JPH07113144A - Nonmagnetic stainless steel excellent in surface property and production thereof - Google Patents

Abstract

PURPOSE:To develop a nonmagnetic stainless steel in which the occurrence of cracking is prevented at the time of hot working and excellent in strength, toughness and surface properties by reducing the content of S in an Ni-Cr based nonmagnetic stainless steel and controlling the conditions in cold rolling. CONSTITUTION:An austenitic stainless steel having a compsn. contg. by weight, <0.08% C, <3.0% Si, <2.0% Mn, <0.003% S, 9.5 to 11.5% Ni, 16.0 to 20.0% Cr and 0.1 to 0.25% N or furthermore contg. one or >= two kinds among <1.0% Cu, <3.0% Mo, <0.010% B, <0.10%Al, <0.050% Ca, <0.050% rare earth elements and <0.030% Y and having 14.5 to 19.0 nonmagnetic property stability index expressed by the formula I is subjected to cold rolling at 40 to 70% draft while the hot sheet is held to 40 to 180 deg.C. Next, it is subjected to aging treatment of holding to <=600 deg.C for <=1hr. The nonmagnetic stainless steel sheet free from the occurrence of cracking at the time of hot rolling, excellent in surface properties and having >380 hardness HV and <1.05 permeability mucan be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-magnetic stainless steel suitable as a material for spring parts used in a strong magnetic field and a method for producing the same.

[0002]

2. Description of the Related Art Practical research on a linear motor car utilizing magnetic levitation by a superconducting magnet has been actively conducted. The linear motor car moves by using the repulsive force of the superconducting coils attached to the concrete panel and the linear motor car as propulsive force. Superconducting coils are attached to concrete panels using non-magnetic bolts and nuts. The bolts are fixed via disc springs and washers so that they will not loosen due to impact when the vehicle passes, thermal expansion of the coils, and the like.

The disc springs and washers used are of high strength because they are designed to have a large tightening torque so that the bolts will not loosen, and are non-magnetic so as not to affect the magnetic field generated by the superconducting coil. In addition, it is required to have excellent corrosion resistance. In addition, a strong magnetic field is generated even in the power transformer of a normal power device. The spring components used in this type of device can also be made non-magnetic to reduce power loss due to heat generation and the like. Austenitic stainless steels such as SUS304 and SUS316 have been conventionally used as materials having such required characteristics. Further, in recent years, high N austenitic stainless steels such as SUS304N1 and SUS304N2 have come to be used.

[0004]

However, the conventional austenitic stainless steel cannot be said to be a material sufficiently satisfying various characteristics required for use in a strong magnetic field. For example, in SUS304, a work-induced martensite phase is generated when cold working with a work ratio of about 20% or more is performed. As a result, it becomes magnetized and the magnetic permeability μ exceeds 1.05. SUS316 maintains low magnetic permeability even after cold working, but cannot secure high strength. Further, since the contents of Ni and Mo as alloy components are high, it is an expensive material. Austenitic stainless steels such as SUS304N1 and SUS304N2 are likely to have mountain-shaped surface defects during hot rolling. The generated surface defects often remain without being removed even if the surface of the steel sheet after hot rolling is ground with a grinder or the like, and are not removed by barrel polishing after product processing. The remaining surface flaws adversely affect the product characteristics such as corrosion resistance and thus cause a reduction in product yield.

Various methods have been proposed in order to prevent surface defects after hot rolling. For example, JP-A-57-1615
In the publication No. 3, δ _cal = 3 (Cr + Mo + 1.5 × Si
+ 0.5 × Nb) -2.8 (Ni + 0.5 × Mn + 0.
5 × Cu) −84 (C + N) −19.8 presented δ
The calculated value of ferrite is set to 4.0 or less. δ _cal ≤
Although 4.0 has some improvement in hot workability, δ _cal for many types of steel is taken into consideration when considering that stainless steel is designed as a component in consideration of mechanical properties and corrosion resistance. It is difficult to adjust to a constant value of 0 or less. Even when stainless steel is melted, it is difficult to obtain high accuracy with respect to target setting components. JP-A-57
In Japanese Patent Publication No. 127506, the occurrence of surface defects is suppressed by adjusting the heating temperature during hot rolling according to the product of the superheat temperature ΔT (superheat) of molten steel during continuous casting and the N content. However, the superheat temperature ΔT and the N content change variously during continuous casting. In addition, considering the heating furnace operation in an actual line that simultaneously heats multiple slabs,
It becomes difficult to control the heating temperature during hot rolling for all slabs.

