JP2024129165A - Stainless steel with excellent cold forgeability, corrosion resistance and non-magnetic properties - Google Patents

Abstract

The present invention provides a stainless steel that is excellent in cold forgeability, corrosion resistance, and non-magnetic properties. The stainless steel contains, by mass%, 0.0010-0.15% C, 0.01-2.00% Si, 0.01-10.00% Mn, 8.00-30.00% Ni, 9.0-21.0% Cr, 0.01-3.00% Mo, 0.01-5.00% Cu, 0.0010-0.10% N, and 0.0001-0.05% B, and has an A value represented by formula (a) of -100 or less and a B grain boundary occupancy rate of 1% or more. A value = 551-462 (C + N) - 9.2Si - 8.1Mn - 29 (Ni + Cu) - 13.7Cr - 18.5Mo (a) [Selection diagram] None

Description

The present invention relates to stainless steel, and in particular to non-magnetic stainless steel that has excellent corrosion resistance and cold forgeability.

Non-Patent Document 1 presents Md30 as an evaluation index for the austenite stability of austenitic stainless steel. Md30 is the temperature (°C) at which the structure transforms to 50% martensite phase when a tensile true strain of 0.30 is applied to a single-phase austenite sample. The higher this value, the more unstable the material is. Non-Patent Document 1 presents the formula for Md30 as a function of the component composition.

Conventionally, austenitic stainless steels such as SUS316 and SUS316L are generally used as stainless steels for nonmagnetic parts. In contrast, Patent Documents 1 and 2 disclose a low-cost, high-hardness nonmagnetic steel that has excellent hydrogen embrittlement resistance, mechanical properties, and corrosion resistance, and is characterized in that the steel has a predetermined composition including C: 0.15-0.80%, Ni: 8.0-20.0%, Cr: 8.0-18.0%, Mo: 0.05-0.50%, V: 0.50-3.00%, and Al: 0.001-1.000%, and that the value of formula (3) obtained by modifying the above Md30 formula described in Non-Patent Document ¹ is set to -100 or less, and 50 or more V(C,N) precipitates of 50 nm or less are dispersed and present in an area of 3.5×10 ⁻² μm 2.

Patent Document 3 discloses tilt rolling. In tilt rolling, three work rolls are arranged on a roll axis that is twisted and tilted in the same direction around the material to be rolled. Each work roll revolves around the material to be rolled while rotating on its own axis. As a result, the material to be rolled is rolled in a spiral shape while moving forward.

JP 2019-49036 A JP 2016-183372 A Japanese Patent Application Publication No. 05-329510

Nohara et al., "Dependence of Deformation-Induced Martensitic Transformation on Composition and Grain Size in Metastable Austenitic Stainless Steels," Iron and Steel, Vol. 5, 63 (1977), pp. 772-782

By using SUS316, SUS316L, or the austenitic stainless steels described in Patent Documents 1 and 2, non-magnetic steels with excellent mechanical strength have been realized. However, it has been found that it is difficult for these conventionally known steels to simultaneously satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working. In particular, in conventional technology, sensitization occurs due to high C, which deteriorates corrosion resistance. In addition, because the material strength before cold forging is high, tool life is short and the forging load for large-diameter steel bars increases. It has been found that these factors cause the cold forgeability to deteriorate. Furthermore, it has been found that processing at high strain such as cold forging causes material processing limits (cracks) in conventional steels.

The third objective of the present invention is to provide a stainless steel that can improve corrosion resistance, reduce tensile strength, improve cold forgeability, and further improve non-magnetic properties after cold working.

In the invention of the original application, three inventions were created: a first invention corresponding to the first object, a second invention corresponding to the second object, and a third invention corresponding to the third object. In the invention of the present application, which is a divisional application of the original application, the following third invention is defined.
That is, the gist of the present invention is as follows.

