JPH05271768A - Manufacture of non-magnetic stainless steel thick plate - Google Patents

Abstract

PURPOSE:To obtain a non-magnetic steel thick plate excellent in spring properties and strength by controlling the cold rolling temp. and cold draft in cold rolling subjected to steel contg. specified C, Si, Mn, Ni, Cr and N. CONSTITUTION:Stainless steel contg., by weight, <=0.08% C, <=3% Si, 2.5 to 5% Mn, 11.5 to 13.5% Ni, 16 to 20% Cr and 0.1 to 0.3% N and having 16 to 21 Ni equivalent (Nieq) is smelted. This Ni equivalent is defined by Nieq=Ni+0.6Mn+9.69(C+N)+0.18Cr-0.11Si<2>. Next, the steel is finished to 6 to 12mm plate thickness and is thereafter subjected to finish annealing. Then, the steel is subjected to cold rolling at 40 to 70% draft while its temp. is controlled above the A-A line in figure as well as to <=100 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Vickers hardness HV used as a thick plate spring component for structural members related to superconductivity.
The present invention relates to a method for producing non-magnetic stainless steel having a hardness of 400 or more.

[0002]

2. Description of the Related Art Cr-Ni type austenitic stainless steel represented by SUS304 has good corrosion resistance and has a non-magnetic austenitic structure in the annealed state, so that it is used as a non-magnetic steel for electric and precision instrument parts. There is.
Among them, SUS304N and SUS3 with large N content
16N steel is used as spring steel.

When N is added, the strength of austenitic stainless steel is improved. By subjecting this austenitic stainless steel to cold working and low temperature annealing treatment, spring characteristics are imparted. However, this kind of austenitic stainless steel was originally developed as a thin plate high-strength material, and there is a problem in using it as a non-magnetic material.

[0004] For example, SUS304N steel is used as spring steel by cold working and work hardening, but a ferromagnetic martensite phase is induced when it is highly cold worked. As a result, it becomes magnetic after cold working and cannot be used as non-magnetic steel. Especially when cold rolling an annealed material having a plate thickness of 6 to 12 mm, since the plate thickness is large, thermal energy due to processing strain is accumulated in the material, and the material temperature may reach about 200 ° C. .. The increase in material temperature releases the processing strain introduced by rolling. As a result, the cold-rolled material obtained from the thick plate has a lower hardness than the case where the thin plate material is cold-rolled at the same rolling rate, and the desired strength cannot be obtained.

Since the SUS316N steel has a stable austenite phase, it does not become magnetized by cold working. However, although Mo as an alloy component exerts an excellent corrosion resistance effect, it does not contribute much to nonmagnetic properties and spring properties. Further, since it contains a large amount of expensive Ni and Mo, the SUS316N-based steel is an unsuitable material as a non-magnetic steel, although it is expensive.

[0006] Therefore, the applicant has determined that the Ni content is 11.
A work-hardening type non-magnetic stainless steel having a relatively small amount of 5 to 13.5% by weight and no need to contain Mo is disclosed in Japanese Patent Laid-Open No.
Introduced in the 1-26143 publication. This work hardening type non-magnetic stainless steel has a nickel equivalent of Ni _eq (= Ni + 0.
60 Mn + 9.69 (C + N) + 0.18Cr-0.1
By maintaining 1Si ² ) in the range of 16.0 to 21.0, non-magnetism can be maintained even when subjected to cold rolling.

[0007]

However, Japanese Unexamined Patent Publication No. Sho 61-61
Even in the case of the stainless steel introduced in Japanese Patent Publication No. 26143, when a thick plate is cold-rolled, the strength may be insufficient depending on the rolling conditions. The insufficient strength depends on the plate thickness of the material to be rolled and causes the mechanical strength required for the cold rolled material not to be satisfied.

The insufficient strength observed when cold rolling a thick plate is presumed to be due to the following phenomenon. When a thick plate is cold-rolled, a large work strain is accumulated as heat energy in the material to be rolled as compared with a thin plate. as a result,
The material temperature rises, the work strain is released, and the strength decreases.

The present invention has been devised to solve such a problem. Vickers hardness HV40 is obtained by work hardening by cold rolling under specified conditions.
The object is to secure a strength of 0 or more and to stably obtain a non-magnetic stainless steel having a magnetic permeability of μ1.005 or less.