By adding a special component such as Ti,
The prevention of surface defects is also partly done. However, when Ti or the like is added, inclusions increase, and a drawback that stringer flaws increase appears. Moreover, since the special additive element is generally expensive, it also causes an increase in the cost of the steel material. The present invention has been devised to solve such a problem, and satisfies the strength and toughness by controlling the conditions of cold rolling and reduces the S content in the specified component system to reduce the hot rolling time. To prevent cracking
An object is to provide a non-magnetic stainless steel having a good surface property.

[0007]

The non-magnetic stainless steel of the present invention has a C: 0.08% by weight in order to achieve its object.
Hereinafter, Si: 3.0 wt% or less, Mn: 2.0 wt% or less, S: less than 0.003 wt%, Ni: 9.5 to 11.
5% by weight, Cr: 16.0 to 20.0% by weight and N:
0.1 to 0.25% by weight, the nonmagnetic stability index X defined by the formula (1) is in the range of 14.5 to 19.0,
In addition, the magnetic permeability μ is 1.05 or less.
This non-magnetic stainless steel further has Cu: 1.0 wt% or less, Mo: 3.0 wt% or less, B: 0.010 wt% or less, Al: 0.10 wt% or less, Ca: 0.050 wt%. % Or less, rare earth element: 0.050% by weight or less and Y:
One or more of 0.030% by weight or less can be contained. X = Ni + 0.60 × Mn + 9.69 × (C + N) + 0.18 × Cr-0.11Si ² (1) The non-magnetic stainless steel of the present invention is a stainless steel having the above-mentioned components and composition, It is manufactured by performing cold rolling at a rolling rate of 40 to 70% while maintaining the material temperature in the range of 40 to 180 ° C. After cold rolling, an aging treatment may be performed in which the temperature is kept at 600 ° C. or lower for 1 hour or less.

[0008]

The present inventors investigated and studied the influence of alloying elements, cold rolling and heat treatment on the spring characteristics of Cr-Ni austenitic stainless steel from a wide range of viewpoints. As a result, when controlling the rolling temperature and rolling rate of cold rolling to be applied to the stainless steel in which the alloying elements and their contents are specified,
It has been found that the target strength and toughness are maintained. Further, it was found that cracks generated during hot rolling become mountain-shaped surface defects in the coil after hot rolling, and it was found that there is a close correlation between the occurrence of cracks and the amount of S in the slab. As a result, it has been found that the surface texture is improved by reducing the S content in the stainless steel whose component system is specified. Hereinafter, alloying elements contained in the non-magnetic stainless steel of the present invention and their contents will be described. C: 0.08% by weight or less Strongly acts as a solid solution in the matrix to cure the material,
It is an effective alloying element for stabilizing the austenite phase and improving the spring characteristics. However, when a large amount of C is contained in excess of 0.08% by weight, the corrosion resistance decreases.

Si: 3.0 wt% or less It is an effective alloying element for obtaining high strength. But S
As the i content increases, the magnetic permeability after cold working sharply increases. As a result, non-magnetism cannot be maintained. At this point, the upper limit of the Si content was set to 3.0% by weight. Mn: 2.0 wt% or less Similar to Ni, it exhibits an action of stabilizing the austenite phase and suppresses an increase in magnetic permeability due to cold working. But,
When a large amount of Mn exceeding 2.0% by weight is contained, the manufacturability and the like in the steelmaking stage deteriorates and the production cost rises.

