JPH0762173B2 - Method for manufacturing high strength non-magnetic prestressed concrete steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention requires that a magnetic levitation type railway guideway, a nuclear fusion reactor, a building such as a hospital with a magnetic resonance diagnostic apparatus installed, or the like is not affected by magnetism. The present invention relates to a method for producing a high-strength non-magnetic prestressed concrete steel material having high strength and excellent ductility as a reinforcing material for a concrete structure.

(Prior art) PC steel wire used for prestressed concrete,
JIS G3536, G35 for PC stranded wire, PC hard steel wire, and PC steel rod, respectively.
As seen in 38 and G3109, a large amount of carbon steel is used as the base material. However, in the field of application of the present invention, in addition to the characteristics possessed by these steel materials, they are placed in a magnetic field, so that the steel materials must also have non-magnetic characteristics so as not to exert their influence. This traditional PC
Steel materials, that is, carbon steels, belong to the category of ferromagnetic materials, which is the opposite of non-magnetic properties, and do not meet the purpose at all.

On the other hand, as a non-magnetic steel, the internal structure of the steel material is composed only of an austenite phase. For example, there are high-Mn steels such as SUS304 and SUS316 which are made of stainless steel in a solid solution state.

Among these steel materials, first of all, SUS304 has a defect that an austenite structure is induced by cold working to transform into a martensite structure in order to improve strength, the magnetic permeability indicating the degree of magnetism becomes large, and the nonmagnetic property is lost.

Furthermore, SUS316 has more stable non-magnetic properties than SUS304, but it has the drawback that it is difficult to obtain the high strength properties required for PC steel wire and PC steel stranded wire, and the economic burden increases. .

Regarding conventionally known high Mn steels, several types have been proposed as described below, but high strength can be obtained due to high work hardening, and elongation sharply decreases with increasing strength. Therefore, JIS G3536, which is the object of the present invention,
It is difficult to satisfy the following requirements: elongation rate 3.5% or more and tensile strength 175 kgf / mm ² or more, which are specified in the mechanical properties of.

Japanese Patent Publication No. Sho 58-44725 This invention is C; ≤1.5%, Si; 0.1-1.5%, Mn; 5-30%,
N; 0.5% or less, containing the balance Fe and unavoidable impurities, 100/9
(C + N) + 2Mn ≧ 25% steel or C; ≦ 1.5%, Si; 0.1
~ 1.5%, Mn; 5-30%, N; 0.5% or less, further Cu; 5% or less,
Contains Ni; 5% or less, Cr; 9% or less, 1 or more, 100/9
The steel of (C + N) +2 (Mn + Cu + 2Ni) ≧ 25% is processed by 90% or less in the temperature range of 600 ° C. or less, and also has the above-mentioned non-magnetic characteristics.

Japanese Patent Publication No. 61-53408 This invention is C; 0.2-1.5%, Si; 0.1-2%, Mn; 4-25
%, Balance Fe and unavoidable impurities or C; 0.2 to
After solution treatment of high Mn steel with 1.5%, Mn; 4 to 25%, Si; 0.1 to 2%, Cr; 0.1 to 10%, balance Fe, and after cold working, 200 to 7
In the area of 00 ° C, a tensile force of a load smaller than the breaking load at this temperature is applied, and a method of cooling is adopted.
As a result, it satisfies the tensile strength, elongation and relaxation values specified in JIS and has non-magnetic characteristics.

From the above, these conventional PC steel materials have defects and instabilities in any of strength, ductility and non-magnetic property, and are unsuitable for prestressed concrete structures that are magnetically severely used.

(Problems to be Solved by the Invention) As described above, the above-mentioned inventions that have been conventionally proposed have Fe-Mn steel as their basic component, but when applied as a concrete tension material, I have left the challenge.

When the tensile strength becomes high, it becomes brittle and the elongation decreases.

There is a thin decarburized layer on the surface which can lead to the loss of non-magnetism due to the generation of ferromagnetic phases in the layer during wire drawing. In other words, this steel shows non-magnetic properties and the permeability μ is 1.01.
Although it is 5 or less, since the hot-rolled material is used as a cover, it has a thin decarburized layer on the surface.Therefore, if a part of the decarburized layer is bent during use, martensitic transformation occurs in the decarburized layer. The magnetic phase appears, the magnetic permeability increases, and it is easily attracted to the magnet.