[14] <Third Invention>
The chemical composition, in mass%, is
C: 0.0010-0.15%, Si: 0.01-2.00%, Mn: 0.01-10.00%, Ni: 8.00-30.00%, Cr: 9.0-21.0%, Mo: 0.01-3.00%, Cu: 0.01-5.00%, N: 0.0010-0.10%, B: 0.000 1-0.05%,
containing Al: 0-2.0%, Ti: 0-2.00%, Nb: 0-2.00%, Sn: 0-2.5%, V: 0-2.0%, W: 0-3.0%, Ga: 0-0.05%, Co: 0-2.5%, Sb: 0-2.5%, Ta: 0-2.5%, Ca: 0-0.05%, Mg: 0-0.012%, Zr: 0-0.012%, REM: 0-0.05%, Pb: 0-0.30%, Se: 0-0.80%, Te: 0-0.30%, Bi: 0-0.50%, S: 0-0.50%, P: 0-0.30%, and the balance being Fe and impurities;
The A value represented by the following formula (a) is −100 or less,
Stainless steel with a grain boundary occupancy rate of 1% or more.
A value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
In the formula (a), the symbol of an element means the content (mass%) of the element in the steel. When the content of an element in the formula (a) is 0%, "0" is substituted for the corresponding symbol in the calculation.

[15] The chemical composition further comprises, in mass%,
Group A includes one or more selected from Al: 0.001 to 2.0%, Ti: 0.01 to 2.00%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001 to 2.0%, W: 0.05 to 3.0%, Ga: 0.0004 to 0.05%, Co: 0.05 to 2.5%, Sb: 0.01 to 2.5%, and Ta: 0.01 to 2.5%;
As group B, one or more selected from Ca: 0.0002 to 0.05%, Mg: 0.0002 to 0.012%, Zr: 0.0002 to 0.012%, and REM: 0.0002 to 0.05%;
Group C: one or more selected from Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, Bi: 0.0001 to 0.50%, S: 0.0001 to 0.50%, and P: 0.0001 to 0.30%;
The stainless steel according to [14], containing one or more of groups A to C.

[16] The stainless steel according to [14] or [15], having a pitting potential of 0.05 V or more.
[17] The stainless steel according to any one of [14] to [16], having a tensile strength of 700 MPa or less.
[18] The stainless steel according to any one of [14] to [17], having a limit compression rate of 60% or more.
[19] The stainless steel according to any one of [14] to [18], having a relative magnetic permeability of 1.10 or less after cold working.

The stainless steel of the third invention contains specified components and has a B grain boundary occupancy rate of 1% or more, which makes it possible to satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working.

The stainless steel of the present invention can be applied in either a bar shape or a plate shape. In particular, it is suitable for use as bar-shaped steel material. Bar-shaped steel material includes "steel bar," "wire rod," "steel wire," "deformed wire," "deformed steel bar," and the like. The stainless steel of the present invention is an austenitic stainless steel.

The third invention aims to provide stainless steel, particularly bar-shaped steel, that can satisfy the requirements for corrosion resistance, cold forgeability, and non-magnetic properties after cold working, as described above.

Regarding cold forgeability, a φ8 x 12 mm test piece is used, and an end-constrained compression test (processing temperature: RT (room temperature), strain rate: 10/s) is performed. The maximum compression ratio at which no cracks appear on the side of the test piece after compression processing is defined as the limit compression ratio, and the target limit compression ratio is 60% or more.

For corrosion resistance, the center of the L-section of a φ20 x 30 mm test piece (20 width x 30 length x 1 mm thickness) was used as the evaluation surface, and passivation treatment was performed on the evaluation surface, including the evaluation surface, by immersing it in 15% nitric acid for 30 minutes. After that, a pitting potential test was performed on the evaluation surface according to JIS G 0577 (3.5% NaCl, 30°C, average of N = 3, V vs Ag/AgCl, saturated KCl) to measure the pitting potential. The goal of the third invention is to achieve a pitting potential of 0.05 V or more. In the examples, the pitting potential of the comparative material without passivation treatment was measured immediately after polishing.

Regarding the non-magnetic properties after cold working, first, a solution heat treatment is performed at 1100°C for 30 minutes (water cooling), and then a cold working sample with a cold working rate (reduction of area) of 80% is prepared, and the relative permeability at 1000 [Oe] is measured. The goal of the third invention is to achieve a relative permeability of 1.10 or less.

The details of the third invention are explained below.