S: 0.003% by weight or less This is a harmful element that deteriorates hot workability and causes minute cracks on the surface of the slab during hot rolling. The generated minute cracks appear as mountain-shaped flaws or whisker-like surface flaws on the coil surface after hot rolling. These surface defects are S in the γ grain boundary.
Is considered to be generated by segregation and increase the crack susceptibility of grain boundaries. For example, C: 0.02 to 0.
08% by weight, Si: 0.5 to 3.0% by weight, Mn: 0.
5 to 2.0 wt%, S: 0.0005 to 0.009 wt%, Ni: 8.5 to 12.0 wt% and N: 0.1
12 various steels containing alloying components in the range of 0.25% by weight
After heating to 30 ° C. and hot rolling, the number of bald spots generated in the hot coil was compared with the S content, and the relationship shown in FIG. 1 was established. As shown in FIG. 1, the bald defects are reduced as the S content is reduced. From this, S
The lower the content, the better, but for economic reasons, the upper limit of the S content is less than 0.003% by weight, preferably 0.00
It was set to 2% by weight.

Ni: 9.5 to 11.5% by weight It is an essential alloying component for stabilizing the austenite phase. In order to ensure non-magnetism with a magnetic permeability of μ1.05 or less after cold working, it is necessary to contain 9.5 wt% or more of Ni. However, containing a large amount of expensive Ni in excess of 11.5% by weight causes an increase in cost and makes it impossible to provide an inexpensive austenitic stainless steel. Cr: 16.0 to 20.0% by weight It is a basic component of stainless steel, and in order to obtain excellent corrosion resistance, it is necessary to contain 16.0% by weight or more of Cr. However, when a large amount of Cr exceeding 20.0% by weight is contained, a large amount of δ ferrite is generated, and it becomes impossible to secure non-magnetism.

N: 0.1 to 0.25% by weight It is an alloying element which has an effect of improving the hardness and strength of austenitic stainless steel and stabilizes the austenitic phase. These effects are 0.10% by weight
The above N content remarkably appears. However, when a large amount of N exceeding 0.25% by weight is contained, blowholes are likely to occur and it becomes difficult to obtain a sound steel ingot. The non-magnetic stainless steel of the present invention further comprises Cu, Mo, B, Al and C.
One or two or more of a, a rare earth element, and Y can be contained if necessary. The action of these optional components is
It is as follows. Cu: 1.0 wt% or less Contributes to the stabilization of the austenite phase and maintains non-magnetic properties after cold working. However, a large amount of C exceeding 1.0% by weight
When u is contained, the hot cracking susceptibility during welding becomes high.

Mo: 3.0% by weight or less The hardness after aging treatment is remarkably increased, and it also works effectively for improving the corrosion resistance. However, if a large amount of Mo exceeding 3.0% by weight is contained, the amount of δ ferrite produced increases, and it becomes impossible to maintain non-magnetism. B: 0.010 wt% or less Not only does it improve toughness, but also exerts a remarkable effect in improving hot workability. However, if the content of B exceeds 0.010% by weight, a large amount of precipitates are generated and the toughness deteriorates. Al: 0.10 wt% or less It is an element effective as a deoxidizing agent. However, if it is added so as to remain in the steel in a large amount, inclusions are formed and the fatigue strength is significantly reduced. In this respect, the upper limit of the Al content is set to 0.
It was set to 10% by weight.

Ca: 0.050 wt% or less Not only does it improve the toughness, but also exerts a remarkable effect in improving the hot workability. However, Ca exceeding 0.050% by weight
The inclusion causes the formation of a large amount of precipitates and deteriorates the toughness. Rare earth element: 0.050 wt% or less Not only does it improve toughness, but also exerts a remarkable effect in improving hot workability. However, if the rare earth element is contained in an amount of more than 0.050% by weight, a large amount of precipitates will be generated and the toughness will be deteriorated. Y: 0.030% by weight or less Not only does the toughness be improved as in the case of the rare earth element, but also exhibits a remarkable effect in improving the hot workability. But 0.030
If the rare earth element is contained in an amount of more than wt%, a large amount of precipitates will be generated, and the toughness will be deteriorated.