Since the high Mn steel is an austenitic alloy, the yield ratio YR (yield stress / tensile strength) of the wire is low, and as a result, the relaxation value is high.

Therefore, in the high Mn steel according to the present invention, in order to solve the above-mentioned problems, the following means were tried and new findings were obtained. That is, a new relationship was found in which the tensile strength at an elongation of 3.5% was 175 kgf / mm ² or more when the'component (C + N) ≧ 0.55%.

′ Material with Cr: 11% or more did not have a decarburized layer and showed non-magnetism with a magnetic permeability of ≦ 1.015 even after wire drawing.

When heat treating a steel material having a composition satisfying the relation of'Cr / 7 + (C + N) /3≧2.0% in the range of 300 ° C to 550 ° C, Cr
Carbides having a complex shape of ₇ (CN) ₃ were precipitated, the yield ratio YR increased, and the relaxation value was 3% or less.

The present invention has been invented in view of the above-mentioned viewpoint, and an object thereof is to provide a non-magnetic steel material for prestressed concrete, particularly a non-magnetic PC steel wire and a non-magnetic PC steel wire, magnetic permeability: 1.015% Tensile strength: 175 kgf / mm ² or more Elongation: 3.5% or more Relaxation value (10hr): 3.0% or less Stable non-magnetic properties and high strength.

(Means for Solving the Problems) As a means for achieving the above-mentioned object, the present invention is C; 0.3
~ 0.7%, Si; 0.2 ~ 2.0%, Mn; 13.0 ~ 22.0%, Cr; 11.0 ~
20.0%, Ni; 0.1 to 0.5%, N; 0.10 to 0.30%, balance Fe and unavoidable impurities are contained, and (C + N) ≧ 0.55%, Cr / 7 + between C and N and C, n and Cr. (C + N) /3≧2.0% of steel having a relation of cold working, and then subjected to heat treatment in a temperature range of 300 to 550 ° C., which employs a method for producing a steel material for high strength non-magnetic prestressed concrete. is there.

(Operation) The present invention is a method for producing a steel material for high-strength nonmagnetic prestressed concrete as described above, and the reason why the constituent components of the steel material obtained by the method are as described above is as follows.

That is, tensile strength: 175 kgf / mm ² or more, elongation: 3.5% or more, relaxation value (10 hr); 3.0% or less, magnetic permeability μ; 1.015
% Or less (nonmagnetic reference value).

Specifically, regarding C, C is a component that has an important function of stabilizing and strengthening austenite, and if it is 0.3% or less, strengthening is small and austenite tends to be unstable.
% Or more is required.

Further, if it exceeds 0.7%, carbides precipitate during hot rolling at high temperature, causing rolling cracks and embrittlement. Therefore, it is preferable to set the content to 0.7% or less.

It is necessary for steelmaking as a Si deoxidizer of 0.2% or more for Si, but if the content exceeds 2%, Si not only makes austenite unstable because it is a ferrite-forming element, but also embrittles austenite itself. Therefore, the upper limit of the content is set to 2%.

About Mn Mn is a strong austenite stabilizing element like C, and unless added in an amount of 13% or more, it does not become austenite at room temperature and loses non-magnetism.

However, if the content is 22% or more, the melting loss of the refractory in the furnace during melting is severe, the hot workability is deteriorated, and the practicality is lost, so the upper limit was made 22%.

About Cr Cr is an important element to improve the oxidation resistance of high Mn steel, and in order to produce non-magnetic steel material for prestressed concrete, hot-rolled samurai wire rods with a temperature of 900-1200 ℃ It is exposed to the atmosphere in a high temperature state such that it is necessary to heat it at a temperature for a predetermined time to align the structure (crystal grains) in a stable state.

Therefore, the steel is more or less oxidized, carbon in the steel is lost from the surface, and a decarburized layer is formed.

Therefore, a martensite phase is generated in the decarburized layer during the wire drawing process performed in the next step, and the martensite phase is ferromagnetic, so that the non-magnetism of the steel material itself is lost.