[0010]

In the present invention, weight C: 0.08% by weight or less, Si: 3.0% by weight or less, M
n: 2.0 to 5.0% by weight, Ni: 11.5-13.5
Wt%, Cr: 16.0 to 20.0 wt% and N: 0.
Stainless steel containing 10 to 0.30% by weight is used. In this stainless steel, the nickel equivalent Ni _eq defined by the formula (1) was adjusted to 16.0 to 2 by adjusting the composition.
Keep in the range of 1.0. _{Ni eq = Ni + 0.60Mn + 9.69} (C + N) + 0.18Cr-0.11Si 2 ···· (1)

After finishing this stainless steel to a plate thickness of 6 to 12 mm, it is subjected to finish annealing, and then 100 at a rolling ratio set above the line AA in FIG. 1 and in the range of 40 to 70%.
Perform cold rolling controlled to a temperature of ℃ or below. The cold-rolled thick plate may be further subjected to a heat treatment of heating it to a temperature of 600 ° C. or lower within 1 hour.

[0012]

The present inventors investigated and studied the effects of alloying elements, cold working and heat treatment on the spring properties of thick Cr-Ni austenitic stainless steel. As a result, by controlling the cold rolling temperature and rolling rate applied to stainless steel with specified alloy components and their contents, excellent spring characteristics and high strength after cold working are exhibited, and non-magnetic properties are exhibited. The present invention has been completed by finding that the above can be maintained.

In the present invention, the rolling rate that gives work hardening is set in the shaded area in FIG. When cold rolling is performed at this rolling rate, a given work strain is applied to the thick plate, and the strength of the stainless steel increases. However, as described above, it is impossible to obtain sufficient strength simply by controlling the rolling rate. Therefore, the rolling conditions are adjusted so that the material temperature does not exceed 100 ° C. during cold rolling. For example, when the material temperature tends to rise, the rolling speed is slowed down and the material temperature is lowered by using a large amount of rolling oil. By this temperature control, the processing strain introduced during cold rolling is not released, and the hardness after cold rolling is maintained at Vickers hardness HV400 or higher.

The method of manufacturing a non-magnetic stainless steel thick plate having excellent spring characteristics will be specifically described below. The components of stainless steel used as a raw material in the present invention and the content thereof will be described. It is an austenite phase stabilizing element similar to C: N and is an element effective for improving spring characteristics. However, a large amount of C content reduces the corrosion resistance. Therefore, the upper limit of the C content is specified to be 0.08% by weight.

Si: An element useful for obtaining high strength. However, as the Si content increases, the magnetic permeability after cold working sharply increases. As a result, it becomes impossible to maintain non-magnetism. Therefore, the upper limit of the Si content is specified to be 3.0% by weight.

Like Mn: Ni, it is an austenite stabilizing element and suppresses the increase in magnetic permeability due to cold working.
Mn is an alloy element that is effective in increasing the solid solubility of N. In order to exert these performances, a Mn content of 2.0% by weight or more is necessary. Further, in order to maintain the non-magnetic property after cold working, it is necessary to adjust the Mn content together with the Ni content. However, even if Mn is contained in an amount of more than 5.0% by weight, an effect commensurate with that is not seen. Therefore, the Mn content is set to 5.0% by weight or less.

Cr: It is a basic component of stainless steel and must be contained in an amount of 16.0% by weight or more in order to obtain excellent corrosion resistance. However, if a large amount of Cr is contained, a large amount of δ-ferrite is generated, and it becomes impossible to secure non-magnetism. Therefore, the Cr content is set in the range of 16.0 to 20.0%.

Ni: A basic component of austenitic steel, and an alloying element essential for stabilizing the austenitic phase. In order to secure the magnetic permeability μ after cold working to be 1.005 or less, it is necessary to contain 11.5% by weight or more of Ni. However, a large amount of Ni exceeding 13.5% by weight
If included, the strength increasing effect due to cold working is reduced. Therefore, the Ni content is set to 11.5-13.5.
It was set within the range of weight%.