X: 14.5 to 19.0 The nonmagnetic stability index X defined by the equation (1) is an index derived from the experimental results by the present inventors. The austenitic stainless steel whose composition is adjusted by containing Si, N, and Mn, which is the object of the present invention, can obtain high hardness, that is, high strength, as well as permeability by cold working or further heat treatment. Rises. The nonmagnetic stability index X is used as an index of hardness and magnetic permeability of such a material. The larger the non-magnetic stability index X, the lower the magnetic permeability and Vickers hardness of the steel material obtained after cold working.
In order to obtain a relatively inexpensive austenitic stainless steel having a hardness of HV 380 or higher, X ≦ 19.0 is required in the component system of the present invention in which the Ni content is limited. However, if the non-magnetic stability index X is less than 14.5, δ ferrite is likely to be generated, and it becomes difficult to stably obtain non-magnetism having a magnetic permeability of μ1.05 or less after cold rolling.

Cold rolling reduction rate: 40 to 70% The cold rolling applied to the finish annealed material is HV with Vickers hardness.
In order to obtain 380 or more, the rolling rate is 40 to 70%. If the rolling ratio is less than 40%, the required strength cannot be obtained. On the other hand, when the rolling ratio exceeds 70%, the hardness hardly increases in proportion to the increase in the rolling ratio, and rather, the shape and formability after cold rolling tend to deteriorate. Material temperature during cold rolling: 40 to 180 ° C. When austenite phase is stable and cold-rolled component stainless steel that is non-magnetic even if cold rolled is obtained,
The required strength may not be obtained due to an increase in material temperature during rolling. In the present invention, by maintaining the material temperature during rolling within the range of 40 to 180 ° C, stable and high strength is obtained. In particular, in order to develop high strength, the lower the cold rolling rate, the lower the material temperature is set.

Aging treatment: When cold-rolled stainless steel is subjected to an aging treatment within 1 hour, higher strength can be obtained. However, when the aging temperature exceeds 600 ° C, the strength tends to decrease. The lower limit of the aging treatment temperature is not particularly specified. However, in order to raise the hardness to the required level, a lower temperature requires a longer heat treatment. Therefore, in actual operation, 300 to 60
Aging treatment is preferably performed in the range of 0 ° C.

[0018]

EXAMPLES The components of the stainless steel used in this example are
It shows in Table 1. In Table 1, Group B is a stainless steel that satisfies the requirements specified in the present invention. Group A is a conventional material, A1 is SUS304, A2 is SUS304
316. Although the group C satisfies the requirements stipulated in the present invention with respect to individual alloy components and contents,
A steel having a non-magnetic stability index X outside the range of the present invention.

[0019]

[Table 1]

Each stainless steel was melted in a high-frequency induction melting furnace of 30 kg, and hot rolled to obtain a plate thickness of 10 mm and a plate width of 10 mm.
It was a 0 mm hot rolled plate. The hot-rolled sheet was soaked at 1050 ° C. for 5 minutes, and then cold-rolled at a rolling rate of 0 to 80%. Some steels were hot-rolled and cold-rolled in a real line. Also, some of the test materials were subjected to an aging treatment. Table 2 shows the conditions of cold rolling and heat treatment at this time. In Table 2, the production method D is the method of the present invention, and the production method E is the comparison method.

[0021]

[Table 2]

Occurrence of bald spots was investigated on the cold-rolled sheet after annealing and pickling. There are 4 levels of surface texture, with 1 having frequent bald defects, 2 having blistering defects, 3 having slight blistering defects, and 4 having no blistering defects. evaluated. Further, a test piece was cut out from the obtained cold-rolled sheet and the Vickers hardness (HV) was measured. Furthermore,
Using a magnetic balance, under a magnetic field of 1000 Oersted,
The magnetic permeability of each test piece was measured. Table 3 shows the results of these surveys.
Shown in.