Therefore, in order to maintain non-magnetism, it is necessary to suppress the above decarburization.

In order to pursue this point, an experiment was conducted on the relationship between the change in magnetic permeability and Cr when the wire rod was heated to 20 Min and cooled in the air at 900 to 1200 ° C under the conditions of wire drawing with a surface reduction rate of 60%. However, the results shown in FIG. 1 were obtained. That is, as shown in the figure, when the Cr content is 11% or more, decarburization is suppressed,
It was found that the Cr content satisfying the nonmagnetic reference value μ ≦ 1.015 is required to be 11% or more.

However, if the Cr content is 2% or more, the hot deformation resistance of the steel material remarkably increases and the hot workability deteriorates, which is not preferable, and therefore the upper limit is 20%.

About Ni Ni is necessary to improve the hot working of austenite, and will suppress the occurrence of scratches during slabbing and wire rolling, and improve the yield.

The content must be 0.1% or more, but if it is too large, the effect is the same and the cost will increase, so the upper limit is 0.5%.
Is preferred. Therefore, the Ni content is 0.1-0.5%
And the range.

About N N does not only stabilize the austenite phase like C, but also strengthens the austenite phase.
The above is necessary, but if it exceeds 0.3%, bubbles are often generated during the ingot making, which hinders hot workability.

Therefore, the upper limit of the N content is 0.3%.

The reasons for limiting the constituent components of the non-magnetic PC steel material of the present invention are as described above, but in these components, the elements C and N have a strong influence on the stability and strengthening of the austenite phase.

These elements have an extremely strong correlation with the work hardening coefficient during wire drawing, and the work hardening coefficient increases as the content of these elements increases.

Therefore, the strength is greatly increased while the ductility is not deteriorated so much, and a combination of high strength and high ductility is realized.

The graph in Fig. 2 shows the result of pursuing the relationship between the tensile strength and the C + N of the processed steel when it shows an elongation of 3.5%. The elongation is 3.5% or more and the tensile strength is 175 kgf / mm ² or more. In order to satisfy the value of, it is necessary to satisfy the relationship of (C + N) ≧ 0.55% (1).

Satisfying the relationship shown in the above equation is an important matter in the present invention.

Furthermore, since the high Mn non-magnetic steel has an austenitic structure,
The yield ratio YR (yield stress / tensile stress) is small in the as-drawn state, and the relaxation value of 3% or less specified in JIS cannot be satisfied.

Therefore, in the present invention, when the relationship between C, N, Cr and the relaxation value was examined, the results shown in FIG. 3 were found.

The graph of FIG. 3 is an experiment of how the relaxation value of the processed steel material changes depending on the processing temperature and factors such as Cr / 7 + (C + N) / 3. That is, it was found that it is necessary to heat treat the steel material of Cr / 7 + (C + N) /3≧2.0% (2) formula at a temperature of 300 to 550 ° C.

The reason that the equation (2) is related to the relaxation value is as follows in the micrograph showing the metal structure of FIG.
Carbide called Cr ₇ (CN) ₃ is finely dispersed and yield ratio (Y
This is because R) rises.

In the invention of the conventional example, "when the relaxation property is important, it is preferable to perform annealing at 200 to 700 ° C after processing.

In particular, when the above-mentioned components of the front group (N; 0.5% or less, Cr; 9% or less, etc.) are included, precipitation hardening occurs due to this annealing .... " The relationship between the aging temperature and the aging temperature is quantified in association with the relaxation value, and Cr is also in the high Cr region, which is different from the above viewpoint.

(Example) Hereinafter, the Example of this invention is described.

Manufacture hot-rolled wire rod (6mm) of each component shown in Table 1,
The wire is continuously heated in a tunnel furnace maintained at 1100 ° C.

Then, after water cooling, pickling and coating treatment were performed.

Next, a wire of 4.22 mm diameter and 4.40 mm diameter is manufactured by a continuous wire drawing machine. Then, a 7-strand PC steel wire having an outer diameter of 12.4 mm was manufactured by using the wire of 4.40 mm diameter as a core wire and the wire of 4.22 mm diameter as a side wire. Finally, the PC strand was heat-treated at a temperature of 500 ° C. for a heating time of 10 minutes.