N: An alloying element which is an essential component for increasing the strength of stainless steel and which stabilizes the austenite phase. In order to exert these performances, it is necessary to contain N in an amount of 0.10% by weight or more. Also, Mn
Since it contains 2.0 to 5.0% of N, the solid solubility of N is increased. However, if the N content exceeds 0.30%, bubbles and the like are generated, and a sound steel ingot cannot be obtained.
Therefore, the N content is specified in the range of 0.10 to 0.30% by weight.

Ni _eq : Nickel equivalent Ni _eq Equation (1)
Is derived from the experimental results of the present inventors,
FIG. 1 is also based on the experimental results. That is, excellent spring characteristics can be obtained by cold-working Cr-Ni austenitic stainless steel containing Si, N and Mn and adjusting the composition, or by heat-treating it thereafter. As an index at this time, Ni _eq 21.0 or less is necessary to obtain Vickers hardness HV400 or more. Further, as shown in FIG. 2, Ni _eq 16.0 or more is required to suppress the magnetic permeability after cold rolling to 1.005 or less.

Cold rolling: As the rolling ratio applied to the finish annealed material increases, the hardness of the cold rolled thick plate increases. In order to obtain a high hardness of Vickers hardness HV400 or more, a cold working rate of 40 to 70% is necessary. 70%
Even if cold working is carried out at a rolling ratio of more than 100%, almost no increase in hardness commensurate with the increase in rolling ratio is observed, and on the contrary, the surface properties and formability of the steel sheet after cold rolling deteriorate.

Furthermore, when cold rolling a thick plate having a composition in which the austenite phase is stable and non-magnetic is maintained even when cold rolling is performed, the desired strength may not be obtained due to an increase in the material temperature. Occurs. Therefore, in the present invention, the temperature of the strip material is controlled to 100 ° C. or lower. From the viewpoint of developing high strength, the material temperature is controlled to be lower as the cold rolling rate is smaller.

Heat treatment: By performing heat treatment within 1 hour after cold rolling, higher strength can be obtained. At this time, if the heat treatment is performed at a temperature higher than 600 ° C., the strength is reduced. If the heat treatment temperature is lower than 400 ° C, the hardness improving effect cannot be obtained. Therefore, 400
It is desirable to carry out the heat treatment in the temperature range of up to 600 ° C. Further, the heat treatment time is set longer as the heat treatment temperature is lower.

[0024]

EXAMPLES The components of the stainless steel used in this example are shown in Table 1. Sample No. A1 to A3 are conventional steels, A1 is S
US304, A2 is SUS304N, A3 is SUS30
It is 5. C1 and C2 are within the ranges specified in the present invention with respect to alloy components and contents, but nickel equivalent Ni
_eq is a comparative steel having a value outside the range specified in the present invention. Sample No. B1 to B5 are steels in which both the alloy component content and the nickel equivalent Ni _eq satisfy the requirements of the present invention.

[0025]

[Table 1]

Each of the steels was melted in a high frequency induction melting furnace of 30 kg, forged and hot rolled to have a plate thickness of 9 mm and a plate width of 1.
It was a hot rolled sheet of 00 mm. The hot-rolled sheet was subjected to soaking annealing by heating at 1000 ° C. for 5 minutes, and then cold-rolled at a rolling rate of 30 to 80%.

A test piece was cut out from the obtained cold-rolled sheet, and 9
The Vickers hardness HV was measured with a load of 8N. Also, 7
The magnetic permeability was measured under a magnetic field of 9.6 × 10 ³ A / m. The measurement results are shown in Table 2.

[0028]

[Table 2]

As is clear from Table 2, the magnetic permeability of A1 rapidly increased by cold rolling until it could not be used as a non-magnetic steel. A2 has a smaller increase in magnetic permeability due to cold working than A1, but has a magnetic permeability of 1. after cold rolling.
It exceeds 005 and lacks reliability as a non-magnetic material. A3 also has a large increase in magnetic permeability due to cold rolling, the magnetic permeability after cold rolling exceeds 1.005, and the hardness after cold rolling is less than HV400.

On the other hand, in Comparative Steel C1, when the rolling ratio is 70%, which is the upper limit of the range of the present invention, the magnetic permeability after rolling is 1. 0 even though the rolling temperature is 100 ° C. which is the upper limit. 0
It is over 05. Further, the comparative steel C2 has a rolling ratio of 70% of the upper limit value of the range of the present invention, and further 500 after cold rolling.
The hardness is less than HV400, even though the heat treatment of heating to ℃ for 1 hour is performed.