[0023]

[Table 3]

As is clear from Table 3, the conventional steel A
Conventional steel A1 manufactured by applying the method of the present invention to A1 and A2
The magnetic permeability rapidly increased due to cold rolling and could not be used as non-magnetic steel. Similarly, in the conventional steel A2, although the magnetic permeability as a non-magnetic steel was secured even when cold-rolled at a rolling rate of 70%, the obtained hardness did not satisfy HV380. In contrast, B produced according to the method of the present invention
The invention steels of the group have a magnetic permeability of 1.05 even after cold rolling.
Below, the austenite phase after cold rolling was stable. The Vickers hardness after cold rolling is H
It was V380 or more, and it was found that both the non-magnetic property and the strength were excellent. Regarding bald defects, comparative steels C1 and C2 having an S content outside the range of the present invention had bald defects on the coil surface after cold rolling, which caused a decrease in yield. On the other hand, in the B group steels of the present invention in which the S content was regulated to less than 0.003% by weight, the tendency of occurrence of bald defects was remarkably improved. Among them, the S content is 0.
The steels of B6 to B11 with 002% by weight or less did not have any bald defects. Also, B, rare earth elements, Ca, Y
The steel containing S and the like has an S content of 0.002 to 0.002.
S content of 0.002% by weight even within the range of 9% by weight
As with the following steels, no bald spots were observed.

Even if the steel compositionally satisfies the scope of the present invention, the steel B1 of the present invention was cold-rolled at a rolling ratio of more than 70% as shown in the comparison method of Table 2 and FIG. If
The magnetic permeability exceeded 1.05. On the contrary, in cold rolling with a rolling ratio of less than 30%, even if the heat treatment of heating at 550 ° C. for 30 minutes is performed, the Vickers hardness is only about HV340,
It was well below the target HV380. As shown in FIG. 3, the material temperature during cold rolling affected the hardness after aging treatment of the cold rolled steel material. In order to obtain the target Vickers hardness HV380 or higher, the material temperature during cold rolling is maintained at 40 to 180 ° C., and the rolling ratio is 4
It can be seen from FIG. 3 that cold rolling needs to be performed at 0% or more. Further, even if the present invention steel B2 is subjected to cold rolling within the range of the present invention, the Vickers hardness of HV is HV when subjected to heat treatment of 700 ° C. × 1 hour outside the range of the present invention after cold rolling.
It fell significantly to 362 and did not reach the target value HV380.

In the comparative steel C1 having the non-magnetic stability index X below the range of the present invention, even when cold rolling was performed at a rolling rate of 40% lower limit and a material temperature of 180 ° C. upper limit, which can be expected to have low magnetic permeability, cold rolling was performed. The magnetic permeability after rolling exceeded 1.05.
Comparative steel C having a non-magnetic stability index X exceeding the range of the present invention
In No. 2, even if the material is cold-rolled at a rolling rate of 70% with an upper limit of 40% and a material temperature of 40 ° C as a lower limit, which is expected to increase in hardness, and the heat treatment of heating at 500 ° C for 1 hour is performed, the hardness after the heat treatment is H
It was less than V380. As is clear from the above comparison, in the case where the steel compositionally satisfying the range specified in the present invention is subjected to cold rolling and aging treatment within the range of the present invention, Vickers hardness HV380 or more and magnetic permeability μ1 .0
A non-magnetic stainless steel of 5 or less could be stably obtained.

[0027]

As described above, in the present invention, the alloy components are combined under the specified conditions, and the cold rolling condition and the aging treatment condition are controlled, whereby the Vickers hardness HV380 or more and the magnetic permeability are obtained. A non-magnetic stainless steel having a μ of 1.05 or less can be stably obtained. This non-magnetic stainless steel is an inexpensive and high-strength material having excellent surface properties, and is used as a disc spring, a washer, etc., especially in a strong magnetic field.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between S content and bald spots.