When the mechanical properties and magnetism of the PC steel wire thus produced were investigated, the results shown in Table 2 were obtained.

As can be seen from the results shown in Table ² , it was confirmed that Sample 1 did not satisfy the 175 kgf / mm ² prescribed in JIS standard because the tensile strength was lowered due to the low C content.

In the sample 2, the individual values of C and N are within the range set by the present invention, but the total amount of C + N is lower than 0.55, the tensile strength is
175kgf / mm ² is not satisfied.

Sample 3 has a high magnetic permeability due to the low Mn and loses the non-magnetic property.

Sample 4 has low Cr, and the value of Cr / 7 + (C + N) / 3 is 2.0%
It is lower, has a higher relaxation value and higher magnetic permeability, and cannot be said to be non-magnetic.

Samples 5 and 6 have C, N, and Cr within the setting range of the present invention, but the yield ratio is low because the value of Cr / 7 + (C + N) / 3 is lower than 2.0%. As a result, the relaxation value is Becomes large and becomes impractical.

Samples 7 and 8 are steel materials obtained by the method of the present invention, which have a high strength, an elongation of 3.5 or more, a low relaxation value, and sufficiently exhibit the non-magnetic characteristics (small μ), It has excellent properties that are comparable to PC steel strands manufactured using JIS G 3536 high carbon steel wire rods currently used as PC steel strands used in strong magnetic fields. I understand.

The physical properties shown in Table 2 are the values for PC steel strands,
A wire rod having a diameter of 7 mm including the components of sample 7 and sample 8 shown in the table is annealed at a temperature of 1100 ° C. for an annealing time of 5 minutes, and then water-cooled, pickled and coated. After that, wire drawing was performed to 5 mm, and after linear processing, heat treatment was performed at 500 ° C for 10 minutes, whereby a high strength non-magnetic PC steel wire could be manufactured.

The results are shown in Table 3.

(Effect of the invention) The present invention, as described above, C; 0.3-0.7%, Si; 0.2-2.0
%, Mn; 13.0 to 22.0%, Cr; 11.0 to 20.0%, Ni; 0.1 to 0.5
%, N; 0.10 to 0.30%, balance Fe and unavoidable impurities are contained, and (C + N) ≧ 0.55 between C and N and C and N and Cr.
%, Cr / 7 + (C + N) /3≧2.0%, by cold working, heat treating in the temperature range of 300 to 550 ° C. to obtain high ductility and high strength. Therefore, in a strong magnetic field, as a reinforcement for high-strength non-magnetic prestressed concrete, it can be replaced with PC steel stranded wire manufactured using the high-carbon steel wire stipulated in the existing JIS G3536 standard. It is possible to put the non-magnetic steel material of the above into practical use.

The method of the present invention can be sufficiently used in the production of PC steel rods, and the effects are the same as described above.

[Brief description of drawings]

Fig. 1 is a graph showing the relationship between the change in magnetic permeability and Cr.
The figure shows the relationship between the tensile strength and the C + N of the processed steel when the elongation is 3.5%. Figure 3 shows the relaxation value of the processed steel and the treatment temperature and Cr / 7 + (C + N).
Graph showing the relationship with the value of / 3, Fig. 4 shows Cr ₇ (CN)
It is a microscope picture of the metal structure which shows the precipitation state of ₃ carbide.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akito Shiina 7-3-3, Goshikiyama, Tarumi-ku, Kobe City, Hyogo Prefecture (56) References JP-A 64-254 (JP, A) JP-A 60-197853 (JP, A) JP-A-53-100122 (JP, A)

Claims (1)

[Claims] 1. C: 0.3-0.7%, Si; 0.2-2.0%, Mn; 13.0-
22.0%, Cr; 11.0 to 20.0%, Ni; 0.1 to 0.5%, N; 0.10 to 0.
30%, containing the balance Fe and unavoidable impurities, and (C + N) ≧ 0.55% between C and N and C, N and Cr, Cr / 7 + (C +
N) /3≧2.0%, after cold working of steel having a relationship of 300 to
A method for producing a high-strength non-magnetic prestressed concrete steel material, characterized by heat-treating at a temperature range of 550 ° C.