<Third Invention>
<<B grain boundary occupancy rate of the stainless steel according to the third invention>>
The present inventors came up with the idea of controlling the B grain boundary occupancy rate of a steel material as a means for satisfying the corrosion resistance, cold forgeability, and non-magnetic properties after cold working in stainless steel, particularly in bar-shaped steel material. The B grain boundary occupancy rate (%) is the ratio (B/A×100) of the grain boundaries (B) where a finite amount of B exists to the total grain boundaries (A). It was conceived that a large B grain boundary occupancy rate promotes passivation of Cr-deficient regions due to grain boundary Cr-based precipitates, improving corrosion resistance, facilitating plastic deformation at the grain boundaries, improving cold forgeability, and suppressing local deformation at the grain boundaries, thereby suppressing the generation of processing-induced α' martensite in the magnetic phase and maintaining non-magnetic properties.

The B grain boundary occupancy rate was evaluated using EPMA analysis. The total length (A) of the grain boundaries in an arbitrary field of view was measured in the L section of the steel (a section including the center line in the case of a bar-shaped steel), and then an area analysis of the B concentration was performed in the same field of view. Grain boundaries with a higher B concentration than the parent phase within the grains were defined as B grain boundary occupancy, the length of the B grain boundary occupancy (B) was calculated, and the B grain boundary occupancy rate was calculated using the above formula.

It was also found that if the B grain boundary occupancy rate of the steel is 1% or more, the above-mentioned target corrosion resistance, cold forgeability, and non-magnetic properties after cold working can be satisfied. It is more preferable for the B grain boundary occupancy rate to be 5% or more on average, even more preferable for it to be 15% or more, and even more preferable for it to be 20% or more.

<<Composition of the Stainless Steel of the Third Invention>>
Next, the composition of the stainless steel according to the third invention will be described. In the composition, % means mass %.

(C: 0.0010-0.15%)
C suppresses the formation of processing-induced martensite and enhances non-magnetic properties, so the content is set to 0.0010% or more. Excessive addition of C reduces the B grain boundary occupancy rate and reduces corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the C content is set to 0.15%, preferably 0.12% or less, more preferably 0.05% or less, and even more preferably 0.02% or less. The upper limit of C is preferably set to less than 0.15%.

(Si: 0.01-2.00%)
Silicon is added as a deoxidizing element, and is set to 0.01% or more. Excessive addition of silicon reduces the B grain boundary occupancy rate, and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the silicon content is The upper limit of the amount is set to 2.0%, preferably 1.2% or less, more preferably 0.6% or less, and even more preferably 0.5% or less.

(Mn: 0.01-10.00%)
Mn suppresses the formation of processing-induced martensite and enhances non-magnetic properties, so the content is set to 0.01% or more. Excessive addition of Mn reduces the B grain boundary occupancy rate and reduces the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Mn content is set to 10.0%, preferably 2.5% or less, more preferably 1.5% or less, and even more preferably 1.0% or less. do.

(Ni: 8.00-30.00%)
Ni suppresses the formation of processing-induced martensite and enhances non-magnetic properties. In addition, in order to enhance cold forgeability, the Ni content is set to 8.00% or more. Preferably, it is 10.00% or more. The content is more preferably 13.00% or more, and even more preferably 15.00% or more. Excessive addition of Ni reduces the B grain boundary occupancy rate, and deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ni content is set to 30.00%, and preferably to 25.00% or less.

(Cr:9.0~21.0%)
Cr suppresses the formation of processing-induced martensite and improves non-magnetic properties. In order to improve corrosion resistance, the Cr content is set to 9.0% or more, preferably 10.5% or more. Excessive Cr content is not recommended. Addition of Cr reduces the B grain boundary occupancy rate, and deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Cr content is set to 21.0%, and preferably 19.5% or less. , and more preferably 15.0% or less.

(Mo: 0.01-3.00%)
Mo suppresses the formation of processing-induced martensite and enhances non-magnetic properties. In addition, Mo content is set to 0.01% or more in order to enhance the corrosion resistance and cold forgeability. Addition of Mo reduces the B grain boundary occupancy rate and deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Mo content is set to 3.0%, and preferably 2.8% or less. , more preferably 2.5% or less, and even more preferably 1.0% or less.

