounts of C and Mn, and that the above formula (IV) can be applied. According to the present invention as explained above, both the high coefficient of thermal expansion and low yield strength, which are the unavoidable drawbacks of conventional high-Mn nonmagnetic steels, can be solved, and the steel can be made to the same level as ordinary steel without adding special alloying elements. or higher (this makes it possible to lower the coefficient of thermal expansion and increase the yield point, and achieve these objectives at low cost. In addition, it is possible to reduce the surface area, which has been a problem in the production of high-Mn steel. Industrial C is a highly effective invention because it completely solves problems such as the occurrence of defects and can be used as a non-magnetic steel for thick plates, shaped steel, and steel bars that are required in large quantities. be.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and Figure 1 is a diagram showing the balance relationship between the amount of C and the amount of Mn in the present invention, and Figure 2 is a diagram showing the balance relationship between the amount of C and the amount of Mn in the present invention, and Figure 2 shows the balance between C and Mn in order to obtain a stable austenite phase.
Figure 3 is a graph showing the relationship between the balance of Mn content, Figure 3 is a graph showing the relationship between tensile temperature, strain rate, and reduction of area in a high-temperature tensile test, and Figure 4 is a graph showing the relationship between finishing rolling temperature and yield strength (0.2% proof stress). A diagram showing some examples of the relationship C, Figure 5 is 0.2
A diagram summarizing the relationship between the amount of C exerted on the % proof stress ζ and the finishing temperature of rolling, Figure 6 is a diagram of constant thermal expansion coefficient in the stable austenite region, and Figure 7 is the coefficient of thermal expansion and N amount of high Mn steel. FIG. 8 is a chart showing the relationship between the coefficient of thermal expansion and the amount of Cr in high Mn steel. However, what is appended in these drawings is the steel type specifically used, and the numbers appended to each line in FIG. 6 are the average coefficient of thermal expansion from 0 to 100°C: 'C.

Claims (1)

[Claims] 1 C: 0.5% or less, Si: 2% or less, Mn: 20~
Steel containing 30%, N: 0.005 to 0.04%, the remainder consisting of iron and unavoidable impurities, and satisfying the relationship of the following inequality between C and Mn, is heated to 1220 ° C. or less. A method for producing a nonmagnetic steel with a low coefficient of thermal expansion and a high yield point, characterized in that hot rolling is carried out at a temperature of 800° C. + 400° C.×C (%) or less depending on the amount of C. Mn (%) > 16 x C (%) + 18 Mn (%) > -12 x C (%) + 21.5