FIG. 2 is a graph showing the effect of cold rolling reduction on magnetic permeability.

FIG. 3 is a graph showing the influence of the rolling ratio and material temperature during cold rolling on the hardness after aging treatment.

Claims (6)

[Claims] 1. C: 0.08 wt% or less, Si: 3.0
Wt% or less, Mn: 2.0 wt% or less, S: 0.003
Less than wt%, Ni: 9.5 to 11.5 wt%, Cr: 1
The nonmagnetic stability index X defined by the formula (1) is 1 including 6.0 to 20.0% by weight and N: 0.1 to 0.25% by weight.
It is in the range of 4.5 to 19.0 and the magnetic permeability μ is 1.0.
A non-magnetic stainless steel with excellent surface properties of 5 or less. X = Ni + 0.60 × Mn + 9.69 × (C + N) + 0.18 × Cr-0.11Si 2 ···· (1) 2. C: 0.08 wt% or less, Si: 3.0
Wt% or less, Mn: 2.0 wt% or less, S: 0.003
Less than wt%, Ni: 9.5 to 11.5 wt%, Cr: 1
6.0 to 20.0 wt% and N: 0.1 to 0.25 wt%, further Cu: 1.0 wt% or less, Mo: 3.0
Wt% or less, B: 0.010 wt% or less, Al: 0.1
0% by weight or less, Ca: 0.050% by weight or less, rare earth element: 0.050% by weight or less, and Y: 0.030% by weight or less, including one or more types and defined by the formula (1). A non-magnetic stainless steel having an excellent non-magnetic stability index X in the range of 14.5 to 19.0 and a magnetic permeability μ of 1.05 or less. X = Ni + 0.60 × Mn + 9.69 × (C + N) + 0.18 × Cr-0.11Si 2 ···· (1) 3. C: 0.08 wt% or less, Si: 3.0
Wt% or less, Mn: 2.0 wt% or less, S: 0.003
Less than wt%, Ni: 9.5 to 11.5 wt%, Cr: 1
The nonmagnetic stability index X defined by the formula (1) is 1 including 6.0 to 20.0% by weight and N: 0.1 to 0.25% by weight.
For stainless steel in the range of 4.5 to 19.0, rolling rate 40 to 40 while maintaining the material temperature in the range of 40 to 180 ° C.
Hardness HV38 characterized by 70% cold rolling
A method for producing non-magnetic stainless steel having a surface property of 0 or more and a permeability of μ1.05 or less. X = Ni + 0.60 × Mn + 9.69 × (C + N) + 0.18 × Cr-0.11Si 2 ···· (1) 4. After the cold rolling according to claim 3, 600 ° C.
A method for producing a non-magnetic stainless steel, which is subjected to an aging treatment in which the temperature is maintained at the temperature below for 1 hour. 5. C: 0.08 wt% or less, Si: 3.0
Wt% or less, Mn: 2.0 wt% or less, S: 0.003
Less than wt%, Ni: 9.5 to 11.5 wt%, Cr: 1
6.0 to 20.0 wt% and N: 0.1 to 0.25 wt%, further Cu: 1.0 wt% or less, Mo: 3.0
Wt% or less, B: 0.010 wt% or less, Al: 0.1
0% by weight or less, Ca: 0.050% by weight or less, rare earth element: 0.050% by weight or less, and Y: 0.030% by weight or less, including one or more types and defined by the formula (1). The non-magnetic stability index X is subjected to cold rolling at a rolling rate of 40 to 70% while maintaining the material temperature in the range of 40 to 180 ° C. A method for producing a non-magnetic stainless steel having a hardness HV of 380 or more and a magnetic permeability of μ1.05 or less and having excellent surface properties. X = Ni + 0.60 × Mn + 9.69 × (C + N) + 0.18 × Cr-0.11Si 2 ···· (1) 6. After the cold rolling according to claim 5, 600 ° C.
A method for producing a non-magnetic stainless steel, which is subjected to an aging treatment in which the temperature is maintained at the temperature below for 1 hour.