JPH1150203A - Martensitic stainless steel having surface high hardness, high corrosion resistance and high toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a surface-modified martensitic stainless steel for structures such as buildings, building materials or the like excellent in surface hardness, corrosion resistance and toughness. SOLUTION: This martensitic stainless steel is the one in which the compsn. of the matrix is composed of, by weight, 0.05 to 0.40% C, <=0.12% N, 12.0 to 16.0% Cr, 0.1%<=C+N<=0.40%, and the balance substantial Fe with inevitable impurities. In this case, the grain size in the surface layer is regulated to about 30 to 100 μm, the concn. of N in the surface layer is regulated to 0.2 to 0.8 wt.%, and the content of carbon nitride in the surface layer is regulated to <=0.7 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-modified martensitic stainless steel having excellent surface hardness, corrosion resistance and toughness, and used as a structural material such as construction and building materials.

[0002]

2. Description of the Related Art The addition of nitrogen is effective for improving the corrosion resistance of stainless steel. However, when attempting to add a large amount of nitrogen to martensitic stainless steel, a δ ferrite structure is formed during solidification, so that the solubility of nitrogen is low and blowholes are generated. For this reason, the limit is about 0.1% even if nitrogen is added to the material.

On the other hand, martensitic stainless steel has an austenitic structure at about 900 to 1300 ° C., at which temperature the solubility of nitrogen is high. For this reason, it is known that when a martensitic stainless steel is heat-treated at that temperature in an atmosphere mainly composed of nitrogen, nitrogen penetrates into the material from the surface layer and the surface is hardened (nitrided) (Japanese Patent Laid-Open No. 52-7 / 1982).
No. 7836).

[0004] In order to increase the toughness of martensitic stainless steel, crystal grains are refined using fine carbonitrides (see Japanese Patent Application Laid-Open No. 8-67950).

[0005]

However, depending on the nitriding conditions, poor hardening, poor corrosion resistance, or poor toughness may occur on the surface, and it is necessary to obtain surface characteristics having all of high surface hardness, high corrosion resistance and high toughness. Could not. Further, along with the refinement of crystal grains, a large amount of grain boundary carbonitride was generated, and the toughness was sometimes deteriorated. An object of the present invention is to solve these problems and to provide a martensitic stainless steel material having high surface hardness, high corrosion resistance and high toughness.

[0006]

Means for Solving the Problems The present inventors have conducted various studies to solve the above problems, and as a result, in martensitic stainless steel, limited the components of the matrix,
Further, it has been found that a martensitic stainless steel having high surface hardness, high corrosion resistance and high toughness can be obtained by limiting the crystal grain size, N amount (%), and precipitate amount of the surface. The present invention has been made based on this finding.

That is, in the present invention, the composition of the matrix is as follows: C: 0.05 to 0.40%, N: 0.1% or less, Cr: 12.0 to 16.0%, 0.1% ≦ C + N ≦ 0.4%, the balance being substantially Fe and martensitic stainless steel which is an unavoidable impurity, and having a surface grain size of about 30 to 100 μm; It is a martensitic stainless steel with high corrosion resistance and high toughness. Further, the present invention is a martensitic stainless steel having high surface hardness, high corrosion resistance, and high toughness, wherein the amount of carbonitride in the surface layer of the steel described above is 0.7% or less by weight. Further, the present invention is a martensitic stainless steel having high surface hardness, high corrosion resistance, and high toughness, wherein the N content of the surface layer in each of the steels described above is 0.2 to 0.8% by weight. .

Further, there is provided a martensitic stainless steel having high hardness, high corrosion resistance and high toughness, wherein the composition of each of the above-mentioned steels further contains 3% by weight or less of Mo. Further, the steel composition described above is a martensitic stainless steel having high hardness, high corrosion resistance and high toughness, characterized in that the composition of the steel further contains not more than 3.5% by weight of Ni. Further, the composition of each of the above-mentioned steels may further comprise 0.2% by weight or less of Ti, or 0.1% by weight.
A martensitic stainless steel having high surface hardness, high corrosion resistance and high toughness, characterized by containing one or more of Nb of 2% or less.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the range of components of steel of the matrix of the present invention will be described. C is added in an amount of 0.05% or more in order to secure a Vickers hardness of 550 or more in the surface layer of the martensitic stainless steel after nitriding. But,
If added in excess of 0.4%, coarse grain boundary carbides precipitate, deteriorating corrosion resistance and toughness.
%.