On the other hand, the steels of the test Nos. B1 to B5 produced according to the present invention have a magnetic permeability of 1.005 or less even after cold rolling, indicating that the austenite phase is extremely stable. Further, the hardness was remarkably increased by the cold rolling and the Vickers hardness was HV400 or more, and the excellent properties in strength were shown.

However, even if stainless steel, which has the composition including nickel equivalent Ni _eq and satisfies the requirements specified in the present invention, when a rolling ratio outside the range of the present invention is given, as shown in the comparative method. In addition, the hardness is less than HV400 even if a heat treatment of heating at 550 ° C. for 5 minutes after cold rolling is performed. If the rolling temperature exceeds 100 ° C, the Vickers hardness is less than HV400. Further, when the stainless steel of test No. B2 cold-rolled at a rolling ratio within the range of the present invention was subjected to a heat treatment at a high temperature outside the range of the present invention, a remarkable decrease in hardness was observed, and the Vickers hardness HV was 400 or less. Fell. Also in the test No. B4, the magnetic permeability exceeds 1.005 by applying the rolling rate exceeding the range of the present invention.

Table 3 shows the temperature control during cold rolling when the stainless steels of tests No. B6 and B7 were used as the material to be rolled, and the thickness before cold rolling was 1.5 mm and 9 mm. The Vickers hardness HV of the cold-rolled steel sheet obtained by performing the ordinary rolling method at a rolling rate of 60% is shown. As is clear from Table 3, when stainless steel having a large original plate thickness of more than 4 mm was rolled, a hardness of HV 400 or higher could not be obtained by the ordinary cold rolling method.

[0034]

[Table 3]

[0035]

As described above, according to the method of the present invention, the plate thickness is 2 to 4 mm, the Vickers hardness is HV400.
A nonmagnetic stainless steel thick plate having a magnetic permeability of 1.005 or less can be obtained. This stainless steel thick plate is utilized as a material for electronic equipment parts, devices, etc., including spring parts for superconducting-related structural members such as linear motor cars, by taking advantage of the unprecedented plate thickness.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the minimum nickel equivalent Ni _eq required to maintain non-magnetism and the rolling rate.

FIG. 2 Permeability and nickel equivalent Ni _{eq of} a cold rolled steel sheet cold-rolled at a material temperature of 100 ° C. and a rolling rate of 40 to 70%.
Graph showing the relationship with

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[Procedure amendment]

[Submission date] April 20, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiaki Hori 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Nisshin Steel Co., Ltd.

Claims (2)

[Claims] 1. A weight C: 0.08% by weight or less, Si:
3.0 wt% or less, Mn: 2.0 to 5.0 wt%, N
i: 11.5-13.5% by weight, Cr: 16.0-2
0.0 wt% and N: 0.10 to 0.30 wt% and has the formula Ni _eq = Ni + 0.60 Mn + 9.69 (C
+ N) + After 0.18Cr-0.11Si ² nickel equivalent Ni _eq defined in the finished stainless steel which satisfies the 16.0 to 21.0 in the thickness 6 to 12 mm, subjected to finish annealing, followed by 1 Above line A-A and 40-70%
The method for producing a non-magnetic stainless steel thick plate, characterized in that cold rolling is performed with the rolling rate set in the range to control the temperature to 100 ° C. or less. 2. A weight C: 0.08 wt% or less, Si:
3.0 wt% or less, Mn: 2.0 to 5.0 wt%, N
i: 11.5-13.5% by weight, Cr: 16.0-2
0.0 wt% and N: 0.10 to 0.30 wt% and has the formula Ni _eq = Ni + 0.60 Mn + 9.69 (C
+ N) + After 0.18Cr-0.11Si ² nickel equivalent Ni _eq defined in the finished stainless steel which satisfies the 16.0 to 21.0 in the thickness 6 to 12 mm, subjected to finish annealing, followed by 1 Above line A-A and 40-70%
A method for producing a non-magnetic stainless steel thick plate, characterized in that cold rolling is performed at a rolling rate set to the range of 100 ° C. or less and the temperature is controlled to 100 ° C. or less, and further heat treatment is performed at a temperature of 600 ° C. or less for 1 hour or less. ..