(Cu: 0.01-5.00%)
Cu suppresses the formation of processing-induced martensite and enhances non-magnetic properties. In addition, in order to enhance cold forgeability, the Cu content is set to 0.01% or more. Preferably, it is 1.00% or more. The content of Cu is more preferably 2.00% or more. Excessive Cu addition reduces the B grain boundary occupancy rate, deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties, and also causes hot brittleness. The upper limit of the Cu content is set to 5.00%, and preferably to 3.50%.

(N: 0.0010-0.10%)
N suppresses the formation of processing-induced martensite and enhances non-magnetic properties, so the content is set to 0.0010% or more. Excessive addition of N reduces the B grain boundary occupancy rate and reduces corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the N content is set to 0.10%, preferably 0.08% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. do.

(B: 0.0001-0.05%)
B is a main element that increases the B grain boundary occupancy rate, and in order to improve corrosion resistance, cold forgeability, and non-magnetic properties, the content is set to 0.0001% or more, preferably 0.0005% or more. When B is added, coarse B-based precipitates are formed, which in turn deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the B content is set at 0.05%, and preferably 0.02%. The content is preferably 0.015% or less, and more preferably 0.015% or less.

The stainless steel of the third invention contains the above components, with the remainder being Fe and impurities. It may further contain one or more components selected from the following components:

(Al: 0-2.0%)
Al may be added to fix N, which reduces the B grain boundary occupancy rate. On the other hand, excessive addition of Al will form coarse Al-based precipitates, which will result in deterioration of corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Al content is set to 2.0%, preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.05% or less. The preferred lower limit is 0.001%.

(Ti: 0-2.00%)
Ti may be added to fix C and N, which lowers the B grain boundary occupancy rate. On the other hand, excessive addition of Ti forms coarse Ti-based precipitates, which deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ti content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. The lower limit of Ti is preferably 0.01% or more, and more preferably 0.05% or more.

(Nb: 0-2.00%)
Nb may be added to fix C and N, which lowers the B grain boundary occupancy rate. On the other hand, excessive addition of Nb results in the formation of coarse Nb-based precipitates, which deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Nb content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, and further preferably 0.5% or less. The lower limit of Nb is preferably 0.01% or more, and more preferably 0.05% or more.

(Sn: 0-2.5%)
Sn is an effective element for improving corrosion resistance, so it may be contained. However, if an excessive amount of Sn is contained, the effect is saturated and, conversely, the corrosion resistance, cold forgeability, and non-magnetic properties are deteriorated. Therefore, the upper limit of the Sn content is set to 2.5%, more preferably 1.0% or less, and even more preferably 0.2% or less. In order to realize this, the Sn content is preferably 0.0001% or more, more preferably 0.01% or more, and even more preferably 0.05% or more.

(V: 0-2.0%)
V may be added to fix C and N, which lowers the B grain boundary occupancy rate. On the other hand, excessive V addition will form coarse V-based precipitates, which will result in poor corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the V content is set to 2.0%, preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. The lower limit of V is preferably 0.001%.

(W: 0-3.0%)
W may be contained since it is an effective element for improving corrosion resistance. However, if W is contained in excess, the effect is saturated and, conversely, the corrosion resistance, cold forgeability, and non-magnetic properties are deteriorated. Therefore, the upper limit of the W content is set to 3.0%, more preferably 2.0% or less, and even more preferably 1.5% or less. In order to realize this, the W content is preferably 0.05% or more, and more preferably 0.10% or more.

(Ga: 0-0.05%)
Ga is an effective element for improving corrosion resistance, so it may be contained. However, if Ga is contained in excess, the effect is saturated and, conversely, the corrosion resistance, cold forgeability, and non-magnetic properties are deteriorated. Therefore, the upper limit of Ga content is set to 0.05%. In order to obtain the above-mentioned effect, the Ga content is preferably set to 0.0004% or more.

(Co: 0-2.5%)
Co may be contained since it has the effect of improving corrosion resistance. However, if an excessive amount of Co is contained, the effect becomes saturated and there is a risk that the corrosion resistance, cold forgeability, and non-magnetic properties may be deteriorated. Therefore, the upper limit of Co content is set to 2.5%, more preferably 1.0% or less, and further preferably 0.8% or less. The Co content is preferably 0.05% or more, and more preferably 0.10% or more.