[0010] In order to impart corrosion resistance, Cr is used in an amount of 12.0%.
% Or more. However, if added over 16%,
The upper limit was limited to 16.0% because a δ ferrite structure was present and deteriorated corrosion resistance and toughness.

N is added to secure a Vickers hardness of 550 or more in the surface layer of the martensitic stainless steel after nitriding. However, if added in excess of 0.1%, bubbles are generated during coagulation and the productivity is significantly reduced, so the upper limit was limited to 0.1%.

C + N is set to 0.1% or more in order to secure Vickers hardness of 550 or more on the surface of the martensitic stainless steel after nitriding and to reduce the crystal grain size of the surface layer during nitriding to 100 μm or less. However, if added in excess of 0.4%, a large amount of carbonitrides are formed at the grain boundaries during cooling, deteriorating the toughness, so the upper limit was limited to 0.4%.

Mo is added as necessary to impart corrosion resistance. However, if Mo is added in excess of 3.0%, the effect is not only saturated, but also δ ferrite is formed, and the corrosion resistance and toughness are deteriorated. For this purpose, the upper limit is limited to 3.0%.

Ni is added as necessary to enhance the toughness of the martensite structure. However, if it exceeds 3.5%, residual γ is generated in the surface layer during nitriding, and the hardness of the surface layer is reduced. The upper limit was limited to 3.5%.

[0015] Ti and Nb are formed by Cr at the grain boundary during cooling.
One or two kinds as necessary to suppress carbonitrides, improve corrosion resistance and toughness, and prevent coarsening of crystal grains.
Although seeds are added, if added at 0.2% or more, not only the effect is saturated, but also the toughness is deteriorated. For this reason, the upper limit was limited to 0.2%. Mn, Si, S,
Elements such as P are usually contained in martensitic stainless steel.

The present invention comprises the above-mentioned matrix components and the following surface characteristics. The crystal grains of the surface layer after nitriding are
The thickness is limited to 100 μm or less in order to promote nitriding by diffusion of nitrogen at the grain boundary and secure toughness. However, 30 μm
In order to make it smaller, it is necessary to use carbonitride pinning.In the case of nitriding, grain boundary carbonitride is formed starting from the carbonitride used for pinning during cooling, and the toughness is deteriorated. The lower limit was limited to 30 μm.

The amount of carbonitride deposited on the surface layer after nitriding is limited to 0.7% or less by weight in order to ensure toughness and corrosion resistance.

The N content (%) of the surface layer after nitriding is 0.2% in order to make the corrosion resistance and the hardness of the surface layer 550 or more in Hv.
%. However, if it exceeds 0.8%, the surface layer C
The upper limit is limited to 0.8% because the amount of nitrides increases and the corrosion resistance and toughness deteriorate. Here, the surface layer of N amount (%) of the surface layer means, for example, a range of about 0.2 mm from the surface.

The grain size of the surface layer, the carbonitride and toughness of the surface layer,
FIG. 1 shows the relationship between the corrosion resistance. The hunted area in the figure is the scope of the present invention. FIG. 2 shows the relationship between the crystal grain size of the surface layer, the N content (%) of the surface layer, and the hardness, toughness, and corrosion resistance.
The hunted area in the figure is the scope of the present invention.
Within the range of the present invention, high hardness, high corrosion resistance and high toughness can be obtained. The martensitic stainless steel having the above components and surface properties has high surface hardness, high corrosion resistance and high toughness.

[0020]

EXAMPLES The present invention will be described more specifically below with reference to examples. Table 1 shows the components of the inventive steels A to M and comparative steels N to U. In Table 1, M less than 0.2%
The amounts of o and Cr and the amounts of Ti and Nb less than 0.01% are not particularly added, but show the analysis values contained in the steel.

The steels A to D of the present invention and the comparative steels N to P are 12.1
The amount of C (%) and the amount of N (%), which affect the strength and crystal grain size of martensite, are changed based on% Cr system as a basic component. Inventive steels B, E, and F and comparative steels Q and R contain 0.15% C-0.03% N as a basic component and have a Cr content (%) that affects the corrosion resistance of martensite.
Is changed. The steels E, G, H of the present invention and the comparative steel S are based on 0.16% C-13% Cr-0.04% N and have a Mo content that affects the corrosion resistance of martensite.
The amount (%) was changed.

The steels G, H, I, and J of the present invention and the comparative steel T have a content of 0.1%.
15% C-12.5% Cr-2% Mo-0.06% N-base is used as a basic component, and the amount of Ni (%) affecting the toughness of martensite is changed. Invention Steel G,
K, L, M and Comparative Steel U are 0.15% C-13% Cr-1
% Mo-0.05% N based on the Ti content (%) and N, which have an effect on the refinement of the crystal grains of martensite, as a basic component.
The amount (%) of b was changed.

The above-mentioned steel of the present invention and the comparative steel were melted in the usual stainless steel wire manufacturing process, hot-rolled to a diameter of 7 mm, and hot-rolled at 1000 ° C. The obtained hot-rolled wire was annealed, pickled, and cold-drawn to 5.5 mm in diameter. Thereafter, the steel wire was subjected to annealing and pickling to form a steel wire having a diameter of 5.5 mm. The above steel wire is φ5.5mm × 20mm for hardness evaluation test
And the end faces were mirror-polished. Also, it was processed into a test piece of φ5.5mm × 100mm for corrosion resistance evaluation test,
The specimen was processed into a 5.5 × 2 × 55 mm, 1 mm deep U-notched test piece for a toughness evaluation test.

Thereafter, the test piece was subjected to 1000 to 1200
Nitriding was performed in a nitrogen gas atmosphere at a temperature of 200 ° C., cooled to a room temperature by a nitrogen gas at a cooling rate of about 60 ° C./min, and low-temperature tempered at 200 ° C. As a result, a surface-modified martensitic stainless steel was obtained.

Next, tests were conducted to obtain the surface hardness, corrosion resistance and toughness of the heat-treated material. Surface hardness is JIS Z
According to 2244, the surface hardness of the test piece end face was measured. The hardness rank of the example of the present invention was 550 or more in Vickers hardness. Corrosion resistance of the surface layer is # 500 according to JIS Z2371.
After polishing for 240 hours, a test was conducted for 240 hours. According to the rust condition of the present invention, no flow rust was generated. Toughness is 5.5 × 2 size according to JIS Z2202
Test at room temperature with U-notch of × 55mm, depth 1mm,
Evaluation was made based on the Charpy impact value at that time. The Charpy impact value of the present invention was set to 60 J / cm ² or more.

Table 2 shows the test results. As is clear from Table 2, all of the examples of the present invention satisfy the above-described characteristic ranks, whereas Comparative Examples No. 14 is C content (%) and C
The + N amount (%) was low and the surface hardness was poor. Comparative Example N
o. Sample No. 15 had a high C content (%) and C + N content (%) and was inferior not only in corrosion resistance and toughness due to precipitation of grain boundary carbonitrides, but also inferior in surface hardness due to surface layer residual γ. Comparative Example No. In No. 16, the N content (%) of the matrix was high, bubbles were generated during casting, and the productivity was poor.

Comparative Example No. 17 has a low Cr content (%),
Poor corrosion resistance. Comparative Example No. 18 is Cr content (%)
And the amount of grain boundary carbonitride was large due to the formation of δ ferrite, and the corrosion resistance and toughness were poor. Comparative Example No. 19
Has a high Mo content (%), a large amount of grain boundary carbonitride due to the formation of δ ferrite, and is inferior in corrosion resistance and toughness.

Comparative Example No. 20 has a high Ni content (%),
The surface layer was inferior in hardness due to generation of residual γ in the surface layer. Comparative Example No. 21 has high Ti content (%) and Nb content (%),
The amount of precipitates was large and the toughness was poor. As can be seen from the above examples, the superiority of the steel of the present invention is clear.

Table 3 shows Examples in which the present invention and Comparative Examples are compared with respect to the crystal grain size, carbonitride content, and N content (%) of the surface layer after nitriding. In these examples, steel wires of the components of the steels C and I of the present invention shown in Table 1 were processed into the test pieces, and nitrided and quenched under the conditions shown in Table 3, and then 200 ° C.
Tempered.