(Sb: 0-2.5%)
Sb may be contained since it has the effect of improving corrosion resistance. However, if an excessive amount of Sb is contained, the effect becomes saturated and there is a risk that the corrosion resistance, cold forgeability, and non-magnetic properties may be deteriorated. Therefore, the upper limit of Sb content is set to 2.5%, more preferably 1.0% or less, and further preferably 0.8% or less. The Sb content is preferably 0.01% or more, and more preferably 0.05% or more.

(Ta: 0-2.5%)
Ta may be added to fix C and N, which lowers the B grain boundary occupancy rate. On the other hand, excessive Ta addition will form coarse Ta-based precipitates, which will result in poor corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ta content is set to 2.5%, preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. The preferred lower limit of Ta is 0.01%.

(Ca: 0-0.05%)
Ca may be added as necessary for deoxidization. On the other hand, excessive addition of Ca forms coarse Ca-based inclusions, which deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. The upper limit of the Ca content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. The lower limit of Ca is preferably 0.0002%.

(Mg: 0-0.012%)
Mg may be added as necessary for deoxidization. On the other hand, excessive addition of Mg forms coarse Mg-based inclusions, which deteriorates cold forgeability and hydrogen embrittlement resistance. The upper limit of the Mg content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. The lower limit of Mg is preferably 0.0002%.

(Zr: 0-0.012%)
Zr may be added as necessary for deoxidization. On the other hand, excessive addition of Zr causes the formation of coarse Zr-based inclusions, which deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. The upper limit of the Zr content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. The lower limit of Zr is preferably 0.0002%.

(REM: 0-0.05%)
REM may be added as necessary for deoxidization. However, excessive addition of REM will cause the formation of coarse REM inclusions, which will deteriorate the corrosion resistance, cold forgeability, and non-magnetic properties. The upper limit of the content is set to 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. The preferable lower limit of REM is 0.0002%.

(Pb: 0-0.30%)
Pb is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Pb deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Pb content is set to 0. The Pb content is preferably 0.30%, more preferably 0.10% or less, and even more preferably 0.05% or less. The lower limit of Pb is preferably 0.0001%.

(Se: 0-0.80%)
Se is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Se deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Se content is set to 0. The content of Se is preferably 0.80%, more preferably 0.1% or less, and even more preferably 0.05% or less. The lower limit of Se is preferably 0.0001%.

(Te: 0-0.30%)
Te is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Te deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Te content is set to 0. The content of Te is preferably 0.30%, more preferably 0.1% or less, and even more preferably 0.05% or less. The lower limit of Te is preferably 0.0001%.

(Bi: 0-0.50%)
Bi is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Bi deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Bi content is set to 0. The Bi content is preferably 0.50%, more preferably 0.1% or less, and even more preferably 0.05% or less. The lower limit of Bi is preferably 0.0001%.

(S: 0-0.50%)
S is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of S deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the S content is set to 0. The content of S is set to 50%, preferably 0.1% or less, and more preferably 0.05% or less. The preferred lower limit of S is 0.0001%. S is an impurity that is mixed in from the steelmaking raw materials. It is usually contained in steel.

(P: 0-0.30%)
P is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of P deteriorates the corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the P content is set to 0. The P content is preferably 0.30%, more preferably 0.1% or less, and even more preferably 0.05% or less. The lower limit of P is preferably 0.0001%.

<Common to the first to third inventions>
"A value of formula (a)"
Based on the formula for Md30 described in Non-Patent Document 1, the following formula (a) was introduced.
A value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
The element symbol in formula (a) means the content (mass%) of the element in the steel. When the content of an element in formula (a) is 0%, the corresponding symbol is substituted with "0" for the calculation. The above formula (a) corresponds to the formula of Md30 described in Non-Patent Document 1 with the Nb term deleted. The reason for deleting the Nb term is that the addition rate of Nb is small and its contribution to Md30 is small.
In the first to third inventions, the A value shown in the above formula (a) is -100 or less. By making the A value -100 or less, the formation of processing-induced martensite is suppressed, and work hardening is reduced, resulting in softening and suppressing the occurrence of cracks, thereby improving cold forgeability. Furthermore, the first invention can obtain the effect of reducing microstrain and improving hydrogen embrittlement resistance. The second invention can obtain the effect of reducing cutting resistance and improving machinability by softening. As for hydrogen embrittlement resistance, processing-induced martensite at the fracture origin is reduced, improving hydrogen embrittlement resistance. The third invention can obtain the effect of improving non-magnetic properties.

<Quality of the steel materials according to the first to third inventions>
The stainless steel of the present invention, particularly the bar-shaped steel material, can achieve the following qualities as a result of having the above-mentioned component composition, and further as a result of the first invention having a microstrain of the steel material surface layer to D/4, as a result of the second invention having an amount of precipitated B as borides and an aspect ratio of sulfides, and as a result of the third invention having a B grain boundary occupancy rate of the steel material.

<Common to the first to third inventions>
It may be stainless steel having a tensile strength of 700 MPa or less.
<Common to the first to third inventions>
The stainless steel may have a limit compression ratio of 60% or more. Here, for the evaluation of the limit compression ratio, the shape of the test piece, the content of the compression test, and the definition of the limit compression ratio are all the same as those described above.

<Third Invention>
The material may be stainless steel having a pitting potential of 0.05V or more.

<Third Invention>
The stainless steel may have a relative magnetic permeability of 1.10 or less after cold working.
Here, the cold working has a cold working rate (area reduction rate) of 80%.

<<Method of manufacturing steel material according to the third invention>>
The method for producing the steel material according to the third invention will now be described.

<Third Invention>
In producing the stainless steel of the third invention, particularly a bar-shaped steel material, it is preferable to subject the material to heating, hot rolling (tilt rolling, BD, bar rolling, etc.), heat treatment, pickling, etc., but it is particularly preferable to control the rough rolling entry temperature and the average time between rough rolling stands and to perform a passivation treatment.

The rough rolling entry temperature of the steel material is specified to be 1000 to 1400°C, and the rough rolling inter-stand average time of the rolling material is set to be within the range of 0.01 to 30 seconds. It is more preferable that the rough rolling entry temperature is within the range of 1000 to 1300°C and the rough rolling inter-stand average time is within the range of 0.03 to 10 seconds. The rough rolling entry temperature is more preferably 1050 to 1300°C, and even more preferably 1100 to 1300°C. The rough rolling inter-stand average time is more preferably 0.05 to 5 seconds, and even more preferably 0.1 to 2 seconds. If the rough rolling entry temperature is less than 1000°C, strain accumulates in the steel material during hot rolling, B-based precipitates are formed within the grains, and the B grain boundary occupancy rate decreases, resulting in deterioration of corrosion resistance, cold forgeability, and non-magnetic properties. If the rough rolling entry temperature exceeds 1400°C, the B present at the grain boundaries diffuses into the grains and forms as intragranular B precipitates during rolling, resulting in a small B grain boundary occupancy rate. In addition, high-temperature heating causes the steel material to oxidize, resulting in a decrease in yield, or the steel material creeps during passage, resulting in poor rolling. In addition, if the rough rolling stand-to-stand average time is less than 0.01 seconds, strain accumulates in the steel material during hot rolling, B-based precipitates form within the grains, resulting in a small B grain boundary occupancy rate, resulting in poor corrosion resistance, cold forgeability, and non-magnetic properties. If the rough rolling stand-to-stand average time exceeds 30 seconds, the B present at the grain boundaries diffuses into the grains and forms as intragranular B precipitates during rolling, resulting in a small B grain boundary occupancy rate. In addition, high-temperature heating causes the steel material to oxidize, resulting in a decrease in yield, or the steel material creeps during passage, resulting in poor rolling.

When the rolled material controlled under the above conditions is heat-treated and the surface scale is removed, the passivation of the Cr-deficient regions due to grain boundary Cr-based precipitates is promoted, improving corrosion resistance. In addition, in the rolled and heat-treated material under the above conditions, plastic deformation at the grain boundaries is facilitated, improving cold forgeability, and local deformation at the grain boundaries is suppressed, suppressing the formation of processing-induced α' martensite in the magnetic phase, thereby maintaining non-magnetic properties. Here, the passivation treatment is a process in which the material is immersed in a solution such as nitric acid, and may be a single treatment or one of the acidic processes. This is effective when treating stainless steel (especially bar-shaped steel), and also has the same effect when treating products that have been secondary processed (drawn, forged, cut, etc.) from the bar-shaped steel.

<Third Invention>
(Example 3-1)
The steel was produced assuming AOD production, an inexpensive process for producing stainless steel, by melting the steel in a 100 kg vacuum melting furnace and casting it into a slab with a diameter of 180 mm. Then, stainless steel bars with a diameter of 20.0 mm and the chemical compositions shown in Tables 1 and 2 were produced under the following production conditions. In Tables 1 to 5, items outside the range of the third invention and items outside the preferred production conditions of the third invention are underlined.

The cast slab was heated, tilt rolled, and inline heat treated, the rough rolling entry temperature was adjusted to 1130°C, and rough rolling was performed. The average stand time for rough rolling was 1.8 s. After that, bar wire rolling was performed, followed by heat treatment at 1100°C for 30 minutes (water cooling) as a solution treatment, and pickling to produce a bar-shaped steel material with a diameter of 20.0 mm. From this bar-shaped steel material (φ20 mm), a φ20 x 30 mm L cross section was taken for corrosion resistance evaluation, and a φ8 x 12 mm test piece was taken for end restraint compression testing, with a length of 12 mm in the L direction, from the D (diameter)/4 position of the steel material C cross section.

The method for measuring the B-grain boundary occupancy rate of the bar-shaped steel material was performed using the L-section of a solution-treated bar-shaped steel material (φ20 mm) and the method described above. For corrosion resistance, a test piece with a diameter of φ20 x 30 mm was used and the method described above was used. For tensile strength, a solution-treated bar-shaped steel material (φ20 mm) was used and evaluated by a normal method. For cold forgeability, a test piece with a diameter of φ8 x 12 mm was used and the method described above was used to measure the limit compression ratio. For evaluation of the relative magnetic permeability after cold working, a bar-shaped steel material with a diameter of φ9 mm was cold-drawn with an area reduction rate of 80% from the above solution-treated bar-shaped steel material and the method described above was used.

The B grain boundary occupancy rate was rated as AA when it was 15% or more, A when it was 5% or more and less than 15%, B when it was 1% or more and less than 5%, and C when it was less than 1%.
Regarding corrosion resistance, 0.20V or more was rated as AA, 0.10V or more and less than 0.20V was rated as A, 0.05V or more and less than 0.10V was rated as B, and less than 0.05V was rated as C.
The tensile strength was rated as AA for 500 MPa or less, A for more than 500 MPa and 620 MPa or less, B for more than 620 MPa and 700 MPa or less, and C for over 700 MPa.
The limiting compression ratio was rated as AA for 80% or more, A for 70% or more and less than 80%, B for 60% or more and less than 70%, and C for less than 60%.
With regard to the relative permeability after cold working, 1.03 or less was rated as AA, more than 1.03 and less than 1.05 was rated as A, more than 1.05 and less than 1.10 was rated as B, and more than 1.10 was rated as C.
The evaluation results are shown in Tables 3 and 4.

The bar-shaped steel materials described in invention examples No. 1 to 39 have the composition and B grain boundary occupancy rate specified in the third invention, and the corrosion resistance, tensile strength, limit compression ratio, and relative magnetic permeability after cold working were all either AA, A, or B, which were good.

On the other hand, for Comparative Examples No. 40 to 54, any of the components was outside the range of the third invention, and the B grain boundary occupancy rate was outside the range of the third invention, and as a result, the corrosion resistance, tensile strength, limit compression ratio, and relative permeability after cold working were all C.

(Example 3-2)
Steel type P in Table 1 was used as the component composition, and the conditions shown in Table 4 were changed by varying the rough rolling entry temperature, the average time between rough rolling stands, and whether or not the test pieces were passivated during corrosion resistance evaluation (if no passivation treatment was performed, the test pieces were as polished). The other manufacturing conditions were the same as those in Example 3-1 above, and bar-shaped steel materials were manufactured and test pieces were prepared.

As shown in Table 5, invention examples No. 55 to 64 were manufactured under the preferred conditions of the third invention, had the component composition and B grain boundary occupancy rate specified in the third invention, and were excellent in corrosion resistance, tensile strength, limit compression ratio, and relative permeability after cold working, all of which were either AA, A, or B.

On the other hand, for Comparative Examples 65-68 and 70, one of the manufacturing conditions was outside the preferred range of the third invention, and the B grain boundary occupancy rate was outside the range of the third invention, resulting in a C for all of the corrosion resistance, tensile strength, limit compression ratio, and relative permeability after cold working. For Comparative Example 69, no passivation treatment was performed, and passivation in the Cr-deficient region was not promoted, resulting in a B grain boundary occupancy rate outside the range of the third invention, resulting in a C for all of the corrosion resistance, tensile strength, limit compression ratio, and relative permeability after cold working.

Claims (17)

The chemical composition, in mass%, is
C: 0.0010-0.15%,
Si: 0.01-2.00%,
Mn: 0.01 to 10.00%,
Ni: 8.00-30.00%,
Cr: 9.0-21.0%,
Mo: 0.01-3.00%,
Cu: 0.01 to 5.00%,
N: 0.0010-0.10%,
B: 0.0001-0.05%,
Al: 0-2.0%,
Ti: 0-2.00%,
Nb: 0 to 2.00%,
Sn: 0 to 2.5%,
V: 0 to 2.0%,
W: 0 to 3.0%,
Ga: 0 to 0.05%,
Co: 0 to 2.5%,
Sb: 0 to 2.5%,
Ta: 0 to 2.5%,
Ca: 0-0.05%,
Mg: 0 to 0.012%,
Zr: 0 to 0.012%,
REM: 0-0.05%,
Pb: 0 to 0.30%,
Se: 0-0.80%,
Te: 0 to 0.30%,
Bi: 0-0.50%,
S: 0-0.50%,
P: 0-0.30%,
and the balance being Fe and impurities;
The A value represented by the following formula (a) is −100 or less,
Stainless steel with a grain boundary occupancy rate of 1% or more.
A value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
In the formula (a), the symbol of an element means the content (mass%) of the element in the steel. When the content of an element in the formula (a) is 0%, "0" is substituted for the corresponding symbol in the calculation. The chemical composition further comprises, in mass %,
As group A,
Al: 0.001-2.0%,
Ti: 0.01-2.00%,
Nb: 0.01-2.00%,
Sn: 0.0001-2.5%,
V: 0.001-2.0%,
W: 0.05-3.0%,
Ga: 0.0004-0.05%,
Co: 0.05-2.5%,
Sb: 0.01 to 2.5%, and Ta: 0.01 to 2.5%,
One or more selected from
As group B,
Ca: 0.0002-0.05%,
Mg: 0.0002-0.012%,
Zr: 0.0002-0.012%, and REM: 0.0002-0.05%,
One or more selected from
As group C,
Pb: 0.0001 to 0.30%,
Se: 0.0001 to 0.80%,
Te: 0.0001 to 0.30%,
Bi: 0.0001 to 0.50%,
S: 0.0001-0.50%, and P: 0.0001-0.30%,
One or more selected from
Contains one or more of groups A to C of
The stainless steel of claim 1. The stainless steel according to claim 1 or 2, having a pitting potential of 0.05 V or more. The stainless steel according to claim 1 or 2, having a tensile strength of 700 MPa or less. The stainless steel according to claim 1 or 2, having a limiting compressibility of 60% or more. The stainless steel according to claim 1 or 2, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 3, having a tensile strength of 700 MPa or less. The stainless steel according to claim 3, having a limiting compressibility of 60% or more. The stainless steel according to claim 4, having a limiting compressibility of 60% or more. The stainless steel according to claim 7, having a limiting compressibility of 60% or more. The stainless steel according to claim 3, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 4, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 5, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 7, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 8, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 9, having a relative magnetic permeability of 1.10 or less after cold working. The stainless steel according to claim 10, having a relative magnetic permeability of 1.10 or less after cold working.