In these examples, the hardness, corrosion resistance and toughness of the surface layer described above were evaluated. As is clear from Table 3, all of the examples of the present invention satisfy the above-mentioned characteristic rank, whereas N
o. Sample No. 26 had a small crystal grain size, precipitated grain boundary carbides, and was inferior in corrosion resistance and toughness. No. 27 is the surface N
The amount (%) was low and the corrosion resistance was poor.

No. No. 28 has large crystal grains and is inferior in toughness. No. No. 29 had a small grain size, a large amount of grain boundary carbonitrides, and was inferior in corrosion resistance and toughness. No. In No. 30, the N content (%) of the surface layer was low, and the surface layer was inferior in hardness. No. 31
Had a large grain size and poor toughness. No. Sample No. 32 had a high N content on the surface and was inferior in corrosion resistance and toughness.

Table 4 shows examples in which the present invention and comparative examples are compared with respect to the crystal grain size of the surface layer when the nitriding time is changed. In these examples, a steel wire having the composition of the steel C of the present invention shown in Table 1 was processed into the test piece, nitrided and quenched under the conditions shown in Table 4, and then tempered at 200 ° C.

In these examples, the hardness, corrosion resistance and toughness of the surface layer described above were evaluated. As is clear from Table 4, all of the examples of the present invention satisfy the above-mentioned characteristic rank, whereas
o. No. 35 had a small crystal grain size, a large amount of carbonitride, and was inferior in corrosion resistance and toughness. No. No. 36 had a large crystal grain size and was inferior in toughness.

Table 5 shows examples in which the present invention and comparative examples are compared with respect to the amount of carbonitride in the surface layer when the cooling conditions after nitriding are changed. In these examples, steel wires of the components of the steels C and I of the present invention shown in Table 1 were processed into the test pieces.
Nitriding quenching was performed under the conditions shown in FIG.
Tempered at ° C.

In these examples, the hardness, corrosion resistance and toughness of the surface layer described above were evaluated. As is clear from Table 5, all of the examples of the present invention satisfy the above-mentioned characteristic rank, whereas
o. Nos. 39 and 40 had a large amount of carbonitride and were inferior in corrosion resistance and toughness. As can be seen from the above examples, the superiority of the steel of the present invention is clear.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

As is apparent from the above embodiments, the present invention provides a surface-modified martensitic stainless steel having excellent surface hardness, corrosion resistance and toughness and having a surface-modified structure for construction and building materials at low cost. And is very useful industrially.

[Brief description of the drawings]

FIG. 1 shows the crystal grain size of the surface layer, Cr carbonitride and toughness of the surface layer,
Shows the relationship between corrosion resistance.

FIG. 2 shows the relationship between the crystal grain size of the surface layer, the N content (%) of the surface layer, and the hardness, toughness, and corrosion resistance.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Koichi Yoshimura 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Yoshinobu Tada 3434 Shimada, Hikari-shi, Hikari-shi, Yamaguchi New Nippon Steel Corporation Hikari Works

Claims (6)

[Claims] 1. A composition of a matrix containing, by weight%, C: 0.05 to 0.40%, N: 0.1% or less, Cr: 12.0 to 16.0%, and 0.1% ≦ C + N ≦ 0.4%, the balance being substantially Fe and martensitic stainless steel which is an unavoidable impurity, and having a surface grain size of about 30 to 100 μm;
High corrosion resistance and high toughness martensitic stainless steel. 2. The surface hardness according to claim 1, wherein the amount of carbonitride in the surface layer is 0.7% or less by weight.
High corrosion resistance and high toughness martensitic stainless steel. 3. The N content (%) of the surface layer is 0.2 to 0.2% by weight.
The martensitic stainless steel having a high surface hardness, high corrosion resistance and high toughness according to claim 1 or 2, characterized by being 0.8%. 4. The surface height of the martensitic stainless steel according to claim 1, wherein the composition further comprises 3% by weight or less of Mo. Hardness, high corrosion resistance and high toughness martensitic stainless steel. 5. The martensitic stainless steel according to claim 1, wherein the composition of the steel further contains 3.5% by weight or less of Ni. Martensitic stainless steel with high surface hardness, high corrosion resistance and high toughness. 6. The martensitic stainless steel according to any one of claims 1 to 5, wherein the composition of the steel is 0.2% or less by weight of Ti or 0.2% or less. A high surface hardness, high corrosion resistance and high toughness martensitic stainless steel comprising at least one of the following Nb: